How to fork a Claude Code or Codex session to try another approach

The agent has a plan, and it's halfway through. You want to see what the other approach looks like, the one you talked it out of an hour ago. If you ask in the same session, you pollute the context it has built up. If you start a new session, you lose that context. A fork gives you both. It copies the conversation so far into a new session, and the original stays exactly where it was: Claude Code, inside a session: /branch. You're moved into the copy, and the original can be resumed later. Claude Code, from the shell: claude --resume <session-id> --fork-session. Codex, inside a session: /fork. From the shell: codex fork <session-id>, or codex fork --last. The catch: a fork copies the conversation, not your files. Two sessions editing the same checkout will step on each other. Give the fork its own git worktree. Fork when you want to keep the current thread intact: Trying a different approach. The fork explores the alternative. If it doesn't work out, the original session still has the plan it was following, untouched. Comparing two approaches. Fork, then let each session build its version in its own worktree. Compare the diffs, not the arguments. Asking a side question. A tangent that isn't worth keeping in the main context: how some library works, or a quick review of one file. If you just want to undo the last few steps, you don't need a fork. Claude Code's /rewind (also /checkpoint) restores the conversation, the code, or both to an earlier message. Inside a running session, type: /branch try-event-queue Claude Code copies the conversation into a new session and switches you into it. The name is optional. The fork's title ends in "(Branch)", so it's easy to tell apart in /resume. The confirmation message includes the command to get back to the original, claude -r <original-session-id>. From the shell, add --fork-session to a resume: # Fork the most recent session in this directory claude --continue --fork-session # Fork a specific session claude --resume <session-id> --fork-session Without --fork-session, both commands reopen the original session and keep adding to it. Inside a running session, type /fork. From the shell: # Pick a session to fork from a list codex fork # Fork the most recent session in this directory codex fork --last # Fork a specific session codex fork <session-id> Like codex resume, the picker and --last only look at sessions from the current directory. Add --all to see every directory. Codex has two more ways to branch. Press Esc twice to step back through your earlier prompts, pick one, and press Enter. Codex forks the conversation just before that prompt and puts the prompt back in the composer for you to edit. The original session keeps the full history. For a quick tangent, /side starts a side conversation in a temporary fork. No. A fork copies the conversation. Both sessions still work in the same directory, on the same files. If the original agent is editing src/queue.ts while the fork rewrites it, you get a mess that neither of them planned. Give the fork its own git worktree. A new worktree starts from your last commit, not from the files on disk, so uncommitted edits and new files don't come along. If the agent has been working for a while, commit first: git add -A && git commit -m "wip: before fork" You can squash or amend that commit later. Without it, the fork remembers changes that its worktree doesn't have. If you've used claude --worktree in this repo before, add .claude/worktrees/ to .gitignore first, or git add -A will pick up those worktree folders too. Then fork into a worktree. Both CLIs can create one for you: # Claude Code: fork into a new worktree under .claude/worktrees/try-event-queue claude --worktree try-event-queue --resume <session-id> --fork-session # Codex: fork into a new worktree under ~/.codex/worktrees codex fork --worktree <session-id> Claude Code puts its worktree on a new branch named worktree-<name>. Codex needs an explicit session ID here, since --last doesn't work with --worktree, and its worktree starts on a detached HEAD, so create a branch there (git switch -c try-event-queue) before you commit anything you want to keep. Or create the worktree yourself and fork from inside it: git worktree add ../myapp-event-queue -b try-event-queue cd ../myapp-event-queue claude --resume <session-id> --fork-session The fork still has the whole conversation, even though it now runs in a different directory. When one approach wins, merge its branch and remove the other worktree by path, for example git worktree remove ../myapp-event-queue. git worktree list shows the paths, including the ones Claude Code and Codex created. Forks look alike in a session list, since they start from the same conversation. Name them when you create them: Claude Code: /branch <name>, or /rename inside any session. Codex: /rename inside the session. A name that says what the fork is trying, such as "event queue" or "keep polling", makes the right session easy to find in claude --resume or codex resume a day later. Yes. In PonyMux, select a Terminal that's running Claude Code or Codex, open the Inspector, and click Fork Session in the Session section. The same action is in each session's ⋯ menu in the Sessions panel. The Inspector's Session section for a Claude Code session. Fork Session is right above Handoff. PonyMux asks where to put the fork: Here ends whatever is running in the current Terminal and starts the fork in its place. New Terminal opens the fork next to the original, in the same list. It runs claude --resume … --fork-session or codex fork … for you, in the session's working directory. A Terminal that holds a fork shows an Origin section in the Inspector, with Go to Source Session to jump back to the conversation it came from. PonyMux doesn't create a worktree for the fork, and it always starts the fork in the original session's working directory. If the two sessions will edit files, commit first as described above, create the worktree yourself, open a Terminal in it, and run claude --resume <session-id> --fork-session there, or use codex fork --worktree. A session needs at least one saved exchange before it can be forked. You also don't need to remember which session was which. Each PonyMux Terminal keeps a timeline of every Claude Code and Codex session that has run in it. Open the Sessions panel to see them, with their titles, recaps, and when each was last used. Click Resume on any row to go back to that exact session, not just the most recent one. The Sessions panel lists every conversation that has run in this Terminal. Resume reopens the one you pick. Resume needs the agent's transcript to still be on disk. Claude Code deletes transcripts after 30 days by default; see Claude Code conversation history for how to keep them longer. For the details of Resume, Fork, and Handoff, see the Sessions docs. To move a task to the other agent instead of copying it, see How to hand off a task from Claude Code to Codex. Originally published on ponymux.com, where the demos are interactive.