diff --git a/tests/Support/AssertsToolErrors.php b/tests/Support/AssertsToolErrors.php
new file mode 100644
index 0000000..2f8aa66
--- /dev/null
+++ b/tests/Support/AssertsToolErrors.php
@@ -0,0 +1,47 @@
+<?php
+
+declare(strict_types=1);
+
+namespace LIT\MCP\Tests\Support;
+
+use PHPUnit\Framework\Assert;
+use WP_Error;
+
+/**
+ * Assertions on the errors abilities return (Errors::make(): WP_Error code "lit_<code>", the
+ * message carries the client JSON {code, message, hint, fields?, …}).
+ */
+trait AssertsToolErrors
+{
+    /**
+     * Asserts a tool error with $code as the client sees it (without "lit_"): the WP_Error code
+     * and the code in the JSON payload.
+     *
+     * @return array<string, mixed> The client payload.
+     */
+    public static function assertToolError(mixed $result, string $code, string $message = ''): array
+    {
+        Assert::assertInstanceOf(WP_Error::class, $result, $message);
+        Assert::assertSame('lit_' . $code, $result->get_error_code(), $message);
+        $payload = self::clientError($result);
+        Assert::assertSame($code, $payload['code'] ?? null, $message);
+
+        return $payload;
+    }
+
+    /** @return array<string, mixed> The structured error the client sees (message JSON, spec §8.2). */
+    public static function clientError(WP_Error $error): array
+    {
+        return (array) json_decode($error->get_error_message(), true);
+    }
+
+    /** @return array<string, string> The error's fields (input path => problem); asserts it has some. */
+    public static function errorFields(WP_Error $error): array
+    {
+        $data = $error->get_error_data();
+        Assert::assertIsArray($data, 'error data');
+        Assert::assertIsArray($data['fields'] ?? null, 'error fields');
+
+        return $data['fields'];
+    }
+}
diff --git a/tests/Unit/Abilities/AbilityRegistrarTest.php b/tests/Unit/Abilities/AbilityRegistrarTest.php
index 98a43df..feaded8 100644
--- a/tests/Unit/Abilities/AbilityRegistrarTest.php
+++ b/tests/Unit/Abilities/AbilityRegistrarTest.php
@@ -96,7 +96,7 @@ final class AbilityRegistrarTest extends TestCase
         $result = ($this->registrarFor($ability)->argsFor($ability)['permission_callback'])([]);
 
         self::assertInstanceOf(WP_Error::class, $result);
-        self::assertSame('lit_forbidden', $result->get_error_code());
+        self::assertToolError($result, 'forbidden');
     }
 
     public function testPermissionDeniedOutsideOAuthServerRequest(): void
@@ -109,7 +109,7 @@ final class AbilityRegistrarTest extends TestCase
         $result = ($this->registrarFor($ability)->argsFor($ability)['permission_callback'])([]);
 
         self::assertInstanceOf(WP_Error::class, $result);
-        self::assertSame('lit_forbidden', $result->get_error_code());
+        self::assertToolError($result, 'forbidden');
     }
 
     public function testPermissionDelegatesToAbilityWhenGatePasses(): void
@@ -149,7 +149,7 @@ final class AbilityRegistrarTest extends TestCase
         }
 
         self::assertInstanceOf(WP_Error::class, $result);
-        self::assertSame('lit_internal_error', $result->get_error_code());
+        self::assertToolError($result, 'internal_error');
         self::assertStringNotContainsString('/var/www', $result->get_error_message());
         self::assertStringContainsString('lit/test failed', (string) file_get_contents($log));
         unlink($log);
@@ -170,7 +170,7 @@ final class AbilityRegistrarTest extends TestCase
         }
 
         self::assertInstanceOf(WP_Error::class, $result);
-        self::assertSame('lit_internal_error', $result->get_error_code());
+        self::assertToolError($result, 'internal_error');
         self::assertStringNotContainsString('/var/www', $result->get_error_message());
         self::assertStringContainsString('lit/test failed', (string) file_get_contents($log));
         unlink($log);
@@ -196,7 +196,7 @@ final class AbilityRegistrarTest extends TestCase
             ini_set('error_log', (string) $previous);
         }
 
-        self::assertSame('lit_partial_write', $result->get_error_code());
+        self::assertToolError($result, 'partial_write');
         self::assertStringNotContainsString('/var/www', $result->get_error_message());
         self::assertStringContainsString('meta /var/www', (string) file_get_contents($log));
         unlink($log);
diff --git a/tests/Unit/Abilities/CursorTest.php b/tests/Unit/Abilities/CursorTest.php
index 1b819e8..fae2be8 100644
--- a/tests/Unit/Abilities/CursorTest.php
+++ b/tests/Unit/Abilities/CursorTest.php
@@ -46,7 +46,7 @@ final class CursorTest extends TestCase
 
     public function testInvalidCursorErrorIsAValidationError(): void
     {
-        self::assertSame('lit_validation_failed', Cursor::invalid()->get_error_code());
+        self::assertToolError(Cursor::invalid(), 'validation_failed');
     }
 
     public function testInputPropertiesDescribeLimitAndCursor(): void
diff --git a/tests/Unit/Abilities/WriteFailedTest.php b/tests/Unit/Abilities/WriteFailedTest.php
index b4fd958..3d14b42 100644
--- a/tests/Unit/Abilities/WriteFailedTest.php
+++ b/tests/Unit/Abilities/WriteFailedTest.php
@@ -18,7 +18,7 @@ final class WriteFailedTest extends TestCase
         self::assertTrue($failure->rolledBack);
         self::assertSame($cause, $failure->getPrevious());
         self::assertStringContainsString('meta kaputt', $failure->getMessage());
-        self::assertSame('lit_write_failed', $failure->toError()->get_error_code());
+        self::assertToolError($failure->toError(), 'write_failed');
     }
 
     public function testRolledBackHintReplacesTheDefaultHintOfWriteFailedOnly(): void
@@ -35,7 +35,7 @@ final class WriteFailedTest extends TestCase
     {
         $error = WriteFailed::partial('Felder konnten nicht gespeichert werden.', postId: 5, revisionId: 811)->toError();
 
-        self::assertSame('lit_partial_write', $error->get_error_code());
+        self::assertToolError($error, 'partial_write');
         self::assertSame(5, $error->get_error_data()['post_id']);
         self::assertSame(811, $error->get_error_data()['revision_id']);
     }
@@ -49,7 +49,7 @@ final class WriteFailedTest extends TestCase
             details: ['menu_id' => 3]
         )->toError();
 
-        self::assertSame('lit_partial_write', $error->get_error_code());
+        self::assertToolError($error, 'partial_write');
         self::assertStringContainsString('Das Menü ist teilweise geändert.', $error->get_error_message());
         self::assertStringNotContainsString('Entwurf', $error->get_error_message());
         self::assertSame('Mit lit/menu-get prüfen.', $error->get_error_data()['hint']);
@@ -71,7 +71,7 @@ final class WriteFailedTest extends TestCase
     {
         $error = WriteFailed::rolledBack('Nicht übernommen.', details: ['backup_id' => 88])->toError();
 
-        self::assertSame('lit_write_failed', $error->get_error_code());
+        self::assertToolError($error, 'write_failed');
         self::assertSame(88, $error->get_error_data()['backup_id']);
         self::assertStringContainsString('"backup_id":88', $error->get_error_message());
     }
diff --git a/tests/Unit/Integrations/Core/GuideAbilityTest.php b/tests/Unit/Integrations/Core/GuideAbilityTest.php
index 24c9387..a669e86 100644
--- a/tests/Unit/Integrations/Core/GuideAbilityTest.php
+++ b/tests/Unit/Integrations/Core/GuideAbilityTest.php
@@ -40,7 +40,7 @@ final class GuideAbilityTest extends TestCase
         $result = $this->ability->execute(['topic' => '../../wp-config']);
 
         self::assertInstanceOf(WP_Error::class, $result);
-        self::assertSame('lit_not_found', $result->get_error_code());
+        self::assertToolError($result, 'not_found');
         self::assertStringContainsString('divi-shortform', $result->get_error_message());
     }
 
diff --git a/tests/Unit/Integrations/Divi/Design/DiviDesignSystemAbilityTest.php b/tests/Unit/Integrations/Divi/Design/DiviDesignSystemAbilityTest.php
index 3a5da0b..6ef4d0e 100644
--- a/tests/Unit/Integrations/Divi/Design/DiviDesignSystemAbilityTest.php
+++ b/tests/Unit/Integrations/Divi/Design/DiviDesignSystemAbilityTest.php
@@ -39,6 +39,6 @@ final class DiviDesignSystemAbilityTest extends TestCase
 
         $result = (new DiviDesignSystemAbility(Mockery::mock(DesignSystemReader::class)))->permission([]);
 
-        self::assertSame('lit_forbidden', $result->get_error_code());
+        self::assertToolError($result, 'forbidden');
     }
 }
diff --git a/tests/Unit/Integrations/Divi/Pages/DiviContentComposerTest.php b/tests/Unit/Integrations/Divi/Pages/DiviContentComposerTest.php
index 881d169..d46eb8b 100644
--- a/tests/Unit/Integrations/Divi/Pages/DiviContentComposerTest.php
+++ b/tests/Unit/Integrations/Divi/Pages/DiviContentComposerTest.php
@@ -97,16 +97,16 @@ final class DiviContentComposerTest extends TestCase
     {
         $disabled = $this->composer(false)->check('post', [['type' => 'section']]);
         self::assertInstanceOf(WP_Error::class, $disabled);
-        self::assertSame('lit_validation_failed', $disabled->get_error_code());
-        self::assertArrayHasKey('divi', $disabled->get_error_data()['fields']);
+        self::assertToolError($disabled, 'validation_failed');
+        self::assertArrayHasKey('divi', self::errorFields($disabled));
 
         $invalid = $this->composer()->check('page', [
             ['type' => 'section', 'modules' => [['type' => 'button', 'attrs' => ['x.y' => ['desktop' => 1]]]]],
             ['type' => 'section', 'modules' => [['type' => 'button', 'url' => 'javascript:alert(1)', 'text' => 'x']]],
         ]);
         self::assertInstanceOf(WP_Error::class, $invalid);
-        self::assertSame('lit_invalid_attr_path', $invalid->get_error_code());
-        $fields = $invalid->get_error_data()['fields'];
+        self::assertToolError($invalid, 'invalid_attr_path');
+        $fields = self::errorFields($invalid);
         self::assertArrayHasKey('divi[0].modules[0].modules[0][0].attrs.x.y', $fields);
         self::assertGreaterThanOrEqual(2, count($fields), 'every error of every section comes back');
         self::assertSame([], $this->saved);
@@ -130,7 +130,7 @@ final class DiviContentComposerTest extends TestCase
         $written = $this->composer(true, $registry)->write(42, 'page', $sections);
 
         self::assertInstanceOf(WP_Error::class, $written);
-        self::assertSame('lit_validation_failed', $written->get_error_code());
+        self::assertToolError($written, 'validation_failed');
         self::assertSame([], $this->saved);
     }
 
diff --git a/tests/Unit/Integrations/Divi/Pages/DiviPageLoaderTest.php b/tests/Unit/Integrations/Divi/Pages/DiviPageLoaderTest.php
index 1afb1c6..46739de 100644
--- a/tests/Unit/Integrations/Divi/Pages/DiviPageLoaderTest.php
+++ b/tests/Unit/Integrations/Divi/Pages/DiviPageLoaderTest.php
@@ -34,7 +34,7 @@ final class DiviPageLoaderTest extends TestCase
         $result = (new DiviPageLoader(DiviFixtures::schema()))->load(7, false);
 
         self::assertInstanceOf(WP_Error::class, $result);
-        self::assertSame('lit_unsupported', $result->get_error_code());
+        self::assertToolError($result, 'unsupported');
         self::assertStringContainsString('Divi-4', $result->get_error_message());
     }
 
@@ -50,7 +50,7 @@ final class DiviPageLoaderTest extends TestCase
             ini_set('error_log', (string) $previous);
         }
 
-        self::assertSame('lit_unsupported', $result->get_error_code());
+        self::assertToolError($result, 'unsupported');
         self::assertStringContainsString('kein gültiges Block-Markup', $result->get_error_message());
         self::assertStringContainsString('divi parse of post 7 failed', (string) file_get_contents($log));
         unlink($log);
@@ -62,7 +62,7 @@ final class DiviPageLoaderTest extends TestCase
         $loader = new DiviPageLoader(DiviFixtures::schema());
 
         self::assertInstanceOf(DiviPage::class, $loader->load(7, false));
-        self::assertSame('lit_validation_failed', $loader->load(7, true)->get_error_code());
+        self::assertToolError($loader->load(7, true), 'validation_failed');
     }
 
     public function testWritableChecks(): void
@@ -70,7 +70,7 @@ final class DiviPageLoaderTest extends TestCase
         $loader = new DiviPageLoader(DiviFixtures::schema(false));
         DiviFixtures::givenPost('');
         $empty = $loader->load(7, true);
-        self::assertSame('lit_unsupported', $loader->writable($empty)->get_error_code());
+        self::assertToolError($loader->writable($empty), 'unsupported');
 
         $loader = new DiviPageLoader(DiviFixtures::schema());
         DiviFixtures::givenPost("<!-- wp:paragraph -->\n<p>Gutenberg</p>\n<!-- /wp:paragraph -->");
@@ -81,6 +81,6 @@ final class DiviPageLoaderTest extends TestCase
         DiviFixtures::givenPost('', ['post_status' => 'future', 'post_date_gmt' => '2020-01-01 00:00:00']);
         $missed = $loader->load(7, true);
         self::assertTrue($missed->isDiviLayout());
-        self::assertSame('lit_validation_failed', $loader->writable($missed)->get_error_code());
+        self::assertToolError($loader->writable($missed), 'validation_failed');
     }
 }
diff --git a/tests/Unit/Integrations/Divi/Pages/DiviReadAbilitiesTest.php b/tests/Unit/Integrations/Divi/Pages/DiviReadAbilitiesTest.php
index c748c83..11e7f0d 100644
--- a/tests/Unit/Integrations/Divi/Pages/DiviReadAbilitiesTest.php
+++ b/tests/Unit/Integrations/Divi/Pages/DiviReadAbilitiesTest.php
@@ -65,7 +65,7 @@ final class DiviReadAbilitiesTest extends TestCase
         self::assertSame(['s1', 's1.r0'], array_column($result['nodes'], 'id'));
 
         $missing = $ability->execute(['post_id' => 7, 'id' => 's7']);
-        self::assertSame('lit_not_found', $missing->get_error_code());
+        self::assertToolError($missing, 'not_found');
         self::assertStringContainsString('tree_rev', $missing->get_error_message());
     }
 
@@ -120,7 +120,7 @@ final class DiviReadAbilitiesTest extends TestCase
         DiviFixtures::givenPost(DiviFixtures::markup('synthetic-basic.html'));
         $ability = new DiviNodeGetAbility(new DiviPageLoader(DiviFixtures::schema()), $this->design());
 
