### Check for existing issues

- [x] I have searched the existing issues and checked that my issue is not a duplicate.

### What happened?

Hi team,

I think there is a bug in the policy pipeline test endpoint.

I am using LiteLLM 1.83.10. When I call /policies/test-pipeline, the pipeline itself appears to run successfully, but the endpoint still returns 500.

From the logs, I can see that the pipeline step completed normally first:

Pipeline 'test-pipeline' step 5: guardrail=SSN_PII Native Block, outcome=pass, action=next

Right after that, FastAPI fails while serializing the response:

TypeError: 'InstrumentationInfo' object is not iterable

During handling of the above exception, another exception occurred:

TypeError: vars() argument must have __dict__ attribute

...

File "/app/.venv/lib/python3.13/site-packages/fastapi/routing.py", line 305, in serialize_response return jsonable_encoder(response_content)

...

ValueError: [TypeError("'InstrumentationInfo' object is not iterable"), TypeError('vars() argument must have __dict__ attribute')]

So this does not look like a guardrail decision failure. It looks like the endpoint runs the pipeline, but then crashes while building…