diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0452e772b..436b7972a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -117,6 +117,7 @@
 - Logging: tolerate `EIO` from console writes to avoid gateway crashes. (#925, fixes #878) — thanks @grp06.
 - Sandbox: restore `docker.binds` config validation and preserve configured PATH for `docker exec`. (#873) — thanks @akonyer.
 - Google: downgrade unsigned thinking blocks before send to avoid missing signature errors.
+- Agents: preserve Antigravity Claude signatures and skip Gemini downgrades. (#959) — thanks @rdev.
 
 #### macOS / Apps
 - macOS: ensure launchd log directory exists with a test-only override. (#909) — thanks @roshanasingh4.
diff --git a/src/agents/pi-embedded-runner/google.ts b/src/agents/pi-embedded-runner/google.ts
index be6e66a90..8dc3dac4b 100644
--- a/src/agents/pi-embedded-runner/google.ts
+++ b/src/agents/pi-embedded-runner/google.ts
@@ -157,14 +157,14 @@ export async function sanitizeSessionHistory(params: {
   sessionManager: SessionManager;
   sessionId: string;
 }): Promise<AgentMessage[]> {
+  const isAntigravityClaudeModel = isAntigravityClaude(params.modelApi, params.modelId);
   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),
+    preserveSignatures: params.modelApi === "google-antigravity" && isAntigravityClaudeModel,
   });
   const repairedTools = sanitizeToolUseResultPairing(sanitizedImages);
-  const shouldDowngradeGemini =
-    isGoogleModelApi(params.modelApi) && !isAntigravityClaude(params.modelId);
+  const shouldDowngradeGemini = isGoogleModelApi(params.modelApi) && !isAntigravityClaudeModel;
   // Gemini rejects unsigned thinking blocks; downgrade them before send to avoid INVALID_ARGUMENT.
   const downgradedThinking = shouldDowngradeGemini
     ? downgradeGeminiThinkingBlocks(repairedTools)