diff --git a/src/Integrations/Divi/BlockTree/AttrMap.php b/src/Integrations/Divi/BlockTree/AttrMap.php
index aa784f6..dd977ad 100644
--- a/src/Integrations/Divi/BlockTree/AttrMap.php
+++ b/src/Integrations/Divi/BlockTree/AttrMap.php
@@ -4,6 +4,7 @@ declare(strict_types=1);
 
 namespace LIT\MCP\Integrations\Divi\BlockTree;
 
+use InvalidArgumentException;
 use stdClass;
 
 /**
@@ -52,6 +53,9 @@ final class AttrMap
     {
         $value = $attrs;
         foreach (explode('.', $path) as $segment) {
+            if ($value instanceof stdClass) {
+                $value = (array) $value;
+            }
             if (! is_array($value) || ! array_key_exists($segment, $value)) {
                 return null;
             }
@@ -76,10 +80,15 @@ final class AttrMap
     /**
      * @param array<string, mixed> $attrs
      * @return array<string, mixed>
+     * @throws InvalidArgumentException When $path has an empty segment ("a..b", ".a", "a.").
      */
     public static function set(array $attrs, string $path, mixed $value): array
     {
         $segments = explode('.', $path);
+        if (in_array('', $segments, true)) {
+            throw new InvalidArgumentException(sprintf('"%s" has an empty path segment.', $path));
+        }
+
         $key = (string) array_shift($segments);
         if ($segments === []) {
             $attrs[$key] = $value;
@@ -87,9 +96,13 @@ final class AttrMap
             return $attrs;
         }
 
-        $child = $attrs[$key] ?? [];
-        $child = $child instanceof stdClass ? (array) $child : (is_array($child) ? $child : []);
-        $attrs[$key] = self::set($child, implode('.', $segments), $value);
+        $existing = $attrs[$key] ?? null;
+        $wasObject = $existing instanceof stdClass;
+        $child = $wasObject ? (array) $existing : (is_array($existing) ? $existing : []);
+        $child = self::set($child, implode('.', $segments), $value);
+        // Keep an object an object (spec §5.2/Json.php): a numeric-looking key like "0" would
+        // otherwise silently turn $child into a PHP list and re-encode as a JSON array.
+        $attrs[$key] = $wasObject ? (object) $child : $child;
 
         return $attrs;
     }
diff --git a/src/Integrations/Divi/BlockTree/Block.php b/src/Integrations/Divi/BlockTree/Block.php
index 532f8ae..e7212e2 100644
--- a/src/Integrations/Divi/BlockTree/Block.php
+++ b/src/Integrations/Divi/BlockTree/Block.php
@@ -4,6 +4,10 @@ declare(strict_types=1);
 
 namespace LIT\MCP\Integrations\Divi\BlockTree;
 
+use InvalidArgumentException;
+use LogicException;
+use stdClass;
+
 /**
  * One block of the tree. A parsed block keeps its original opener/closer bytes; they are
  * dropped only when the block itself changes (setAttrs(), a void block getting children),
@@ -15,6 +19,9 @@ namespace LIT\MCP\Integrations\Divi\BlockTree;
  */
 final class Block extends Container
 {
+    /** A valid block name: "namespace/block-name" (WordPress' own block name convention). */
+    private const NAME_PATTERN = '/^[a-z][a-z0-9_-]*\/[a-z][a-z0-9_-]*$/';
+
     /**
      * @param array<string, mixed> $attrs
      */
@@ -42,6 +49,10 @@ final class Block extends Container
      */
     public static function create(string $name, array $attrs = [], array $children = []): self
     {
+        if (preg_match(self::NAME_PATTERN, $name) !== 1) {
+            throw new InvalidArgumentException(sprintf('"%s" is not a valid block name.', $name));
+        }
+
         $block = new self($name, $attrs, null, null, false, false);
         foreach ($children as $child) {
             $block->parts[] = $child;
@@ -77,6 +88,10 @@ final class Block extends Container
     /** @param array<string, mixed> $attrs */
     public function setAttrs(array $attrs): void
     {
+        if ($this->opaque) {
+            throw new LogicException('Cannot set attributes of an opaque block.');
+        }
+
         $this->attrs = $attrs;
         $this->rawOpen = null;
     }
@@ -125,6 +140,28 @@ final class Block extends Container
                 $this->parts[$position] = clone $part;
             }
         }
+        $this->attrs = self::deepCloneAttrs($this->attrs);
+    }
+
+    /**
+     * PHP's own array-copy-on-write already deep-copies nested arrays, but stdClass values
+     * (Json.php keeps empty/list-like JSON objects as stdClass) are objects – clone() alone
+     * would leave the copy's attrs() sharing them with the original.
+     *
+     * @param array<string, mixed> $attrs
+     * @return array<string, mixed>
+     */
+    private static function deepCloneAttrs(array $attrs): array
+    {
+        foreach ($attrs as $key => $value) {
+            if ($value instanceof stdClass) {
+                $attrs[$key] = (object) self::deepCloneAttrs((array) $value);
+            } elseif (is_array($value)) {
+                $attrs[$key] = self::deepCloneAttrs($value);
+            }
+        }
+
+        return $attrs;
     }
 