-        self::assertSame('lit_validation_failed', $ability->execute(['post_id' => 7, 'id' => 'hero'])->get_error_code());
-        self::assertSame('lit_not_found', $ability->execute(['post_id' => 7, 'id' => 's5.r0'])->get_error_code());
+        self::assertToolError($ability->execute(['post_id' => 7, 'id' => 'hero']), 'validation_failed');
+        self::assertToolError($ability->execute(['post_id' => 7, 'id' => 's5.r0']), 'not_found');
     }
 }
diff --git a/tests/Unit/Integrations/Divi/Pages/DiviReplaceTextAbilityTest.php b/tests/Unit/Integrations/Divi/Pages/DiviReplaceTextAbilityTest.php
index ba92f14..bcdb0c8 100644
--- a/tests/Unit/Integrations/Divi/Pages/DiviReplaceTextAbilityTest.php
+++ b/tests/Unit/Integrations/Divi/Pages/DiviReplaceTextAbilityTest.php
@@ -107,12 +107,6 @@ final class DiviReplaceTextAbilityTest extends TestCase
         return $block === null ? null : AttrMap::desktop($block->attrs(), $path);
     }
 
-    /** @return array<string, mixed> */
-    private static function clientError(WP_Error $error): array
-    {
-        return (array) json_decode($error->get_error_message(), true);
-    }
-
     public function testDryRunListsHitsWithTreeRevAndChangesNothing(): void
     {
         $ability = $this->ability();
@@ -177,13 +171,13 @@ final class DiviReplaceTextAbilityTest extends TestCase
     {
         $ability = $this->ability();
 
-        self::assertSame('lit_validation_failed', $ability->execute(['search' => 'Termin', 'replace' => 'X', 'dry_run' => false])->get_error_code());
-        self::assertSame('lit_validation_failed', $ability->execute([
+        self::assertToolError($ability->execute(['search' => 'Termin', 'replace' => 'X', 'dry_run' => false]), 'validation_failed');
+        self::assertToolError($ability->execute([
             'search' => 'Termin', 'replace' => 'X', 'dry_run' => false, 'confirm' => true, 'targets' => [['post_id' => 7]],
-        ])->get_error_code());
-        self::assertSame('lit_confirm_required', $ability->execute([
+        ]), 'validation_failed');
+        self::assertToolError($ability->execute([
             'search' => 'Termin', 'replace' => 'X', 'dry_run' => false, 'targets' => [['post_id' => 7, 'tree_rev' => 'abc']],
-        ])->get_error_code());
+        ]), 'confirm_required');
         self::assertSame([], $this->saved);
     }
 
@@ -225,9 +219,9 @@ final class DiviReplaceTextAbilityTest extends TestCase
         $short = $ability->execute(['search' => 'T', 'replace' => 'X']);
         $reference = $ability->execute(['search' => 'Termin', 'replace' => self::REF, 'dry_run' => false, 'confirm' => true, 'targets' => [['post_id' => 7, 'tree_rev' => 'x']]]);
 
-        self::assertSame('lit_validation_failed', $short->get_error_code());
+        self::assertToolError($short, 'validation_failed');
         self::assertArrayHasKey('search', self::clientError($short)['fields']);
-        self::assertSame('lit_validation_failed', $reference->get_error_code());
+        self::assertToolError($reference, 'validation_failed');
         self::assertArrayHasKey('replace', self::clientError($reference)['fields']);
         self::assertSame([], $this->queries);
         self::assertSame([], $this->saved);
@@ -357,7 +351,7 @@ final class DiviReplaceTextAbilityTest extends TestCase
 
         self::assertSame([], $this->saved);
         if ($result instanceof WP_Error) {
-            self::assertSame('lit_validation_failed', $result->get_error_code());
+            self::assertToolError($result, 'validation_failed');
             self::assertInstanceOf(WP_Error::class, $dry);
 
             return;
@@ -454,7 +448,7 @@ final class DiviReplaceTextAbilityTest extends TestCase
 
         foreach (['', "Ter\0min", "Ter\x0Bmin"] as $search) {
             $error = $ability->execute(['search' => $search, 'replace' => 'X']);
-            self::assertSame('lit_validation_failed', $error->get_error_code(), json_encode($search));
+            self::assertToolError($error, 'validation_failed', json_encode($search));
             self::assertArrayHasKey('search', self::clientError($error)['fields']);
         }
 
@@ -478,7 +472,7 @@ final class DiviReplaceTextAbilityTest extends TestCase
         self::assertStringContainsString('LIMIT 3 OFFSET 0', $this->queries[0]);
         self::assertStringContainsString('LIMIT 3 OFFSET 2', $this->queries[1]);
         self::assertIsString($second['next_cursor'], 'the fake query returns three rows again');
-        self::assertSame('lit_validation_failed', $invalid->get_error_code());
+        self::assertToolError($invalid, 'validation_failed');
         self::assertStringContainsString('LIMIT 101 OFFSET 0', $this->queries[2]);
         self::assertCount(3, $capped['matches']);
     }
diff --git a/tests/Unit/Integrations/Divi/Pages/DiviSaverTest.php b/tests/Unit/Integrations/Divi/Pages/DiviSaverTest.php
index a7d7498..d6840c2 100644
--- a/tests/Unit/Integrations/Divi/Pages/DiviSaverTest.php
+++ b/tests/Unit/Integrations/Divi/Pages/DiviSaverTest.php
@@ -171,7 +171,7 @@ final class DiviSaverTest extends TestCase
             self::fail('WriteFailed expected');
         } catch (WriteFailed $e) {
             self::assertFalse($e->rolledBack);
-            self::assertSame('lit_partial_write', $e->toError()->get_error_code());
+            self::assertToolError($e->toError(), 'partial_write');
         }
     }
 
diff --git a/tests/Unit/Integrations/Divi/Pages/DiviWriteAbilitiesTest.php b/tests/Unit/Integrations/Divi/Pages/DiviWriteAbilitiesTest.php
index 4092095..3678ef2 100644
--- a/tests/Unit/Integrations/Divi/Pages/DiviWriteAbilitiesTest.php
+++ b/tests/Unit/Integrations/Divi/Pages/DiviWriteAbilitiesTest.php
@@ -71,12 +71,6 @@ final class DiviWriteAbilitiesTest extends TestCase
         return new DiviDuplicateNodeAbility($this->writer);
     }
 
-    /** @return array<string, mixed> The structured error the client sees (message JSON, spec §8.2). */
-    private static function clientError(WP_Error $error): array
-    {
-        return (array) json_decode($error->get_error_message(), true);
-    }
-
     public function testInsertComposesAndSavesWithTreeRevAndPostRev(): void
     {
         $result = $this->insert()->execute([
@@ -106,7 +100,7 @@ final class DiviWriteAbilitiesTest extends TestCase
     public function testStaleTreeRevAndInvalidNodesWriteNothing(): void
     {
         $stale = $this->insert()->execute(['post_id' => 7, 'tree_rev' => '000000000000', 'nodes' => [['type' => 'section']]]);
-        self::assertSame('lit_rev_mismatch', $stale->get_error_code());
+        self::assertToolError($stale, 'rev_mismatch');
         self::assertSame('rev_mismatch', self::clientError($stale)['code'], 'the client sees the code without lit_');
         self::assertStringContainsString('lit-divi-outline', $stale->get_error_data()['hint']);
         self::assertSame($stale->get_error_code(), $this->insert()->permission(['post_id' => 7, 'tree_rev' => 'x'])->get_error_code());
@@ -120,7 +114,7 @@ final class DiviWriteAbilitiesTest extends TestCase
                 ['type' => 'button', 'url' => 'javascript:alert(1)', 'text' => 'x', 'nope' => 1],
             ],
         ]);
-        self::assertSame('lit_invalid_attr_path', $invalid->get_error_code());
+        self::assertToolError($invalid, 'invalid_attr_path');
         $client = self::clientError($invalid);
         self::assertSame('invalid_attr_path', $client['code']);
         self::assertArrayHasKey('nodes[0].attrs.title.fnt', $client['fields']);
@@ -128,7 +122,7 @@ final class DiviWriteAbilitiesTest extends TestCase
         self::assertStringContainsString('lit-divi-catalog', $client['hint']);
 
         $wrongPlace = $this->insert()->execute(['post_id' => 7, 'tree_rev' => $this->treeRev(), 'nodes' => [['type' => 'button']]]);
-        self::assertSame('lit_validation_failed', $wrongPlace->get_error_code());
+        self::assertToolError($wrongPlace, 'validation_failed');
 
         $badPosition = $this->insert()->execute([
             'post_id' => 7,
@@ -137,7 +131,7 @@ final class DiviWriteAbilitiesTest extends TestCase
             'after' => 's0.r0.c1.m1',
             'nodes' => [['type' => 'text']],
         ]);
-        self::assertSame('lit_validation_failed', $badPosition->get_error_code());
+        self::assertToolError($badPosition, 'validation_failed');
         self::assertArrayHasKey('position', self::clientError($badPosition)['fields']);
 
         $unknown = $this->insert()->execute([
@@ -146,7 +140,7 @@ final class DiviWriteAbilitiesTest extends TestCase
             'parent' => 's9',
             'nodes' => [['type' => 'section']],
         ]);
-        self::assertSame('lit_validation_failed', $unknown->get_error_code());
+        self::assertToolError($unknown, 'validation_failed');
         self::assertSame([], $this->saved);
         self::assertSame($this->original, $this->post->post_content);
     }
@@ -177,7 +171,7 @@ final class DiviWriteAbilitiesTest extends TestCase
             ['id' => 's0.r0.c1.m0', 'set' => ['level' => 'h9']],
             ['id' => 's0.r0.c1.m9', 'label' => 'x'],
         ]]);
-        self::assertSame('lit_validation_failed', $failed->get_error_code());
+        self::assertToolError($failed, 'validation_failed');
         $fields = self::clientError($failed)['fields'];
         self::assertArrayHasKey('changes[1].set.level', $fields);
         self::assertArrayHasKey('changes[2].id', $fields, 'every error of the call in one response (S39)');
@@ -220,7 +214,7 @@ final class DiviWriteAbilitiesTest extends TestCase
             'changes' => [['id' => 's0.r0.c1.m2', 'label' => 'x']],
         ]);
 
-        self::assertSame('lit_rev_mismatch', $result->get_error_code());
+        self::assertToolError($result, 'rev_mismatch');
         self::assertSame([], $this->saved);
     }
 
@@ -251,14 +245,14 @@ final class DiviWriteAbilitiesTest extends TestCase
         $intoSection = $this->move()->execute([
             'post_id' => 7, 'tree_rev' => $treeRev, 'id' => 's0.c0.m0', 'parent' => 's1',
         ]);
-        self::assertSame('lit_validation_failed', $intoSection->get_error_code());
+        self::assertToolError($intoSection, 'validation_failed');
         self::assertStringContainsString('Zeilen (row)', self::clientError($intoSection)['fields']['position']);
 
         // s0.c1 is a specialty column (specialtyColumns set, holds a row-inner): refuses every child.
         $intoSpecialtyColumn = $this->move()->execute([
             'post_id' => 7, 'tree_rev' => $treeRev, 'id' => 's0.c0.m0', 'parent' => 's0.c1',
         ]);
-        self::assertSame('lit_validation_failed', $intoSpecialtyColumn->get_error_code());
+        self::assertToolError($intoSpecialtyColumn, 'validation_failed');
         self::assertStringContainsString('Spezialspalten', self::clientError($intoSpecialtyColumn)['fields']['position']);
 
         self::assertSame([], $this->saved);
@@ -271,7 +265,7 @@ final class DiviWriteAbilitiesTest extends TestCase
             'post_id' => 7, 'tree_rev' => $this->treeRev(), 'id' => 's0', 'parent' => 's0.r0.c1',
         ]);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
         self::assertStringContainsString(
             'nicht in sich selbst verschoben',
             self::clientError($result)['fields']['position']
@@ -285,7 +279,7 @@ final class DiviWriteAbilitiesTest extends TestCase
             'post_id' => 7, 'tree_rev' => '000000000000', 'id' => 's0.r0.c1.m2', 'parent' => 's1.r0.c0',
         ]);
 
-        self::assertSame('lit_rev_mismatch', $result->get_error_code());
+        self::assertToolError($result, 'rev_mismatch');
         self::assertSame('rev_mismatch', self::clientError($result)['code'], 'the client sees the code without lit_');
         self::assertSame([], $this->saved);
     }
@@ -349,7 +343,7 @@ final class DiviWriteAbilitiesTest extends TestCase
             'post_id' => 7, 'tree_rev' => $this->treeRev(), 'ids' => ['s9'], 'confirm' => true,
         ]);
 
-        self::assertSame('lit_not_found', $result->get_error_code());
+        self::assertToolError($result, 'not_found');
         self::assertSame('not_found', self::clientError($result)['code']);
         self::assertSame([], $this->saved);
     }
@@ -371,10 +365,10 @@ final class DiviWriteAbilitiesTest extends TestCase
 
         $delete = $this->delete();
         $ask = $delete->execute(['post_id' => 7, 'tree_rev' => Rev::of($this->saved[0]), 'ids' => ['s2', 's2.r0']]);
-        self::assertSame('lit_confirm_required', $ask->get_error_code());
+        self::assertToolError($ask, 'confirm_required');
         self::assertSame(
             '1 Divi-Knoten würden samt Inhalt entfernt: s2 section "Leistungen (Kopie)" (1 Kind-Knoten).',
-            json_decode($ask->get_error_message(), true)['message']
+            self::clientError($ask)['message']
         );
 
         $deleted = $delete->execute(['post_id' => 7, 'tree_rev' => Rev::of($this->saved[0]), 'ids' => ['s2'], 'confirm' => true]);
@@ -490,7 +484,7 @@ final class DiviWriteAbilitiesTest extends TestCase
         foreach ([$this->move(), $this->delete(), $this->duplicate()] as $ability) {
             $result = $ability->permission(['post_id' => 7, 'tree_rev' => $this->treeRev()]);
             self::assertInstanceOf(WP_Error::class, $result, $ability->name());
-            self::assertSame('lit_forbidden', $result->get_error_code(), $ability->name());
+            self::assertToolError($result, 'forbidden', $ability->name());
         }
         self::assertSame([], $this->saved);
     }
@@ -505,7 +499,7 @@ final class DiviWriteAbilitiesTest extends TestCase
             'changes' => [['id' => 's1.g1', 'label' => 'x']],
         ]);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
         self::assertStringContainsString('nur lesbar', $result->get_error_message());
         self::assertSame([], $this->saved);
     }
@@ -518,7 +512,7 @@ final class DiviWriteAbilitiesTest extends TestCase
             ['id' => 's0.r0.c1.m2', 'set' => ['url' => '/zwei']],
         ]]);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
         self::assertStringContainsString('changes[0]', self::clientError($result)['fields']['changes[1].id']);
         self::assertSame([], $this->saved);
     }
@@ -532,7 +526,7 @@ final class DiviWriteAbilitiesTest extends TestCase
             'attrs' => ['button.innerContent' => ['desktop' => ['linkUrl' => '/neu']]],
         ]]]);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
         self::assertArrayHasKey('changes[0].attrs.button.innerContent', self::clientError($result)['fields']);
         self::assertSame([], $this->saved);
     }
@@ -551,7 +545,7 @@ final class DiviWriteAbilitiesTest extends TestCase
             ]],
         ]);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
         self::assertArrayHasKey('nodes[0].attrs.button.innerContent', self::clientError($result)['fields']);
         self::assertSame([], $this->saved);
     }
@@ -564,7 +558,7 @@ final class DiviWriteAbilitiesTest extends TestCase
         $broken = $this->update()->execute(['post_id' => 7, 'tree_rev' => $this->treeRev(), 'changes' => [
             ['id' => 's0.r0.c0', 'attrs' => $sizing('12_24')],
         ]]);
-        self::assertSame('lit_validation_failed', $broken->get_error_code());
+        self::assertToolError($broken, 'validation_failed');
         self::assertArrayHasKey('changes[0].attrs.module.decoration.sizing', self::clientError($broken)['fields']);
         self::assertSame([], $this->saved);
 
