---
title: "Compaction"
description: ""
---

Compaction replaces the active model context from an older part of a session
with a generated checkpoint. The checkpoint contains a structured summary and
a serialized tail of recent context, so the agent can continue with more room
in the model's context window.

Compaction is lossy, but it does not delete the earlier durable session
messages. After a successful compaction, V2 builds model requests from the
latest completed checkpoint and the messages that follow it.

## Automatic compaction

Automatic compaction is enabled by default. Before a model call, V2 estimates
the size of the final system prompt, messages, and advertised tools. It starts
compaction when:

```text
estimated tokens > context limit - max(requested output tokens, buffer)
```

The estimate is approximate: V2 JSON-serializes the request and assumes four
characters per token. When compaction succeeds, V2 rebuilds the request from
the new checkpoint and retries the step without promoting the input again.

V2 also recognizes provider errors classified as context overflow. If an
overflow occurs before the provider produces assistant output or other retry
evidence, V2 can compact and retry that step once. This recovery is attempted
even when `auto` is `false`; `auto` controls only the preflight size check. A
second overflow after recovery is returned as an error.

## Manual compaction

In the TUI, run:

```text
/compact
```

`/summarize` is an alias. The default keybind is `<leader>c`, configured as
`session_compact`.

A manual request is durably admitted and wakes the session runner. It can
compact short histories that would not trigger automatic compaction. If the
session is busy, compaction runs at the next safe drain boundary before later
steered or queued prompts are promoted. Repeated requests while one is pending
coalesce into that pending request. Whether compaction completes or fails, the
barrier is then settled so later prompts can proceed.

The CLI has no separate `compact` subcommand. Use the TUI command or the server
API. For example:

```bash
opencode2 api v2.session.compact \
  --param sessionID=ses_example \
  --data '{}'
```

The equivalent raw request is:

```bash
opencode2 api post /api/session/ses_example/compact --data '{}'
```

`POST /api/session/:sessionID/compact` returns the admitted compaction input;
it does not wait for summary generation. Clients can call
`client.session.compact({ sessionID })` and then wait for the session or follow
the `session.compaction.*` events. Supplying an optional message `id` makes an
exact retry idempotent, but reusing an ID owned by another record returns a
conflict.

## Configuration

Add `compaction` to any [OpenCode configuration file](/config):

```jsonc title="opencode.jsonc"
{
  "$schema": "https://opencode.ai/config.json",
  "compaction": {
    "auto": true,
    "keep": {
      "tokens": 15000,
    },
    "buffer": 20000,
  },
}
```

| Field         | Default | V2 behavior                                                                                                                                   |
| ------------- | ------: | --------------------------------------------------------------------------------------------------------------------------------------------- |
| `auto`        |  `true` | Runs the preflight context-size check. It does not disable manual compaction or one-shot provider-overflow recovery.                          |
| `keep.tokens` | `15000` | Approximate number of tokens from the newest serialized conversation context to retain beside the summary.                                    |
| `buffer`      | `20000` | Safety reserve below an explicit input limit. Without one, it is the minimum context reserve and the model output allowance wins when larger. |

`keep.tokens` and `buffer` accept non-negative integers. Larger `keep.tokens`
preserves more recent detail but leaves less room for future work. Larger
`buffer` triggers preflight compaction earlier.

## Checkpoint contents

V2 uses the session's selected or default model to generate the summary, with
tools disabled and at most 4096 output tokens. The summary records the
objective, important details, completed and active work, blockers, next moves,
and relevant files.

The newest serialized context up to `keep.tokens` is retained separately. This
is not a byte-for-byte transcript: tool output is limited to 2000 characters,
and file or media attachments become textual descriptors rather than embedded
data. On later compactions, V2 updates the previous summary and carries forward
its retained recent context before selecting a new tail.

The completed compaction is presented to the model as historical conversation
context, explicitly not as new instructions. Running and failed compactions are
not included in model context.

## Compaction advances the instruction epoch

Conversation compaction and instruction synchronization are separate. Before
promoting pending input, V2 compares live instruction sources with the latest
admitted values. Ordinary changes become durable value deltas; their
model-facing System messages are derived during request assembly rather than
persisted.

Completed compaction advances the instruction epoch at the exact ended-event
sequence and makes the currently admitted values initial. It does not reread
sources or publish an instruction event. Session movement and committed revert
clear the instruction fold so the next safe boundary requires one complete
source read. See [Instructions](/instructions) for source ordering and update
behavior.

## Current limitations

- Compaction requires a resolvable model with a positive catalog context limit.
  There is no separate compaction-model setting or fallback model.
- Summary generation can fail if the summary prompt itself cannot fit beside
  its output allowance, the model returns no summary, or the provider fails.
- Automatic and overflow compaction need older conversation context that can be
  replaced. A provider overflow can still surface when there is no compressible
  head or fixed instructions and tool schemas dominate the request.
- Overflow recovery retries only once per step. Token estimation is heuristic,
  so it cannot prevent every provider-specific overflow.
- Earlier durable messages remain stored even though they are no longer in the
  active model context.

V1 used additional tail-turn and pruning behavior. Those V1 details are only
migration context; the settings and behavior on this page describe V2.