     protected function touched(): void
diff --git a/src/Integrations/Divi/BlockTree/Container.php b/src/Integrations/Divi/BlockTree/Container.php
index 08cd5f1..610e58b 100644
--- a/src/Integrations/Divi/BlockTree/Container.php
+++ b/src/Integrations/Divi/BlockTree/Container.php
@@ -5,6 +5,7 @@ declare(strict_types=1);
 namespace LIT\MCP\Integrations\Divi\BlockTree;
 
 use InvalidArgumentException;
+use LogicException;
 
 /**
  * Ordered parts of a document or of a block's inner content: blocks and the raw text
@@ -28,9 +29,21 @@ abstract class Container
         return array_values(array_filter($this->parts, static fn (string|Block $part): bool => $part instanceof Block));
     }
 
-    /** Used by the Parser: adds a part without marking the container as changed. */
+    /**
+     * Adds a part without marking the container as changed – the Parser's own way of
+     * building the tree (a block is appended to its container immediately, before parsing
+     * knows whether it stays void).
+     *
+     * @internal Parser only. Any other caller adding a child must use insertChild(), which
+     *           also converts a void block so the new child is not silently dropped again
+     *           (Serializer::block() never descends into a void block's parts()).
+     */
     public function append(string|Block $part): void
     {
+        if ($this instanceof Block && $this->isVoid()) {
+            throw new LogicException('Cannot append to a void block; use insertChild() instead.');
+        }
+
         $this->parts[] = $part;
     }
 
@@ -48,6 +61,7 @@ abstract class Container
      */
     public function insertChild(int $index, Block $block): void
     {
+        $this->guardMutable();
         $children = $this->children();
         $index = max(0, min($index, count($children)));
         if ($index < count($children)) {
@@ -65,6 +79,7 @@ abstract class Container
     /** Removes the child block; the text around it stays. Returns its former child index. */
     public function removeChild(Block $block): int
     {
+        $this->guardMutable();
         $index = $this->indexOf($block);
         if ($index === null) {
             throw new InvalidArgumentException('Block is not a child of this container.');
@@ -81,6 +96,14 @@ abstract class Container
     {
     }
 
+    /** insertChild()/removeChild() are the tools' structural edits – never on an opaque block. */
+    private function guardMutable(): void
+    {
+        if ($this instanceof Block && $this->isOpaque()) {
+            throw new LogicException('Cannot modify the children of an opaque block.');
+        }
+    }
+
     private function partPosition(Block $block): int
     {
         $position = array_search($block, $this->parts, true);
diff --git a/src/Integrations/Divi/BlockTree/Json.php b/src/Integrations/Divi/BlockTree/Json.php
index f865d34..e479df9 100644
--- a/src/Integrations/Divi/BlockTree/Json.php
+++ b/src/Integrations/Divi/BlockTree/Json.php
@@ -15,7 +15,11 @@ use stdClass;
  */
 final class Json
 {
-    /** @return array<string, mixed>|null Null when $json is not a JSON object. */
+    /**
+     * @return array<string, mixed>|null Null when $json is not a JSON object, or its top level
+     *      is one only nominally (keys "0", "1", … – see the class docblock): re-encoding those
+     *      keys as a PHP array would silently turn the object back into a JSON list.
+     */
     public static function decodeObject(string $json): ?array
     {
         try {
@@ -28,6 +32,11 @@ final class Json
             return null;
         }
 