@@ -636,9 +630,9 @@ final class DiviWriteAbilitiesTest extends TestCase
     public function testUpdateKeepsExactRowsStrict(): void
     {
         $broken = $this->updateOn($this->original, [['id' => 's0.r0.c0', 'attrs' => self::sizing(null)]]);
-        self::assertSame('lit_validation_failed', $broken->get_error_code());
+        self::assertToolError($broken, 'validation_failed');
         $broken = $this->updateOn($this->original, [['id' => 's0.r0.c0', 'attrs' => self::sizing('10_24')]]);
-        self::assertSame('lit_validation_failed', $broken->get_error_code());
+        self::assertToolError($broken, 'validation_failed');
         self::assertSame([], $this->saved);
     }
 
@@ -650,7 +644,7 @@ final class DiviWriteAbilitiesTest extends TestCase
             'attrs' => ['module.decoration.attributes' => ['desktop' => ['attributes' => [['name' => 'onclick', 'value' => 'x']]]]],
         ]]]);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
         $client = self::clientError($result);
         self::assertSame('validation_failed', $client['code']);
         self::assertStringContainsString('nur im Divi-Builder', $client['fields']['changes[0].attrs.module.decoration.attributes']);
diff --git a/tests/Unit/Integrations/Divi/Pages/DiviWriterTest.php b/tests/Unit/Integrations/Divi/Pages/DiviWriterTest.php
index 28b338c..89ca407 100644
--- a/tests/Unit/Integrations/Divi/Pages/DiviWriterTest.php
+++ b/tests/Unit/Integrations/Divi/Pages/DiviWriterTest.php
@@ -54,9 +54,9 @@ final class DiviWriterTest extends TestCase
         $writer = $this->writer(Mockery::mock(DiviSaver::class), FakeChangeLog::forbidden());
 
         self::assertInstanceOf(DiviPage::class, $writer->page(['post_id' => 7, 'tree_rev' => Rev::of($content)]));
-        self::assertSame('lit_rev_mismatch', $writer->page(['post_id' => 7])->get_error_code());
+        self::assertToolError($writer->page(['post_id' => 7]), 'rev_mismatch');
         $stale = $writer->page(['post_id' => 7, 'tree_rev' => Rev::ofPost(DiviFixtures::givenPost($content))]);
-        self::assertSame('lit_rev_mismatch', $stale->get_error_code(), 'the post rev is no tree_rev');
+        self::assertToolError($stale, 'rev_mismatch', 'the post rev is no tree_rev');
         self::assertStringContainsString('nichts wurde', $stale->get_error_message());
     }
 
@@ -72,7 +72,7 @@ final class DiviWriterTest extends TestCase
             Mockery::mock(DiviComposition::class)
         );
 
-        self::assertSame('lit_unsupported', $writer->page(['post_id' => 7, 'tree_rev' => Rev::of($content)])->get_error_code());
+        self::assertToolError($writer->page(['post_id' => 7, 'tree_rev' => Rev::of($content)]), 'unsupported');
     }
 
     public function testSectionLabelOfANode(): void
diff --git a/tests/Unit/Integrations/Divi/Schema/DiviCatalogAbilityTest.php b/tests/Unit/Integrations/Divi/Schema/DiviCatalogAbilityTest.php
index 50cf66b..21836c4 100644
--- a/tests/Unit/Integrations/Divi/Schema/DiviCatalogAbilityTest.php
+++ b/tests/Unit/Integrations/Divi/Schema/DiviCatalogAbilityTest.php
@@ -78,10 +78,10 @@ final class DiviCatalogAbilityTest extends TestCase
 
     public function testUnknownModuleAndPermission(): void
     {
-        self::assertSame('lit_not_found', $this->ability()->execute(['module' => 'slider-pro'])->get_error_code());
+        self::assertToolError($this->ability()->execute(['module' => 'slider-pro']), 'not_found');
 
         Functions\when('current_user_can')->justReturn(false);
-        self::assertSame('lit_forbidden', $this->ability()->permission([])->get_error_code());
+        self::assertToolError($this->ability()->permission([]), 'forbidden');
         self::assertTrue($this->ability()->annotations()['readonly']);
         self::assertTrue($this->ability()->isDirect());
     }
diff --git a/tests/Unit/Integrations/Media/MediaDeleteAbilityTest.php b/tests/Unit/Integrations/Media/MediaDeleteAbilityTest.php
index 8917ae3..9862cff 100644
--- a/tests/Unit/Integrations/Media/MediaDeleteAbilityTest.php
+++ b/tests/Unit/Integrations/Media/MediaDeleteAbilityTest.php
@@ -41,7 +41,7 @@ final class MediaDeleteAbilityTest extends TestCase
 
         $result = $this->ability(self::USAGE, FakeChangeLog::forbidden())->execute(['id' => 33]);
 
-        self::assertSame('lit_confirm_required', $result->get_error_code());
+        self::assertToolError($result, 'confirm_required');
         self::assertStringContainsString('endgültig', $result->get_error_message());
         self::assertStringContainsString('1 Stelle', $result->get_error_message());
         self::assertSame(self::USAGE, $result->get_error_data()['usages']);
@@ -51,7 +51,7 @@ final class MediaDeleteAbilityTest extends TestCase
     {
         $result = $this->ability([], FakeChangeLog::forbidden(), true)->execute(['id' => 33, 'confirm' => false]);
 
-        self::assertSame('lit_confirm_required', $result->get_error_code());
+        self::assertToolError($result, 'confirm_required');
         self::assertStringContainsString('Papierkorb', $result->get_error_message());
     }
 
@@ -61,7 +61,7 @@ final class MediaDeleteAbilityTest extends TestCase
 
         $result = $this->ability(self::USAGE, FakeChangeLog::forbidden())->execute(['id' => 33, 'confirm' => true]);
 
-        self::assertSame('lit_in_use', $result->get_error_code());
+        self::assertToolError($result, 'in_use');
         self::assertSame(self::USAGE, $result->get_error_data()['usages']);
     }
 
@@ -71,7 +71,7 @@ final class MediaDeleteAbilityTest extends TestCase
 
         $result = $this->ability(self::POSSIBLE, FakeChangeLog::forbidden())->execute(['id' => 33, 'confirm' => true]);
 
-        self::assertSame('lit_in_use', $result->get_error_code());
+        self::assertToolError($result, 'in_use');
     }
 
     public function testForceDeletesUsedMedia(): void
@@ -115,7 +115,7 @@ final class MediaDeleteAbilityTest extends TestCase
 
         $result = $this->ability([], FakeChangeLog::forbidden())->execute(['id' => 33, 'confirm' => true]);
 
-        self::assertSame('lit_write_failed', $result->get_error_code());
+        self::assertToolError($result, 'write_failed');
     }
 
     public function testRefusedDeleteThatRemovedDataIsAPartialWrite(): void
@@ -133,7 +133,7 @@ final class MediaDeleteAbilityTest extends TestCase
 
         $result = $this->ability([], $log)->execute(['id' => 33, 'confirm' => true]);
 
-        self::assertSame('lit_partial_write', $result->get_error_code());
+        self::assertToolError($result, 'partial_write');
         self::assertSame(33, $result->get_error_data()['attachment_id']);
         $log->assertPartials(['ability' => 'lit/media-delete', 'object_type' => 'attachment', 'object_id' => '33']);
     }
@@ -162,7 +162,7 @@ final class MediaDeleteAbilityTest extends TestCase
             unlink($file);
         }
 
-        self::assertSame('lit_in_use', $result->get_error_code());
+        self::assertToolError($result, 'in_use');
         self::assertSame(MediaUsageFinder::FAILED_TITLE, $result->get_error_data()['usages'][0]['title']);
     }
 
@@ -193,8 +193,8 @@ final class MediaDeleteAbilityTest extends TestCase
 
         $ability = $this->ability([], FakeChangeLog::forbidden());
 
-        self::assertSame('lit_validation_failed', $ability->permission(['id' => 33])->get_error_code());
-        self::assertSame('lit_validation_failed', $ability->execute(['id' => 33, 'confirm' => true])->get_error_code());
+        self::assertToolError($ability->permission(['id' => 33]), 'validation_failed');
+        self::assertToolError($ability->execute(['id' => 33, 'confirm' => true]), 'validation_failed');
     }
 
     public function testChecksDeletePostOnTheAttachment(): void
@@ -203,7 +203,7 @@ final class MediaDeleteAbilityTest extends TestCase
 
         $result = $this->ability([], FakeChangeLog::forbidden())->permission(['id' => 33]);
 
-        self::assertSame('lit_forbidden', $result->get_error_code());
+        self::assertToolError($result, 'forbidden');
     }
 
     public function testIsDestructiveAndDescribesWhatTheUsageCheckMisses(): void
diff --git a/tests/Unit/Integrations/Media/MediaDownloadAbilityTest.php b/tests/Unit/Integrations/Media/MediaDownloadAbilityTest.php
index 7817abf..d6edfbc 100644
--- a/tests/Unit/Integrations/Media/MediaDownloadAbilityTest.php
+++ b/tests/Unit/Integrations/Media/MediaDownloadAbilityTest.php
@@ -59,7 +59,7 @@ final class MediaDownloadAbilityTest extends TestCase
 
         $result = (new MediaDownloadAbility(new Limits()))->execute(['id' => 40]);
 
-        self::assertSame('lit_unsupported', $result->get_error_code());
+        self::assertToolError($result, 'unsupported');
     }
 
     public function testDownloadsAreCappedAtTenMegabytesEvenWithAHigherUploadLimit(): void
@@ -69,7 +69,7 @@ final class MediaDownloadAbilityTest extends TestCase
 
         $result = (new MediaDownloadAbility(new Limits()))->execute(['id' => 40]);
 
-        self::assertSame('lit_unsupported', $result->get_error_code());
+        self::assertToolError($result, 'unsupported');
         self::assertStringContainsString('Limit 10 MB', $result->get_error_message());
         self::assertStringContainsString('URL', (string) $result->get_error_data()['hint']);
     }
@@ -81,7 +81,7 @@ final class MediaDownloadAbilityTest extends TestCase
 
         $result = (new MediaDownloadAbility(new Limits()))->execute(['id' => 40, 'size' => 'huge']);
 
-        self::assertSame('lit_not_found', $result->get_error_code());
+        self::assertToolError($result, 'not_found');
         self::assertNull((new MediaDownloadAbility(new Limits()))->outputSchema());
     }
 
@@ -92,6 +92,6 @@ final class MediaDownloadAbilityTest extends TestCase
 
         $result = (new MediaDownloadAbility(new Limits()))->execute(['id' => 40]);
 
-        self::assertSame('lit_not_found', $result->get_error_code());
+        self::assertToolError($result, 'not_found');
     }
 }
diff --git a/tests/Unit/Integrations/Media/MediaGetAbilityTest.php b/tests/Unit/Integrations/Media/MediaGetAbilityTest.php
index 91cfa1b..e501511 100644
--- a/tests/Unit/Integrations/Media/MediaGetAbilityTest.php
+++ b/tests/Unit/Integrations/Media/MediaGetAbilityTest.php
@@ -45,7 +45,7 @@ final class MediaGetAbilityTest extends TestCase
         Functions\when('get_post')->justReturn(new WP_Post(['ID' => 5, 'post_type' => 'page']));
         $ability = new MediaGetAbility(Mockery::mock(MediaPresenter::class), Mockery::mock(MediaUsageFinder::class));
 
-        self::assertSame('lit_not_found', $ability->permission(['id' => 5])->get_error_code());
+        self::assertToolError($ability->permission(['id' => 5]), 'not_found');
     }
 
     public function testIsIndirectAndReadonly(): void
diff --git a/tests/Unit/Integrations/Media/MediaGuardTest.php b/tests/Unit/Integrations/Media/MediaGuardTest.php
index e8cacdc..8e49fa3 100644
--- a/tests/Unit/Integrations/Media/MediaGuardTest.php
+++ b/tests/Unit/Integrations/Media/MediaGuardTest.php
@@ -13,17 +13,17 @@ final class MediaGuardTest extends TestCase
 {
     public function testInvalidIdIsAValidationError(): void
     {
-        self::assertSame('lit_validation_failed', MediaGuard::attachment('abc')->get_error_code());
-        self::assertSame('lit_validation_failed', MediaGuard::attachment(0)->get_error_code());
+        self::assertToolError(MediaGuard::attachment('abc'), 'validation_failed');
+        self::assertToolError(MediaGuard::attachment(0), 'validation_failed');
     }
 
     public function testMissingOrNonAttachmentIsNotFound(): void
     {
         Functions\when('get_post')->justReturn(null);
-        self::assertSame('lit_not_found', MediaGuard::attachment(33)->get_error_code());
+        self::assertToolError(MediaGuard::attachment(33), 'not_found');
 
         Functions\when('get_post')->justReturn(new WP_Post(['ID' => 33, 'post_type' => 'page']));
-        self::assertSame('lit_not_found', MediaGuard::attachment(33)->get_error_code());
+        self::assertToolError(MediaGuard::attachment(33), 'not_found');
     }
 
     public function testChecksTheRequestedCapabilityOnTheAttachment(): void
@@ -32,7 +32,7 @@ final class MediaGuardTest extends TestCase
         Functions\when('get_post')->justReturn($attachment);
         Functions\expect('current_user_can')->once()->with('delete_post', 33)->andReturn(false);
 
-        self::assertSame('lit_forbidden', MediaGuard::attachment('33', 'delete_post')->get_error_code());
+        self::assertToolError(MediaGuard::attachment('33', 'delete_post'), 'forbidden');
     }
 
     public function testRefusesTrashedAttachmentsUnlessAllowed(): void
@@ -42,7 +42,7 @@ final class MediaGuardTest extends TestCase
         Functions\when('current_user_can')->justReturn(true);
 
