Agent Client Protocol (ACP)

ACP connects editors to coding agents. It defines no on-disk session format, and a session id means something only to the agent that issued it, so ACP leaves moving work between agents out of scope; a checkpoint covers that case.

Passing a checkpoint to an agent

When an editor starts a session with a new agent to continue earlier work, it can include the checkpoint as a resource content block in the first prompt, with media type application/vnd.opencheckpoint+json. The agent restores it as untrusted input.

Producing a checkpoint from an ACP session

An editor or agent can build a checkpoint from what ACP already exposes:

ACP	OpenCheckpoint
Plan entries (content, priority, status)	progress.completed for completed entries, progress.focus for the in-progress entry, progress.next for pending entries
Tool call with status: in_progress at the boundary	operation with status: uncertain and an explicit effect
Tool call locations	resource entries for the files, with application_context selections for line ranges
Permission request awaiting a response	operation with kind: approval, status: awaiting
Session id	source.session_refs with protocol: acp

The software development example shows the result.