DeepSeek Anthropic-compatible endpoint doesn't support `system` role in messages array

[yohanchen1]

Environment

• claude-agent-acp version: 0.38.0
• API endpoint: DeepSeek Anthropic-compatible (https://api.deepseek.com/anthropic)
• Claude Code CLI works fine with the same endpoint

Error

API Error: 400 - messages[1].role: unknown variant system, expected user or assistant

Cause

SDK 0.3.154 puts system role into the messages array, but DeepSeek's /anthropic
endpoint only supports system as a top-level field, not inside the messages array.

Previous working version

0.36.1 (SDK 0.3.143) - works correctly

Note

Claude Code CLI (v2.1.150) with the same DeepSeek endpoint works fine,
suggesting the issue is specific to how claude-agent-acp constructs messages.

[strcpy-s]

@yohanchen1 If you use CC Switch, you can refer to this link for a solution.

[yohanchen1]

@strcpy-s Thanks, but the new version has already fixed this issue.

[slonopotamus]

Thanks, but the new version has already fixed this issue.

Could you please be more specific, which version and version of what?

[slonopotamus]

I am still observing that claude-agent-acp 0.40.0 is sending multiple system messages when running from JetBrains Rider. Running claude code (currently, version v2.1.152) standalone from terminal does not exhibit this issue, it is only specific to ACP.

UPD: Still broken with claude-agent-acp 0.41.0