         $error = MediaGuard::attachment(33);
-        self::assertSame('lit_validation_failed', $error->get_error_code());
+        self::assertToolError($error, 'validation_failed');
         self::assertStringContainsString('Papierkorb', $error->get_error_message());
         self::assertSame($attachment, MediaGuard::attachment(33, 'read_post', true));
     }
diff --git a/tests/Unit/Integrations/Media/MediaImporterTest.php b/tests/Unit/Integrations/Media/MediaImporterTest.php
index 6724102..af8d17f 100644
--- a/tests/Unit/Integrations/Media/MediaImporterTest.php
+++ b/tests/Unit/Integrations/Media/MediaImporterTest.php
@@ -136,7 +136,7 @@ final class MediaImporterTest extends TestCase
     {
         $result = $this->importer()->fromBase64(str_repeat('A', 2 * 1024 * 1024), 'big.png', []);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
         self::assertStringContainsString('Limit 1 MB', $result->get_error_message());
         self::assertSame([], $this->tempFiles, 'no temp file may be written for an oversized upload');
     }
@@ -147,7 +147,7 @@ final class MediaImporterTest extends TestCase
 
         $result = $this->importer()->fromBase64(str_repeat('A', 16 * 1024 * 1024), 'big.png', []);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
         self::assertStringContainsString('Limit 10 MB', $result->get_error_message());
         self::assertStringContainsString('lit/media-upload-from-url', (string) $result->get_error_data()['hint']);
         self::assertSame([], $this->tempFiles);
@@ -157,7 +157,7 @@ final class MediaImporterTest extends TestCase
     {
         $result = $this->importer()->fromBase64('***', 'x.png', []);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
         self::assertSame([], $this->tempFiles);
     }
 
@@ -168,7 +168,7 @@ final class MediaImporterTest extends TestCase
 
         $result = $this->importer()->fromBase64(base64_encode('<?php evil();'), 'bild.png', []);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
         self::assertNotSame([], $this->tempFiles);
         $this->assertTempFilesRemoved();
     }
@@ -182,7 +182,7 @@ final class MediaImporterTest extends TestCase
 
         $result = $this->importer()->fromBase64(base64_encode('PNGDATA'), 'logo.png', []);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
         self::assertStringContainsString('png', $result->get_error_message());
     }
 
@@ -198,7 +198,7 @@ final class MediaImporterTest extends TestCase
         $result = $this->importer()->fromBase64(base64_encode('PNGDATA'), 'logo.png', []);
         $logged = (string) file_get_contents($this->errorLog);
 
-        self::assertSame('lit_write_failed', $result->get_error_code());
+        self::assertToolError($result, 'write_failed');
         self::assertStringNotContainsString('/var/www', $result->get_error_message());
         self::assertStringNotContainsString('/var/www', (string) json_encode($result->get_error_data()));
         self::assertStringContainsString('WordPress hat die Datei nicht übernommen', $result->get_error_message());
@@ -238,7 +238,7 @@ final class MediaImporterTest extends TestCase
             self::fail('WriteFailed expected');
         } catch (WriteFailed $e) {
             self::assertTrue($e->rolledBack);
-            self::assertSame('lit_write_failed', $e->toError()->get_error_code());
+            self::assertToolError($e->toError(), 'write_failed');
         }
         $this->assertTempFilesRemoved();
     }
@@ -258,7 +258,7 @@ final class MediaImporterTest extends TestCase
             self::assertFalse($e->rolledBack);
             self::assertSame(55, $e->postId);
             $error = $e->toError();
-            self::assertSame('lit_partial_write', $error->get_error_code());
+            self::assertToolError($error, 'partial_write');
             self::assertStringContainsString('Medium 55', $error->get_error_message());
             self::assertSame(55, $error->get_error_data()['attachment_id']);
         }
@@ -282,7 +282,7 @@ final class MediaImporterTest extends TestCase
 
         $result = $this->importer()->fromUrl('http://127.0.0.1/wp-config.php', null, []);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
     }
 
     public function testOnlyHttpUrlsAreFetched(): void
@@ -292,7 +292,7 @@ final class MediaImporterTest extends TestCase
 
         $result = $this->importer()->fromUrl('ftp://example.com/logo.png', null, []);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
     }
 
     public function testUrlImportDownloadsSafelyWithTheSizeLimit(): void
@@ -332,7 +332,7 @@ final class MediaImporterTest extends TestCase
 
         $result = $this->importer()->fromUrl('https://example.com/fehlt.jpg', null, []);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
         self::assertStringContainsString('Not Found', $result->get_error_message());
         $this->assertTempFilesRemoved();
     }
@@ -344,7 +344,7 @@ final class MediaImporterTest extends TestCase
 
         $result = $this->importer()->fromUrl('https://example.com/fehlt.jpg', null, []);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
         self::assertStringContainsString('404', $result->get_error_message());
         $this->assertTempFilesRemoved();
     }
@@ -356,7 +356,7 @@ final class MediaImporterTest extends TestCase
 
         $result = $this->importer()->fromUrl('https://example.com/gross.jpg', null, []);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
         self::assertStringContainsString('Limit 1 MB', $result->get_error_message());
         $this->assertTempFilesRemoved();
     }
@@ -368,7 +368,7 @@ final class MediaImporterTest extends TestCase
 
         $result = $this->importer()->fromUrl('https://example.com/gross.jpg', null, []);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
         self::assertStringContainsString('Limit 1 MB', $result->get_error_message());
         $this->assertTempFilesRemoved();
     }
@@ -396,7 +396,7 @@ final class MediaImporterTest extends TestCase
 
         $result = $this->importer()->fromBase64(base64_encode('<html></html>'), 'seite.html', []);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
         $this->assertTempFilesRemoved();
     }
 
@@ -416,7 +416,7 @@ final class MediaImporterTest extends TestCase
 
         $result = $this->importer()->fromBase64(base64_encode('PNGDATA'), 'logo.png', []);
 
-        self::assertSame('lit_write_failed', $result->get_error_code());
+        self::assertToolError($result, 'write_failed');
         self::assertFileDoesNotExist($moved);
         self::assertFalse(has_filter('wp_handle_upload'));
     }
@@ -462,6 +462,6 @@ final class MediaImporterTest extends TestCase
 
         $result = $this->importer()->fromUrl('https://example.com/logo.png', null, []);
 
-        self::assertSame('lit_write_failed', $result->get_error_code());
+        self::assertToolError($result, 'write_failed');
     }
 }
diff --git a/tests/Unit/Integrations/Media/MediaPreviewAbilityTest.php b/tests/Unit/Integrations/Media/MediaPreviewAbilityTest.php
index 572c62b..d0fbe73 100644
--- a/tests/Unit/Integrations/Media/MediaPreviewAbilityTest.php
+++ b/tests/Unit/Integrations/Media/MediaPreviewAbilityTest.php
@@ -68,7 +68,7 @@ final class MediaPreviewAbilityTest extends TestCase
 
         $result = (new MediaPreviewAbility())->execute(['id' => 33]);
 
-        self::assertSame('lit_unsupported', $result->get_error_code());
+        self::assertToolError($result, 'unsupported');
         self::assertStringContainsString(
             'lit/media-download (direkt: lit-media-download)',
             (string) $result->get_error_data()['hint']
@@ -82,7 +82,7 @@ final class MediaPreviewAbilityTest extends TestCase
 
         $result = (new MediaPreviewAbility())->execute(['id' => 33]);
 
-        self::assertSame('lit_not_found', $result->get_error_code());
+        self::assertToolError($result, 'not_found');
     }
 
     public function testIsADirectToolWithoutOutputSchema(): void
diff --git a/tests/Unit/Integrations/Media/MediaSearchAbilityTest.php b/tests/Unit/Integrations/Media/MediaSearchAbilityTest.php
index a99cff8..f78ceb5 100644
--- a/tests/Unit/Integrations/Media/MediaSearchAbilityTest.php
+++ b/tests/Unit/Integrations/Media/MediaSearchAbilityTest.php
@@ -68,7 +68,7 @@ final class MediaSearchAbilityTest extends TestCase
         $result = (new MediaSearchAbility($this->presenter(), Mockery::mock(MediaUsageFinder::class)))
             ->execute(['missing_alt' => true, 'mime' => 'application/pdf']);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
     }
 
     public function testMissingAltAllowsAnImageSubtypeMime(): void
@@ -108,7 +108,7 @@ final class MediaSearchAbilityTest extends TestCase
         $result = (new MediaSearchAbility($this->presenter(), Mockery::mock(MediaUsageFinder::class)))
             ->execute(['cursor' => '???']);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
     }
 
     public function testExcludesAttachmentsTheUserCannotEdit(): void
@@ -303,7 +303,7 @@ final class MediaSearchAbilityTest extends TestCase
         Functions\when('current_user_can')->justReturn(false);
         $ability = new MediaSearchAbility($this->presenter(), Mockery::mock(MediaUsageFinder::class));
 
-        self::assertSame('lit_forbidden', $ability->permission([])->get_error_code());
+        self::assertToolError($ability->permission([]), 'forbidden');
         self::assertTrue($ability->isDirect());
     }
 }
diff --git a/tests/Unit/Integrations/Media/MediaUpdateAbilityTest.php b/tests/Unit/Integrations/Media/MediaUpdateAbilityTest.php
index 0895521..eff33b2 100644
--- a/tests/Unit/Integrations/Media/MediaUpdateAbilityTest.php
+++ b/tests/Unit/Integrations/Media/MediaUpdateAbilityTest.php
@@ -92,7 +92,7 @@ final class MediaUpdateAbilityTest extends TestCase
         ]]);
 
         self::assertInstanceOf(WP_Error::class, $result);
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
         self::assertStringContainsString('3 von 4', $result->get_error_message());
         self::assertStringContainsString('nichts wurde gespeichert', $result->get_error_message());
         $failures = $result->get_error_data()['failures'];
@@ -109,7 +109,7 @@ final class MediaUpdateAbilityTest extends TestCase
     {
         $result = (new MediaUpdateAbility($this->log()))->execute(['items' => [['id' => '99', 'alt' => 'x']]]);
 
-        self::assertSame('lit_not_found', $result->get_error_code());
+        self::assertToolError($result, 'not_found');
         self::assertSame(99, $result->get_error_data()['failures'][0]['id']);
     }
 
@@ -138,7 +138,7 @@ final class MediaUpdateAbilityTest extends TestCase
             ['id' => 34, 'alt' => 'neu', 'modified' => '2026-09-24 07:00:00'],
         ]]);
 
-        self::assertSame('lit_rev_mismatch', $result->get_error_code());
+        self::assertToolError($result, 'rev_mismatch');
         self::assertSame(1, $result->get_error_data()['failures'][0]['index']);
         self::assertStringContainsString('lit/media-get', $result->get_error_data()['hint']);
         self::assertSame([], $this->altUpdates);
@@ -162,7 +162,7 @@ final class MediaUpdateAbilityTest extends TestCase
             self::fail('WriteFailed expected');
         } catch (WriteFailed $e) {
             self::assertTrue($e->rolledBack);
-            self::assertSame('lit_write_failed', $e->toError()->get_error_code());
+            self::assertToolError($e->toError(), 'write_failed');
             self::assertStringContainsString('neu lesen', $e->toError()->get_error_data()['hint']);
             self::assertStringContainsString('Datenbankfehler', $e->getMessage());
         }
@@ -232,7 +232,7 @@ final class MediaUpdateAbilityTest extends TestCase
             self::fail('WriteFailed expected');
         } catch (WriteFailed $e) {
             $error = $e->toError();
-            self::assertSame('lit_partial_write', $error->get_error_code());
+            self::assertToolError($error, 'partial_write');
             self::assertSame([33], $error->get_error_data()['changed_ids']);
             self::assertStringContainsString('33', $error->get_error_message());
         }
diff --git a/tests/Unit/Integrations/Media/MediaUploadAbilitiesTest.php b/tests/Unit/Integrations/Media/MediaUploadAbilitiesTest.php
index 938c8fe..30d2d2e 100644
--- a/tests/Unit/Integrations/Media/MediaUploadAbilitiesTest.php
+++ b/tests/Unit/Integrations/Media/MediaUploadAbilitiesTest.php
@@ -106,7 +106,7 @@ final class MediaUploadAbilitiesTest extends TestCase
         $result = (new MediaUploadAbility($importer, $this->uploads(), new Limits()))
             ->execute(['filename' => 'x.png', 'data' => 'AAAA']);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
     }
 
     public function testAPartialUploadIsLoggedAsPartialAndRethrown(): void
@@ -136,7 +136,7 @@ final class MediaUploadAbilitiesTest extends TestCase
         $ability = new MediaUploadFromUrlAbility(Mockery::mock(MediaImporter::class), $this->uploads());
 
         self::assertTrue($ability->permission(['url' => 'https://example.com/a.png']));
-        self::assertSame('lit_forbidden', $ability->permission(['url' => 'https://example.com/a.png', 'post_id' => 5])->get_error_code());
+        self::assertToolError($ability->permission(['url' => 'https://example.com/a.png', 'post_id' => 5]), 'forbidden');
     }
 
     public function testPermissionRefusesUsersWithoutUploadRights(): void
@@ -144,7 +144,7 @@ final class MediaUploadAbilitiesTest extends TestCase
         Functions\when('current_user_can')->justReturn(false);
         $ability = new MediaUploadAbility(Mockery::mock(MediaImporter::class), $this->uploads(), new Limits());
 
-        self::assertSame('lit_forbidden', $ability->permission(['filename' => 'a.png', 'data' => 'AAAA'])->get_error_code());
+        self::assertToolError($ability->permission(['filename' => 'a.png', 'data' => 'AAAA']), 'forbidden');
     }
 
     public function testUploadsForAParentThatDoesNotExistAreRefused(): void
@@ -153,7 +153,7 @@ final class MediaUploadAbilitiesTest extends TestCase
         Functions\when('get_post')->justReturn(null);
         $ability = new MediaUploadAbility(Mockery::mock(MediaImporter::class), $this->uploads(), new Limits());
 
-        self::assertSame('lit_not_found', $ability->permission(['filename' => 'a.png', 'data' => 'AAAA', 'post_id' => 9])->get_error_code());
+        self::assertToolError($ability->permission(['filename' => 'a.png', 'data' => 'AAAA', 'post_id' => 9]), 'not_found');
     }
 
     public function testDescriptionNamesTheConfiguredLimit(): void
diff --git a/tests/Unit/Integrations/WordPress/Content/ContentCreateAbilityTest.php b/tests/Unit/Integrations/WordPress/Content/ContentCreateAbilityTest.php
index 512a8d5..80bb101 100644
--- a/tests/Unit/Integrations/WordPress/Content/ContentCreateAbilityTest.php
+++ b/tests/Unit/Integrations/WordPress/Content/ContentCreateAbilityTest.php
@@ -76,7 +76,7 @@ final class ContentCreateAbilityTest extends TestCase
         $result = (new ContentCreateAbility($this->schema(), $writer, FakeChangeLog::forbidden()))
             ->execute(['post_type' => 'lit_person', 'title' => 'Anna']);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
     }
 
     public function testCreatesADraftWritesExtrasLogsAndReturnsAToolResult(): void
@@ -131,7 +131,7 @@ final class ContentCreateAbilityTest extends TestCase
 
         self::assertTrue($failure->rolledBack);
         self::assertStringContainsString('Pods could not save the fields.', $failure->getMessage());
-        self::assertSame('lit_write_failed', $failure->toError()->get_error_code());
+        self::assertToolError($failure->toError(), 'write_failed');
     }
 
     public function testMetaFieldsTheUserMayNotAddAreRefusedAndTheDraftIsRemoved(): void
@@ -149,8 +149,8 @@ final class ContentCreateAbilityTest extends TestCase
 
