ACP v2 Draft Released

A draft of Agent Client Protocol (ACP) v2 has been released. Since the v1 launch, features have been added through more than 15 RFDs (Request for Discussion), but the point has come where some breaking changes are needed.

There are three core changes in v2.

• Moving away from the turn concept: Previously, the structure centered on request-response turns based on user messages, but as scenarios where agents work in the background for extended periods have increased, session/update notifications have been changed to be freely sendable outside of turns as well. A prompt response now signifies 'message acknowledgment' rather than 'turn end,' and agents can explicitly signal an 'idle' state.
• Unified message streaming and updates: A stable ID-based patch approach is now consistently applied across user/agent messages, tool calls, and terminal output. Tool call content can now also be streamed, eliminating the need to repeatedly send the entire content.
• Diff structure overhaul: Instead of the existing oldText/newText approach, this has been replaced with a structured file change format that can express file additions, deletions, modifications, moves, copies, and binary changes all together. A git_patch can also optionally be provided.

New features will be sequentially reflected in both v1 and v2 through the existing RFD process, with v2 focusing solely on core behavioral changes.