Add manual compaction support

[felixarntz]

ACP v2 can report that compaction happened, but it does not provide a standard way for a Client to request compaction. This leaves clients without a portable way to compact a long-running session before starting another turn or when approaching a context limit.

Clients need a portable way to request compaction, provide optional instructions for the summary, and observe when the operation completes. A standard ACP method would let Clients offer the same control across Agents.

Proposed shape

Add an optional, capability-gated session/compact method:

{
  "sessionId": "sess_123",
  "instructions": "Preserve the decisions, unresolved errors, and file paths needed for the next turn."
}

The Agent would advertise support through the existing v2 capability tree. The request could return once compaction is accepted, with completion and failure reported through session/update. The update should identify the compaction boundary and remain associated with the same session so Clients can update their local history and continue normally.

Optional instructions should guide the summary without changing the session's user-visible conversation. The protocol should define whether compaction is permitted only between turns or may interrupt an active turn, and how a Client observes completion before sending the next prompt.

Open questions

• What capability shape should advertise manual compaction, and should it include support for custom instructions?
• Should session/compact be rejected while a turn is active, or should it queue until a safe boundary?
• Should the method be synchronous, or should it return an operation identifier with lifecycle updates?
• What should the Agent report when it auto-compacts during the same period as a manual request?
• Should completion include a summary, a message boundary, token usage, or only a compaction event?
• How should Agents report unsupported requests or compaction failures?

Harness support

All lists are non-comprehensive and just samples of the ecosystem.

Support for manual compaction:

• Claude Code
• OpenCode
• Pi

No support for manual compaction:

• Cline
• Codex
• Cursor
• Deep Agents
• fx
• GitHub Copilot
• Grok Build
• ACP profiles without a profile-specific compaction implementation

Related discussion

#871 discusses exposing compaction summaries and boundaries so ACP-only Clients can persist resumable context. This proposal complements that work by defining how a Client asks an Agent to perform compaction in the first place.

Looking for

Feedback on the method name, capability shape, instruction semantics, and lifecycle events. If this direction fits ACP v2, we can draft an RFD and test it against multiple ACP harnesses.