         $result = (new ContentCreateAbility($this->schema(), $writer, $log))->execute(self::INPUT);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
-        self::assertArrayHasKey('fields.city', $result->get_error_data()['fields']);
+        self::assertToolError($result, 'validation_failed');
+        self::assertArrayHasKey('fields.city', self::errorFields($result));
     }
 
     public function testARefusedDraftThatCannotBeDeletedIsAPartialWrite(): void
@@ -219,7 +219,7 @@ final class ContentCreateAbilityTest extends TestCase
 
         self::assertFalse($failure->rolledBack);
         self::assertNull($failure->postId);
-        self::assertSame('lit_partial_write', $failure->toError()->get_error_code());
+        self::assertToolError($failure->toError(), 'partial_write');
         $log->assertPartials(['ability' => 'lit/content-create', 'object_type' => 'lit_person', 'object_id' => '']);
     }
 
@@ -261,6 +261,6 @@ final class ContentCreateAbilityTest extends TestCase
         self::assertTrue($ability->permission(['post_type' => 'page']));
 
         Functions\when('current_user_can')->justReturn(false);
-        self::assertSame('lit_forbidden', $ability->permission(['post_type' => 'page'])->get_error_code());
+        self::assertToolError($ability->permission(['post_type' => 'page']), 'forbidden');
     }
 }
diff --git a/tests/Unit/Integrations/WordPress/Content/ContentCreateDiviTest.php b/tests/Unit/Integrations/WordPress/Content/ContentCreateDiviTest.php
index de1e24d..510e947 100644
--- a/tests/Unit/Integrations/WordPress/Content/ContentCreateDiviTest.php
+++ b/tests/Unit/Integrations/WordPress/Content/ContentCreateDiviTest.php
@@ -128,14 +128,14 @@ final class ContentCreateDiviTest extends TestCase
         Functions\expect('wp_insert_post')->never();
 
         $inactive = $this->ability($this->composer(false))->execute(self::INPUT);
-        self::assertSame('lit_unsupported', $inactive->get_error_code());
+        self::assertToolError($inactive, 'unsupported');
 
         $none = $this->ability(null)->execute(self::INPUT);
-        self::assertSame('lit_unsupported', $none->get_error_code());
+        self::assertToolError($none, 'unsupported');
 
         $both = $this->ability($this->composer())->execute(self::INPUT + ['content' => '<p>x</p>']);
-        self::assertSame('lit_validation_failed', $both->get_error_code());
-        self::assertArrayHasKey('content', $both->get_error_data()['fields']);
+        self::assertToolError($both, 'validation_failed');
+        self::assertArrayHasKey('content', self::errorFields($both));
     }
 
     public function testEveryValidationErrorComesBackAndNothingIsCreated(): void
@@ -149,10 +149,10 @@ final class ContentCreateDiviTest extends TestCase
 
         $error = $this->ability($composer, $writer)->execute(self::INPUT);
 
-        self::assertSame('lit_invalid_attr_path', $error->get_error_code());
+        self::assertToolError($error, 'invalid_attr_path');
         self::assertSame(
             ['fields.role' => 'Pflichtfeld fehlt.', 'divi[0].attrs.x' => 'unbekannt'],
-            $error->get_error_data()['fields']
+            self::errorFields($error)
         );
     }
 
@@ -181,8 +181,8 @@ final class ContentCreateDiviTest extends TestCase
         $error = $this->ability($composer)->execute(self::INPUT);
 
         self::assertInstanceOf(WP_Error::class, $error);
-        self::assertSame('lit_validation_failed', $error->get_error_code());
-        self::assertArrayHasKey('divi[0].modules[0].text', $error->get_error_data()['fields']);
+        self::assertToolError($error, 'validation_failed');
+        self::assertArrayHasKey('divi[0].modules[0].text', self::errorFields($error));
     }
 
     public function testARefusedLayoutWhoseDraftCannotBeDeletedIsAPartialWrite(): void
@@ -254,8 +254,8 @@ final class ContentCreateDiviTest extends TestCase
             'attrs' => ['module.meta.adminLabel' => ['desktop' => 'Lang']],
             'modules' => [['type' => 'button', 'text' => 'Los', 'attrs' => ['button.innerContent' => ['desktop' => ['text' => 'x']]]]],
         ]]]);
-        self::assertSame('lit_validation_failed', $overlap->get_error_code());
-        $fields = $overlap->get_error_data()['fields'];
+        self::assertToolError($overlap, 'validation_failed');
+        $fields = self::errorFields($overlap);
         self::assertArrayHasKey('divi[0].attrs.module.meta.adminLabel', $fields, 'S41: label and attrs label');
         self::assertArrayHasKey('divi[0].modules[0].modules[0][0].attrs.button.innerContent', $fields, 'S40');
         self::assertSame([], $saved);
diff --git a/tests/Unit/Integrations/WordPress/Content/ContentGetAbilityTest.php b/tests/Unit/Integrations/WordPress/Content/ContentGetAbilityTest.php
index 0d1fea8..e21d765 100644
--- a/tests/Unit/Integrations/WordPress/Content/ContentGetAbilityTest.php
+++ b/tests/Unit/Integrations/WordPress/Content/ContentGetAbilityTest.php
@@ -47,7 +47,7 @@ final class ContentGetAbilityTest extends TestCase
         $schema = Mockery::mock(ContentSchema::class);
         $ability = new ContentGetAbility($schema, Mockery::mock(PostPresenter::class));
 
-        self::assertSame('lit_not_found', $ability->permission(['post_id' => 99])->get_error_code());
+        self::assertToolError($ability->permission(['post_id' => 99]), 'not_found');
     }
 
     public function testOutputSchemaAllowsNullContentForDivi(): void
diff --git a/tests/Unit/Integrations/WordPress/Content/ContentQueryAbilityTest.php b/tests/Unit/Integrations/WordPress/Content/ContentQueryAbilityTest.php
index 50bbf4e..f38cd57 100644
--- a/tests/Unit/Integrations/WordPress/Content/ContentQueryAbilityTest.php
+++ b/tests/Unit/Integrations/WordPress/Content/ContentQueryAbilityTest.php
@@ -96,7 +96,7 @@ final class ContentQueryAbilityTest extends TestCase
     {
         $result = $this->ability()->execute(['post_type' => 'page', 'cursor' => '???']);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
     }
 
     public function testQueryIsRestrictedToOwnPostsWithoutEditOthersPosts(): void
@@ -193,7 +193,7 @@ final class ContentQueryAbilityTest extends TestCase
 
         $result = $this->ability($schema)->execute(['post_type' => 'page', 'field' => ['key' => 'ghost']]);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
         self::assertStringContainsString('role', $result->get_error_message());
     }
 
@@ -209,9 +209,9 @@ final class ContentQueryAbilityTest extends TestCase
         $password = $this->ability($schema)->execute(['post_type' => 'page', 'field' => ['key' => 'secret']]);
         $protected = $this->ability($schema)->execute(['post_type' => 'page', 'field' => ['key' => '_internal']]);
 
-        self::assertSame('lit_validation_failed', $password->get_error_code());
+        self::assertToolError($password, 'validation_failed');
         self::assertSame('Verfügbare Felder: role.', $password->get_error_data()['hint']);
-        self::assertSame('lit_validation_failed', $protected->get_error_code());
+        self::assertToolError($protected, 'validation_failed');
         self::assertSame('Verfügbare Felder: role.', $protected->get_error_data()['hint']);
     }
 
@@ -225,7 +225,7 @@ final class ContentQueryAbilityTest extends TestCase
             'taxonomy' => ['taxonomy' => 'ghost', 'terms' => [1]],
         ]);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
         self::assertStringContainsString('category', $result->get_error_message());
     }
 
@@ -235,7 +235,7 @@ final class ContentQueryAbilityTest extends TestCase
         $schema->allows('isManaged')->andReturn(false);
         $ability = new ContentQueryAbility($schema, Mockery::mock(PostPresenter::class));
 
-        self::assertSame('lit_not_found', $ability->permission(['post_type' => 'secret'])->get_error_code());
+        self::assertToolError($ability->permission(['post_type' => 'secret']), 'not_found');
     }
 
     public function testPermissionIsForbiddenWithoutEditRights(): void
@@ -245,7 +245,7 @@ final class ContentQueryAbilityTest extends TestCase
         $schema = SchemaFixture::managed();
         $ability = new ContentQueryAbility($schema, Mockery::mock(PostPresenter::class));
 
-        self::assertSame('lit_forbidden', $ability->permission(['post_type' => 'page'])->get_error_code());
+        self::assertToolError($ability->permission(['post_type' => 'page']), 'forbidden');
     }
 
     public function testInputSchemaLimitsSearchLengthAndTaxonomyTermsCount(): void
diff --git a/tests/Unit/Integrations/WordPress/Content/ContentTypesAbilityTest.php b/tests/Unit/Integrations/WordPress/Content/ContentTypesAbilityTest.php
index 38e4067..7b34756 100644
--- a/tests/Unit/Integrations/WordPress/Content/ContentTypesAbilityTest.php
+++ b/tests/Unit/Integrations/WordPress/Content/ContentTypesAbilityTest.php
@@ -65,7 +65,7 @@ final class ContentTypesAbilityTest extends TestCase
         $result = (new ContentTypesAbility($schema))->execute(['post_type' => 'nope']);
 
         self::assertInstanceOf(WP_Error::class, $result);
-        self::assertSame('lit_not_found', $result->get_error_code());
+        self::assertToolError($result, 'not_found');
         self::assertStringContainsString('page, post', $result->get_error_message());
     }
 
@@ -95,7 +95,7 @@ final class ContentTypesAbilityTest extends TestCase
         $result = (new ContentTypesAbility($schema))->execute(['post_type' => 'page']);
 
         self::assertInstanceOf(WP_Error::class, $result);
-        self::assertSame('lit_forbidden', $result->get_error_code());
+        self::assertToolError($result, 'forbidden');
     }
 
     public function testPermissionRequiresEditPosts(): void
@@ -103,7 +103,7 @@ final class ContentTypesAbilityTest extends TestCase
         Functions\when('current_user_can')->justReturn(false);
         $ability = new ContentTypesAbility(Mockery::mock(ContentSchema::class));
 
-        self::assertSame('lit_forbidden', $ability->permission([])->get_error_code());
+        self::assertToolError($ability->permission([]), 'forbidden');
     }
 
     public function testPermissionForbidsANonEditableSpecificType(): void
@@ -115,7 +115,7 @@ final class ContentTypesAbilityTest extends TestCase
         $result = (new ContentTypesAbility($schema))->permission(['post_type' => 'lit_person']);
 
         self::assertInstanceOf(WP_Error::class, $result);
-        self::assertSame('lit_forbidden', $result->get_error_code());
+        self::assertToolError($result, 'forbidden');
     }
 
     public function testPermissionAllowsAnEditableSpecificType(): void
diff --git a/tests/Unit/Integrations/WordPress/Content/ContentUpdateAbilityTest.php b/tests/Unit/Integrations/WordPress/Content/ContentUpdateAbilityTest.php
index 4ab5b12..8d34db9 100644
--- a/tests/Unit/Integrations/WordPress/Content/ContentUpdateAbilityTest.php
+++ b/tests/Unit/Integrations/WordPress/Content/ContentUpdateAbilityTest.php
@@ -71,7 +71,7 @@ final class ContentUpdateAbilityTest extends TestCase
         $result = $this->ability(Mockery::mock(PostWriter::class))
             ->execute(['post_id' => 5, 'rev' => Rev::ofPost($this->post), 'title' => 'Neu']);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
     }
 
     public function testAMissedScheduleIsRefusedBecauseSavingWouldPublish(): void
@@ -84,8 +84,8 @@ final class ContentUpdateAbilityTest extends TestCase
         $result = $this->ability(Mockery::mock(PostWriter::class))
             ->execute(['post_id' => 5, 'rev' => Rev::ofPost($this->post), 'title' => 'Neu']);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
-        self::assertArrayHasKey('post_id', $result->get_error_data()['fields']);
+        self::assertToolError($result, 'validation_failed');
+        self::assertArrayHasKey('post_id', self::errorFields($result));
         self::assertStringContainsString('lit/post-schedule', $result->get_error_data()['hint']);
     }
 
@@ -96,7 +96,7 @@ final class ContentUpdateAbilityTest extends TestCase
         $result = $this->ability(Mockery::mock(PostWriter::class))
             ->execute(['post_id' => 5, 'rev' => 'stale', 'title' => 'Neu']);
 
-        self::assertSame('lit_rev_mismatch', $result->get_error_code());
+        self::assertToolError($result, 'rev_mismatch');
         self::assertStringContainsString('lit-content-get', $result->get_error_data()['hint']);
     }
 
@@ -105,7 +105,7 @@ final class ContentUpdateAbilityTest extends TestCase
         $result = $this->ability(Mockery::mock(PostWriter::class))
             ->execute(['post_id' => 5, 'rev' => Rev::of('<p>Alt</p>'), 'title' => 'Neu']);
 
-        self::assertSame('lit_rev_mismatch', $result->get_error_code());
+        self::assertToolError($result, 'rev_mismatch');
     }
 
     public function testRawContentOnDiviPagesIsUnsupported(): void
@@ -117,7 +117,7 @@ final class ContentUpdateAbilityTest extends TestCase
         $result = $this->ability($writer)
             ->execute(['post_id' => 5, 'rev' => Rev::ofPost($this->post), 'content' => '<p>Neu</p>']);
 
-        self::assertSame('lit_unsupported', $result->get_error_code());
+        self::assertToolError($result, 'unsupported');
         self::assertStringContainsString(PostPresenter::DIVI_INACTIVE_NOTE, $result->get_error_data()['hint']);
     }
 
@@ -129,7 +129,7 @@ final class ContentUpdateAbilityTest extends TestCase
 
         $result = $this->ability($writer)->execute(['post_id' => 5, 'rev' => Rev::ofPost($this->post)]);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
     }
 
     public function testUpdateKeepsTheStatusAndCapturesARestorePointFirst(): void
@@ -236,7 +236,7 @@ final class ContentUpdateAbilityTest extends TestCase
         $failure = $this->executeExpectingFailure($writer, $input);
 
         self::assertTrue($failure->rolledBack);
-        self::assertSame('lit_write_failed', $failure->toError()->get_error_code());
+        self::assertToolError($failure->toError(), 'write_failed');
         $message = $failure->toError()->get_error_message();
         self::assertStringNotContainsString('Nichts wurde gespeichert', $message);
         self::assertStringContainsString('Die Änderung wurde nicht gespeichert (Revision 811 bleibt erhalten).', $message);
@@ -326,7 +326,7 @@ final class ContentUpdateAbilityTest extends TestCase
         self::assertFalse($failure->rolledBack);
         self::assertStringContainsString('restore kaputt', $failure->getMessage());
         $error = $failure->toError();
-        self::assertSame('lit_partial_write', $error->get_error_code());
+        self::assertToolError($error, 'partial_write');
         self::assertSame(5, $error->get_error_data()['post_id']);
         self::assertSame(811, $error->get_error_data()['revision_id']);
         $log->assertPartials([
diff --git a/tests/Unit/Integrations/WordPress/Content/PostWriterTest.php b/tests/Unit/Integrations/WordPress/Content/PostWriterTest.php
index 8bd7146..ac7d603 100644
--- a/tests/Unit/Integrations/WordPress/Content/PostWriterTest.php
+++ b/tests/Unit/Integrations/WordPress/Content/PostWriterTest.php
@@ -48,8 +48,8 @@ final class PostWriterTest extends TestCase
             'template' => 'fehlt.php',
         ], true);
 
