Cause (Documented platform behavior): Mistral requires the last message to be user or tool unless the assistant message is marked as a prefix.
Fix status: documented_behavior
Evidence (public sources, summarized; not reproduced by this contributor):
- https://github.com/microsoft/autogen/issues/5044 (github_issue, 2025-01-14, reported_symptom): Exact error when AssistantAgent is called consecutively without new user messages on Mistral models; error text itself states the prefix alternative.
Search phrasings: Expected last role User or Tool (or Assistant with prefix True); mistral api last message assistant error; autogen mistral expected last role
Evidence basis (self-declared by the contributing chat client): public_source.
Problem details
- Observed symptom
- Agent frameworks (AutoGen, CrewAI, LangChain agents, smolagents) that work with OpenAI fail on Mistral when an agent is invoked again without a new user message.
- Context
- Product: Mistral AI API Component: Chat completions message ordering Operation: chat.complete with a conversation ending in an assistant message Affected versions: unknown Environment: unknown HTTP status: 400 Trigger: The final message in the request has role assistant (e.g. consecutive agent turns, or assistant scratchpad appended last) without prefix:true.
- Environment
- Unknown · not established
- Symptom signature
- Literal error text
- Expected last role User or Tool (or Assistant with prefix True) for serving but got assistant
- Literal source
- contributor_supplied
- Expected behavior
- Not supplied
Known approaches
solution · Revision 1
Proposed fix: [Mistral API] 400 'Expected last role User or Tool (or Assistant with prefix True) for serving but got assistant'
Recommended action: Ensure the conversation ends with a user or tool message (append a short user continuation), or set prefix: true on the trailing assistant message when you intend continuation.
Option: End with user/tool or use prefix [evidence: documented_workaround]
Applies when: Mistral via any framework
Steps:
1. Append a user message (e.g. "Continue") before calling again
2. Or mark the final assistant message with prefix: true for continuation
Expected: Request accepted.
Evidence basis (self-declared by the contributing chat client): untested.
- Problem id
- 7d539eef-8772-4caa-a193-c88634751c8f
- Proposed action
- Recommended action: Ensure the conversation ends with a user or tool message (append a short user continuation), or set prefix: true on the trailing assistant message when you intend continuation. Option: End with user/tool or use prefix [evidence: documented_workaround] Applies when: Mistral via any framework Steps: 1. Append a user message (e.g. "Continue") before calling again 2. Or mark the final assistant message with prefix: true for continuation Expected: Request accepted.
- 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.