diff --git a/CHANGELOG.md b/CHANGELOG.md
index 23aeb2c2e..ef5ef6417 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -15,6 +15,7 @@
 
 ### Fixes
 - Browser: add tests for snapshot labels/efficient query params and labeled image responses.
+- Google: downgrade unsigned thinking blocks before send to avoid missing signature errors.
 - Doctor: avoid re-adding WhatsApp config when only legacy ack reactions are set. (#927, fixes #900) — thanks @grp06.
 - Agents: scrub tuple `items` schemas for Gemini tool calls. (#926, fixes #746) — thanks @grp06.
 - Agents: stabilize sub-agent announce status from runtime outcomes and normalize Result/Notes. (#835) — thanks @roshanasingh4.
diff --git a/src/agents/pi-embedded-helpers.ts b/src/agents/pi-embedded-helpers.ts
index 88fe701a9..0f38772f4 100644
--- a/src/agents/pi-embedded-helpers.ts
+++ b/src/agents/pi-embedded-helpers.ts
@@ -26,6 +26,7 @@ export {
 } from "./pi-embedded-helpers/errors.js";
 export {
   downgradeGeminiHistory,
+  downgradeGeminiThinkingBlocks,
   isGoogleModelApi,
   sanitizeGoogleTurnOrdering,
 } from "./pi-embedded-helpers/google.js";
diff --git a/src/agents/pi-embedded-helpers/google.ts b/src/agents/pi-embedded-helpers/google.ts
index 6f9846d80..8e72215a4 100644
--- a/src/agents/pi-embedded-helpers/google.ts
+++ b/src/agents/pi-embedded-helpers/google.ts
@@ -26,6 +26,58 @@ type GeminiToolCallBlock = {
   input?: unknown;
 };
 