-        $fields = $result->get_error_data()['fields'];
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        $fields = self::errorFields($result);
+        self::assertToolError($result, 'validation_failed');
         self::assertSame('Pflichtfeld fehlt.', $fields['fields.role']);
         self::assertSame('Taxonomie gehört nicht zu diesem Inhaltstyp.', $fields['terms.genre']);
         self::assertSame('Inhaltstyp unterstützt kein Beitragsbild.', $fields['featured_image']);
@@ -72,7 +72,7 @@ final class PostWriterTest extends TestCase
         $result = $writer->prepare('page', $input, false);
         self::assertFalse(is_array($result), $path . ' should be rejected');
 
-        return (string) $result->get_error_data()['fields'][$path];
+        return (string) self::errorFields($result)[$path];
     }
 
     public function testParentMustBeAReadableNonDescendantEntryOfAHierarchicalType(): void
@@ -152,7 +152,7 @@ final class PostWriterTest extends TestCase
             false
         );
 
-        self::assertSame(['fields.secret' => 'Keine Berechtigung, dieses Feld zu ändern.'], $result->get_error_data()['fields']);
+        self::assertSame(['fields.secret' => 'Keine Berechtigung, dieses Feld zu ändern.'], self::errorFields($result));
         self::assertSame([['edit_post_meta', 5, 'city'], ['edit_post_meta', 5, 'secret']], $checked);
     }
 
diff --git a/tests/Unit/Integrations/WordPress/Menus/MenuAssignLocationAbilityTest.php b/tests/Unit/Integrations/WordPress/Menus/MenuAssignLocationAbilityTest.php
index a1b0fe6..0f01a60 100644
--- a/tests/Unit/Integrations/WordPress/Menus/MenuAssignLocationAbilityTest.php
+++ b/tests/Unit/Integrations/WordPress/Menus/MenuAssignLocationAbilityTest.php
@@ -50,10 +50,10 @@ final class MenuAssignLocationAbilityTest extends TestCase
         $result = (new MenuAssignLocationAbility(FakeChangeLog::forbidden()))
             ->execute(['location' => 'primary-menu', 'menu_id' => 0]);
 
-        self::assertSame('lit_confirm_required', $result->get_error_code());
+        self::assertToolError($result, 'confirm_required');
         self::assertStringContainsString(
             '"Hauptmenü" (ID 3)',
-            json_decode($result->get_error_message(), true)['message']
+            self::clientError($result)['message']
         );
     }
 
@@ -104,7 +104,7 @@ final class MenuAssignLocationAbilityTest extends TestCase
         $result = (new MenuAssignLocationAbility(FakeChangeLog::forbidden()))
             ->execute(['location' => 'primary-menu', 'menu_id' => 99]);
 
-        self::assertSame('lit_not_found', $result->get_error_code());
+        self::assertToolError($result, 'not_found');
     }
 
     public function testPermissionNeedsThemeOptionRights(): void
@@ -114,7 +114,7 @@ final class MenuAssignLocationAbilityTest extends TestCase
         $ability = new MenuAssignLocationAbility(FakeChangeLog::forbidden());
         $input = ['location' => 'primary-menu', 'menu_id' => 8];
 
-        self::assertSame('lit_forbidden', $ability->permission($input)->get_error_code());
+        self::assertToolError($ability->permission($input), 'forbidden');
     }
 
     public function testIsDestructiveBecauseItReplacesTheMenuOfTheLocation(): void
diff --git a/tests/Unit/Integrations/WordPress/Menus/MenuCreateAbilityTest.php b/tests/Unit/Integrations/WordPress/Menus/MenuCreateAbilityTest.php
index b75ab3c..65bf163 100644
--- a/tests/Unit/Integrations/WordPress/Menus/MenuCreateAbilityTest.php
+++ b/tests/Unit/Integrations/WordPress/Menus/MenuCreateAbilityTest.php
@@ -20,7 +20,7 @@ final class MenuCreateAbilityTest extends TestCase
         $ability = new MenuCreateAbility(FakeChangeLog::forbidden());
         $result = $ability->execute(['name' => 'Neu', 'location' => 'nope']);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
         self::assertStringContainsString('primary-menu', $result->get_error_message());
     }
 
@@ -32,7 +32,7 @@ final class MenuCreateAbilityTest extends TestCase
 
         $result = (new MenuCreateAbility(FakeChangeLog::forbidden()))->execute(['name' => 'Hauptmenü']);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
     }
 
     public function testCreatesAndAssignsTheLocation(): void
@@ -65,6 +65,6 @@ final class MenuCreateAbilityTest extends TestCase
 
         $ability = new MenuCreateAbility(FakeChangeLog::forbidden());
 
-        self::assertSame('lit_forbidden', $ability->permission(['name' => 'Neu'])->get_error_code());
+        self::assertToolError($ability->permission(['name' => 'Neu']), 'forbidden');
     }
 }
diff --git a/tests/Unit/Integrations/WordPress/Menus/MenuOperationsTest.php b/tests/Unit/Integrations/WordPress/Menus/MenuOperationsTest.php
index 68df89a..6cc6362 100644
--- a/tests/Unit/Integrations/WordPress/Menus/MenuOperationsTest.php
+++ b/tests/Unit/Integrations/WordPress/Menus/MenuOperationsTest.php
@@ -48,9 +48,9 @@ final class MenuOperationsTest extends TestCase
         $cycle = $operations->apply($this->tree(), [['op' => 'move', 'id' => 1, 'parent_id' => 2]]);
         $badOp = $operations->apply($this->tree(), [['op' => 'rename', 'id' => 1]]);
 
-        self::assertArrayHasKey('operations.0', $unknown->get_error_data()['fields']);
-        self::assertStringContainsString('unter sich selbst', $cycle->get_error_data()['fields']['operations.0']);
-        self::assertStringContainsString('add, move oder remove', $badOp->get_error_data()['fields']['operations.0']);
+        self::assertArrayHasKey('operations.0', self::errorFields($unknown));
+        self::assertStringContainsString('unter sich selbst', self::errorFields($cycle)['operations.0']);
+        self::assertStringContainsString('add, move oder remove', self::errorFields($badOp)['operations.0']);
     }
 
     public function testExistingNodesKeepOnlyTheirIdSoTheWriterKeepsTheirData(): void
diff --git a/tests/Unit/Integrations/WordPress/Menus/MenuReadAbilitiesTest.php b/tests/Unit/Integrations/WordPress/Menus/MenuReadAbilitiesTest.php
index d9d3a6f..1dfe760 100644
--- a/tests/Unit/Integrations/WordPress/Menus/MenuReadAbilitiesTest.php
+++ b/tests/Unit/Integrations/WordPress/Menus/MenuReadAbilitiesTest.php
@@ -51,14 +51,14 @@ final class MenuReadAbilitiesTest extends TestCase
         self::assertSame('Start', $result['items'][0]['title']);
         self::assertSame(Rev::ofMenu([self::item(10, 0, 'Start')]), $result['menu_rev']);
         self::assertContains('menu_rev', $ability->outputSchema()['required']);
-        self::assertSame('lit_not_found', $ability->execute(['menu_id' => 9])->get_error_code());
+        self::assertToolError($ability->execute(['menu_id' => 9]), 'not_found');
     }
 
     public function testReadToolsNeedThemeOptionRights(): void
     {
         Functions\when('current_user_can')->justReturn(false);
 
-        self::assertSame('lit_forbidden', (new MenuListAbility())->permission([])->get_error_code());
-        self::assertSame('lit_forbidden', (new MenuGetAbility())->permission(['menu_id' => 3])->get_error_code());
+        self::assertToolError((new MenuListAbility())->permission([]), 'forbidden');
+        self::assertToolError((new MenuGetAbility())->permission(['menu_id' => 3]), 'forbidden');
     }
 }
diff --git a/tests/Unit/Integrations/WordPress/Menus/MenuSetItemsAbilityTest.php b/tests/Unit/Integrations/WordPress/Menus/MenuSetItemsAbilityTest.php
index 51a4798..7a476f8 100644
--- a/tests/Unit/Integrations/WordPress/Menus/MenuSetItemsAbilityTest.php
+++ b/tests/Unit/Integrations/WordPress/Menus/MenuSetItemsAbilityTest.php
@@ -42,10 +42,10 @@ final class MenuSetItemsAbilityTest extends TestCase
         $stale = $ability->execute(['menu_id' => 3, 'menu_rev' => 'abcdefabcdef', 'items' => [['id' => 10]]]);
         $missing = $ability->execute(['menu_id' => 3, 'items' => [['id' => 10]]]);
 
-        self::assertSame('lit_rev_mismatch', $stale->get_error_code());
+        self::assertToolError($stale, 'rev_mismatch');
         self::assertStringContainsString('lit/menu-get', $stale->get_error_data()['hint']);
         self::assertStringContainsString('Menü', $stale->get_error_message());
-        self::assertSame('lit_rev_mismatch', $missing->get_error_code());
+        self::assertToolError($missing, 'rev_mismatch');
         self::assertContains('menu_rev', $ability->inputSchema()['required']);
         self::assertStringContainsString('menu_rev', $ability->description());
     }
@@ -68,7 +68,7 @@ final class MenuSetItemsAbilityTest extends TestCase
     {
         $ability = new MenuSetItemsAbility(new MenuWriter(), new MenuOperations(), FakeChangeLog::forbidden());
 
-        self::assertSame('lit_validation_failed', $ability->execute(['menu_id' => 3, 'menu_rev' => self::rev()])->get_error_code());
+        self::assertToolError($ability->execute(['menu_id' => 3, 'menu_rev' => self::rev()]), 'validation_failed');
         self::assertSame(
             'lit_validation_failed',
             $ability->execute(['menu_id' => 3, 'menu_rev' => self::rev(), 'items' => [], 'operations' => [['op' => 'remove', 'id' => 10]]])
@@ -81,7 +81,7 @@ final class MenuSetItemsAbilityTest extends TestCase
         Functions\when('wp_get_nav_menu_object')->justReturn(false);
         $ability = new MenuSetItemsAbility(new MenuWriter(), new MenuOperations(), FakeChangeLog::forbidden());
 
-        self::assertSame('lit_not_found', $ability->execute(['menu_id' => 9, 'items' => []])->get_error_code());
+        self::assertToolError($ability->execute(['menu_id' => 9, 'items' => []]), 'not_found');
     }
 
     public function testRemovalsNeedConfirmationAndNothingIsWritten(): void
@@ -92,7 +92,7 @@ final class MenuSetItemsAbilityTest extends TestCase
 
         $result = $ability->execute(['menu_id' => 3, 'menu_rev' => self::rev(), 'items' => []]);
 
-        self::assertSame('lit_confirm_required', $result->get_error_code());
+        self::assertToolError($result, 'confirm_required');
         self::assertSame([['id' => 10, 'title' => 'Home']], $result->get_error_data()['removals']);
     }
 
@@ -109,8 +109,8 @@ final class MenuSetItemsAbilityTest extends TestCase
             'items' => [['id' => 10], ['type' => 'custom', 'url' => 'javascript:alert(1)', 'title' => 'X']],
         ]);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
-        self::assertArrayHasKey('items.1', $result->get_error_data()['fields']);
+        self::assertToolError($result, 'validation_failed');
+        self::assertArrayHasKey('items.1', self::errorFields($result));
     }
 
     public function testInvalidOperationsWriteNothing(): void
@@ -121,7 +121,7 @@ final class MenuSetItemsAbilityTest extends TestCase
 
         $result = $ability->execute(['menu_id' => 3, 'menu_rev' => self::rev(), 'operations' => [['op' => 'move', 'id' => 99]]]);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
     }
 
     public function testAppliesAndReturnsTheTreeBeforeAndAfter(): void
@@ -190,7 +190,7 @@ final class MenuSetItemsAbilityTest extends TestCase
             ['menu_id' => 3, 'menu_rev' => self::rev(), 'items' => [['type' => 'post', 'object_id' => 7]], 'allow_unpublished' => true]
         );
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
         self::assertArrayHasKey('allow_unpublished', $ability->inputSchema()['properties']);
     }
 
@@ -209,8 +209,8 @@ final class MenuSetItemsAbilityTest extends TestCase
         $refused = $ability->execute($input);
         $allowed = $ability->execute($input + ['allow_unpublished' => true]);
 
-        self::assertSame('lit_validation_failed', $refused->get_error_code());
-        self::assertStringContainsString('allow_unpublished', $refused->get_error_data()['fields']['items.1']);
+        self::assertToolError($refused, 'validation_failed');
+        self::assertStringContainsString('allow_unpublished', self::errorFields($refused)['items.1']);
         self::assertStringContainsString('1 neu', $allowed['summary']);
         self::assertStringContainsString('sofort im Menü sichtbar', $allowed['warnings'][0]);
     }
@@ -230,8 +230,8 @@ final class MenuSetItemsAbilityTest extends TestCase
             'operations' => [['op' => 'add', 'item' => ['type' => 'post', 'object_id' => 7]]],
         ]);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
-        self::assertStringContainsString('Private Inhalte', $result->get_error_data()['fields']['items.1']);
+        self::assertToolError($result, 'validation_failed');
+        self::assertStringContainsString('Private Inhalte', self::errorFields($result)['items.1']);
     }
 
     public function testAWriteFailureIsPassedOnAsWriteFailed(): void
@@ -290,7 +290,7 @@ final class MenuSetItemsAbilityTest extends TestCase
         Functions\when('current_user_can')->justReturn(false);
         $ability = new MenuSetItemsAbility(new MenuWriter(), new MenuOperations(), FakeChangeLog::forbidden());
 
-        self::assertSame('lit_forbidden', $ability->permission(['menu_id' => 3, 'menu_rev' => self::rev()])->get_error_code());
+        self::assertToolError($ability->permission(['menu_id' => 3, 'menu_rev' => self::rev()]), 'forbidden');
     }
 
     public function testIsDestructiveAndDescribesWhenConfirmIsNeeded(): void
diff --git a/tests/Unit/Integrations/WordPress/Menus/MenuWriterTest.php b/tests/Unit/Integrations/WordPress/Menus/MenuWriterTest.php
index b36fcb8..4edcfe2 100644
--- a/tests/Unit/Integrations/WordPress/Menus/MenuWriterTest.php
+++ b/tests/Unit/Integrations/WordPress/Menus/MenuWriterTest.php
@@ -334,7 +334,7 @@ final class MenuWriterTest extends TestCase
         } catch (WriteFailed $e) {
             self::assertTrue($e->rolledBack);
             self::assertStringContainsString('kaputt', $e->getMessage());
-            self::assertSame('lit_write_failed', $e->toError()->get_error_code());
+            self::assertToolError($e->toError(), 'write_failed');
         }
 
