diff --git a/tests/mock_gemini_cli.py b/tests/mock_gemini_cli.py
index 93a4676..d6a5e99 100644
--- a/tests/mock_gemini_cli.py
+++ b/tests/mock_gemini_cli.py
@@ -7,6 +7,18 @@ def main() -> None:
  sys.stderr.write(f"DEBUG: GEMINI_CLI_HOOK_CONTEXT: {os.environ.get('GEMINI_CLI_HOOK_CONTEXT')}\n")
  sys.stderr.flush()
 
+ mock_mode = os.environ.get("MOCK_MODE", "success")
+ if mock_mode == "malformed_json":
+  print("{broken_json: ", flush=True)
+  sys.exit(1)
+ elif mock_mode == "error_result":
+  print(json.dumps({"type": "result", "status": "error", "error": "Mock simulated error"}), flush=True)
+  sys.exit(1)
+ elif mock_mode == "timeout":
+  import time
+  time.sleep(120)
+  sys.exit(1)
+
  # Read prompt from stdin
  try:
   prompt = sys.stdin.read()