fix: sanitize antigravity thinking signatures

src/agents/pi-embedded-runner.google-sanitize-thinking.test.ts

@@ -86,7 +86,7 @@ describe("sanitizeSessionHistory (google thinking)", () => {
     expect(assistant.content?.[0]?.thinking).toBe("reasoning");
   });
 
-  it("keeps unsigned thinking blocks for Antigravity Claude", async () => {
+  it("drops unsigned thinking blocks for Antigravity Claude", async () => {
     const sessionManager = SessionManager.inMemory();
     const input = [
       {
@@ -107,11 +107,37 @@ describe("sanitizeSessionHistory (google thinking)", () => {
       sessionId: "session:antigravity-claude",
     });
 
+    const assistant = out.find((msg) => (msg as { role?: string }).role === "assistant");
+    expect(assistant).toBeUndefined();
+  });
+
+  it("maps base64 signatures to thinkingSignature for Antigravity Claude", async () => {
+    const sessionManager = SessionManager.inMemory();
+    const input = [
+      {
+        role: "user",
+        content: "hi",
+      },
+      {
+        role: "assistant",
+        content: [{ type: "thinking", thinking: "reasoning", signature: "c2ln" }],
+      },
+    ] 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 }>;
+      content?: Array<{ type?: string; thinking?: string; thinkingSignature?: string }>;
     };
     expect(assistant.content?.map((block) => block.type)).toEqual(["thinking"]);
     expect(assistant.content?.[0]?.thinking).toBe("reasoning");
+    expect(assistant.content?.[0]?.thinkingSignature).toBe("c2ln");
   });
 
   it("preserves order for mixed assistant content", async () => {

src/agents/pi-embedded-runner/google.ts

@@ -55,6 +55,15 @@ const MISTRAL_MODEL_HINTS = [
   "ministral",
   "mistralai",
 ];
+const ANTIGRAVITY_SIGNATURE_RE = /^[A-Za-z0-9+/]+={0,2}$/;
+
+function isValidAntigravitySignature(value: unknown): value is string {
+  if (typeof value !== "string") return false;
+  const trimmed = value.trim();
+  if (!trimmed) return false;
+  if (trimmed.length % 4 !== 0) return false;
+  return ANTIGRAVITY_SIGNATURE_RE.test(trimmed);
+}
 
 function shouldSanitizeToolCallIds(modelApi?: string | null): boolean {
   if (!modelApi) return false;
@@ -69,6 +78,61 @@ function isMistralModel(params: { provider?: string | null; modelId?: string | n
   return MISTRAL_MODEL_HINTS.some((hint) => modelId.includes(hint));
 }
 
+function sanitizeAntigravityThinkingBlocks(messages: AgentMessage[]): AgentMessage[] {
+  let touched = false;
+  const out: AgentMessage[] = [];
+  for (const msg of messages) {
+    if (!msg || typeof msg !== "object" || msg.role !== "assistant") {
+      out.push(msg);
+      continue;
+    }
+    const assistant = msg as Extract<AgentMessage, { role: "assistant" }>;
+    if (!Array.isArray(assistant.content)) {
+      out.push(msg);
+      continue;
+    }
+    const nextContent = [];
+    let contentChanged = false;
+    for (const block of assistant.content) {
+      if (
+        !block ||
+        typeof block !== "object" ||
+        (block as { type?: unknown }).type !== "thinking"
+      ) {
+        nextContent.push(block);
+        continue;
+      }
+      const rec = block as {
+        thinkingSignature?: unknown;
+        signature?: unknown;
+        thought_signature?: unknown;
+        thoughtSignature?: unknown;
+      };
+      const candidate =
+        rec.thinkingSignature ?? rec.signature ?? rec.thought_signature ?? rec.thoughtSignature;
+      if (!isValidAntigravitySignature(candidate)) {
+        contentChanged = true;
+        continue;
+      }
+      if (rec.thinkingSignature !== candidate) {
+        nextContent.push({ ...rec, thinkingSignature: candidate });
+        contentChanged = true;
+      } else {
+        nextContent.push(block);
+      }
+    }
+    if (contentChanged) {
+      touched = true;
+    }
+    if (nextContent.length === 0) {
+      touched = true;
+      continue;
+    }
+    out.push(contentChanged ? { ...assistant, content: nextContent } : msg);
+  }
+  return touched ? out : messages;
+}
+
 function findUnsupportedSchemaKeywords(schema: unknown, path: string): string[] {
   if (!schema || typeof schema !== "object") return [];
   if (Array.isArray(schema)) {
@@ -226,7 +290,11 @@ export async function sanitizeSessionHistory(params: {
       ? { allowBase64Only: true, includeCamelCase: true }
       : undefined,
   });
-  const repairedTools = sanitizeToolUseResultPairing(sanitizedImages);
+  const sanitizedThinking =
+    params.modelApi === "google-antigravity" && isAntigravityClaudeModel
+      ? sanitizeAntigravityThinkingBlocks(sanitizedImages)
+      : sanitizedImages;
+  const repairedTools = sanitizeToolUseResultPairing(sanitizedThinking);
 
   return applyGoogleTurnOrderingFix({
     messages: repairedTools,