         $restore = $calls[3];
@@ -357,7 +357,7 @@ final class MenuWriterTest extends TestCase
         } catch (WriteFailed $e) {
             self::assertFalse($e->rolledBack);
             $error = $e->toError();
-            self::assertSame('lit_partial_write', $error->get_error_code());
+            self::assertToolError($error, 'partial_write');
             self::assertSame(3, $error->get_error_data()['menu_id']);
             self::assertSame('Home', $error->get_error_data()['previous_items'][0]['title']);
             self::assertStringContainsString('lit/menu-get', $error->get_error_data()['hint']);
diff --git a/tests/Unit/Integrations/WordPress/PostGuardTest.php b/tests/Unit/Integrations/WordPress/PostGuardTest.php
index b88bbe1..7e8fc69 100644
--- a/tests/Unit/Integrations/WordPress/PostGuardTest.php
+++ b/tests/Unit/Integrations/WordPress/PostGuardTest.php
@@ -23,16 +23,16 @@ final class PostGuardTest extends TestCase
 
     public function testRejectsInvalidIds(): void
     {
-        self::assertSame('lit_validation_failed', PostGuard::editable('abc', $this->schema())->get_error_code());
-        self::assertSame('lit_validation_failed', PostGuard::editable(-3, $this->schema())->get_error_code());
+        self::assertToolError(PostGuard::editable('abc', $this->schema()), 'validation_failed');
+        self::assertToolError(PostGuard::editable(-3, $this->schema()), 'validation_failed');
     }
 
     public function testMissingOrUnmanagedPostsAreNotFound(): void
     {
         Functions\when('get_post')->alias(static fn (int $id) => $id === 8 ? new WP_Post(['ID' => 8, 'post_type' => 'et_pb_layout']) : null);
 
-        self::assertSame('lit_not_found', PostGuard::editable(5, $this->schema())->get_error_code());
-        self::assertSame('lit_not_found', PostGuard::editable(8, $this->schema())->get_error_code());
+        self::assertToolError(PostGuard::editable(5, $this->schema()), 'not_found');
+        self::assertToolError(PostGuard::editable(8, $this->schema()), 'not_found');
     }
 
     public function testRequiresEditRightsAndReturnsThePost(): void
@@ -44,7 +44,7 @@ final class PostGuardTest extends TestCase
         self::assertSame($post, PostGuard::editable('5', $this->schema()));
 
         Functions\when('current_user_can')->justReturn(false);
-        self::assertSame('lit_forbidden', PostGuard::editable(5, $this->schema())->get_error_code());
+        self::assertToolError(PostGuard::editable(5, $this->schema()), 'forbidden');
     }
 
     public function testTrashedPostsAreRefusedUnlessAllowed(): void
@@ -54,8 +54,8 @@ final class PostGuardTest extends TestCase
         Functions\when('current_user_can')->justReturn(true);
 
         $error = PostGuard::editable(5, $this->schema(), 'copy_id');
-        self::assertSame('lit_validation_failed', $error->get_error_code());
-        self::assertArrayHasKey('copy_id', $error->get_error_data()['fields']);
+        self::assertToolError($error, 'validation_failed');
+        self::assertArrayHasKey('copy_id', self::errorFields($error));
         self::assertSame($post, PostGuard::editable(5, $this->schema(), 'post_id', true));
     }
 
@@ -107,8 +107,8 @@ final class PostGuardTest extends TestCase
 
         $error = PostGuard::statusStable($post, 'original_id');
 
-        self::assertSame('lit_validation_failed', $error->get_error_code());
-        self::assertArrayHasKey('original_id', $error->get_error_data()['fields']);
+        self::assertToolError($error, 'validation_failed');
+        self::assertArrayHasKey('original_id', self::errorFields($error));
         self::assertStringContainsString('lit-post-publish', $error->get_error_data()['hint']);
         self::assertStringContainsString('lit/post-schedule', $error->get_error_data()['hint']);
         self::assertNull(PostGuard::statusStable(new WP_Post(['post_status' => 'publish'])));
@@ -141,12 +141,12 @@ final class PostGuardTest extends TestCase
         Functions\when('get_post_type_object')->justReturn($object);
         Functions\when('current_user_can')->alias(static fn (string $cap): bool => $cap === 'edit_pages');
 
-        self::assertSame('lit_not_found', PostGuard::postType('et_pb_layout', $this->schema(), 'create_posts')->get_error_code());
-        self::assertSame('lit_not_found', PostGuard::postType(null, $this->schema(), 'create_posts')->get_error_code());
+        self::assertToolError(PostGuard::postType('et_pb_layout', $this->schema(), 'create_posts'), 'not_found');
+        self::assertToolError(PostGuard::postType(null, $this->schema(), 'create_posts'), 'not_found');
         self::assertSame($object, PostGuard::postType('page', $this->schema(), 'create_posts'));
 
         Functions\when('current_user_can')->justReturn(false);
-        self::assertSame('lit_forbidden', PostGuard::postType('page', $this->schema(), 'create_posts')->get_error_code());
+        self::assertToolError(PostGuard::postType('page', $this->schema(), 'create_posts'), 'forbidden');
     }
 
     public function testAssertNoMissedScheduleRefusesAMissedScheduleWithTheCallersMessage(): void
diff --git a/tests/Unit/Integrations/WordPress/Revisions/RevisionRestoreAbilityTest.php b/tests/Unit/Integrations/WordPress/Revisions/RevisionRestoreAbilityTest.php
index 9a6e110..1ec88e4 100644
--- a/tests/Unit/Integrations/WordPress/Revisions/RevisionRestoreAbilityTest.php
+++ b/tests/Unit/Integrations/WordPress/Revisions/RevisionRestoreAbilityTest.php
@@ -50,7 +50,7 @@ final class RevisionRestoreAbilityTest extends TestCase
 
     public function testNonRevisionIdIsNotFound(): void
     {
-        self::assertSame('lit_not_found', $this->ability(FakeChangeLog::forbidden())->permission(['revision_id' => 5])->get_error_code());
+        self::assertToolError($this->ability(FakeChangeLog::forbidden())->permission(['revision_id' => 5]), 'not_found');
     }
 
     public function testRequiresConfirmation(): void
@@ -59,7 +59,7 @@ final class RevisionRestoreAbilityTest extends TestCase
 
         $result = $this->ability(FakeChangeLog::forbidden())->execute(['revision_id' => 810]);
 
-        self::assertSame('lit_confirm_required', $result->get_error_code());
+        self::assertToolError($result, 'confirm_required');
     }
 
     public function testOrphanRevisionYieldsValidationErrorOnRevisionIdField(): void
@@ -71,8 +71,8 @@ final class RevisionRestoreAbilityTest extends TestCase
         $error = $this->ability(FakeChangeLog::forbidden())
             ->permission(['revision_id' => 810, 'rev' => 'irrelevant', 'confirm' => true]);
 
-        self::assertSame('lit_validation_failed', $error->get_error_code());
-        self::assertArrayHasKey('revision_id', $error->get_error_data()['fields']);
+        self::assertToolError($error, 'validation_failed');
+        self::assertArrayHasKey('revision_id', self::errorFields($error));
     }
 
     public function testATrashedEntryIsRefused(): void
@@ -86,8 +86,8 @@ final class RevisionRestoreAbilityTest extends TestCase
 
         $error = $this->ability(FakeChangeLog::forbidden())->execute(['revision_id' => 810, 'rev' => 'x', 'confirm' => true]);
 
-        self::assertSame('lit_validation_failed', $error->get_error_code());
-        self::assertArrayHasKey('revision_id', $error->get_error_data()['fields']);
+        self::assertToolError($error, 'validation_failed');
+        self::assertArrayHasKey('revision_id', self::errorFields($error));
     }
 
     public function testAMissedScheduleIsRefusedBecauseRestoringWouldPublish(): void
@@ -110,7 +110,7 @@ final class RevisionRestoreAbilityTest extends TestCase
             'confirm' => true,
         ]);
 
-        self::assertSame('lit_validation_failed', $error->get_error_code());
+        self::assertToolError($error, 'validation_failed');
         self::assertStringContainsString('lit-post-publish', $error->get_error_data()['hint']);
     }
 
@@ -121,7 +121,7 @@ final class RevisionRestoreAbilityTest extends TestCase
         $result = $this->ability(FakeChangeLog::forbidden())
             ->execute(['revision_id' => 810, 'confirm' => true, 'rev' => 'stale']);
 
-        self::assertSame('lit_rev_mismatch', $result->get_error_code());
+        self::assertToolError($result, 'rev_mismatch');
     }
 
     public function testSavesTheCurrentStateThenRestores(): void
@@ -173,7 +173,7 @@ final class RevisionRestoreAbilityTest extends TestCase
             'rev' => $this->currentRev(),
         ]);
 
-        self::assertSame('lit_write_failed', $result->get_error_code());
+        self::assertToolError($result, 'write_failed');
         self::assertStringNotContainsString('Nichts wurde gespeichert', $result->get_error_message());
         self::assertStringContainsString(
             'Die Änderung wurde nicht gespeichert (Revision 813 bleibt erhalten).',
diff --git a/tests/Unit/Integrations/WordPress/Revisions/RevisionsListAbilityTest.php b/tests/Unit/Integrations/WordPress/Revisions/RevisionsListAbilityTest.php
index 8474782..69aac9e 100644
--- a/tests/Unit/Integrations/WordPress/Revisions/RevisionsListAbilityTest.php
+++ b/tests/Unit/Integrations/WordPress/Revisions/RevisionsListAbilityTest.php
@@ -79,6 +79,6 @@ final class RevisionsListAbilityTest extends TestCase
     {
         $result = $this->ability()->execute(['post_id' => 5, 'cursor' => 'not-a-cursor']);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
     }
 }
diff --git a/tests/Unit/Integrations/WordPress/Settings/WpSettingsAbilitiesTest.php b/tests/Unit/Integrations/WordPress/Settings/WpSettingsAbilitiesTest.php
index 01382d0..9ac1386 100644
--- a/tests/Unit/Integrations/WordPress/Settings/WpSettingsAbilitiesTest.php
+++ b/tests/Unit/Integrations/WordPress/Settings/WpSettingsAbilitiesTest.php
@@ -120,8 +120,8 @@ final class WpSettingsAbilitiesTest extends TestCase
         $result = (new WpSettingsUpdateAbility(new SettingsWhitelist(), FakeChangeLog::forbidden()))
             ->execute(['settings' => ['blogname' => 'Neu', 'admin_email' => 'x@y.de']]);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
-        self::assertArrayHasKey('admin_email', $result->get_error_data()['fields']);
+        self::assertToolError($result, 'validation_failed');
+        self::assertArrayHasKey('admin_email', self::errorFields($result));
     }
 
     public function testRejectsAGenuinelyUnknownKey(): void
@@ -131,8 +131,8 @@ final class WpSettingsAbilitiesTest extends TestCase
         $result = (new WpSettingsUpdateAbility(new SettingsWhitelist(), FakeChangeLog::forbidden()))
             ->execute(['settings' => ['template' => 'twentyten']]);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
-        self::assertArrayHasKey('template', $result->get_error_data()['fields']);
+        self::assertToolError($result, 'validation_failed');
+        self::assertArrayHasKey('template', self::errorFields($result));
     }
 
     public function testUpdatesOnlyChangedValuesAndFlushesPermalinks(): void
@@ -221,7 +221,7 @@ final class WpSettingsAbilitiesTest extends TestCase
             'blogdescription' => 'Auch neu',
         ]);
 
-        self::assertSame('lit_write_failed', $error->get_error_code());
+        self::assertToolError($error, 'write_failed');
         $log->assertRecorded();
         $log->assertPartials();
     }
@@ -252,7 +252,7 @@ final class WpSettingsAbilitiesTest extends TestCase
             'blogdescription' => 'Auch neu',
         ]);
 
-        self::assertSame('lit_partial_write', $error->get_error_code());
+        self::assertToolError($error, 'partial_write');
         self::assertStringContainsString('blogname', $error->get_error_message());
         self::assertStringContainsString('lit/wp-settings-get', $error->get_error_data()['hint']);
         $log->assertPartials([
@@ -278,7 +278,7 @@ final class WpSettingsAbilitiesTest extends TestCase
     {
         Functions\when('current_user_can')->alias(static fn (string $cap): bool => $cap !== 'manage_options');
 
-        self::assertSame('lit_forbidden', (new WpSettingsGetAbility(new SettingsWhitelist()))->permission([])->get_error_code());
+        self::assertToolError((new WpSettingsGetAbility(new SettingsWhitelist()))->permission([]), 'forbidden');
         self::assertSame(
             'lit_forbidden',
             (new WpSettingsUpdateAbility(new SettingsWhitelist(), FakeChangeLog::forbidden()))->permission([])->get_error_code()
diff --git a/tests/Unit/Integrations/WordPress/Status/PostClonerTest.php b/tests/Unit/Integrations/WordPress/Status/PostClonerTest.php
index f2c58c0..55862cb 100644
--- a/tests/Unit/Integrations/WordPress/Status/PostClonerTest.php
+++ b/tests/Unit/Integrations/WordPress/Status/PostClonerTest.php
@@ -230,7 +230,7 @@ final class PostClonerTest extends TestCase
             self::fail('WriteFailed expected');
         } catch (WriteFailed $e) {
             self::assertTrue($e->rolledBack);
-            self::assertSame('lit_write_failed', $e->toError()->get_error_code());
+            self::assertToolError($e->toError(), 'write_failed');
         }
     }
 
diff --git a/tests/Unit/Integrations/WordPress/Status/PostDuplicateAbilityTest.php b/tests/Unit/Integrations/WordPress/Status/PostDuplicateAbilityTest.php
index 4a6ee0f..498925e 100644
--- a/tests/Unit/Integrations/WordPress/Status/PostDuplicateAbilityTest.php
+++ b/tests/Unit/Integrations/WordPress/Status/PostDuplicateAbilityTest.php
@@ -71,7 +71,7 @@ final class PostDuplicateAbilityTest extends TestCase
 
         $result = $this->ability($cloner, FakeChangeLog::forbidden())->execute(['post_id' => 5]);
 
-        self::assertSame('lit_forbidden', $result->get_error_code());
+        self::assertToolError($result, 'forbidden');
     }
 
     public function testATrashedOriginalIsRefused(): void
@@ -82,7 +82,7 @@ final class PostDuplicateAbilityTest extends TestCase
 
