diff --git a/lib/ai/gemini-chat.ts b/lib/ai/gemini-chat.ts
index 0ccb3f3b..d693e9c5 100644
--- a/lib/ai/gemini-chat.ts
+++ b/lib/ai/gemini-chat.ts
@@ -55,20 +55,22 @@ function toGeminiContents(messages: ChatMessage[]) {
       if (msg.content) parts.push({ text: msg.content });
       if (msg.toolCalls?.length) {
         for (const tc of msg.toolCalls) {
-          const fc: any = { name: tc.name, args: tc.args, id: tc.id };
-          if (tc.thoughtSignature) fc.thought_signature = tc.thoughtSignature;
-          parts.push({ functionCall: fc });
+          // thoughtSignature is a SIBLING of functionCall in the part object,
+          // not nested inside it. See: ai.google.dev/gemini-api/docs/thought-signatures
+          const part: any = { functionCall: { name: tc.name, args: tc.args, id: tc.id } };
+          if (tc.thoughtSignature) part.thoughtSignature = tc.thoughtSignature;
+          parts.push(part);
         }
       }
       if (parts.length) contents.push({ role: 'model', parts });
     } else if (msg.role === 'tool') {
-      const fr: any = {
-        name: msg.toolName || 'unknown',
-        id: msg.toolCallId,
-        response: { content: msg.content },
+      const part = {
+        functionResponse: {
+          name: msg.toolName || 'unknown',
+          id: msg.toolCallId,
+          response: { content: msg.content },
+        },
       };
-      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);
@@ -147,7 +149,8 @@ export async function callGeminiChat(opts: {
         id: part.functionCall.id || `tc-${Date.now()}-${Math.random().toString(36).slice(2)}`,
         name: part.functionCall.name,
         args: part.functionCall.args ?? {},
-        thoughtSignature: part.functionCall.thought_signature,
+        // thoughtSignature is a SIBLING of functionCall in the part, not inside it
+        thoughtSignature: part.thoughtSignature,
       });
     }
   }