## Problem
When `Store: true` is set for OpenAI Responses API calls (the new
default), multi-turn conversations with reasoning models fail on the
second message:
```
stream response: bad request: Item 'rs_xxx' of type 'reasoning' was provided
without its required following item.
```
The fantasy library was reconstructing full `OfReasoning` input items
(with encrypted content and summary) when replaying assistant messages.
The API cannot pair these reconstructed reasoning items with the output
items that originally followed them because the output items are sent as
plain `OfMessage` without server-side IDs.
## Fix
Updates the fantasy dependency (`kylecarbs/fantasy@cj/go1.25`) to skip
reasoning parts during conversation replay in `toResponsesPrompt`. With
`Store` enabled, the API already has the reasoning persisted server-side
— it doesn't need to be replayed in the input.
Fantasy PR: https://github.com/charmbracelet/fantasy/pull/181
## Testing
Adds `TestOpenAIReasoningRoundTrip` integration test that:
1. Sends a query to `o4-mini` (reasoning model with `Store: true`)
2. Verifies reasoning content is persisted
3. Sends a follow-up message — this was the failing step
4. Verifies the follow-up completes successfully
Requires `OPENAI_API_KEY` env var to run.
Adds cursor-based pagination to the chat messages endpoint.
## Backend
- New `GetChatMessagesByChatIDPaginated` SQL query: returns messages in
`id DESC` order with a `before_id` keyset cursor and configurable
`limit`
- Handler parses `?before_id=N&limit=N` query params, uses the `LIMIT
N+1` trick to set `has_more` without a separate COUNT query
- Queued messages only returned on the first page (no cursor) since
they're always the most recent
- SDK client updated with `ChatMessagesPaginationOptions`
- Fully backward compatible: omitting params returns the 50 newest
messages
## Frontend
- Switches `getChatMessages` from `useQuery` to `useInfiniteQuery` with
cursor chaining via `getNextPageParam`
- Pages flattened and sorted by `id` ascending for chronological display
- `MessagesPaginationSentinel` component uses `IntersectionObserver`
(200px rootMargin prefetch) inside the existing `flex-col-reverse`
scroll container
- `flex-col-reverse` handles scroll anchoring natively when older
messages are prepended — no manual `scrollTop` adjustment needed (same
pattern as coder/blink)
## Why cursor-based instead of offset/limit
Offset-based pagination breaks when new messages arrive while paginating
backward (offsets shift, causing duplicates or missed messages). The
`before_id` cursor is stable regardless of inserts — each page is
deterministic.
## Summary
Moves the messages response out of `GET /chats/{id}` and into a
dedicated `GET /chats/{id}/messages` endpoint.
### Backend
- `GET /chats/{id}` now returns just the `Chat` object (no messages)
- `GET /chats/{id}/messages` is a new endpoint returning
`ChatMessagesResponse` with `messages` and `queued_messages`
- Added `ChatMessagesResponse` SDK type and `GetChatMessages` client
method
### Frontend
- `getChat()` API method returns `Chat` instead of `ChatWithMessages`
- Added `getChatMessages()` API method for the new endpoint
- Split `chatQuery` into two: `chatQuery` (metadata) and
`chatMessagesQuery` (messages)
- Updated all cache mutations, optimistic updates, and websocket
handlers
- Updated tests and stories
### Files changed
| File | Change |
|---|---|
| `coderd/coderd.go` | Register `GET /messages` route |
| `coderd/chats.go` | Simplify `getChat`, add `getChatMessages` handler
|
| `codersdk/chats.go` | New type + method, update `GetChat` return |
| `site/src/api/api.ts` | New method, update `getChat` |
| `site/src/api/queries/chats.ts` | New query, update cache mutations |
| `site/src/pages/AgentsPage/AgentDetail.tsx` | Use separate queries |
| `site/src/pages/AgentsPage/AgentDetail/ChatContext.ts` | Update types
and cache writes |
| `site/src/pages/AgentsPage/AgentsPage.tsx` | Update websocket cache
handler |
Fixes Anthropic 400 error on multi-turn conversations with web search:
> web_search tool use with id srvtoolu_... was found without a
corresponding web_search_tool_result block
Provider-executed tool results (e.g. `web_search`) had a nil `Result`
field, which serialized as `"result":null`. Fantasy's
`UnmarshalToolResultOutputContent` couldn't deserialize `null` back, so
the entire assistant message became unreadable after persistence. On the
next LLM call, Anthropic rejected the conversation because
`server_tool_use` had no matching `web_search_tool_result`.
**Fix:** Bump the fantasy fork to e4bbc7bb3054 which returns `nil, nil`
for null `Result` JSON instead of erroring.
**Testing:** Added `integration_test.go` with
`TestAnthropicWebSearchRoundTrip` (requires `ANTHROPIC_API_KEY`) that:
- Sends a query triggering web search
- Verifies the persisted assistant message contains all parts the UI
needs: `tool-call(PE)`, `source`, `tool-result(PE)`, and `text`
- Sends a follow-up to confirm the round-trip works with Anthropic
Michael Suchacz