/rewind says "conversation will be forked" but never creates a resumable fork

Bug Description

/rewind promises "The conversation will be forked" but mutates the session in place. After rewinding, the original session disappears from /resume and claude --resume. No fork is created.

CLI text shown at rewind time

  Rewind

  Confirm you want to restore to the point before you sent this message:

  │ did you save the transcript anywhere? if not get it again and put it
  │ in the inbox next to the report but don't commit it.
  │ (9m ago)

  The conversation will be forked.
  The code will be unchanged.

"The conversation will be forked" promises two things:

1. The rewound conversation gets a new session ID
2. The original session stays unchanged and resumable

The docs make the same promise for --fork-session:

This creates a new session ID while preserving the conversation history up to that point. The original session remains unchanged. — How Claude Code works → Resume or fork sessions

/rewind honors neither.

What actually happens

1. /rewind resets the branch pointer to an earlier message within the same JSONL file
2. No new session file or session ID is created
3. The original end-state vanishes from /resume and claude --resume
4. Post-rewind messages remain in the JSONL (append-only) but no UI can reach them

JSONL evidence

Session 08bbfa97-c2f2-4220-a4a4-d9ca2f10fe42:

• 204 lines, every one marked isSidechain: false — zero isSidechain: true entries
• Every message carries the same sessionId; no second session ID exists
• last-prompt / leafUuid entries advance through messages but never branch into a separate session
• No other JSONL file in the project directory references this session ID

A real fork would produce one of:

• A new JSONL file with a new session ID (containing the rewound conversation), or
• The abandoned branch marked isSidechain: true, with new messages appended as isSidechain: false

Neither occurred.

Expected behavior

When /rewind says "The conversation will be forked":

1. Create a new session with a new session ID for the rewound conversation
2. Leave the original session unchanged and visible in /resume
3. Make both independently resumable

Environment

• Claude Code version: 2.1.126
• OS: macOS 26.4.1 (Darwin, arm64)
• Platform: CLI

Related issues

• #24471 — isSidechain never set on rewound branches; compaction severs the tree. Same root cause, different symptom: that issue covers the rewind picker losing history; this one covers the original session vanishing from /resume.
• #9279 (closed) — Requests rewind without forking. Its description confirms rewind is designed to fork every time, making the current behavior a bug.
• #47987 — "Summarize from here" after rewind drops pre-rewind messages. Same class of data loss, different trigger.