diff --git a/packages/core/src/core/geminiChat.test.ts b/packages/core/src/core/geminiChat.test.ts
index 83d97e59d9..1524ef3b61 100644
--- a/packages/core/src/core/geminiChat.test.ts
+++ b/packages/core/src/core/geminiChat.test.ts
@@ -2447,6 +2447,37 @@ describe('GeminiChat', () => {
       expect(newContents[5]?.parts?.[1]).not.toHaveProperty('thoughtSignature');
     });
 
+    it('should skip a user turn that has text alongside a functionResponse when locating the active loop', () => {
+      const chat = new GeminiChat(mockConfig, '', [], []);
+      // `coalesceConsecutiveRoles` can merge a function response turn with the
+      // prompt that follows it, producing a user turn holding both.
+      const history: Content[] = [
+        { role: 'user', parts: [{ text: 'First prompt' }] },
+        {
+          role: 'model',
+          parts: [
+            { text: 'Working on it' },
+            { functionCall: { name: 'some_tool', args: {} } },
+          ],
+        },
+        {
+          role: 'user',
+          parts: [
+            { functionResponse: { name: 'some_tool', response: {} } },
+            { text: 'Second prompt' },
+          ],
+        },
+      ];
+
+      const newContents = chat.ensureActiveLoopHasThoughtSignatures(history);
+
+      // The merged turn must not be taken as the loop start, otherwise the
+      // model turn before it is left unsigned while the API still validates it.
+      expect(newContents[1]?.parts?.[1]?.thoughtSignature).toBe(
+        SYNTHETIC_THOUGHT_SIGNATURE,
+      );
+    });
+
     it('should not modify contents if there is no user text message', () => {
       const chat = new GeminiChat(mockConfig, '', [], []);
       const history: Content[] = [
diff --git a/packages/core/src/core/geminiChat.ts b/packages/core/src/core/geminiChat.ts
index e573a69060..56480c70f0 100644
--- a/packages/core/src/core/geminiChat.ts
+++ b/packages/core/src/core/geminiChat.ts
@@ -1044,11 +1044,19 @@ export class GeminiChat {
     requestContents: readonly Content[],
   ): readonly Content[] {
     // First, find the start of the active loop by finding the last user turn
-    // with a text message, i.e. that is not a function response.
+    // with a text message, i.e. that is not a function response. Testing for
+    // text alone is not enough: `coalesceConsecutiveRoles` can merge a function
+    // response turn with the prompt that follows it, and starting the loop at
+    // such a turn starts it later than the API starts the turn, leaving earlier
+    // function calls unsigned but still validated.
     let activeLoopStartIndex = -1;
     for (let i = requestContents.length - 1; i >= 0; i--) {
       const content = requestContents[i];
-      if (content.role === 'user' && content.parts?.some((part) => part.text)) {
+      if (
+        content.role === 'user' &&
+        content.parts?.some((part) => part.text) &&
+        !content.parts?.some((part) => part.functionResponse)
+      ) {
         activeLoopStartIndex = i;
         break;
       }