diff --git a/src/agents/pi-embedded-helpers/google.ts b/src/agents/pi-embedded-helpers/google.ts
index fe3290412..e3c6200bd 100644
--- a/src/agents/pi-embedded-helpers/google.ts
+++ b/src/agents/pi-embedded-helpers/google.ts
@@ -8,6 +8,12 @@ export function isGoogleModelApi(api?: string | null): boolean {
   );
 }
 
+export function isAntigravityClaude(api?: string | null, modelId?: string): boolean {
+  if (api !== "google-antigravity") return false;
+  return modelId?.toLowerCase().includes("claude") ?? false;
+}
+
+
 export { sanitizeGoogleTurnOrdering };
 
 /**
diff --git a/src/agents/pi-embedded-helpers/images.ts b/src/agents/pi-embedded-helpers/images.ts
index 56130abec..bd390b842 100644
--- a/src/agents/pi-embedded-helpers/images.ts
+++ b/src/agents/pi-embedded-helpers/images.ts
@@ -30,7 +30,7 @@ function isEmptyAssistantErrorMessage(
 export async function sanitizeSessionMessagesImages(
   messages: AgentMessage[],
   label: string,
-  options?: { sanitizeToolCallIds?: boolean; enforceToolCallLast?: boolean },
+  options?: { sanitizeToolCallIds?: boolean; enforceToolCallLast?: boolean; preserveSignatures?: boolean },
 ): Promise<AgentMessage[]> {
   // We sanitize historical session messages because Anthropic can reject a request
   // if the transcript contains oversized base64 images (see MAX_IMAGE_DIMENSION_PX).
@@ -76,7 +76,10 @@ export async function sanitizeSessionMessagesImages(
       }
       const content = assistantMsg.content;
       if (Array.isArray(content)) {
-        const strippedContent = stripThoughtSignatures(content);
+        const strippedContent = options?.preserveSignatures
+          ? content  // Keep signatures for Antigravity Claude
+          : stripThoughtSignatures(content);  // Strip for Gemini
+
         const filteredContent = strippedContent.filter((block) => {
           if (!block || typeof block !== "object") return true;
           const rec = block as { type?: unknown; text?: unknown };
diff --git a/src/agents/pi-embedded-runner.google-sanitize-thinking.test.ts b/src/agents/pi-embedded-runner.google-sanitize-thinking.test.ts
index 0f4aa90ff..900679b9c 100644
--- a/src/agents/pi-embedded-runner.google-sanitize-thinking.test.ts
+++ b/src/agents/pi-embedded-runner.google-sanitize-thinking.test.ts
@@ -59,6 +59,34 @@ describe("sanitizeSessionHistory (google thinking)", () => {
     expect(assistant.content?.[0]?.thinkingSignature).toBe("sig");
   });
 
+  it("keeps unsigned thinking blocks for Antigravity Claude", async () => {
+    const sessionManager = SessionManager.inMemory();
+    const input = [
+      {
+        role: "user",
+        content: "hi",
+      },
+      {
+        role: "assistant",
+        content: [{ type: "thinking", thinking: "reasoning" }],
+      },
+    ] satisfies AgentMessage[];
+
+    const out = await sanitizeSessionHistory({
+      messages: input,
+      modelApi: "google-antigravity",
+      modelId: "anthropic/claude-3.5-sonnet",
+      sessionManager,
+      sessionId: "session:antigravity-claude",
+    });
+
+    const assistant = out.find((msg) => (msg as { role?: string }).role === "assistant") as {
+      content?: Array<{ type?: string; thinking?: string }>;
+    };
+    expect(assistant.content?.map((block) => block.type)).toEqual(["thinking"]);
+    expect(assistant.content?.[0]?.thinking).toBe("reasoning");
+  });
+
   it("preserves order when downgrading mixed assistant content", async () => {
     const sessionManager = SessionManager.inMemory();
     const input = [
diff --git a/src/agents/pi-embedded-runner/compact.ts b/src/agents/pi-embedded-runner/compact.ts
index 8c88c1945..de871ebee 100644
--- a/src/agents/pi-embedded-runner/compact.ts
+++ b/src/agents/pi-embedded-runner/compact.ts
@@ -302,6 +302,7 @@ export async function compactEmbeddedPiSession(params: {
             const prior = await sanitizeSessionHistory({
               messages: session.messages,
               modelApi: model.api,
+              modelId,
               sessionManager,
               sessionId: params.sessionId,
             });
diff --git a/src/agents/pi-embedded-runner/google.ts b/src/agents/pi-embedded-runner/google.ts
index 4a3a7c3f6..be6e66a90 100644
--- a/src/agents/pi-embedded-runner/google.ts
+++ b/src/agents/pi-embedded-runner/google.ts
@@ -13,6 +13,7 @@ import {
 import { sanitizeToolUseResultPairing } from "../session-transcript-repair.js";
 import { log } from "./logger.js";
 import { describeUnknownError } from "./utils.js";
+import { isAntigravityClaude } from "../pi-embedded-helpers/google.js";
 
 const GOOGLE_TURN_ORDERING_CUSTOM_TYPE = "google-turn-ordering-bootstrap";
 const GOOGLE_SCHEMA_UNSUPPORTED_KEYWORDS = new Set([
@@ -152,19 +153,23 @@ export function applyGoogleTurnOrderingFix(params: {
 export async function sanitizeSessionHistory(params: {
   messages: AgentMessage[];
   modelApi?: string | null;
+  modelId?: string;
   sessionManager: SessionManager;
   sessionId: string;
 }): Promise<AgentMessage[]> {
   const sanitizedImages = await sanitizeSessionMessagesImages(params.messages, "session:history", {
     sanitizeToolCallIds: shouldSanitizeToolCallIds(params.modelApi),
     enforceToolCallLast: params.modelApi === "anthropic-messages",
+    preserveSignatures: params.modelApi === "google-antigravity" && isAntigravityClaude(params.modelId),
   });
   const repairedTools = sanitizeToolUseResultPairing(sanitizedImages);
+  const shouldDowngradeGemini =
+    isGoogleModelApi(params.modelApi) && !isAntigravityClaude(params.modelId);
   // Gemini rejects unsigned thinking blocks; downgrade them before send to avoid INVALID_ARGUMENT.
-  const downgradedThinking = isGoogleModelApi(params.modelApi)
+  const downgradedThinking = shouldDowngradeGemini
     ? downgradeGeminiThinkingBlocks(repairedTools)
     : repairedTools;
-  const downgraded = isGoogleModelApi(params.modelApi)
+  const downgraded = shouldDowngradeGemini
     ? downgradeGeminiHistory(downgradedThinking)
     : downgradedThinking;
 
diff --git a/src/agents/pi-embedded-runner/run/attempt.ts b/src/agents/pi-embedded-runner/run/attempt.ts
index 93e881c59..3d5313bdc 100644
--- a/src/agents/pi-embedded-runner/run/attempt.ts
+++ b/src/agents/pi-embedded-runner/run/attempt.ts
@@ -303,6 +303,7 @@ export async function runEmbeddedAttempt(
         const prior = await sanitizeSessionHistory({
           messages: activeSession.messages,
           modelApi: params.model.api,
+          modelId: params.modelId,
           sessionManager,
           sessionId: params.sessionId,
         });