Git.Backport

Your task

Back-port the commit given in $ARGUMENTS to a release branch using git cherry-pick. Your arguments are in the format <commit-sha> <target-branch>.

Parse $ARGUMENTS now: the first token is COMMIT_SHA, the second is TARGET_BRANCH.

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Step 0 — Record the starting branch

Before touching anything, capture where the user currently is so you can return them there at the end:

git rev-parse --abbrev-ref HEAD

Store this as ORIGINAL_BRANCH. You will check out this branch at the very end, regardless of whether the backport succeeds or fails.

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Step 1 — Validate inputs

1. Fetch from origin to ensure remote refs are up to date:

   git fetch origin

2. Confirm COMMIT_SHA resolves using the remote ref (do NOT check it out locally):

   git rev-parse --verify <COMMIT_SHA>^{commit}

If it still fails after fetching, stop and tell the user the SHA could not be resolved. Checkout ORIGINAL_BRANCH before stopping.

3. Confirm origin/<TARGET_BRANCH> exists on the remote:

   git rev-parse --verify origin/<TARGET_BRANCH>

Always use the remote ref (origin/<TARGET_BRANCH>) as the source of truth — do not rely on a local checkout of the target branch. If the remote ref does not exist, stop and tell the user. Checkout ORIGINAL_BRANCH before stopping.

4. Check the working tree is clean (git status --porcelain). If it is not clean, stop and tell the user to stash or commit their in-progress work before proceeding. Do NOT checkout ORIGINAL_BRANCH in this case (they're already on it and have local changes).

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Step 2 — Summarize the commit

Show the user what they are about to cherry-pick. Use the remote ref for all inspection — never check out the source commit locally:

git show --stat <COMMIT_SHA>

Print:

• The commit subject
• The author and date
• The list of files changed with their stat line

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Step 3 — Derive source links

Extract the repo's GitHub URL from the remote:

git remote get-url origin

Convert SSH form (git@github.com:org/repo.git) or HTTPS form to a base URL: https://github.com/org/repo.

Build two links:

• Commit link: https://github.com/org/repo/commit/<COMMIT_SHA>
• Original PR link: Look for a (#NNN) pattern in the commit subject line. If found, build https://github.com/org/repo/pull/NNN. If the pattern is absent, omit the PR link and note that it could not be determined automatically.

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Step 4 — Create the backport branch

Derive SHORT_SHA = first 8 chars of COMMIT_SHA. Check the backport branch does not already exist:

git rev-parse --verify backport/<SHORT_SHA>-to-<TARGET_BRANCH>

If it exists locally or on origin, stop and tell the user rather than overwriting it. Checkout ORIGINAL_BRANCH before stopping.

Create the backport branch directly from the remote ref (no local checkout of target branch needed):

git checkout -b backport/<SHORT_SHA>-to-<TARGET_BRANCH> origin/<TARGET_BRANCH>

Tell the user the branch name.

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Step 5 — Attempt the cherry-pick

Run:

git cherry-pick <COMMIT_SHA>

Capture the exit code and stdout/stderr.

If the cherry-pick succeeds (exit code 0) — proceed to Step 6

If the cherry-pick fails (exit code non-zero) — CONFLICT PATH

Immediately abort the cherry-pick:

git cherry-pick --abort

Then proceed to Step 7 (conflict diagnosis). Return to ORIGINAL_BRANCH after reporting.

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Step 6 — Push and open a PR (default on success)

6a — Push the backport branch

git push -u origin backport/<SHORT_SHA>-to-<TARGET_BRANCH>

6b — Build the PR body

## Summary

Backport of commit [`<SHORT_SHA>`](commit-link) from `master`[, originally merged via [#NNN](pr-link)].

<one or two plain-language bullet points summarising what the commit does, derived from the commit message and changed files — do not just copy the commit message verbatim>

## Test plan

- [ ] Verify no CI steps in the `<TARGET_BRANCH>` pipeline depend on anything removed or changed by this commit

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Omit the "originally merged via" clause if the original PR number could not be determined.

6c — Open the PR

gh pr create \
  --base <TARGET_BRANCH> \
  --head backport/<SHORT_SHA>-to-<TARGET_BRANCH> \
  --title "<original commit subject> (backport to <TARGET_BRANCH>)" \
  --body "<body from 6b>"

If gh pr create fails due to missing authentication:

• Tell the user that gh is not authenticated for this host
• Print the URL that git push echoed (looks like https://github.com/org/repo/pull/new/<branch>) so they can open the PR manually
• Print the suggested PR title and body so they can paste them in

6d — Return to original branch

git checkout <ORIGINAL_BRANCH>

Then report to the user:

• The new commit SHA on the backport branch (git rev-parse backport/<SHORT_SHA>-to-<TARGET_BRANCH>)
• The PR URL (or the manual URL + body if gh was not authenticated)
• That they are back on <ORIGINAL_BRANCH>

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Step 7 — Diagnose the conflict (do NOT resolve it)

Your goal here is to explain _why_ the conflict happened so the user can resolve it themselves. Do NOT attempt to modify any files or re-run the cherry-pick. Use remote refs for all file inspection — never check out anything locally.

7a — Identify what the commit changed

git show --name-only <COMMIT_SHA>

Collect the list of files the commit touches.

7b — Find the merge base

git merge-base <COMMIT_SHA> origin/<TARGET_BRANCH>

This gives you MERGE_BASE.

7c — Compare the relevant files across three points in history

For each file in the cherry-picked commit, inspect using remote refs only:

1. What the commit changed (the patch being applied):

   git show <COMMIT_SHA> -- <file>

2. How the file looks on TARGET_BRANCH (the destination — use remote ref):

   git show origin/<TARGET_BRANCH>:<file>

3. How the file looked at the merge base:

   git show <MERGE_BASE>:<file>

4. What has diverged on TARGET_BRANCH since the merge base:

   git diff <MERGE_BASE>..origin/<TARGET_BRANCH> -- <file>

7d — Reason about the conflict

Determine the most likely cause:

7e — Return to original branch, then report

git checkout <ORIGINAL_BRANCH>

Then write the conflict report:

## Backport conflict detected

Cherry-pick of <COMMIT_SHA> onto <TARGET_BRANCH> produced conflicts.
The cherry-pick has been aborted and you are back on `<ORIGINAL_BRANCH>`.

### Conflicting files
<list each file>

### Diagnosis

For each file:
- What the cherry-picked commit was trying to do to this file
- What the current state of this file is on TARGET_BRANCH
- Why those two things conflict (pick the most precise cause from the table above)
- A concrete suggested approach for manual resolution (e.g., "the function was renamed from X to Y on release-8.x — apply the logic change to the renamed function")

Be specific. Quote relevant line ranges or symbol names. Give the user enough context to know exactly where to look and what to do.

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Important rules

• Record `ORIGINAL_BRANCH` at the very start and always return to it at the end — success, failure, or early stop (except when stopping due to a dirty working tree, since you haven't moved).
• Never resolve conflicts autonomously. In the conflict path, diagnose and explain only.
• Always abort a failed cherry-pick before reporting.
• Always push and open a PR on success — this is the default, not optional.
• Never check out the source commit or the target branch locally. Use origin/<TARGET_BRANCH> and inspect commits via git show / git diff. The only local branches you should create or switch to are the backport branch and the return to ORIGINAL_BRANCH.
• If the backport branch already exists, stop and return to ORIGINAL_BRANCH rather than overwriting.
• Do not amend or force-push any branch.