Mistral API error with Devstral via LiteLLM: "Expected last role User or Tool (or Assistant with prefix True)"

[nanosek]

Description

When using Devstral (Mistral model) via LiteLLM with @ai-sdk/openai-compatible,
Mistral returns error 3230: "Expected last role User or Tool (or Assistant with prefix True)"

(Opencode diagnosed this itself with devstral model base on the 1.3.2 codebase)

The issue occurs because:

1. Agent performs tool calls (ends with tool role)
2. Agent continues with assistant message (no user after)
3. Mistral expects either a user/tool message OR an assistant with prefix=true
   OpenCode's Mistral transformation (transform.ts lines 90-134) handles tool→user
   transitions but NOT assistant→(end) transitions.
   Expected: OpenCode should either insert a continuation user message OR add
   prefix:true to the last assistant message when sending to Mistral.

Stuff works if I am using the mistral provider explicitly, but then the token usage (context) is not working.

So basically 2 routes to resolution for me: either mistral provider can deal with the context / cost informations from litellm (like openai provider) or the openai provider is able to fix this ordering issue for models which have mistral / devstral in its name

Plugins

No response

OpenCode version

No response

Steps to reproduce

No response

Screenshot and/or share link

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Operating System

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Terminal

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[github-actions]

This issue might be a duplicate of existing issues. Please check:

• Mistral Small 4 "conversation roles must alternate user and assistant..." #18998: Same Mistral conversation role ordering error ("roles must alternate") when using an OpenAI-compatible provider with a Mistral model — very similar root cause and setup.

[nanosek]

similar, but the concrete constrain of mistral is a different one here it seems

[ziouf]

I think it's related to this #16636 and so, to this #16638