diff --git a/docs/ARCHITECTURE.md b/docs/ARCHITECTURE.md
index 07cf44b6..f6635276 100644
--- a/docs/ARCHITECTURE.md
+++ b/docs/ARCHITECTURE.md
@@ -340,6 +340,25 @@ $embeddings = AiClient::generateEmbeddings(
 );
 ```
 
+### Function declaration annotations
+
+`FunctionDeclaration` accepts optional annotations for consumer-specific hints that do not belong to the function's parameter schema:
+
+```php
+$function = new FunctionDeclaration(
+    'get_weather',
+    'Gets the weather',
+    null,
+    ['deferredLoading' => true]
+);
+
+$annotations = $function->getAnnotations();
+```
+
+Annotations are an `array<string, mixed>` whose values should be JSON-serializable. The core SDK preserves annotations without interpreting their names or values. Providers and other consumers define the annotations they recognize and ignore unknown annotations. Provider-specific annotations should use namespaced keys or a nested provider-specific map to avoid collisions, and must not be blindly merged into provider requests.
+
+Empty annotations are omitted from serialized declarations, preserving the existing shape for callers that do not use them. Missing annotations are restored as an empty array. Annotations are not authorization; applications must still validate tool execution.
+
 ## Class diagrams
 
 This section shows comprehensive class diagrams for the proposed architecture. For explanation on specific terms, see the [glossary](./GLOSSARY.md).
@@ -806,6 +825,7 @@ direction LR
             +getName() string
             +getDescription() string
             +getParameters() mixed
+            +getAnnotations() array< string, mixed >
             +getJsonSchema() array< string, mixed >$
         }
         class FunctionResponse {
@@ -1229,6 +1249,7 @@ direction LR
             +getName() string
             +getDescription() string
             +getParameters() mixed
+            +getAnnotations() array< string, mixed >
             +getJsonSchema() array< string, mixed >$
         }
         class Tool {
diff --git a/src/Tools/DTO/FunctionDeclaration.php b/src/Tools/DTO/FunctionDeclaration.php
index 5b08ce80..2b148636 100644
--- a/src/Tools/DTO/FunctionDeclaration.php
+++ b/src/Tools/DTO/FunctionDeclaration.php
@@ -10,14 +10,15 @@
  * Represents a function declaration for AI models.
  *
  * This DTO describes a function that can be called by the AI model,
- * including its name, description, and parameter schema.
+ * including its name, description, parameter schema, and optional annotations.
  *
  * @since 0.1.0
  *
  * @phpstan-type FunctionDeclarationArrayShape array{
  *     name: string,
  *     description: string,
- *     parameters?: array<string, mixed>
+ *     parameters?: array<string, mixed>,
+ *     annotations?: array<string, mixed>
  * }
  *
  * @extends AbstractDataTransferObject<FunctionDeclarationArrayShape>
@@ -27,6 +28,7 @@ class FunctionDeclaration extends AbstractDataTransferObject
     public const KEY_NAME = 'name';
     public const KEY_DESCRIPTION = 'description';
     public const KEY_PARAMETERS = 'parameters';
+    public const KEY_ANNOTATIONS = 'annotations';
     /**
      * @var string The name of the function.
      */
@@ -42,20 +44,32 @@ class FunctionDeclaration extends AbstractDataTransferObject
      */
     private ?array $parameters;
 
+    /**
+     * @var array<string, mixed> The function annotations.
+     */
+    private array $annotations;
+
     /**
      * Constructor.
      *
      * @since 0.1.0
+     * @since n.e.x.t Adds the optional $annotations parameter.
      *
      * @param string $name The name of the function.
      * @param string $description A description of what the function does.
      * @param array<string, mixed>|null $parameters The JSON schema for the function parameters.
+     * @param array<string, mixed> $annotations Optional annotations with JSON-serializable values.
      */
-    public function __construct(string $name, string $description, ?array $parameters = null)
-    {
+    public function __construct(
+        string $name,
+        string $description,
+        ?array $parameters = null,
+        array $annotations = []
+    ) {
         $this->name = $name;
         $this->description = $description;
         $this->parameters = $parameters;
+        $this->annotations = $annotations;
     }
 
     /**
@@ -94,6 +108,18 @@ public function getParameters(): ?array
         return $this->parameters;
     }
 
+    /**
+     * Gets the function annotations.
+     *
+     * @since n.e.x.t
+     *
+     * @return array<string, mixed> The annotations, or an empty array if none were provided.