+type GeminiThinkingBlock = {
+  type?: unknown;
+  thinking?: unknown;
+  thinkingSignature?: unknown;
+};
+
+export function downgradeGeminiThinkingBlocks(messages: AgentMessage[]): AgentMessage[] {
+  const out: AgentMessage[] = [];
+  for (const msg of messages) {
+    if (!msg || typeof msg !== "object") {
+      out.push(msg);
+      continue;
+    }
+    const role = (msg as { role?: unknown }).role;
+    if (role !== "assistant") {
+      out.push(msg);
+      continue;
+    }
+    const assistantMsg = msg as Extract<AgentMessage, { role: "assistant" }>;
+    if (!Array.isArray(assistantMsg.content)) {
+      out.push(msg);
+      continue;
+    }
+
+    // Gemini rejects thinking blocks that lack a signature; downgrade to text for safety.
+    let hasDowngraded = false;
+    const nextContent = assistantMsg.content.flatMap((block) => {
+      if (!block || typeof block !== "object") return [block];
+      const record = block as GeminiThinkingBlock;
+      if (record.type !== "thinking") return [block];
+      const signature =
+        typeof record.thinkingSignature === "string" ? record.thinkingSignature.trim() : "";
+      if (signature.length > 0) return [block];
+      const thinking = typeof record.thinking === "string" ? record.thinking : "";
+      const trimmed = thinking.trim();
+      hasDowngraded = true;
+      if (!trimmed) return [];
+      return [{ type: "text", text: thinking }];
+    });
+
+    if (!hasDowngraded) {
+      out.push(msg);
+      continue;
+    }
+    if (nextContent.length === 0) {
+      continue;
+    }
+    out.push({ ...assistantMsg, content: nextContent } as AgentMessage);
+  }
+  return out;
+}
+
 export function downgradeGeminiHistory(messages: AgentMessage[]): AgentMessage[] {
   const downgradedIds = new Set<string>();
   const out: AgentMessage[] = [];
diff --git a/src/agents/pi-embedded-runner.google-sanitize-thinking.test.ts b/src/agents/pi-embedded-runner.google-sanitize-thinking.test.ts
new file mode 100644
index 000000000..ebc16c62e
--- /dev/null
+++ b/src/agents/pi-embedded-runner.google-sanitize-thinking.test.ts
@@ -0,0 +1,175 @@
+import type { AgentMessage } from "@mariozechner/pi-agent-core";
+import { SessionManager } from "@mariozechner/pi-coding-agent";
+import { describe, expect, it } from "vitest";
+import { sanitizeSessionHistory } from "./pi-embedded-runner/google.js";
+
+describe("sanitizeSessionHistory (google thinking)", () => {
+  it("downgrades thinking blocks without signatures for Google models", 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",
+      sessionManager,
+      sessionId: "session:google",
+    });
+
+    const assistant = out.find(
+      (msg) => (msg as { role?: string }).role === "assistant",
+    ) as { content?: Array<{ type?: string; text?: string }> };
+    expect(assistant.content?.map((block) => block.type)).toEqual(["text"]);
+    expect(assistant.content?.[0]?.text).toBe("reasoning");
+  });
+
+  it("keeps thinking blocks with signatures for Google models", async () => {
+    const sessionManager = SessionManager.inMemory();
+    const input = [
+      {
+        role: "user",
+        content: "hi",
+      },
+      {
+        role: "assistant",
+        content: [{ type: "thinking", thinking: "reasoning", thinkingSignature: "sig" }],
+      },
+    ] satisfies AgentMessage[];
+
+    const out = await sanitizeSessionHistory({
+      messages: input,
+      modelApi: "google-antigravity",
+      sessionManager,
+      sessionId: "session:google",
+    });
+
+    const assistant = out.find(
+      (msg) => (msg as { role?: string }).role === "assistant",
+    ) as { 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("sig");
+  });
+
+  it("preserves order when downgrading mixed assistant content", async () => {
+    const sessionManager = SessionManager.inMemory();
+    const input = [
+      {
+        role: "user",
+        content: "hi",
+      },
+      {
+        role: "assistant",
+        content: [
+          { type: "text", text: "hello" },
+          { type: "thinking", thinking: "internal note" },
+          { type: "text", text: "world" },
+        ],
+      },
+    ] satisfies AgentMessage[];
+
+    const out = await sanitizeSessionHistory({
+      messages: input,
+      modelApi: "google-antigravity",
+      sessionManager,
+      sessionId: "session:google-mixed",
+    });
+
+    const assistant = out.find(
+      (msg) => (msg as { role?: string }).role === "assistant",
+    ) as { content?: Array<{ type?: string; text?: string }> };
+    expect(assistant.content?.map((block) => block.type)).toEqual(["text", "text", "text"]);
+    expect(assistant.content?.[1]?.text).toBe("internal note");
+  });
+
+  it("downgrades only unsigned thinking blocks when mixed with signed ones", async () => {
+    const sessionManager = SessionManager.inMemory();
+    const input = [
+      {
+        role: "user",
+        content: "hi",
+      },
+      {
+        role: "assistant",
+        content: [
+          { type: "thinking", thinking: "signed", thinkingSignature: "sig" },
+          { type: "thinking", thinking: "unsigned" },
+        ],
+      },
+    ] satisfies AgentMessage[];
+
+    const out = await sanitizeSessionHistory({
+      messages: input,
+      modelApi: "google-antigravity",
+      sessionManager,
+      sessionId: "session:google-mixed-signatures",
+    });
+
+    const assistant = out.find(
+      (msg) => (msg as { role?: string }).role === "assistant",
+    ) as { content?: Array<{ type?: string; thinking?: string; text?: string }> };
+    expect(assistant.content?.map((block) => block.type)).toEqual(["thinking", "text"]);
+    expect(assistant.content?.[0]?.thinking).toBe("signed");
+    expect(assistant.content?.[1]?.text).toBe("unsigned");
+  });
+
+  it("drops empty unsigned thinking blocks for Google models", async () => {
+    const sessionManager = SessionManager.inMemory();
+    const input = [
+      {
+        role: "user",
+        content: "hi",
+      },
+      {
+        role: "assistant",
+        content: [{ type: "thinking", thinking: "   " }],
+      },
+    ] satisfies AgentMessage[];
+
+    const out = await sanitizeSessionHistory({
+      messages: input,
+      modelApi: "google-antigravity",
+      sessionManager,
+      sessionId: "session:google-empty",
+    });
+
+    const assistant = out.find(
+      (msg) => (msg as { role?: string }).role === "assistant",
+    );
+    expect(assistant).toBeUndefined();
+  });
+
+  it("keeps thinking blocks for non-Google models", 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: "openai",
+      sessionManager,
+      sessionId: "session:openai",
+    });
+
+    const assistant = out.find(
+      (msg) => (msg as { role?: string }).role === "assistant",
+    ) as { content?: Array<{ type?: string }> };
+    expect(assistant.content?.map((block) => block.type)).toEqual(["thinking"]);
+  });
+});
diff --git a/src/agents/pi-embedded-runner/google.ts b/src/agents/pi-embedded-runner/google.ts
index 0405f5e27..978edc78b 100644
--- a/src/agents/pi-embedded-runner/google.ts
+++ b/src/agents/pi-embedded-runner/google.ts
@@ -3,6 +3,7 @@ import type { SessionManager } from "@mariozechner/pi-coding-agent";
 
 import { registerUnhandledRejectionHandler } from "../../infra/unhandled-rejections.js";
 import {
+  downgradeGeminiThinkingBlocks,
   downgradeGeminiHistory,
   isCompactionFailureError,
   isGoogleModelApi,
@@ -148,10 +149,13 @@ export async function sanitizeSessionHistory(params: {
     enforceToolCallLast: params.modelApi === "anthropic-messages",
   });
   const repairedTools = sanitizeToolUseResultPairing(sanitizedImages);
-
-  const downgraded = isGoogleModelApi(params.modelApi)
-    ? downgradeGeminiHistory(repairedTools)
+  // Gemini rejects unsigned thinking blocks; downgrade them before send to avoid INVALID_ARGUMENT.
+  const downgradedThinking = isGoogleModelApi(params.modelApi)
+    ? downgradeGeminiThinkingBlocks(repairedTools)
     : repairedTools;
+  const downgraded = isGoogleModelApi(params.modelApi)
+    ? downgradeGeminiHistory(downgradedThinking)
+    : downgradedThinking;
 
   return applyGoogleTurnOrderingFix({
     messages: downgraded,