# Objective

Diagnostics published by a language server survive the buffer being
detached from that server, so a file keeps showing errors from a server
that is no longer reading it.

To reproduce:

1. Install the [GitHub Actions
extension](https://zed.dev/extensions/github-actions).
2. Open a composite action definition,
`.github/actions/<name>/action.yml`.
3. Select the GitHub Actions language for it in the language selector.
That server validates the file as a workflow, so it reports `on` and
`jobs` as missing and `inputs`, `outputs` and `runs` as unexpected.
4. Select YAML again, since GitHub Actions was not what you wanted here.
5. The errors are still there. `editor: restart language servers` does
not clear them either. They go away only once the buffer is dropped, in
practice by switching to another project and back.

Two things keep them alive:

- `LocalLspStore::unregister_buffer_from_language_servers` drops the
snapshot bookkeeping and sends `didClose`, but never clears what that
server already published. `LspStore::set_language_for_buffer` calls it,
so a language change detaches the buffer while leaving the old verdict
on it.
- Restarting does not reach the server either.
`stop_local_language_servers_for_buffers` collects servers through
`language_server_ids_for_buffer`, evaluated against the language the
buffer has *now*. The old server no longer matches, so it is never
stopped, and it is typically still running legitimately for other files
in the project. Only `stop_local_language_server` clears diagnostics,
and it is never reached for that server.

## Solution

The first commit cleared the detached server's diagnostics inside
`unregister_buffer_from_language_servers`, on `didClose`. Review pointed
out that publishes are applied by path regardless of registration, so a
running server can legitimately describe a file that is no longer open
in it, and that this clearing also left `diagnostic_summaries` and the
worktree's per-path diagnostics stale.

The second commit reworks this. Detaching a buffer now only drops the
buffer's entry in `buffers_opened_in_servers` and the path's
pull-diagnostics result ids for the servers it left, two pieces of
per-server state that previously leaked. After the buffer is registered
with the servers of its new language, the server tree is rebuilt through
`refresh_server_tree`, the same pruning that runs on settings changes:
every server no registered buffer maps to any more is stopped through
`stop_local_language_server`, whose path already clears buffer
diagnostic sets, `diagnostic_summaries` and worktree diagnostics. A
detached server that other buffers still map to keeps running and keeps
everything it published. An earlier revision judged orphanhood by
`buffers_opened_in_servers`, which is only filled in once a server runs,
so it stopped servers that were still starting;
`test_context_completion_provider_mentions` caught that.

## Testing

Added `test_selecting_a_language_clears_the_old_servers_diagnostics`: a
Rust buffer gets a diagnostic from a fake server, the buffer is switched
to JavaScript, and afterwards both the buffer's diagnostics and the
project-level `diagnostic_summary` must be empty. It fails on `main`,
where the message survives the switch:

```text
< ["unused constant"]
> []
```

The test passes with the rework and fails without it. The `project` test
suite, `script/clippy` and style checks ran green on the reworked change
in CI. Tested on Linux; the change is platform independent.

## Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content adheres to Zed's UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

## Showcase

The buffer is back on YAML, as the status bar shows, while the workflow
schema's complaints from the GitHub Actions server are still on screen:
`on` and `jobs` reported missing, and `inputs`, `outputs` and `runs`
reported as unexpected values.

<img width="2514" height="2008" alt="Screenshot From 2026-08-30
21-52-12"
src="https://github.com/user-attachments/assets/040898a6-cc61-41b6-88c6-f346666e8e13"
/>

---

Release Notes:

- Fixed diagnostics from the previous language server staying on a file
after changing its language.

---------

Co-authored-by: Kirill Bulatov <kirill@zed.dev>