diff --git a/genkit-tools/common/src/types/parts.ts b/genkit-tools/common/src/types/parts.ts
index eef420cc7b..ee460ab071 100644
--- a/genkit-tools/common/src/types/parts.ts
+++ b/genkit-tools/common/src/types/parts.ts
@@ -196,6 +196,7 @@ export type Part = z.infer<typeof PartSchema>;
 export const MultipartToolResponseSchema = z.object({
   output: z.unknown().optional(),
   content: z.array(PartSchema).optional(),
+  metadata: z.record(z.unknown()).optional(),
 });
 
 export type MultipartToolResponse = z.infer<typeof MultipartToolResponseSchema>;
diff --git a/genkit-tools/genkit-schema.json b/genkit-tools/genkit-schema.json
index 26cc4fbf4f..7080fa8fcf 100644
--- a/genkit-tools/genkit-schema.json
+++ b/genkit-tools/genkit-schema.json
@@ -906,6 +906,10 @@
           "items": {
             "$ref": "#/$defs/Part"
           }
+        },
+        "metadata": {
+          "type": "object",
+          "additionalProperties": {}
         }
       },
       "additionalProperties": false
diff --git a/go/ai/gen.go b/go/ai/gen.go
index e391ef2215..eec7cdfd80 100644
--- a/go/ai/gen.go
+++ b/go/ai/gen.go
@@ -326,7 +326,8 @@ type ModelResponseChunk struct {
 // MultipartToolResponse represents a tool response with both structured output and content parts.
 type MultipartToolResponse struct {
 	// Content holds additional message parts providing context or details.
-	Content []*Part `json:"content,omitempty"`
+	Content  []*Part        `json:"content,omitempty"`
+	Metadata map[string]any `json:"metadata,omitempty"`
 	// Output contains the structured output data from the tool.
 	Output any `json:"output,omitempty"`
 }
diff --git a/js/ai/src/generate/resolve-tool-requests.ts b/js/ai/src/generate/resolve-tool-requests.ts
index c72fb096d4..5105ce5d4f 100644
--- a/js/ai/src/generate/resolve-tool-requests.ts
+++ b/js/ai/src/generate/resolve-tool-requests.ts
@@ -132,6 +132,7 @@ export async function resolveToolRequest(
           output: multipartResponse.output,
           content: multipartResponse.content,
         } as ToolResponse,
+        metadata: multipartResponse.metadata,
       });
 
       return { response };
diff --git a/js/ai/src/parts.ts b/js/ai/src/parts.ts
index 53fe2f086f..8985bd2ae9 100644
--- a/js/ai/src/parts.ts
+++ b/js/ai/src/parts.ts
@@ -187,6 +187,7 @@ export type Part = z.infer<typeof PartSchema>;
 export const MultipartToolResponseSchema = z.object({
   output: z.unknown().optional(),
   content: z.array(PartSchema).optional(),
+  metadata: z.record(z.unknown()).optional(),
 });
 
 export type MultipartToolResponse = z.infer<typeof MultipartToolResponseSchema>;
diff --git a/js/ai/src/tool.ts b/js/ai/src/tool.ts
index 585669ecf0..ee09030fd2 100644
--- a/js/ai/src/tool.ts
+++ b/js/ai/src/tool.ts
@@ -301,6 +301,7 @@ export type MultipartToolFn<I extends z.ZodTypeAny, O extends z.ZodTypeAny> = (
 ) => Promise<{
   output?: z.infer<O>;
   content?: Part[];
+  metadata?: Record<string, any>;
 }>;
 
 export function defineTool<I extends z.ZodTypeAny, O extends z.ZodTypeAny>(
diff --git a/js/ai/tests/generate/generate_test.ts b/js/ai/tests/generate/generate_test.ts
index a45c3ab0e8..42d00419a3 100644
--- a/js/ai/tests/generate/generate_test.ts
+++ b/js/ai/tests/generate/generate_test.ts
@@ -632,6 +632,7 @@ describe('generate', () => {
         return {
           output: 'main output',
           content: [{ text: 'part 1' }],
+          metadata: { custom: 'data' },
         };
       }
     );
@@ -699,6 +700,7 @@ describe('generate', () => {
                 },
               ],
             },
+            metadata: { custom: 'data' },
           },
         ],
       },
diff --git a/js/ai/tests/tool_test.ts b/js/ai/tests/tool_test.ts
index e9fa56262e..1bca6e48b1 100644
--- a/js/ai/tests/tool_test.ts
+++ b/js/ai/tests/tool_test.ts
@@ -135,6 +135,28 @@ describe('defineInterrupt', () => {
       });
     });
 
+    it('should define a multipart tool returning metadata', async () => {
+      const t = defineTool(
+        registry,
+        { name: 'test_meta', description: 'test', multipart: true },
+        async () => {
+          return {
+            output: 'main output',
+            content: [{ text: 'part 1' }],
+            metadata: { customField: 123 },
+          };
+        }
+      );
+      assert.equal(t.__action.metadata.type, 'tool.v2');
+      assert.equal(t.__action.actionType, 'tool.v2');
+      const result = await t({});
+      assert.deepStrictEqual(result, {
+        output: 'main output',
+        content: [{ text: 'part 1' }],
+        metadata: { customField: 123 },
+      });
+    });
+
     it('should handle fallback output', async () => {
       const t = defineTool(
         registry,
diff --git a/py/packages/genkit/src/genkit/core/typing.py b/py/packages/genkit/src/genkit/core/typing.py
index 02f5927450..96abb6c3d4 100644
--- a/py/packages/genkit/src/genkit/core/typing.py
+++ b/py/packages/genkit/src/genkit/core/typing.py
@@ -936,6 +936,7 @@ class MultipartToolResponse(BaseModel):
     model_config: ClassVar[ConfigDict] = ConfigDict(alias_generator=to_camel, extra='forbid', populate_by_name=True)
     output: Any | None = None
     content: list[Part] | None = None
+    metadata: dict[str, Any] | None = None
 
 
 class RerankerRequest(BaseModel):