         $result = $this->ability($cloner, FakeChangeLog::forbidden())->execute(['post_id' => 5]);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
     }
 
     public function testIsIndirectAndNotIdempotent(): void
diff --git a/tests/Unit/Integrations/WordPress/Status/PostPublishAbilityTest.php b/tests/Unit/Integrations/WordPress/Status/PostPublishAbilityTest.php
index 024ae10..87fc9d1 100644
--- a/tests/Unit/Integrations/WordPress/Status/PostPublishAbilityTest.php
+++ b/tests/Unit/Integrations/WordPress/Status/PostPublishAbilityTest.php
@@ -95,7 +95,7 @@ final class PostPublishAbilityTest extends TestCase
             self::assertFalse($e->rolledBack);
             self::assertSame(5, $e->postId);
             self::assertStringContainsString('save_post-Hook kaputt', $e->getMessage());
-            self::assertSame('lit_partial_write', $e->toError()->get_error_code());
+            self::assertToolError($e->toError(), 'partial_write');
         }
         $log->assertPartials(['ability' => 'lit/post-publish', 'object_type' => 'page', 'object_id' => '5']);
     }
@@ -107,7 +107,7 @@ final class PostPublishAbilityTest extends TestCase
 
         $result = $this->ability(FakeChangeLog::forbidden())->execute(['post_id' => 5]);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
     }
 
     public function testPermissionNeedsPublishRights(): void
@@ -115,6 +115,6 @@ final class PostPublishAbilityTest extends TestCase
         $this->stubPost(['post_status' => 'draft']);
         Functions\when('current_user_can')->alias(static fn (string $cap): bool => $cap !== 'publish_post');
 
-        self::assertSame('lit_forbidden', $this->ability(FakeChangeLog::forbidden())->permission(['post_id' => 5])->get_error_code());
+        self::assertToolError($this->ability(FakeChangeLog::forbidden())->permission(['post_id' => 5]), 'forbidden');
     }
 }
diff --git a/tests/Unit/Integrations/WordPress/Status/PostReplaceWithCopyAbilityTest.php b/tests/Unit/Integrations/WordPress/Status/PostReplaceWithCopyAbilityTest.php
index cd84fe7..48f7ced 100644
--- a/tests/Unit/Integrations/WordPress/Status/PostReplaceWithCopyAbilityTest.php
+++ b/tests/Unit/Integrations/WordPress/Status/PostReplaceWithCopyAbilityTest.php
@@ -68,7 +68,7 @@ final class PostReplaceWithCopyAbilityTest extends TestCase
         $result = $this->ability($cloner, FakeChangeLog::forbidden())
             ->execute(['original_id' => 5, 'copy_id' => 77, 'rev' => Rev::ofPost($this->original)]);
 
-        self::assertSame('lit_confirm_required', $result->get_error_code());
+        self::assertToolError($result, 'confirm_required');
     }
 
     public function testCopyMustStemFromTheOriginal(): void
@@ -79,7 +79,7 @@ final class PostReplaceWithCopyAbilityTest extends TestCase
 
         $result = $this->ability($cloner, FakeChangeLog::forbidden())->execute($this->input());
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
     }
 
     /** @return list<array{0: string, 1: string}> */
@@ -103,7 +103,7 @@ final class PostReplaceWithCopyAbilityTest extends TestCase
 
         $result = $this->ability($cloner, FakeChangeLog::forbidden())->execute($this->input());
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
     }
 
     public function testAMissedScheduleOfTheOriginalIsRefused(): void
@@ -116,8 +116,8 @@ final class PostReplaceWithCopyAbilityTest extends TestCase
 
         $result = $this->ability($cloner, FakeChangeLog::forbidden())->execute($this->input());
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
-        self::assertArrayHasKey('original_id', $result->get_error_data()['fields']);
+        self::assertToolError($result, 'validation_failed');
+        self::assertArrayHasKey('original_id', self::errorFields($result));
     }
 
     public function testAPendingCopyIsAccepted(): void
@@ -142,7 +142,7 @@ final class PostReplaceWithCopyAbilityTest extends TestCase
 
         $result = $this->ability($cloner, FakeChangeLog::forbidden())->execute($this->input(['rev' => 'veraltet']));
 
-        self::assertSame('lit_rev_mismatch', $result->get_error_code());
+        self::assertToolError($result, 'rev_mismatch');
     }
 
     public function testDisabledTrashIsRefusedBeforeAnyChange(): void
@@ -152,7 +152,7 @@ final class PostReplaceWithCopyAbilityTest extends TestCase
 
         $result = $this->ability($cloner, FakeChangeLog::forbidden(), false)->execute($this->input());
 
-        self::assertSame('lit_unsupported', $result->get_error_code());
+        self::assertToolError($result, 'unsupported');
     }
 
     public function testRevisionThenReplaceThenTrashTheCopy(): void
@@ -232,7 +232,7 @@ final class PostReplaceWithCopyAbilityTest extends TestCase
             self::fail('WriteFailed expected');
         } catch (WriteFailed $e) {
             self::assertTrue($e->rolledBack);
-            self::assertSame('lit_write_failed', $e->toError()->get_error_code());
+            self::assertToolError($e->toError(), 'write_failed');
             self::assertSame(88, $e->toError()->get_error_data()['backup_id']);
             $message = $e->toError()->get_error_message();
             self::assertStringNotContainsString('Nichts wurde gespeichert', $message);
@@ -259,7 +259,7 @@ final class PostReplaceWithCopyAbilityTest extends TestCase
             self::assertSame(5, $e->postId);
             self::assertSame(900, $e->revisionId);
             $error = $e->toError();
-            self::assertSame('lit_partial_write', $error->get_error_code());
+            self::assertToolError($error, 'partial_write');
             self::assertSame(88, $error->get_error_data()['backup_id']);
             self::assertStringContainsString('Sicherung', $error->get_error_message());
             self::assertStringContainsString('88', $error->get_error_message());
@@ -280,8 +280,8 @@ final class PostReplaceWithCopyAbilityTest extends TestCase
         $cloner->expects('replace')->never();
         $ability = $this->ability($cloner, FakeChangeLog::forbidden());
 
-        self::assertSame('lit_forbidden', $ability->permission($this->input())->get_error_code());
-        self::assertSame('lit_forbidden', $ability->execute($this->input())->get_error_code());
+        self::assertToolError($ability->permission($this->input()), 'forbidden');
+        self::assertToolError($ability->execute($this->input()), 'forbidden');
     }
 
     public function testAFailedBackupAbortsBeforeTheOriginalIsTouched(): void
@@ -298,7 +298,7 @@ final class PostReplaceWithCopyAbilityTest extends TestCase
             $this->ability($cloner, FakeChangeLog::forbidden())->execute($this->input());
             self::fail('WriteFailed expected');
         } catch (WriteFailed $e) {
-            self::assertSame('lit_write_failed', $e->toError()->get_error_code());
+            self::assertToolError($e->toError(), 'write_failed');
         }
     }
 
diff --git a/tests/Unit/Integrations/WordPress/Status/PostScheduleAbilityTest.php b/tests/Unit/Integrations/WordPress/Status/PostScheduleAbilityTest.php
index 12fa5a3..e36c3a2 100644
--- a/tests/Unit/Integrations/WordPress/Status/PostScheduleAbilityTest.php
+++ b/tests/Unit/Integrations/WordPress/Status/PostScheduleAbilityTest.php
@@ -92,7 +92,7 @@ final class PostScheduleAbilityTest extends TestCase
             self::assertFalse($e->rolledBack);
             self::assertSame(5, $e->postId);
             self::assertStringContainsString('save_post-Hook kaputt', $e->getMessage());
-            self::assertSame('lit_partial_write', $e->toError()->get_error_code());
+            self::assertToolError($e->toError(), 'partial_write');
         }
         $log->assertPartials(['ability' => 'lit/post-schedule', 'object_type' => 'post', 'object_id' => '5']);
     }
@@ -104,7 +104,7 @@ final class PostScheduleAbilityTest extends TestCase
 
         $result = $this->ability(FakeChangeLog::forbidden())->execute(['post_id' => 5, 'date' => '2099-06-01 09:30']);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
     }
 
     public function testPastAndImpossibleDatesAreRejected(): void
@@ -115,9 +115,9 @@ final class PostScheduleAbilityTest extends TestCase
         $past = $ability->execute(['post_id' => 5, 'date' => '2020-01-01 10:00']);
         $impossible = $ability->execute(['post_id' => 5, 'date' => '2099-13-45 10:00']);
 
-        self::assertSame('lit_validation_failed', $past->get_error_code());
+        self::assertToolError($past, 'validation_failed');
         self::assertStringContainsString('Zukunft', $past->get_error_message());
-        self::assertSame('lit_validation_failed', $impossible->get_error_code());
+        self::assertToolError($impossible, 'validation_failed');
     }
 
     public function testADateWithinTheNextMinuteIsRejectedBecauseWordPressWouldPublishAtOnce(): void
@@ -131,6 +131,6 @@ final class PostScheduleAbilityTest extends TestCase
         $result = $this->ability(FakeChangeLog::forbidden())
             ->execute(['post_id' => 5, 'date' => $soon->format('Y-m-d H:i')]);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
     }
 }
diff --git a/tests/Unit/Integrations/WordPress/Status/PostTrashAbilityTest.php b/tests/Unit/Integrations/WordPress/Status/PostTrashAbilityTest.php
index df680e4..38cfeb0 100644
--- a/tests/Unit/Integrations/WordPress/Status/PostTrashAbilityTest.php
+++ b/tests/Unit/Integrations/WordPress/Status/PostTrashAbilityTest.php
@@ -44,7 +44,7 @@ final class PostTrashAbilityTest extends TestCase
 
         $result = $this->ability(FakeChangeLog::forbidden())->execute(['post_id' => 5]);
 
-        self::assertSame('lit_confirm_required', $result->get_error_code());
+        self::assertToolError($result, 'confirm_required');
     }
 
     public function testMovesToTrashNeverDeletes(): void
@@ -76,7 +76,7 @@ final class PostTrashAbilityTest extends TestCase
 
         $result = $ability->execute(['post_id' => 5, 'confirm' => true]);
 
-        self::assertSame('lit_unsupported', $result->get_error_code());
+        self::assertToolError($result, 'unsupported');
     }
 
     public function testAHookThrowingDuringTheWriteIsAPartialWrite(): void
@@ -93,7 +93,7 @@ final class PostTrashAbilityTest extends TestCase
             self::assertFalse($e->rolledBack);
             self::assertSame(5, $e->postId);
             self::assertStringContainsString('save_post-Hook kaputt', $e->getMessage());
-            self::assertSame('lit_partial_write', $e->toError()->get_error_code());
+            self::assertToolError($e->toError(), 'partial_write');
         }
         $log->assertPartials(['ability' => 'lit/post-trash', 'object_type' => 'page', 'object_id' => '5']);
     }
diff --git a/tests/Unit/Integrations/WordPress/Status/PostUnpublishAbilityTest.php b/tests/Unit/Integrations/WordPress/Status/PostUnpublishAbilityTest.php
index 11eee39..e5bd54f 100644
--- a/tests/Unit/Integrations/WordPress/Status/PostUnpublishAbilityTest.php
+++ b/tests/Unit/Integrations/WordPress/Status/PostUnpublishAbilityTest.php
@@ -52,7 +52,7 @@ final class PostUnpublishAbilityTest extends TestCase
             self::assertFalse($e->rolledBack);
             self::assertSame(5, $e->postId);
             self::assertStringContainsString('save_post-Hook kaputt', $e->getMessage());
-            self::assertSame('lit_partial_write', $e->toError()->get_error_code());
+            self::assertToolError($e->toError(), 'partial_write');
         }
         $log->assertPartials(['ability' => 'lit/post-unpublish', 'object_type' => 'page', 'object_id' => '5']);
     }
@@ -63,7 +63,7 @@ final class PostUnpublishAbilityTest extends TestCase
 
         $result = $this->ability(FakeChangeLog::forbidden(), 'trash')->execute(['post_id' => 5]);
 
-        self::assertSame('lit_validation_failed', $result->get_error_code());
+        self::assertToolError($result, 'validation_failed');
     }
 
     public function testDraftIsANoOp(): void
diff --git a/tests/Unit/Integrations/WordPress/WordPressIntegrationTest.php b/tests/Unit/Integrations/WordPress/WordPressIntegrationTest.php
index ccdd205..b96c3e2 100644
--- a/tests/Unit/Integrations/WordPress/WordPressIntegrationTest.php
+++ b/tests/Unit/Integrations/WordPress/WordPressIntegrationTest.php
@@ -80,6 +80,6 @@ final class WordPressIntegrationTest extends TestCase
         ))[0];
         $result = $create->execute(['post_type' => 'page', 'title' => 'Start', 'divi' => [['type' => 'section']]]);
 
-        self::assertSame('lit_unsupported', $result->get_error_code());
+        self::assertToolError($result, 'unsupported');
     }
 }
diff --git a/tests/Unit/Support/AssertsToolErrorsTest.php b/tests/Unit/Support/AssertsToolErrorsTest.php
new file mode 100644
index 0000000..7590084
--- /dev/null
+++ b/tests/Unit/Support/AssertsToolErrorsTest.php
@@ -0,0 +1,44 @@
+<?php
+
+declare(strict_types=1);
+
+namespace LIT\MCP\Tests\Unit\Support;
+
+use LIT\MCP\Abilities\Errors;
+use LIT\MCP\Tests\Unit\TestCase;
+use PHPUnit\Framework\AssertionFailedError;
+use WP_Error;
+
+final class AssertsToolErrorsTest extends TestCase
+{
+    public function testReturnsTheClientPayloadOfAMatchingError(): void
+    {
+        $payload = self::assertToolError(Errors::invalidInput('Titel fehlt.', ['title' => 'Pflichtfeld']), 'validation_failed');
+
+        self::assertSame('Titel fehlt.', $payload['message']);
+        self::assertSame(['title' => 'Pflichtfeld'], self::errorFields(Errors::invalidInput('x', ['title' => 'Pflichtfeld'])));
+    }
+
+    /** @return array<string, array{callable(): mixed}> Built in the test: Errors needs the stubs of setUp(). */
+    public static function notTheError(): array
+    {
+        return [
+            'another code' => [static fn () => Errors::notFound('weg')],
+            'no JSON payload' => [static fn () => new WP_Error('lit_validation_failed', 'kein JSON')],
+            'no error at all' => [static fn () => ['ok' => true]],
+        ];
+    }
+
+    /** @dataProvider notTheError */
+    public function testFailsForAnythingElse(callable $result): void
+    {
+        $this->expectException(AssertionFailedError::class);
+        self::assertToolError($result(), 'validation_failed');
+    }
+
+    public function testErrorFieldsFailsWithoutFields(): void
+    {
+        $this->expectException(AssertionFailedError::class);
+        self::errorFields(Errors::invalidInput('x'));
+    }
+}
diff --git a/tests/Unit/TestCase.php b/tests/Unit/TestCase.php
index 3636e88..7fa1e2c 100644
--- a/tests/Unit/TestCase.php
+++ b/tests/Unit/TestCase.php
@@ -6,12 +6,14 @@ namespace LIT\MCP\Tests\Unit;
 
 use Brain\Monkey;
 use Brain\Monkey\Functions;
+use LIT\MCP\Tests\Support\AssertsToolErrors;
 use LIT\MCP\Tests\Support\FakeChangeLog;
 use Mockery\Adapter\Phpunit\MockeryPHPUnitIntegration;
 use PHPUnit\Framework\TestCase as BaseTestCase;
 
 abstract class TestCase extends BaseTestCase
 {
+    use AssertsToolErrors;
     use MockeryPHPUnitIntegration;
 
     protected function setUp(): void