Summary

• update the session/fork RFD to propose optional messageId-scoped forks
• gate the new request field behind session.fork.messageId
• define fork semantics as branching immediately before the referenced message
• tie valid anchors to acknowledged/replayed message IDs from the message-id RFD
• clarify that support is coarse-grained and agents may still reject specific anchors

Why

session/fork currently only branches from the current session head, which is not enough for message editing or retry-from-here flows. Reusing the draft messageId work gives ACP a stable branch-point identifier without introducing index-based ambiguity.

Notes

• This is an RFD-only proposal; no protocol schema or runtime implementation is included here.
• This is intended to support workflows like editing a prior user message by forking immediately before that message and replaying the replacement turn.