Surface provider error bodies in image generation failures

[adambalogh]

Problem

A seedream-5.0-lite request through /v1/chat/completions failed with:

{
    "error": "Client error '400 Bad Request' for url 'https://ark.ap-southeast.bytepluses.com/api/v3/images/generations'\nFor more information check: https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/400",
    "exception_type": "HTTPStatusError"
}

The gateway's image-generation path calls resp.raise_for_status(), and httpx's HTTPStatusError message only contains the status code and URL. ByteDance ARK (like every OpenAI-compatible provider) puts the actual rejection reason — invalid parameter, prompt length limit, content-filter refusal — in the JSON response body, which was discarded. That makes 400s like this one undiagnosable from either the enclave logs or the client-facing error payload.

Change

• Add _provider_error_detail() in image_generation.py: extracts a compact detail string from an error response, preferring the {"error": {"code", "message"}} shape (also handles a string error field), falling back to the raw body, capped at 600 chars so a pathological body can't flood logs.
• Add _raise_for_status_with_detail() and use it at both raise_for_status() call sites — the /images/generations / /images/edits request and the hosted-URL fetch. It re-raises httpx.HTTPStatusError (so exception_type bucketing is unchanged) with the provider detail appended and httpx's MDN-link footer dropped.

With this, the same failure will read e.g.:

Client error '400 Bad Request' for url '…/images/generations' — provider response: InvalidParameter: The request parameter `prompt` exceeds the limit.

Testing

• New TestProviderErrorDetail cases pin: error body surfaced from generate_images, JSON error.code/error.message extraction, string error field, non-JSON fallback, empty body, truncation, and the streamed URL-fetch error path.
• Full suite: 178 tests, all passing except the pre-existing test_ohttp import error (pytest not installed in the env, unrelated).
• make lint (ruff + mypy) clean.

🤖 Generated with Claude Code

https://claude.ai/code/session_01Mhie33mj7RR62J61vHYjoF

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