+        $keys = get_object_vars($value);
+        if ($keys !== [] && array_is_list($keys)) {
+            return null;
+        }
+
         $converted = self::fromJson($value);
 
         return is_array($converted) ? $converted : [];
@@ -52,7 +61,13 @@ final class Json
         return $value;
     }
 
-    /** @throws JsonException */
+    /**
+     * @throws JsonException When $value cannot be represented as JSON – including a string
+     *      with an invalid UTF-8 byte sequence (JSON_ERROR_UTF8). 2a only ever encodes
+     *      attributes this plugin already decoded from valid block markup, so that never
+     *      happens here; a caller that later encodes user-supplied strings (2b's writers)
+     *      must catch this and map it to a validation error instead of a fatal.
+     */
     public static function encode(mixed $value): string
     {
         // phpcs:ignore WordPress.WP.AlternativeFunctions.json_encode_json_encode -- pure PHP (spec §4.4).
diff --git a/src/Integrations/Divi/BlockTree/Serializer.php b/src/Integrations/Divi/BlockTree/Serializer.php
index b298530..76d55ee 100644
--- a/src/Integrations/Divi/BlockTree/Serializer.php
+++ b/src/Integrations/Divi/BlockTree/Serializer.php
@@ -8,7 +8,8 @@ namespace LIT\MCP\Integrations\Divi\BlockTree;
  * Turns a Document back into block markup. Unchanged blocks reuse their original
  * delimiters; changed or new blocks get delimiters generated exactly like WordPress'
  * get_comment_delimited_block_content() / serialize_block_attributes() do, which also
- * yields the escaping Divi expects inside $variable(…)$ payloads (" → ").
+ * yields the escaping Divi expects inside $variable(…)$ payloads: json_encode()'s own
+ * \" (an escaped quote inside a JSON string) is replaced by ", see ESCAPES below.
  */
 final class Serializer
 {
diff --git a/tests/Unit/Integrations/Divi/BlockTree/AttrMapTest.php b/tests/Unit/Integrations/Divi/BlockTree/AttrMapTest.php
new file mode 100644
index 0000000..1ae665a
--- /dev/null
+++ b/tests/Unit/Integrations/Divi/BlockTree/AttrMapTest.php
@@ -0,0 +1,59 @@
+<?php
+
+declare(strict_types=1);
+
+namespace LIT\MCP\Tests\Unit\Integrations\Divi\BlockTree;
+
+use InvalidArgumentException;
+use LIT\MCP\Integrations\Divi\BlockTree\AttrMap;
+use PHPUnit\Framework\TestCase;
+use stdClass;
+
+final class AttrMapTest extends TestCase
+{
+    /** @return array<string, array{string}> */
+    public static function emptySegmentPaths(): array
+    {
+        return [
+            'double dot' => ['a..b'],
+            'leading dot' => ['.a'],
+            'trailing dot' => ['a.'],
+            'empty' => [''],
+        ];
+    }
+
+    /** @dataProvider emptySegmentPaths */
+    public function testSetRejectsEmptyPathSegments(string $path): void
+    {
+        $this->expectException(InvalidArgumentException::class);
+
+        AttrMap::set([], $path, 'z');
+    }
+
+    public function testSetKeepsAnObjectAnObjectWhenRewrapping(): void
+    {
+        $attrs = ['a' => new stdClass()];
+
+        $changed = AttrMap::set($attrs, 'a.0', 'z');
+
+        self::assertInstanceOf(stdClass::class, $changed['a']);
+        self::assertSame('z', $changed['a']->{'0'});
+        self::assertSame('{"a":{"0":"z"}}', json_encode($changed));
+    }
+
+    public function testSetStillBuildsPlainArraysWhenNoObjectExisted(): void
+    {
+        $changed = AttrMap::set([], 'a.b', 'z');
+
+        self::assertSame(['a' => ['b' => 'z']], $changed);
+    }
+
+    /** get() must still read back through a value set() kept as an object (regression guard). */
+    public function testGetReadsThroughAnObjectRewrappedBySet(): void
+    {
+        $attrs = ['a' => new stdClass()];
+        $changed = AttrMap::set($attrs, 'a.b', 'z');
+
+        self::assertSame('z', AttrMap::get($changed, 'a.b'));
