diff --git a/packages/typescript/ai-gemini/src/adapters/text.ts b/packages/typescript/ai-gemini/src/adapters/text.ts
index 302409f8..6480d655 100644
--- a/packages/typescript/ai-gemini/src/adapters/text.ts
+++ b/packages/typescript/ai-gemini/src/adapters/text.ts
@@ -271,6 +271,12 @@ export class GeminiTextAdapter<
               }
             }
 
+            // Capture thought signature for Gemini 3.0 compatibility
+            const metadata =
+              'thoughtSignature' in part && part.thoughtSignature
+                ? { thoughtSignature: part.thoughtSignature }
+                : undefined
+
             yield {
               type: 'tool_call',
               id: generateId(this.name),
@@ -283,6 +289,7 @@ export class GeminiTextAdapter<
                   name: toolCallData.name,
                   arguments: toolCallData.args,
                 },
+                metadata,
               },
               index: toolCallData.index,
             }
@@ -323,6 +330,12 @@ export class GeminiTextAdapter<
                   index: nextToolIndex++,
                 })
 
+                // Capture thought signature for Gemini 3.0 compatibility
+                const metadata =
+                  'thoughtSignature' in part && part.thoughtSignature
+                    ? { thoughtSignature: part.thoughtSignature }
+                    : undefined
+
                 yield {
                   type: 'tool_call',
                   id: generateId(this.name),
@@ -338,6 +351,7 @@ export class GeminiTextAdapter<
                           ? functionArgs
                           : JSON.stringify(functionArgs),
                     },
+                    metadata,
                   },
                   index: nextToolIndex - 1,
                 }
@@ -461,12 +475,24 @@ export class GeminiTextAdapter<
             >
           }
 
-          parts.push({
+          const part: Part = {
             functionCall: {
               name: toolCall.function.name,
               args: parsedArgs,
             },
-          })
+          }
+
+          // Include thought signature if present for Gemini 3.0 compatibility
+          if (
+            toolCall.metadata &&
+            typeof toolCall.metadata === 'object' &&
+            'thoughtSignature' in toolCall.metadata &&
+            typeof toolCall.metadata.thoughtSignature === 'string'
+          ) {
+            ;(part as any).thoughtSignature = toolCall.metadata.thoughtSignature
+          }
+
+          parts.push(part)
         }
       }
 
diff --git a/packages/typescript/ai-gemini/tests/gemini-adapter.test.ts b/packages/typescript/ai-gemini/tests/gemini-adapter.test.ts
index 1f000171..90552ce5 100644
--- a/packages/typescript/ai-gemini/tests/gemini-adapter.test.ts
+++ b/packages/typescript/ai-gemini/tests/gemini-adapter.test.ts
@@ -343,4 +343,116 @@ describe('GeminiAdapter through AI', () => {
     expect(payload.config.systemInstruction).toContain('123 tokens')
     expect(result.summary).toBe(summaryText)
   })
+
+  it('captures and preserves thought signatures for Gemini 3.0 compatibility', async () => {
+    const testThoughtSignature = 'thought_signature_abc123'
+    const streamChunks = [
+      {
+        candidates: [
+          {
+            content: {
+              parts: [
+                {
+                  functionCall: {
+                    id: 'tool_call_1',
+                    name: 'lookup_weather',
+                    args: { location: 'Paris' },
+                  },
+                  thoughtSignature: testThoughtSignature,
+                },
+              ],
+            },
+            finishReason: 'UNEXPECTED_TOOL_CALL',
+          },
+        ],
+      },
+    ]
+
+    mocks.generateContentStreamSpy.mockResolvedValue(createStream(streamChunks))
+
+    const adapter = createTextAdapter()
+    const received: StreamChunk[] = []
+
+    // First request - capture thought signature
+    for await (const chunk of chat({
+      adapter,
+      messages: [{ role: 'user', content: 'What is the weather in Paris?' }],
+      tools: [weatherTool],
+    })) {
+      received.push(chunk)
+    }
+
+    // Verify thought signature was captured in metadata
+    const toolCallChunk = received.find((c) => c.type === 'tool_call')
+    expect(toolCallChunk).toBeDefined()
+    if (toolCallChunk && toolCallChunk.type === 'tool_call') {
+      expect(toolCallChunk.toolCall.metadata).toEqual({
+        thoughtSignature: testThoughtSignature,
+      })
+    }
+
+    // Clear mocks for second request
+    vi.clearAllMocks()
+
+    const secondResponseChunks = [
+      {
+        candidates: [
+          {
+            content: {
+              parts: [{ text: 'The weather is sunny.' }],
+            },
+            finishReason: 'STOP',
+          },
+        ],
+      },
+    ]
+
+    mocks.generateContentStreamSpy.mockResolvedValue(
+      createStream(secondResponseChunks),
+    )
+
+    // Second request - verify thought signature is sent back
+    // Use ModelMessage format with toolCalls array
+    for await (const _ of chat({
+      adapter,
+      messages: [
+        { role: 'user', content: 'What is the weather in Paris?' },
+        {
+          role: 'assistant',
+          content: null,
+          toolCalls: [
+            {
+              id: 'tool_call_1',
+              type: 'function',
+              function: {
+                name: 'lookup_weather',
+                arguments: '{"location":"Paris"}',
+              },
+              metadata: { thoughtSignature: testThoughtSignature },
+            },
+          ],
+        },
+        {
+          role: 'tool',
+          content: 'Sunny, 22°C',
+          toolCallId: 'tool_call_1',
+        },
+      ],
+    })) {
+      /* consume stream */
+    }
+
+    // Verify thought signature was included in the request
+    expect(mocks.generateContentStreamSpy).toHaveBeenCalledTimes(1)
+    const [payload] = mocks.generateContentStreamSpy.mock.calls[0]
+    const assistantMessage = payload.contents.find(
+      (msg: any) => msg.role === 'model',
+    )
+    expect(assistantMessage).toBeDefined()
+    const functionCallPart = assistantMessage?.parts?.find(
+      (part: any) => part.functionCall,
+    )
+    expect(functionCallPart).toBeDefined()
+    expect(functionCallPart.thoughtSignature).toBe(testThoughtSignature)
+  })
 })
diff --git a/packages/typescript/ai/src/types.ts b/packages/typescript/ai/src/types.ts
index 7c49d995..5568946d 100644
--- a/packages/typescript/ai/src/types.ts
+++ b/packages/typescript/ai/src/types.ts
@@ -91,6 +91,12 @@ export interface ToolCall {
     name: string
     arguments: string // JSON string
   }
+  /**
+   * Provider-specific metadata associated with this tool call.
+   * Used by adapters to store additional information needed for API compatibility.
+   * For example, Gemini stores thought signatures here for Gemini 3.0 models.
+   */
+  metadata?: unknown
 }
 
 // ============================================================================
@@ -675,14 +681,7 @@ export interface ContentStreamChunk extends BaseStreamChunk {
 
 export interface ToolCallStreamChunk extends BaseStreamChunk {
   type: 'tool_call'
-  toolCall: {
-    id: string
-    type: 'function'
-    function: {
-      name: string
-      arguments: string // Incremental JSON arguments
-    }
-  }
+  toolCall: ToolCall
   index: number
 }