diff --git a/CHANGELOG.md b/CHANGELOG.md
index aa6c85844..35f22385a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -175,6 +175,7 @@ Docs: https://docs.openclaw.ai
 
 ### Fixes
 
+- Agents/Antigravity: preserve unsigned Claude thinking blocks as plain text instead of dropping them during transcript sanitization, preventing reasoning context loss while avoiding `thinking.signature` request rejections.
 - Agents/Google: clean tool JSON Schemas for `google-antigravity` the same as `google-gemini-cli` before Cloud Code Assist requests, preventing Claude tool calls from failing with `patternProperties` 400 errors. (#19860)
 - Tests/Telegram: add regression coverage for command-menu sync that asserts all `setMyCommands` entries are Telegram-safe and hyphen-normalized across native/custom/plugin command sources. (#19703) Thanks @obviyus.
 - Agents/Image: collapse resize diagnostics to one line per image and include visible pixel/byte size details in the log message for faster triage.
diff --git a/src/agents/pi-embedded-runner.google-sanitize-thinking.e2e.test.ts b/src/agents/pi-embedded-runner.google-sanitize-thinking.e2e.test.ts
index 0e44cf3c3..f716ff32a 100644
--- a/src/agents/pi-embedded-runner.google-sanitize-thinking.e2e.test.ts
+++ b/src/agents/pi-embedded-runner.google-sanitize-thinking.e2e.test.ts
@@ -91,7 +91,7 @@ describe("sanitizeSessionHistory (google thinking)", () => {
     expect(assistant.content?.[0]?.thinking).toBe("reasoning");
   });
 
-  it("drops unsigned thinking blocks for Antigravity Claude", async () => {
+  it("converts unsigned thinking blocks to text for Antigravity Claude", async () => {
     const out = await sanitizeSimpleSession({
       modelApi: "google-antigravity",
       modelId: "anthropic/claude-3.5-sonnet",
@@ -99,8 +99,10 @@ describe("sanitizeSessionHistory (google thinking)", () => {
       content: [{ type: "thinking", thinking: "reasoning" }],
     });
 
-    const assistant = out.find((msg) => (msg as { role?: string }).role === "assistant");
-    expect(assistant).toBeUndefined();
+    const assistant = out.find((msg) => (msg as { role?: string }).role === "assistant") as {
+      content?: Array<{ type?: string; text?: string }>;
+    };
+    expect(assistant.content).toEqual([{ type: "text", text: "reasoning" }]);
   });
 
   it("maps base64 signatures to thinkingSignature for Antigravity Claude", async () => {
diff --git a/src/agents/pi-embedded-runner/google.ts b/src/agents/pi-embedded-runner/google.ts
index 7a6a0bf76..9a0263a2d 100644
--- a/src/agents/pi-embedded-runner/google.ts
+++ b/src/agents/pi-embedded-runner/google.ts
@@ -101,6 +101,12 @@ export function sanitizeAntigravityThinkingBlocks(messages: AgentMessage[]): Age
       const candidate =
         rec.thinkingSignature ?? rec.signature ?? rec.thought_signature ?? rec.thoughtSignature;
       if (!isValidAntigravitySignature(candidate)) {
+        // Preserve reasoning content as plain text when signatures are invalid/missing.
+        // Antigravity Claude rejects unsigned thinking blocks, but dropping them loses context.
+        const thinkingText = (block as { thinking?: unknown }).thinking;
+        if (typeof thinkingText === "string" && thinkingText.trim()) {
+          nextContent.push({ type: "text", text: thinkingText } as AssistantContentBlock);
+        }
         contentChanged = true;
         continue;
       }