diff --git a/src/adapters/google.ts b/src/adapters/google.ts
index 746f9490a4..d004b9bf87 100644
--- a/src/adapters/google.ts
+++ b/src/adapters/google.ts
@@ -964,7 +964,9 @@ export function createGoogleAdapter(provider: OcxProviderConfig): ProviderAdapte
         const replaySession = provider.googleMode === "cloud-code-assist" ? antigravitySession : vertexReplaySession;
         if ((provider.googleMode === "cloud-code-assist" || provider.googleMode === "vertex")
           && parts && replayModel && replaySession) {
-          pendingStreamThoughtSig = observeAntigravityReplay(
+          // Observation may scan the whole frame, so use it only for replay-cache side effects.
+          // The source-order loop below exclusively owns stream carry and cannot pair backwards.
+          observeAntigravityReplay(
             replayModel,
             replaySession,
             parts as unknown[],
diff --git a/tests/google-signature-history-roundtrip.test.ts b/tests/google-signature-history-roundtrip.test.ts
index 571ce27c1a..40f23154e8 100644
--- a/tests/google-signature-history-roundtrip.test.ts
+++ b/tests/google-signature-history-roundtrip.test.ts
@@ -33,6 +33,13 @@ const provider = {
   apiKey: "vertex-test-key",
 } as OcxProviderConfig;
 
+const aiStudioProvider = {
+  adapter: "google",
+  googleMode: "ai-studio",
+  baseUrl: "https://generativelanguage.googleapis.com",
+  apiKey: "ai-studio-test-key",
+} as OcxProviderConfig;
+
 
 /**
  * A replay scope is now REQUIRED for the store to remember or return anything: a
@@ -89,6 +96,18 @@ function modelParts(body: string): Record<string, unknown>[] {
   return parsed.contents.find(content => content.role === "model")?.parts ?? [];
 }
 
+/** Build a streaming response whose SSE frames remain distinct transport chunks. */
+function sseResponse(frames: string[]): Response {
+  const stream = new ReadableStream<Uint8Array>({
+    start(controller) {
+      const encoder = new TextEncoder();
+      for (const frame of frames) controller.enqueue(encoder.encode(frame));
+      controller.close();
+    },
+  });
+  return new Response(stream);
+}
+
 describe("#1735 thought signature survives history replay", () => {
   let previousHome: string | undefined;
   let testDir: string;
@@ -175,16 +194,8 @@ describe("#1735 thought signature survives history replay", () => {
       `data: ${JSON.stringify({ usageMetadata: { promptTokenCount: 5, candidatesTokenCount: 2 } })}\n\n`,
     ];
 
-    const stream = new ReadableStream<Uint8Array>({
-      start(controller) {
-        const encoder = new TextEncoder();
-        for (const frame of frames) controller.enqueue(encoder.encode(frame));
-        controller.close();
-      },
-    });
-
     const events: AdapterEvent[] = [];
-    for await (const event of adapter.parseStream(new Response(stream))) {
+    for await (const event of adapter.parseStream(sseResponse(frames))) {
       events.push(event);
     }
 
@@ -199,6 +210,52 @@ describe("#1735 thought signature survives history replay", () => {
       .toBe(SIGNATURE);
   });
 
+  test("streaming signatures only attach to function calls that follow them in the same frame", async () => {
+    const adapter = createGoogleAdapter(provider);
+    await adapter.buildRequest(firstTurn());
+    const frames = [
+      `data: ${JSON.stringify(googleBody([
+        { functionCall: { name: "shell_command", args: { command: "pwd" } } },
+        { text: "thinking...", thought: true, thought_signature: SIGNATURE },
+        { functionCall: { name: "shell_command", args: { command: "ls" } } },
+      ]))}\n\n`,
+    ];
+
+    const events: AdapterEvent[] = [];
+    for await (const event of adapter.parseStream(sseResponse(frames))) events.push(event);
+
+    const signatures = events
+      .filter((event): event is Extract<AdapterEvent, { type: "tool_call_start" }> =>
+        event.type === "tool_call_start")
+      .map(event => event.providerMetadata?.google?.thoughtSignature);
+    expect(signatures).toEqual([undefined, SIGNATURE]);
+  });
+
+  test("AI Studio keeps source-order thought signature carry across stream frames", async () => {
+    const adapter = createGoogleAdapter(aiStudioProvider);
+    await adapter.buildRequest(firstTurn());
+    const frames = [
+      `data: ${JSON.stringify({
+        candidates: [{
+          content: {
+            role: "model",
+            parts: [{ text: "thinking...", thought: true, thought_signature: SIGNATURE }],
+          },
+        }],
+      })}\n\n`,
+      `data: ${JSON.stringify(googleBody([
+        { functionCall: { name: "shell_command", args: { command: "pwd" } } },
+      ]))}\n\n`,
+    ];
+
+    const events: AdapterEvent[] = [];
+    for await (const event of adapter.parseStream(sseResponse(frames))) events.push(event);
+
+    const start = events.find((event): event is Extract<AdapterEvent, { type: "tool_call_start" }> =>
+      event.type === "tool_call_start");
+    expect(start?.providerMetadata?.google?.thoughtSignature).toBe(SIGNATURE);
+  });
+
   test("a signature replayed through Responses history reaches the rebuilt Google part", async () => {
     // No cache is warmed here: this is a cold process replaying client-supplied history.
     const parsed = parseRequestScoped({