pi-compact-before-switch

A pi extension that prompts you to compact context using the outgoing model before switching to a smaller-window model via /model. Ctrl+P cycling and session restore pass through silently.

When you switch from an Opus-1M to a Haiku-200K with a long conversation, you'll see:

Compact before switching? Switching from claude-opus-4-5 (1,000,000 ctx) → claude-haiku-4-5 (200,000 ctx). Current context: 320,000 tokens — exceeds target window minus reserve. This will compact, then re-apply Haiku. Take a few seconds — no API key needed on Opus. (← confirm → cancel)

If you confirm, the extension reverts to Opus, runs pi's default compaction, then re-applies Haiku — all automatic. If you cancel, the switch is reverted and you stay on Opus.

Install

pi install npm:pi-compact-before-switch

That's it. The extension registers automatically on next session start.

Other install paths

pi install /path/to/pi-compact-before-switch    # local path
pi install https://example.com/pi-compact-before-switch.tgz   # URL
pi install -l .                                  # project-local (commits .pi/npm/ path)

When does it trigger?

Conditions all met:

1. Source is /model (i.e. source === "set", not Ctrl+P cycling and not session restore)
2. There was a previous model
3. Not re-selecting the same (provider, id)
4. New model's contextWindow < previous model's contextWindow (any narrowing)
5. currentTokens > targetContextWindow − 16,384 (target fits, current doesn't)

If any condition is false, the switch is silent and immediate.

Behavior

Scenario	Result
/model from large → small, doesn't fit	confirm → revert → compact on outgoing model → re-apply target
/model from large → small, fits within reserve	silent, switch happens immediately
Widening (small → big)	silent
Same model re-selected	silent
Ctrl+P cycling	silent
Session restore	silent
Old model has no API key (revert fails)	stay on target, error toast
Compact mid-flight fails	stay on outgoing model, error toast
Confirm canceled	revert to outgoing model, info toast
30 s timer elapses	warning toast, guard cleared, next press re-prompts

Minimal configuration

There is none. Run-time differences:

• Reserve tokens — 16,384 hardcoded. At or below threshold: switch is silent.
• Guard timeout — 30 s after a successful revert, the extension stops swallowing model_select events. If a compact hangs, the next press of /model works again after that.

Why?

When you Ctrl+P through models you can land on one whose window is far below your current context. Many providers truncate silently. Compaction first avoids that.

Development

git clone https://github.com/erans/pi-compact-before-switch
cd pi-compact-before-switch
npm test
./scripts/smoke.sh   # launches `pi -e extensions/compact-before-switch.ts`

Run tests:

./scripts/test.sh  # or: npm test

31 tests cover the trigger filter (cycle/restore/no-previous/same-model/widening/overflow) and the orchestrator paths (cancel, revert failure, reentrancy guard, compact onError, guard recovery). Tests use Node 22+ built-in node:test with --experimental-strip-types. No transpile step.

Debug mode

Set PI_DEBUG_CBS=1 to make the extension emit a notification on every model_select event with the event shape and the trigger decision:

PI_DEBUG_CBS=1 pi

The toast will say:

CBS DEBUG | source=set | prev=anthropic/claude-opus-4-5 (1000000) | next=anthropic/claude-haiku-4-5 (200000) | tokens=12345 | threshold (next - 16384)=183616 | trigger=true

License

MIT © 2026 Eran Sandler. See LICENSE.

Source layout

pi-compact-before-switch/
├── extensions/
│   └── compact-before-switch.ts    # the single extension file
├── tests/                          # node:test, 31 tests
├── scripts/
│   ├── smoke.sh                    # launch pi -e ...
│   └── test.sh                     # npm test runner
├── verification/                   # per-scenario outcomes
├── docs/                           # spec + design + plan
├── package.json
├── README.md
└── LICENSE

When published, pi's convention auto-discovers extensions/*.ts and loads it as a single extension — no pi manifest needed.