+    }
+}
diff --git a/tests/Unit/Integrations/Divi/BlockTree/BlockTest.php b/tests/Unit/Integrations/Divi/BlockTree/BlockTest.php
new file mode 100644
index 0000000..ba1b31a
--- /dev/null
+++ b/tests/Unit/Integrations/Divi/BlockTree/BlockTest.php
@@ -0,0 +1,69 @@
+<?php
+
+declare(strict_types=1);
+
+namespace LIT\MCP\Tests\Unit\Integrations\Divi\BlockTree;
+
+use InvalidArgumentException;
+use LIT\MCP\Integrations\Divi\BlockTree\Block;
+use LIT\MCP\Integrations\Divi\BlockTree\Parser;
+use LogicException;
+use PHPUnit\Framework\TestCase;
+use stdClass;
+
+final class BlockTest extends TestCase
+{
+    /** @return array<string, array{string}> */
+    public static function invalidNames(): array
+    {
+        return [
+            'no namespace' => ['button'],
+            'trailing markup injection' => ['divi/x --><script>'],
+            'uppercase' => ['Divi/Button'],
+            'empty' => [''],
+        ];
+    }
+
+    /** @dataProvider invalidNames */
+    public function testCreateRejectsInvalidBlockNames(string $name): void
+    {
+        $this->expectException(InvalidArgumentException::class);
+
+        Block::create($name);
+    }
+
+    public function testCreateAcceptsAWellFormedName(): void
+    {
+        self::assertSame('divi/button', Block::create('divi/button')->name());
+    }
+
+    public function testSetAttrsRefusesOnAnOpaqueGlobalModule(): void
+    {
+        $document = (new Parser())->parse('<!-- wp:divi/section {"globalModule":"x"} --><!-- /wp:divi/section -->');
+        $section = $document->children()[0];
+        self::assertTrue($section->isOpaque());
+
+        $this->expectException(LogicException::class);
+        $section->setAttrs(['a' => 1]);
+    }
+
+    public function testSetAttrsRefusesOnABlockWithUndecodableAttrs(): void
+    {
+        $document = (new Parser())->parse('<!-- wp:divi/text {"a":} /-->');
+        $block = $document->children()[0];
+        self::assertTrue($block->isOpaque());
+
+        $this->expectException(LogicException::class);
+        $block->setAttrs(['a' => 1]);
+    }
+
+    public function testCopyDeepClonesStdClassAttributeValues(): void
+    {
+        $block = Block::create('divi/text', ['localAttrs' => new stdClass(), 'nested' => ['obj' => new stdClass()]]);
+        $copy = $block->copy();
+
+        self::assertNotSame($block->attrs()['localAttrs'], $copy->attrs()['localAttrs']);
+        self::assertEquals($block->attrs()['localAttrs'], $copy->attrs()['localAttrs']);
+        self::assertNotSame($block->attrs()['nested']['obj'], $copy->attrs()['nested']['obj']);
+    }
+}
diff --git a/tests/Unit/Integrations/Divi/BlockTree/ContainerTest.php b/tests/Unit/Integrations/Divi/BlockTree/ContainerTest.php
new file mode 100644
index 0000000..8fd85d9
--- /dev/null
+++ b/tests/Unit/Integrations/Divi/BlockTree/ContainerTest.php
@@ -0,0 +1,54 @@
+<?php
+
+declare(strict_types=1);
+
+namespace LIT\MCP\Tests\Unit\Integrations\Divi\BlockTree;
+
+use LIT\MCP\Integrations\Divi\BlockTree\Block;
+use LIT\MCP\Integrations\Divi\BlockTree\Parser;
+use LogicException;
+use PHPUnit\Framework\TestCase;
+
+final class ContainerTest extends TestCase
+{
+    public function testInsertChildRefusesOnAnOpaqueBlock(): void
+    {
+        $document = (new Parser())->parse('<!-- wp:divi/section {"globalModule":"x"} --><!-- /wp:divi/section -->');
+        $section = $document->children()[0];
+        self::assertTrue($section->isOpaque());
+
+        $this->expectException(LogicException::class);
+        $section->insertChild(0, Block::create('divi/text'));
+    }
+
+    public function testInsertChildRefusesOnABlockWithUndecodableAttrs(): void
+    {
+        $document = (new Parser())->parse('<!-- wp:divi/section {"a":} --><!-- /wp:divi/section -->');
+        $section = $document->children()[0];