+     */
+    public function getAnnotations(): array
+    {
+        return $this->annotations;
+    }
+
     /**
      * {@inheritDoc}
      *
@@ -117,6 +143,11 @@ public static function getJsonSchema(): array
                     'description' => 'The JSON schema for the function parameters.',
                     'additionalProperties' => true,
                 ],
+                self::KEY_ANNOTATIONS => [
+                    'type' => 'object',
+                    'description' => 'Optional annotations interpreted by consumers.',
+                    'additionalProperties' => true,
+                ],
             ],
             'required' => [self::KEY_NAME, self::KEY_DESCRIPTION],
         ];
@@ -140,6 +171,10 @@ public function toArray(): array
             $data[self::KEY_PARAMETERS] = $this->parameters;
         }
 
+        if ($this->annotations !== []) {
+            $data[self::KEY_ANNOTATIONS] = $this->annotations;
+        }
+
         return $data;
     }
 
@@ -155,7 +190,8 @@ public static function fromArray(array $array): self
         return new self(
             $array[self::KEY_NAME],
             $array[self::KEY_DESCRIPTION],
-            $array[self::KEY_PARAMETERS] ?? null
+            $array[self::KEY_PARAMETERS] ?? null,
+            $array[self::KEY_ANNOTATIONS] ?? []
         );
     }
 }
diff --git a/tests/unit/Tools/DTO/FunctionDeclarationTest.php b/tests/unit/Tools/DTO/FunctionDeclarationTest.php
index b2782bdb..f6f8bb85 100644
--- a/tests/unit/Tools/DTO/FunctionDeclarationTest.php
+++ b/tests/unit/Tools/DTO/FunctionDeclarationTest.php
@@ -5,6 +5,7 @@
 namespace WordPress\AiClient\Tests\unit\Tools\DTO;
 
 use PHPUnit\Framework\TestCase;
+use WordPress\AiClient\Providers\Models\DTO\ModelConfig;
 use WordPress\AiClient\Tests\traits\ArrayTransformationTestTrait;
 use WordPress\AiClient\Tools\DTO\FunctionDeclaration;
 
@@ -320,4 +321,82 @@ public function testImplementsWithArrayTransformationInterface(): void
         $declaration = new FunctionDeclaration('test', 'test function');
         $this->assertImplementsArrayTransformation($declaration);
     }
+
+    /**
+     * Tests legacy declarations keep their serialized shape without empty annotations.
+     *
+     * @return void
+     */
+    public function testAnnotationsDefaultsPreserveCompatibility(): void
+    {
+        $legacy = ['name' => 'get_weather', 'description' => 'Gets the weather'];
+        $declarations = [
+            new FunctionDeclaration('get_weather', 'Gets the weather'),
+            new FunctionDeclaration('get_weather', 'Gets the weather', null, []),
+            FunctionDeclaration::fromArray($legacy),
+        ];
+        foreach ($declarations as $declaration) {
+            $this->assertSame([], $declaration->getAnnotations());
+            $this->assertSame($legacy, $declaration->toArray());
+            $this->assertSame($legacy, json_decode((string) json_encode($declaration), true));
+        }
+    }
+
+    /**
+     * Tests arbitrary annotations survive array and JSON serialization unchanged.
+     *
+     * @return void
+     */
+    public function testAnnotationsRoundTrip(): void
+    {
+        $annotations = [
+            'deferredLoading' => true,
+            'readOnlyHint' => false,
+            'vendor' => ['labels' => ['weather', 'public'], 'priority' => 0, 'optional' => null],
+        ];
+        $declaration = new FunctionDeclaration('get_weather', 'Gets the weather', null, $annotations);
+        $this->assertSame($annotations, $declaration->getAnnotations());
+        $this->assertNull($declaration->getParameters());
+        $this->assertSame($annotations, $declaration->toArray()['annotations']);
+        $this->assertSame($annotations, FunctionDeclaration::fromArray($declaration->toArray())->getAnnotations());
+
+        $json = json_decode((string) json_encode($declaration), true);
+        $this->assertSame($annotations, FunctionDeclaration::fromArray($json)->getAnnotations());
+    }
+
+    /**
+     * Tests annotations are optional and unconstrained in the declaration schema.
+     *
+     * @return void
+     */
+    public function testAnnotationsSchema(): void
+    {
+        $schema = FunctionDeclaration::getJsonSchema();
+        $this->assertSame('object', $schema['properties']['annotations']['type']);
+        $this->assertTrue($schema['properties']['annotations']['additionalProperties']);
+        $this->assertNotContains('annotations', $schema['required']);
+    }
+
+    /**
+     * Tests model configuration preserves annotations and clones declarations independently.
+     *
+     * @return void
+     */
+    public function testAnnotationsSurviveModelConfigRoundTripAndClone(): void
+    {
+        $annotations = ['vendor' => ['enabled' => false]];
+        $declaration = new FunctionDeclaration('lookup', 'Looks up a record', ['type' => 'object'], $annotations);
+        $config = new ModelConfig();
+        $config->setFunctionDeclarations([$declaration]);
+        $restored = ModelConfig::fromArray($config->toArray());
+        $cloned = clone $config;
+        $this->assertSame($annotations, $restored->getFunctionDeclarations()[0]->getAnnotations());
+        $this->assertSame($annotations, $cloned->getFunctionDeclarations()[0]->getAnnotations());
+        $this->assertNotSame($declaration, $cloned->getFunctionDeclarations()[0]);
+
+        $copy = $cloned->getFunctionDeclarations()[0]->getAnnotations();
+        $copy['vendor']['enabled'] = true;
+        $this->assertSame($annotations, $declaration->getAnnotations());
+        $this->assertSame($annotations, $cloned->getFunctionDeclarations()[0]->getAnnotations());
+    }
 }