opencode-context-pruner

Continuous, verbatim context pruning for OpenCode, powered by TypeSafe Jev.

Adapted from fast-jev-compaction (MIT). Upstream targets Claude Code's session.compact hook. OpenCode (tested: opencode2 0.0.0-beta-19271) has no compaction hook to rely on, so this port prunes the outgoing model request from the context hook instead: stale tool calls/results are judged per call (keep / truncate / drop) and removed from the request view. Persisted history is never modified, so nothing disappears from your session log.

Why

Long coding sessions fill up with stale tool output. Instead of an LLM-written summary that paraphrases details away, every kept message stays verbatim — only old tool calls/results are dropped or truncated, decided by calibrated Jev judgments.

How it works

1. session.hook("context") runs right before each model dispatch
2. messages are converted to the vendor core's shape; each tool call gets two noul questions: keep the call? keep the result?
3. decisions are applied to the request's message view:
   • drop_call removes the call and its result
   • drop_result keeps the call and truncates the result text with a note
   • keep is left untouched
4. decisions are cached per tool_use_id inside the session, so each tool call is judged once — requests without new tool calls make no Jev request at all
5. fail-open: any error leaves the request untouched

Tool call/result pairing is always preserved (a dropped call takes its result with it).

Measured (2026-09-19, a real 530-message session)

messages: 530
judged:   257 calls (7 batches, fitted state 24,974 tokens)
dropped:  257 calls + 257 results, removedMessages: 282
time:     1,135 ms
result:   model replied normally

Requirements

• OpenCode V2 beta 0.0.0-beta-19271 or newer
• TypeSafe API key: TYPESAFE_API_KEY in the environment of the OpenCode server, or ~/.config/opencode/typesafe/api_key

Install

opencode plugin add opencode-context-pruner
export TYPESAFE_API_KEY=...

Or run from a local checkout:

git clone https://github.com/hoshinodis/opencode-context-pruner ~/app/opencode-context-pruner
ln -s ~/app/opencode-context-pruner ~/.config/opencode/plugins/opencode-context-pruner

Options

Plugin options can be passed through OpenCode's plugin config; defaults below.

option	default	meaning
enabled	true	TYPESAFE_COMPACTION=off also disables
model	jev-1.13.0	pinned Jev model
keepThreshold	0.15	noul threshold for keeping a call/result (upstream uses 0.5; see below)
preserveRecentMessages	10	newest messages are never judged
truncateHeadChars	300	head kept when truncating a result
minResultChars	4000	skip entirely when the request has less tool output
maxStateTokens / maxRequestTokens	25000 / 30000	state fitting limits
rejudge	never	always re-judges everything on each request
apiKeyEnv / apiKeyFile	TYPESAFE_API_KEY / ~/.config/opencode/typesafe/api_key	key lookup
logFile	~/.config/opencode/context-pruner/decisions.jsonl	JSONL decision log

Why keepThreshold defaults to 0.15, not 0.5

The two questions Jev answers are "is this call still needed for the assistant's next action?" and "is the full result still needed?". Measured on a realistic transcript, old calls score:

keepCall   0.17 – 0.23
keepResult 0.11 – 0.14

With upstream's 0.5 (or even 0.3) every unpinned call is drop_call. With 0.15 the common outcome becomes drop_result: the call and its input stay, only the bulky result text is truncated. Set keepThreshold: 0.5 if you want the original aggressive behavior.

Non-destructive by construction

Decisions are applied to clones: input messages and tool parts are never mutated (this is enforced by tests). Only the request view is replaced; persisted history and the UI keep every original message. If the runtime rejects the replacement array, the plugin does nothing (fail-open).

Caveats

• Decisions are cached per tool_use_id: a call judged once keeps that verdict for the rest of the session.
• keepThreshold: 0.5 (upstream's default) drops nearly every unpinned call in a long session; the 0.15 default exists to keep the call and truncate only its result.
• Jev can be wrong. The decision log keeps every action so you can audit and re-tune.

Vendored core

src/vendor/ is a copy of fast-jev-compaction src/ (MIT, v0.2.0). See THIRD_PARTY_NOTICES.md. To update, copy the upstream src/ again.

Development

npm install
npm run typecheck
npm test

License

MIT. Not affiliated with TypeSafe, OpenCode, or the fast-jev-compaction author.