session: provider image rejection bricks session — generic 400 error, no recovery path #52042

Description

When a custom OpenAI-compatible provider's model rejects image input, a pasted image bricks the session. Every subsequent request replays the image with the full session history and fails with the same generic Error: Provider request failed with HTTP 400. The provider's actual error body is never surfaced, nothing identifies the image as the cause, and there is no in-app recovery. Switching the session to a model that accepts images revives it (undocumented, undiscoverable workaround); switching back re-breaks it.

Distinct from the tool-result path in #39824, #43388, #41163, and the closed #51873: here the image is a user attachment on a custom provider, so the model capability gate never fires — custom models carry no modality metadata, and the image is sent through to a provider that rejects it. A config field to declare input modalities for custom provider models would let the existing gate handle this client-side, and would also cover the inverse case in #43392.

The provider rejection was verified independently of OpenCode — a direct curl to the endpoint with a standard OpenAI image_url content part (both a real screenshot and a 1×1 control pixel) returns 400 with <model> is not a multimodal model. So the request format is fine; the issue is OpenCode's handling of the rejection: an opaque repeated error with no guided recovery.

Plugins

None

OpenCode version

2.0.19 — behavior verified identical on 2.0.16 and 2.0.19

Steps to reproduce

1. Configure a custom OpenAI-compatible provider whose model does not accept image parts (e.g. a gateway fronting a text-only LLM)
2. Start a chat with that model
3. Paste an image into the chat and send
4. The request fails: the chat shows only "Error: Provider request failed with HTTP 400". The provider's response body — which names the actual cause — is not shown
5. Send any new message, even plain text like "hey": it fails with the same generic error, because the image is replayed with the full session history on every request
6. Restart opencode and resume the session: draining fails the same way (Failed to drain Session in the logs)
7. Switch the session's model to one that accepts images: the session revives and works. Switch back to the text-only model: it fails again

Screenshot and/or share link

Server log, repeated on every resume attempt:

ERROR message="Failed to drain Session" cause="AI.Error: <model> is not a multimodal model"
ERROR message="Failed to drain Session" cause="AI.Error: Provider request failed with HTTP 400"

The failed assistant message persists with finish: "error" and error.type: "provider.invalid-request" — the error data exists, it is just not surfaced usefully in the chat UI.

Operating System

macOS (Darwin 25.6.0, arm64)

Terminal

VS Code integrated terminal (zsh)