Compaction Guard

An OpenCode plugin that prevents and auto-recovers from the error that makes a session permanently unrecoverable after auto-compaction (or after an interrupted tool call):

messages.N: `tool_use` ids were found without `tool_result` blocks immediately
after: toolu_XXXX. Each `tool_use` block must have a corresponding `tool_result`
block in the next message.

When this happens, Anthropic returns a 400 and every subsequent message in the session fails — including continue. Without this plugin your only escape is to abandon the session (open a new terminal) or do manual DB surgery.

The problem

Anthropic's API requires that every tool_use block is immediately followed by its matching tool_result block. Two things in OpenCode can break that pairing:

1. Interrupted / aborted tool calls — a tool left in pending/running state produces a tool_use with no tool_result.
2. Session compaction — when OpenCode trims a long session, the slice boundary can split a tool_use from its tool_result, and on auto-compaction it can re-fire and wedge the session into a non-recoverable loop.

This is a known, still-unreleased-fix bug. See Prior art & references below.

What this plugin does (two layers)

Think of it as a seatbelt and an airbag.

1. Prevent (experimental.chat.messages.transform) — runs before every request and before compaction, on the exact message array opencode is about to convert and send. It's a pairing-normalization pass: it mirrors opencode's own conversion rules (which tool-part states emit a tool_use / tool_result, and the assistant-error skip), then drops any tool part that would emit a tool_use with no matching tool_result (and any dangling result). It never fabricates tool output — it only removes provably unpaired members, so it can't feed the model fake results. The pass is idempotent and order-independent, so it's correct no matter where it runs relative to other plugins. This closes the genuinely-unpaired classes (interrupted/aborted turns, and any future opencode regression where a state stops emitting its counterpart).

2. Recover (event → session.error) — if an orphan still reaches Anthropic and it returns the 400, the plugin parses the offending toolu_ ids out of the error, finds the message that actually holds that orphaned tool_use, and reverts to that turn's clean boundary (the nearest preceding user message) so the orphan is excluded from the next send. A per-orphan-id state machine acts once per distinct orphan and hard-stops with an actionable toast if the same orphan recurs after it acted — so you never sit in a silent 400 loop again. Reverts are reversible with /unrevert.

Why two layers? The post-compaction reorder class of orphan is created inside the AI-SDK message conversion, downstream of the prevention hook, so prevention can't always reach it. The recover layer is the real safety net for that class; prevention covers everything reachable before conversion.

Install

Option A — one line (recommended)

Add the package to the plugin array in your OpenCode config (~/.config/opencode/opencode.json or your project's opencode.json):

{
  "plugin": [
    "opencode-compaction-guard@latest"
  ]
}

Restart OpenCode. Done.

Option B — manual (no npm)

Download plugin/compaction-tool-pair-guard.js into your global plugin folder:

mkdir -p ~/.config/opencode/plugin
curl -fsSL https://raw.githubusercontent.com/viiqswim/opencode-compaction-guard/main/plugin/compaction-tool-pair-guard.js \
  -o ~/.config/opencode/plugin/compaction-tool-pair-guard.js

Restart OpenCode. It loads automatically — no config changes needed, and it applies to every project and every worktree.

The file extension must be .js (or .ts). OpenCode's local-plugin directory scanner only picks up .js/.ts children of ~/.config/opencode/plugin/ — a .mjs file (this plugin's name before v1.1.1) sits there silently unloaded, with no error anywhere. If you installed an older version this way, re-download with the command above and delete the old .mjs copy.

How do I know it's working?

OpenCode captures the plugin's console.log output. When it acts you'll see lines prefixed [compaction-tool-pair-guard], e.g.:

[compaction-tool-pair-guard] dropped 1 unpaired tool part(s) before request
[compaction-tool-pair-guard] detected orphaned tool_use error in <session> (ids: toolu_...)
[compaction-tool-pair-guard] reverted to <id> to recover session <id>
[compaction-tool-pair-guard] orphan(s) toolu_... recurred after recovery in <session>; stopping

…and the recovery path also pops a toast: "Session recovered."

Known limitations (read this)

• The prevent layer covers everything that's unpaired before opencode's message conversion: interrupted/aborted turns and any unpaired tool part in the array it's handed. Note opencode already pairs pending/running parts itself during conversion, so prevention's real job is the genuinely-orphaned parts and future regressions — not the common interrupted case opencode already handles.
• The pure post-compaction reorder orphan class is created inside the AI-SDK message conversion, downstream of the prevention hook, so prevention can't always stop it. For that class the recover layer is the safety net: it reverts to the orphan turn's clean boundary and hard-stops (with a toast) if the same orphan recurs, so you never loop on a silent 400 again.
• Recovery is not free. Reverting to the orphan's turn boundary drops that turn and everything after it from the active context (reversible with /unrevert). It unwedges the session; it does not losslessly preserve the failed tail.
• This is a stopgap, not the upstream fix. The real fix belongs in OpenCode's message-conversion / compaction logic. Track anomalyco/opencode#27594. When a release ships the fix, you can remove this plugin.

Compatibility

• Built against the @opencode-ai/plugin 1.3.x hook API; uses only stable public hooks (event, experimental.chat.messages.transform) and SDK methods (session.messages, session.revert, tui.showToast).
• Most relevant to Anthropic / Claude models, which enforce strict tool_use/tool_result pairing. Harmless on other providers.

Prior art & references

This plugin packages and combines approaches already identified by the community — credit to those investigations:

• The "convert pending/running tool calls to error results" heal is essentially the approach in anomalyco/opencode#8497.
• The "revert on session.error" recovery was described in a comment on anomalyco/opencode#1662.
• Root-cause analysis of the compaction slice / auto-retrigger: #27594, #14367, #17065, #21326, #10616.

License

MIT