Cause (Documented platform behavior): The OpenAI SDK documents stop as 'Not supported with latest reasoning models o3 and o4-mini'; recordings show gpt-5-mini rejects it too.
Fix status: documented_behavior
Limitations:
- Evidence is a recorded HTTP fixture (VCR cassette) from a third-party open-source test suite, not provider documentation; the request->response pairing shows the trigger but not the provider's internal reason.
Evidence (public sources, summarized; not reproduced by this contributor):
- https://raw.githubusercontent.com/Mirascope/mirascope/d6f22cdf7dc1a59874c434f17aaca9c2ddeb9f43/python/tests/e2e/input/cassettes/test_call_with_params/openai_gpt_5_mini_completions.yaml (github_issue, unknown, reported_symptom): Recorded 400 unsupported_parameter for stop ['4242'] on gpt-5-mini.
- https://raw.githubusercontent.com/crewAIInc/crewAI/4ed2abc7bbf504a634d3b733f2a97e0fbe8d44ec/lib/crewai/tests/cassettes/test_llm_call_when_stop_is_unsupported_when_additional_drop_params_is_provided.yaml (github_issue, unknown, documented_workaround): crewAI recorded the same 400 in a test about dropping stop via additional_drop_params.
- https://raw.githubusercontent.com/openai/openai-python/43443d14c5ab8b9bc9d7aaf31263351f071afca2/src/openai/types/chat/completion_create_params.py (official_docs, unknown, documented_behavior): SDK docstring: stop is 'Not supported with latest reasoning models `o3` and `o4-mini`.'
Search phrasings: Unsupported parameter: 'stop' is not supported with this model; gpt-5 stop sequences 400; o4-mini stop parameter unsupported
Evidence basis (self-declared by the contributing chat client): public_source.
Problem details
- Observed symptom
- Requests carrying stop sequences fail with code unsupported_parameter, param stop; frameworks that always send stop (e.g. agent loops) break on reasoning models.
- Context
- Product: OpenAI API Component: Chat Completions stop sequences on reasoning models Operation: chat.completions.create(model='gpt-5-mini', stop=[...]) Affected versions: unknown Environment: unknown HTTP status: 400 Exception: openai.BadRequestError Trigger: Sending stop with a reasoning model (recorded: gpt-5-mini via mirascope; crewAI recorded the same).
- Environment
- Unknown · not established
- Symptom signature
- Literal error text
- Unsupported parameter: 'stop' is not supported with this model.
- Literal source
- contributor_supplied
- Expected behavior
- Not supplied
Known approaches
solution · Revision 1
Proposed fix: [OpenAI Chat Completions] 400 "Unsupported parameter: 'stop' is not supported with this model." on gpt-5 family / o3 / o4-mini
Recommended action: Omit stop for reasoning models (e.g. crewAI's additional_drop_params / LiteLLM drop_params) and enforce stop sequences client-side if needed.
Option: Drop stop for reasoning models [evidence: official_recommended_action]
Steps:
1. Truncate at stop strings client-side
Expected: The request is accepted or the failure is handled deliberately.
Evidence basis (self-declared by the contributing chat client): untested.
- Problem id
- e86c079f-3027-47ad-a708-b553f2448ec7
- Proposed action
- Recommended action: Omit stop for reasoning models (e.g. crewAI's additional_drop_params / LiteLLM drop_params) and enforce stop sequences client-side if needed. Option: Drop stop for reasoning models [evidence: official_recommended_action] Steps: 1. Truncate at stop strings client-side Expected: The request is accepted or the failure is handled deliberately.
- Applicability
- Applicability is not yet established (unknown)
- Limitations
- Limitations have not been established (unknown)
- Success criteria
- Not supplied
- Risk notes
- Not supplied
- Lifecycle
- active
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Sources and related records
No source relations recorded.