Cmux Fork

cmux-fork — Fork Conversation into New Pane

Fork the current Claude Code session into a new cmux pane or workspace, preserving full conversation context.

Language Rules

1. Auto-detect input language → output in the same language
2. Japanese input → Japanese output, use references/fork-guide.ja.md
3. English input → English output, use references/fork-guide.md
4. Explicit override takes priority

Prerequisites

BEFORE forking, verify cmux environment:

echo $CMUX_SOCKET_PATH

• If empty or unset → stop with error:
  • EN: "Error: Not running inside a cmux session. Please start Claude Code from within cmux."
  • JA: "エラー: cmux セッション内で実行されていません。cmux 内で Claude Code を起動してください。"
• If set → proceed

Permission Mode

By default, fork with --dangerously-skip-permissions to avoid re-approving every tool in the forked session. Use interactive mode only when explicitly requested.

Direction Mapping

Parse the user's request to determine fork direction. Use the first match from the table below. If no direction keyword is found, use the default (right).

Execution Steps

Step 1: Determine Direction and Permission Mode

Parse the user's input against the Direction Mapping and Permission Mode tables above.

• Direction default: right
• Permission default: --dangerously-skip-permissions

Step 2: Create Surface

Run the appropriate cmux command based on direction:

For pane split (right or down):

cmux new-split right
# or
cmux new-split down

The command outputs: OK surface:{N} workspace:{N}

Extract the surface handle (e.g., surface:31) from the output.

For new workspace:

cmux new-workspace

The command outputs: OK surface:{N} workspace:{N}

Extract both the surface and workspace handles from the output.

Step 3: Launch Forked Claude Code

Send the fork command to the new surface. Default includes --dangerously-skip-permissions to avoid re-approving tools:

Default (auto-approve — recommended):

cmux send --surface surface:{N} "claude --continue --fork-session --dangerously-skip-permissions\n"

Interactive mode (if user explicitly requested):

cmux send --surface surface:{N} "claude --continue --fork-session\n"

Where surface:{N} is the handle extracted in Step 2.

Step 4: Verify Launch

Wait briefly, then read the new surface to confirm Claude Code started:

sleep 3
cmux read-screen --surface surface:{N}

Check the output for:

• Claude Code startup message or prompt → success
• Error messages or empty screen → may need more time; retry once after sleep 3

Step 5: Report Result

On success, report to the user:

• EN: "Forked session to surface:{N}. The new session has your full conversation context."
• JA: "surface:{N} にセッションをフォークしました。会話コンテキストが引き継がれています。"

On failure, report the error:

• EN: "Failed to start forked session. Check the new pane for errors."
• JA: "フォークしたセッションの起動に失敗しました。新しいペインのエラーを確認してください。"

Error Handling

Notes

• By default, --dangerously-skip-permissions is used so the forked session doesn't require re-approving every tool. Use interactive mode if you want manual approval.
• This skill only forks Claude Code sessions. To launch other agents (Codex, Gemini CLI), use cmux-delegate instead.
• For detailed cmux command reference and troubleshooting, see the reference guide.