[Bugfix] Fix Responses API instructions leaking through previous_response_id

[he-yufeng]

Fixes #37697

What's the problem

When using /v1/responses with previous_response_id, the instructions from the prior response carry over into the new response. Per the OpenAI spec, instructions should NOT carry over:

"When using along with previous_response_id, the instructions from a previous response will not be carried over to the next response."

Root cause

construct_input_messages() in responses/utils.py prepends request_instructions as a system message, then the full messages list (including that system message) gets stored in msg_store. When the next request references previous_response_id, those stored messages — old system message included — are retrieved and extended into the new conversation. The new request also adds its own instructions, so you end up with both old and new system messages.

Fix

Filter out system messages when pulling prev_msg from the store in construct_input_messages(). One-line change: messages.extend(prev_msg) becomes messages.extend(m for m in prev_msg if m.get("role") != "system").

This ensures each request only uses its own instructions, regardless of what the previous response had. Works correctly for all cases: new instructions provided, no instructions provided, or no previous response at all.

Test plan

• Added 4 unit tests in tests/entrypoints/openai/responses/test_responses_utils.py covering:
  • Old system message stripped when new instructions provided
  • Old system message stripped when no instructions provided
  • Non-system messages (user/assistant) preserved correctly
  • Baseline: no previous messages works as before

[gemini-code-assist]

Code Review

This pull request addresses a bug where instructions from a previous response would leak into a new response when using previous_response_id. The change in vllm/entrypoints/openai/responses/utils.py correctly filters out system messages from the previous message history, aligning with the specification that instructions should not be carried over. New unit tests have been added in tests/entrypoints/openai/responses/test_responses_utils.py to validate the fix across various scenarios. The changes appear correct and are appropriately tested.

[chaunceyjiang]

Looks good. Is there any related OpenAI spec documentation for this? Could you share the link?

[he-yufeng]

Sure! From the OpenAI API Reference — Create Response, the instructions parameter description states:

When used along with previous_response_id, the instructions from a previous response will not be carried over to the next response. This makes it simple to swap out system (or developer) messages in new responses.

The Text Generation guide also reinforces this — the instructions parameter only applies to the current response, and instructions from previous turns will not be present in the context when using previous_response_id.