+        self::assertTrue($section->isOpaque());
+
+        $this->expectException(LogicException::class);
+        $section->insertChild(0, Block::create('divi/text'));
+    }
+
+    public function testRemoveChildRefusesOnAnOpaqueBlock(): void
+    {
+        $document = (new Parser())->parse(
+            '<!-- wp:divi/section {"globalModule":"x"} --><!-- wp:divi/row --><!-- /wp:divi/row --><!-- /wp:divi/section -->'
+        );
+        $section = $document->children()[0];
+        $row = $section->children()[0];
+        self::assertTrue($section->isOpaque());
+
+        $this->expectException(LogicException::class);
+        $section->removeChild($row);
+    }
+
+    public function testAppendRefusesOnAVoidBlock(): void
+    {
+        $void = Block::parsed('divi/image', [], '<!-- wp:divi/image /-->', true, false);
+
+        $this->expectException(LogicException::class);
+        $void->append('text');
+    }
+}
diff --git a/tests/Unit/Integrations/Divi/BlockTree/JsonTest.php b/tests/Unit/Integrations/Divi/BlockTree/JsonTest.php
new file mode 100644
index 0000000..8e0a9f9
--- /dev/null
+++ b/tests/Unit/Integrations/Divi/BlockTree/JsonTest.php
@@ -0,0 +1,51 @@
+<?php
+
+declare(strict_types=1);
+
+namespace LIT\MCP\Tests\Unit\Integrations\Divi\BlockTree;
+
+use JsonException;
+use LIT\MCP\Integrations\Divi\BlockTree\Json;
+use PHPUnit\Framework\TestCase;
+use stdClass;
+
+final class JsonTest extends TestCase
+{
+    public function testDecodesAPlainObject(): void
+    {
+        self::assertSame(['a' => 1], Json::decodeObject('{"a":1}'));
+        self::assertSame([], Json::decodeObject('{}'));
+    }
+
+    public function testRejectsNonObjectTopLevelJson(): void
+    {
+        self::assertNull(Json::decodeObject('[1,2,3]'));
+        self::assertNull(Json::decodeObject('"a string"'));
+        self::assertNull(Json::decodeObject('not json'));
+    }
+
+    /** A JSON object whose keys are "0", "1", … decodes to a PHP list – re-encoding it would
+     *  silently turn it back into a JSON array (Json.php docblock, EmptyArrayCorruptionMigration). */
+    public function testTreatsAListLikeTopLevelObjectAsOpaqueRatherThanAnArray(): void
+    {
+        self::assertNull(Json::decodeObject('{"0":"a","1":"b"}'));
+        self::assertNull(Json::decodeObject('{"0":"a"}'));
+    }
+
+    public function testDoesNotFlagAGenuineObjectWithNonSequentialNumericKeys(): void
+    {
+        self::assertSame(['0' => 'a', '2' => 'b'], Json::decodeObject('{"0":"a","2":"b"}'));
+    }
+
+    public function testEncodeThrowsOnInvalidUtf8(): void
+    {
+        $this->expectException(JsonException::class);
+
+        Json::encode("\xB1\x31");
+    }
+
+    public function testEncodeRoundtripsAnEmptyObject(): void
+    {
+        self::assertSame('{}', Json::encode(new stdClass()));
+    }
+}
diff --git a/tests/Unit/Integrations/Divi/BlockTree/ParserTest.php b/tests/Unit/Integrations/Divi/BlockTree/ParserTest.php
index a3904e4..1c44511 100644
--- a/tests/Unit/Integrations/Divi/BlockTree/ParserTest.php
+++ b/tests/Unit/Integrations/Divi/BlockTree/ParserTest.php
@@ -94,6 +94,16 @@ final class ParserTest extends TestCase
         self::assertSame([], $broken->children()[0]->attrs());
     }
 
+    public function testMarksListLikeTopLevelAttributesOpaque(): void
+    {
+        $content = '<!-- wp:divi/text {"0":"a","1":"b"} /-->';
+        $document = (new Parser())->parse($content);
+
+        self::assertTrue($document->children()[0]->isOpaque());
+        self::assertSame([], $document->children()[0]->attrs());
+        self::assertSame($content, Serializer::document($document));
+    }
+
     public function testKeepsEmptyAndListLikeObjectsAsObjects(): void
     {
         $document = (new Parser())->parse($this->fixture('synthetic-mixed.html'));