diff --git a/internal/runtime/executor/antigravity_executor_signature_test.go b/internal/runtime/executor/antigravity_executor_signature_test.go
index 31955d35..226daf5c 100644
--- a/internal/runtime/executor/antigravity_executor_signature_test.go
+++ b/internal/runtime/executor/antigravity_executor_signature_test.go
@@ -21,6 +21,14 @@ func testGeminiSignaturePayload() string {
 	return base64.StdEncoding.EncodeToString(payload)
 }
 
+// testFakeClaudeSignature returns a base64 string starting with 'E' that passes
+// the lightweight hasValidClaudeSignature check but has invalid protobuf content
+// (first decoded byte 0x12 is correct, but no valid protobuf field 2 follows),
+// so it fails deep validation in strict mode.
+func testFakeClaudeSignature() string {
+	return base64.StdEncoding.EncodeToString([]byte{0x12, 0xFF, 0xFE, 0xFD})
+}
+
 func testAntigravityAuth(baseURL string) *cliproxyauth.Auth {
 	return &cliproxyauth.Auth{
 		Attributes: map[string]string{
@@ -40,7 +48,7 @@ func invalidClaudeThinkingPayload() []byte {
 			{
 				"role": "assistant",
 				"content": [
-					{"type": "thinking", "thinking": "bad", "signature": "` + testGeminiSignaturePayload() + `"},
+					{"type": "thinking", "thinking": "bad", "signature": "` + testFakeClaudeSignature() + `"},
 					{"type": "text", "text": "hello"}
 				]
 			}