Anthropic HTTP 400 responses (e.g. "image exceeds 5 MB maximum") were
collapsed in the chat UI to the generic headline "Anthropic returned an
unexpected error (HTTP 400)." with no actionable detail — the upstream
message survived to the processor log but was dropped before reaching
the client.
Add a new optional `Detail` field on `codersdk.ChatStreamError` that
carries the upstream provider message alongside the existing normalized
headline. The backend extracts `error.message` from
`fantasy.ProviderError.ResponseBody` (the JSON envelope shared by
Anthropic and OpenAI), falls back to the trimmed provider message when
the body is absent or unparseable, and caps the result at 500 runes. The
frontend threads `Detail` through `useChatStore`, `liveStatusModel`, and
`ChatStatusCallout`, rendering it as a muted secondary line inside the
existing `AlertDescription`.
Before:
<img width="1552" height="185" alt="image"
src="https://github.com/user-attachments/assets/524b588e-3cee-4fad-bc15-6bf3aec0899d"
/>
After:
<img width="814" height="173" alt="image"
src="https://github.com/user-attachments/assets/eae82a89-3ac1-4a33-8d18-ef9f77263d89"
/>
## Persistence
`Detail` is **not** persisted — it disappears on refresh. Persisting it
would require a DB change (today `chats.last_error` is a single nullable
`TEXT` column), and the shape of persisted chat errors is worth a more
deliberate rethink — e.g. promoting `last_error` to `JSONB` so we can
also retain structured fields like `kind`, `statusCode`, `provider`, and
`retryable` instead of only the normalized headline string. That's a
bigger design discussion than this PR should carry.
In the meantime, seeing the upstream error reason *immediately on
failure* is already a large UX improvement over the status quo, and this
PR gets us there without prejudicing the eventual persistence design.
Tracking persistence in CODAGT-239.
Closes CODAGT-235
Ethan
2295e9d5be