OpenAI-compatible endpoint rejects standard \ efusal\ field and \image_url\ content blocks

Summary

The OpenAI-compatible endpoint (OpenCode Go) returns HTTP 400 on messages that conform to the standard OpenAI Chat Completions schema, specifically:

\\ ext
invalid_request_error:
Extra inputs are not permitted,
field: \messages[N].refusal,
value: None
\\

and:

\\ ext
Failed to deserialize the JSON body into the target type:
messages[N]: unknown variant \image_url, expected \ ext
\\

Root cause (analysis)

1. 
   efusal\ field — the OpenAI SDK automatically attaches
   efusal: None\ to every \ssistant\ message. This is part of the standard OpenAI Chat Completions schema and is accepted by OpenAI's API and most compatible endpoints. The endpoint rejects it because its message deserializer does not permit unknown fields.

2. \image_url\ content variant — a message \content\ can be either a plain string or an array of content blocks ({"type": "text", "text": ...}\ or {"type": "image_url", "image_url": ...}) per the standard OpenAI multimodal schema. The endpoint models \content\ as an enum with only a \ ext\ variant, so it rejects \image_url\ blocks.

Expected behavior

The endpoint should tolerate the standard OpenAI message schema:

• Accept (and ignore) the
  efusal\ field on assistant messages.
• Support the \image_url\ content-block variant, or at minimum degrade gracefully rather than failing the whole request.

Impact

Standard OpenAI-compatible clients (e.g. GitHub Copilot App BYOK) cannot be used against this endpoint without errors, even though the same request works against the official OpenAI API and other compatible endpoints.

Suggested fix

• Relax message deserialization to ignore unknown fields (remove \deny_unknown_fields, or explicitly allow
  efusal: Option).
• Model \content\ as a string-or-blocks union supporting \ ext\ and \image_url\ variants.

Reproduction

Configure the endpoint as an OpenAI-compatible BYOK provider in a standard client, then send any multi-turn conversation (triggers
efusal) or any message containing an image (triggers \image_url).

Related report on the Copilot App side: https://github.com/github/app/issues/2903