diff --git a/src/agents/pi-embedded-helpers.isbillingerrormessage.test.ts b/src/agents/pi-embedded-helpers.isbillingerrormessage.test.ts
index afb60b81d09..37d0f4d80c6 100644
--- a/src/agents/pi-embedded-helpers.isbillingerrormessage.test.ts
+++ b/src/agents/pi-embedded-helpers.isbillingerrormessage.test.ts
@@ -501,6 +501,20 @@ describe("isFailoverErrorMessage", () => {
       expect(isFailoverErrorMessage(sample)).toBe(true);
     }
   });
+
+  it("matches Gemini MALFORMED_RESPONSE stop reason as timeout (#42149)", () => {
+    const samples = [
+      "Unhandled stop reason: MALFORMED_RESPONSE",
+      "Unhandled stop reason: malformed_response",
+      "Unhandled stop reason: MALFORMED_FUNCTION_CALL",
+      "stop reason: MALFORMED_RESPONSE",
+    ];
+    for (const sample of samples) {
+      expect(isTimeoutErrorMessage(sample)).toBe(true);
+      expect(classifyFailoverReason(sample)).toBe("timeout");
+      expect(isFailoverErrorMessage(sample)).toBe(true);
+    }
+  });
 });
 
 describe("parseImageSizeError", () => {
diff --git a/src/agents/pi-embedded-helpers/failover-matches.ts b/src/agents/pi-embedded-helpers/failover-matches.ts
index f2e0e3870ab..b61eb4c79b2 100644
--- a/src/agents/pi-embedded-helpers/failover-matches.ts
+++ b/src/agents/pi-embedded-helpers/failover-matches.ts
@@ -40,9 +40,9 @@ const ERROR_PATTERNS = {
     /\benotfound\b/i,
     /\beai_again\b/i,
     /without sending (?:any )?chunks?/i,
-    /\bstop reason:\s*(?:abort|error)\b/i,
-    /\breason:\s*(?:abort|error)\b/i,
-    /\bunhandled stop reason:\s*(?:abort|error)\b/i,
+    /\bstop reason:\s*(?:abort|error|malformed_\w+)\b/i,
+    /\breason:\s*(?:abort|error|malformed_\w+)\b/i,
+    /\bunhandled stop reason:\s*(?:abort|error|malformed_\w+)\b/i,
   ],
   billing: [
     /["']?(?:status|code)["']?\s*[:=]\s*402\b|\bhttp\s*402\b|\berror(?:\s+code)?\s*[:=]?\s*402\b|\b(?:got|returned|received)\s+(?:a\s+)?402\b|^\s*402\s+payment/i,