Console Go relay: HTTP 400 (empty body) when request combines reasoning_content echo-back with reasoning_effort on deepseek-v4-flash

Summary

POST https://opencode.ai/zen/go/v1/chat/completions with model=deepseek-v4-flash rejects requests that contain BOTH:

• reasoning_content echo-back on historical assistant messages (required by DeepSeek-family thinking models when replaying tool-call turns), AND
• any reasoning_effort value (low/medium/high)

The response is HTTP 400 with an empty error body:

{"object":"error","model":"deepseek-v4-flash"}

No message field, so it is impossible to tell what failed. For agents this is fatal: a long session with many tool calls hits this deterministically at some point and cannot recover.

Repro (bisect by replaying a captured request body)

Captured the failing request body from a real session and replayed it with the same x-opencode-session header:

So the trigger is the combination, not either field alone. Removing either side restores 200.

Expected

Either accept the combination (it is a valid OpenAI-compatible payload), or return a descriptive error message instead of a bare {"object":"error","model":...}.

I believe this is in the Console Go relay layer (the same endpoint serves other models fine; deepseek-v4-pro with the same combination is not affected per other reports). Happy to provide more details or a minimal repro script.