The /model menu and SDK flag changes wait on PreModelSwitch hooks

Model and effort changes from the picker now run hooks first and can be cancelled.

Use it now New Features

JSON All of v2.1.251

Use it nowTier: how much it should matter to you

3Useful: my rating, 1 to 5

3Signal: worth watching, 1 to 5

HooksArea: what it touches

New FeaturesKind: in v2.1.251, 28 Aug 26

What probably matters to youSection of the release

What

Changing model or effort level from the /model menu now runs hooks first, showing a "Running PreModelSwitch hooks…" spinner. A block decision cancels the switch and shows the hook's reason; ask sends you to the confirmation dialog with that reason as the prompt. Any system messages the hook returns are appended to the resulting notice.

Details

The same pass runs when an SDK client changes model or agent through apply_flag_settings; anything other than proceed blocks the switch and reports back over the control channel.
If the session's model changes while the hooks are still running, the request is rejected with "agent switch not applied: the session's model changed while PreModelSwitch hooks ran; send the request again".
Only has an effect if a PreModelSwitch hook is configured.

Evidence

Running PreModelSwitch hooks…, agent switch not applied: the session's model changed while PreModelSwitch hooks ran; send the request again

Strings lifted out of the shipped bundle, so the claim above can be checked against them.

Read from

Names in the bundle/model

Since it was published

The entry above is what we published on the day. These lines were added later, as Anthropic's own pages caught up, and they sit beside the original rather than replacing it.

Confirmed since 10 Sep 2026 Anthropic's documentation has since written up /model, on All settings. Cap the [effort level](/docs/en/model-config#adjust-effort-level) a session can use, leaving lower levels available. Any higher level runs at the cap instead, including one from `/effort`, the `/model` picker, `--effort`, [`CLAUDE_CODE_EFF… settings-reference see the edit

Added since 10 Sep 2026 A small documentation edit on Agent SDK reference - TypeScript touched a line naming /model after this was published. `effortLevel` accepts an [effort level](/docs/en/model-config#adjust-effort-level) name. It also accepts `"ultracode"`, which requests `xhigh` effort with [ultracode](/docs/en/workflows#let-claude-decide-with-ultracode) on. `applyFlagSetti… agent-sdk/typescript see the edit

How sure we are

Two sources agreeTwo things we can check say the same as this entry.

Anthropic's documentation agreesA small documentation edit on Agent SDK reference - TypeScript touched a line naming /model after this was published.

Anthropic's release notes agreeAdded PreModelSwitch and PostModelSwitch hook events (block, confirm, or annotate a model switch); SessionStart resume hooks now receive…

See this entry in the whole of v2.1.251 →

The build it came inv2.1.251

28 Aug 2026 · 373 entries, this one is #78.

Use it now45
You'll notice200
Nothing to try yet36
Under the hood92

Either side of it

v2.1.250Older
v2.1.252Newer

Open the release →

Names in this entry

slash/model
Switch the AI model and save it as your default for new sessions. For models that support it, use left/right arrows to adjust effort level. With no argument, opens a picker; press s on a row to switch for the current session only. See when Claude Code asks you to confirm the switch. Once you confirm the switch, if Claude Code asks, Claude Code applies the change without waiting for the current response to finish. Before v2.1.242, Claude Code decided from a feature flag it fetched from Anthropic whether to run the command mid-turn or queue it until the turn finished, and always queued it in a session that doesn't fetch feature flags, such as on a third-party provider. Also available in non-interactive mode (-p) with a model argument instead of the picker, where it applies to the current session only and isn't saved as your default; requires Claude Code v2.1.205 or later
hookPreModelSwitch
Before Claude Code applies a model switch that you or a client requested. Can block the switch

Anthropic's note

Upstream's own release notes for v2.1.251 name something this entry names, so the two are probably about the same change. Probably: the pairing is made on a shared name rather than on the words around it.

Added PreModelSwitch and PostModelSwitch hook events (block, confirm, or annotate a model switch); SessionStart resume hooks now receive session staleness and the estimated re-cache cost

Is this right?