fix(core): preserve functionCall thoughtSignature when stripping thought parts (#28607)

Co-authored-by: David Pierce <davidapierce@google.com>

packages/core/src/core/geminiChat.test.ts

@@ -3433,6 +3433,75 @@ describe('GeminiChat', () => {
       expect(turns[0].content.parts![0].text).toBe('Question 1');
       expect(turns[0].content.parts![1].text).toBe('Question 2');
     });
+
+    it('should inject a synthetic thoughtSignature onto a functionCall left signature-less after stripping a thought part that carried it (regression test for #28604)', () => {
+      vi.mocked(mockConfig.isContextManagementEnabled).mockReturnValue(false);
+      vi.mocked(mockConfig.getModel).mockReturnValue('gemini-2.5-pro');
+
+      chat.setHistory([
+        { role: 'user', parts: [{ text: 'activate the skill' }] },
+        {
+          role: 'model',
+          parts: [
+            {
+              text: 'internal monologue',
+              thought: true,
+              thoughtSignature: 'real-sig-from-api',
+            } as unknown as Part,
+            {
+              functionCall: { name: 'activate_skill', args: {} },
+            },
+          ],
+        },
+        {
+          role: 'user',
+          parts: [
+            { functionResponse: { name: 'activate_skill', response: {} } },
+          ],
+        },
+      ]);
+
+      const turns = chat.getHistoryTurns(true);
+
+      const modelTurn = turns[1];
+      expect(modelTurn.content.parts).toHaveLength(1);
+      expect(modelTurn.content.parts![0].functionCall?.name).toBe(
+        'activate_skill',
+      );
+      expect(modelTurn.content.parts![0].thoughtSignature).toBe(
+        SYNTHETIC_THOUGHT_SIGNATURE,
+      );
+    });
+
+    it('should leave an existing thoughtSignature on a functionCall untouched when stripping thoughts', () => {
+      vi.mocked(mockConfig.isContextManagementEnabled).mockReturnValue(false);
+      vi.mocked(mockConfig.getModel).mockReturnValue('gemini-2.5-pro');
+
+      chat.setHistory([
+        { role: 'user', parts: [{ text: 'activate the skill' }] },
+        {
+          role: 'model',
+          parts: [
+            {
+              text: 'internal monologue',
+              thought: true,
+              thoughtSignature: 'real-sig-from-api',
+            } as unknown as Part,
+            {
+              functionCall: { name: 'activate_skill', args: {} },
+              thoughtSignature: 'existing-sig-on-call',
+            },
+          ],
+        },
+      ]);
+
+      const turns = chat.getHistoryTurns(true);
+
+      const modelTurn = turns[1];
+      expect(modelTurn.content.parts![0].thoughtSignature).toBe(
+        'existing-sig-on-call',
+      );
+    });
   });
 
   describe('ensureActiveLoopHasThoughtSignatures', () => {

packages/core/src/core/geminiChat.ts

@@ -1687,11 +1687,34 @@ export function stripThoughts(history: HistoryTurn[]): HistoryTurn[] {
       if (!hasThought) return turn;
 
       const nonThoughtParts = turn.content.parts.filter((p) => p && !p.thought);
+
+      // The thoughtSignature the API requires on the first functionCall of a
+      // model turn is sometimes only carried by the thought part we just
+      // removed, not by the functionCall part itself. Without it, replaying
+      // this turn in a later request gets rejected with a 400 "missing
+      // thought_signature" error, so inject a synthetic one if needed.
+      let patchedFirstCall = false;
+      const finalParts =
+        turn.content.role === 'model'
+          ? nonThoughtParts.map((p) => {
+              if (!patchedFirstCall && p.functionCall) {
+                patchedFirstCall = true;
+                if (!p.thoughtSignature) {
+                  return {
+                    ...p,
+                    thoughtSignature: SYNTHETIC_THOUGHT_SIGNATURE,
+                  };
+                }
+              }
+              return p;
+            })
+          : nonThoughtParts;
+
       return {
         ...turn,
         content: {
           ...turn.content,
-          parts: nonThoughtParts,
+          parts: finalParts,
         },
       };
     })