Fork: Dual-Model Implementation

Implement the same task with both Claude Code and OpenAI Codex CLI in parallel git worktrees, then run the best-of skill to compare and select the superior implementation.

Task

$ARGUMENTS

Workflow

Step 1: Validate Prerequisites

Verify both CLIs are available:

command -v claude || { echo "ERROR: claude CLI not found"; exit 1; }
CODEX=$(command -v codex || echo "$HOME/node_modules/.bin/codex")
test -x "$CODEX" || { echo "ERROR: codex CLI not found"; exit 1; }

If either is missing, report the error and stop.

Step 2: Capture Context

Record the current state:

REPO_ROOT=$(git rev-parse --show-toplevel)
BRANCH=$(git branch --show-current)
[ -z "$BRANCH" ] && { echo "ERROR: detached HEAD — checkout a branch first"; exit 1; }
SHORT_SHA=$(git rev-parse --short HEAD)

Run git status --porcelain. If there are uncommitted changes, warn the user and ask whether to proceed — uncommitted changes will not be present in the worktrees — or abort.

Step 3: Create Worktrees

Generate a short unique suffix:

SUFFIX=$(python3 -c "import time,hashlib; print(hashlib.sha256(str(time.time_ns()).encode()).hexdigest()[:8])")

Create a temporary directory and two worktrees branching from the current HEAD:

FORK_DIR=$(mktemp -d "${TMPDIR:-/tmp}/fork-XXXXXX")
git worktree add "$FORK_DIR/claude-$SUFFIX" -b fork/claude-$SUFFIX HEAD
git worktree add "$FORK_DIR/codex-$SUFFIX" -b fork/codex-$SUFFIX HEAD

Record the two worktree absolute paths and branch names — you will need them in every subsequent step.

Step 4: Compose the Prompt

Create a temp file for the implementation prompt:

PROMPT_FILE=$(mktemp /tmp/fork-prompt-XXXXXX)

Write a self-contained implementation prompt to this file. The prompt must include:

1. The user's task from $ARGUMENTS.
2. Any project-level constraints from CLAUDE.md or AGENTS.md at the repo root (read the file and inline its relevant sections).
3. Key context: current branch name, language, framework — anything the task needs.
4. A closing instruction:

Implement this change completely. Commit all your work with a clear commit message when done.

This single prompt file is shared by both models so they receive identical instructions.

Step 5: Run Both Models in Parallel

Launch both implementations simultaneously using two parallel Bash tool calls. Use a 600000 ms (10 minute) timeout on both.

Claude (in worktree 1):

cd <claude-worktree-path> && \
claude -p --dangerously-skip-permissions --verbose < "$PROMPT_FILE"

Codex (in worktree 2):

cd <codex-worktree-path> && \
CODEX=$(command -v codex || echo "$HOME/node_modules/.bin/codex") && \
"$CODEX" exec --full-auto - < "$PROMPT_FILE"

No -m or -c flags for Codex — the user's ~/.codex/config.toml supplies model and reasoning settings.

Both calls MUST be issued in the same message so they execute in parallel.

Step 6: Handle Errors

After both complete, check results:

• Both succeeded: continue to Step 7.
• One failed: report the failure, present the successful implementation, and ask the user if they want to adopt it directly or retry the failed model.
• Both failed: report both errors, clean up (skip to Step 10), and suggest the user try the task manually.

A model "succeeded" if it exited zero AND produced at least one commit in its worktree (check with git log <original-branch>..HEAD --oneline in the worktree).

Step 7: Collect Results

For each worktree, gather the implementation summary. Run these in parallel (two Bash calls):

cd <worktree-path> && \
echo "=== Commits ===" && \
git log --oneline <original-branch>..HEAD && \
echo "=== Stat ===" && \
git diff --stat <original-branch>..HEAD && \
echo "=== Diff ===" && \
git diff <original-branch>..HEAD

Note the results. Present a brief side-by-side summary to the user:

• Number of commits
• Files changed
• Lines added / removed

Step 8: Run Best-Of

Invoke the /best-of skill, passing it:

• The original task prompt ($ARGUMENTS).
• Claude's branch name, worktree path, and the diff from Step 7.
• Codex's branch name, worktree path, and the diff from Step 7.

The best-of skill will compare both implementations — correctness, code quality, test coverage, adherence to the prompt — and recommend which to adopt or how to combine the best parts.

Fallback — if the best-of skill is not installed or fails to load: perform an inline comparison yourself. Read the diffs from Step 7 and evaluate both implementations on:

1. Correctness — does it fulfill the task?
2. Code quality — readability, idiomatic style, no unnecessary changes.
3. Completeness — tests, edge cases, documentation if warranted.
4. Safety — no security issues, no broken imports.

Present the comparison under this structure:

## Claude's Implementation
<brief summary and assessment>

## Codex's Implementation
<brief summary and assessment>

## Verdict
<which is better and why, or how to combine>

Then ask the user: "Which implementation would you like to adopt — Claude's, Codex's, a combination, or neither?"

Step 9: Apply Winner

Based on the best-of result or the user's choice:

1. Switch back to the original branch:

   cd "$REPO_ROOT" && git switch <original-branch>

2. Merge the winning branch:

   git merge <winner-branch> --no-ff -m "Merge fork/<model>-$SUFFIX: <concise task summary>"

3. If the user chose a hybrid (parts from each): cherry-pick or manually apply the recommended combination, then commit.

Always ask for user confirmation before merging.

Step 10: Cleanup

Remove the temp prompt file:

rm -f "$PROMPT_FILE"

Remove both worktrees:

git worktree remove "$FORK_DIR/claude-$SUFFIX" --force
git worktree remove "$FORK_DIR/codex-$SUFFIX" --force
git worktree prune
rm -rf "$FORK_DIR"

Optionally delete the fork branches if the user no longer needs them:

git branch -D fork/claude-$SUFFIX fork/codex-$SUFFIX

If the user wants to keep a worktree for inspection, skip its removal and report the path.

Error Handling

• Dirty working tree: Warn and ask before proceeding — worktrees won't include uncommitted changes.
• Worktree creation fails: Clean up any partial state and report the error.
• Model timeout: If either model exceeds 10 minutes, kill the process, note the timeout, and proceed with whichever model finished.
• Merge conflict: If the winning branch conflicts with the original, report the conflicts and let the user resolve them.

Constraints

• Parallel execution: Both models MUST run simultaneously, not sequentially.
• Isolation: Each model works in its own worktree and branch — no cross-contamination.
• Identical starting point: Both worktrees branch from the same HEAD commit.
• Identical prompt: Both models receive the exact same prompt file.
• No bias: Present results from both models without favoring either.
• Cleanup: Always remove worktrees and temp files unless the user explicitly asks to keep them.
• User confirms merge: Never merge a result without explicit user approval.