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# Objective
Add a new `agent.compaction_model` setting that lets users specify a
separate language model for context compaction (`/compact` and
auto-compaction), independent of the thread's active conversation model.
Compaction is just summarization — there's no reason to pay Opus prices
for it when a cheaper model does the job faster. We've also seen
reasoning models misbehave on this task (empty responses, repetition
loops at high effort), so picking a dedicated non-reasoning model for
compaction is useful.
## Solution
- New `compaction_model: Option<LanguageModelSelection>` field in
`AgentSettingsContent` and `AgentSettings`, mirroring
`thread_summary_model`.
- New `compaction_model: Option<ConfiguredModel>` slot on
`LanguageModelRegistry` with `select_/set_/compaction_model()` trio,
mirroring the existing pattern. The setter deliberately does **not**
emit a registry event in v1; callers read the slot lazily at compaction
time.
- New `Thread::compaction_model(&self, cx: &App)` helper that returns
the configured model or falls back to `self.model()`. Two call sites —
`Thread::compact` and `perform_compaction_if_needed` — now go through
this helper instead of reading `self.model()` directly.
- `build_compaction_telemetry` accepts the compaction model explicitly
so the `model` field reflects the model that actually streamed the
request. `max_tokens` still derives from `thread.model()` (threshold
semantics are unchanged).
- Documentation updated at `docs/src/ai/agent-settings.md` (new
user-facing setting).
**Example:**
```json
{
"agent": {
"default_model": {
"provider": "anthropic",
"model": "claude-opus-4-6"
},
"compaction_model": {
"provider": "anthropic",
"model": "claude-sonnet-4-5"
}
}
}
```
**Resolution chain:**
```
agent.compaction_model (if set & available)
→ thread.model() (always available, current behavior)
```
**Behavior change:**
| Trigger | Before | After |
| --- | --- | --- |
| Manual `/compact` | Uses `thread.model()` | Uses
`agent.compaction_model` if set & available; else `thread.model()` |
| Auto-compaction | Uses `thread.model()` | Same as above |
| `/compact` when thread has no model | `NoModelConfiguredError` |
Succeeds if `compaction_model` resolves |
| `compaction_model` configured but provider missing / model id unknown
| n/a | Falls back to `thread.model()` and logs a one-time warning |
**Explicit non-goals:**
- No `Event::CompactionModelChanged`(no consumer; the `_cx` parameter on
`set_compaction_model` is intentionally accepted for future use).
- No GUI selector (consistent with all other feature-specific models).
- No per-profile override.
- No runtime API-error fallback — only config-time failure (provider not
registered, model id not in `provided_models`) triggers fallback. This
matches every other feature-specific model.
- No change to threshold calculation, auto-compact trigger, or
`COMPACTION_PROMPT`.
- No propagation to subagent threads.
## Testing
3 new unit tests in `crates/agent/src/thread.rs::tests`:
- `test_compaction_uses_configured_compaction_model` — manual `/compact`
routes to the configured model; thread's primary model receives no
request; telemetry reflects the configured model.
- `test_compaction_falls_back_when_compaction_model_unavailable` —
configured-but-unresolvable falls back to `thread.model()`; telemetry
reflects the fallback model.
- `test_auto_compaction_uses_compaction_model` — auto-compaction
triggered by threshold honors the same setting.
All 14 existing compaction tests still pass. Test suites in
`crates/agent_settings`, `crates/language_model`,
`crates/settings_content`, `crates/agent_ui` unchanged. `cargo clippy`
clean on the changed crates.
**How reviewers can test:**
1. Add `agent.compaction_model` to `settings.json` with a cheaper model
than the thread's primary model, run `/compact`, observe the cheaper
model receives the request.
2. Set `agent.compaction_model` to a non-existent provider/model id, run
`/compact`, observe fallback to thread model and a `log::warn!` line.
3. Trigger auto-compaction by reaching the threshold, observe it uses
`compaction_model`.
**Platforms tested:** local Linux (cargo check + cargo test on agent /
agent_settings / language_model / settings_content / agent_ui crates).
## Self-Review Checklist:
- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments — no `unsafe`
blocks introduced
- [x] The content adheres to Zed's UI standards — N/A: settings-only
change, no UI touched
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable — model
resolution is an O(1) registry lookup, not on any hot path; no new work
in the streaming loop
Release Notes:
- agent: Add support for specifying which model is used for compaction
(`agent.compaction_model`)
---------
Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>
Co-authored-by: Bennet Bo Fenner <bennet@zed.dev>
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