Fix Gemini 3.1 Pro thought_signature in tool calls

Thinking models attach a thought_signature to functionCall parts.
Must be echoed back in functionResponse or API returns 400.
Carry it through ToolCall -> ChatMessage -> toGeminiContents().

Made-with: Cursor

app/api/chat/route.ts

@@ -183,6 +183,8 @@ export async function POST(request: Request) {
               content: result,
               toolCallId: tc.id,
               toolName: tc.name,
+              // Echo thought_signature back — required by Gemini thinking models
+              thoughtSignature: tc.thoughtSignature,
             };
             messages.push(toolMsg);
           }

lib/ai/gemini-chat.ts

@@ -4,6 +4,11 @@
  * Uses the Gemini API (generativelanguage.googleapis.com) with the
  * existing GOOGLE_API_KEY. Drop-in upgrade to Vertex AI when needed
  * by swapping GEMINI_BASE_URL.
+ *
+ * NOTE: Gemini thinking models (2.5+, 3.x) attach a `thought_signature`
+ * to functionCall parts. This signature MUST be echoed back in the
+ * functionResponse or the API returns a 400. We carry it through our
+ * ToolCall type and re-attach it when building contents[].
  */
 
 const GEMINI_API_KEY = process.env.GOOGLE_API_KEY || '';
@@ -18,12 +23,16 @@ export interface ChatMessage {
   /** Populated when role === 'tool' */
   toolCallId?: string;
   toolName?: string;
+  /** Echo thought_signature back for tool responses (thinking models) */
+  thoughtSignature?: string;
 }
 
 export interface ToolCall {
   id: string;
   name: string;
   args: Record<string, unknown>;
+  /** Gemini thinking-model signature — must be echoed in functionResponse */
+  thoughtSignature?: string;
 }
 
 export interface ToolDefinition {
@@ -51,20 +60,23 @@ function toGeminiContents(messages: ChatMessage[]) {
       if (msg.content) parts.push({ text: msg.content });
       if (msg.toolCalls?.length) {
         for (const tc of msg.toolCalls) {
-          parts.push({ functionCall: { name: tc.name, args: tc.args, id: tc.id } });
+          const fc: any = { name: tc.name, args: tc.args, id: tc.id };
+          if (tc.thoughtSignature) fc.thought_signature = tc.thoughtSignature;
+          parts.push({ functionCall: fc });
         }
       }
       if (parts.length) contents.push({ role: 'model', parts });
     } else if (msg.role === 'tool') {
-      // Tool results must be bundled after the model turn that requested them
-      const last = contents[contents.length - 1];
-      const part = {
-        functionResponse: {
-          name: msg.toolName || 'unknown',
-          id: msg.toolCallId,
-          response: { content: msg.content },
-        },
+      const fr: any = {
+        name: msg.toolName || 'unknown',
+        id: msg.toolCallId,
+        response: { content: msg.content },
       };
+      // Echo the thought_signature back — required for Gemini thinking models
+      if (msg.thoughtSignature) fr.thought_signature = msg.thoughtSignature;
+
+      const part = { functionResponse: fr };
+      const last = contents[contents.length - 1];
       if (last?.role === 'user') {
         last.parts.push(part);
       } else {
@@ -178,6 +190,8 @@ export async function* streamGeminiChat(opts: {
                 id: part.functionCall.id || `tc-${Date.now()}`,
                 name: part.functionCall.name,
                 args: part.functionCall.args ?? {},
+                // Carry the thought_signature so the chat route can echo it back
+                thoughtSignature: part.functionCall.thought_signature,
               },
             };
           }