Stuck trying to resolve git error using coding agent

[mcg]

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Copilot Coding Agent

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We are in the beginning steps of trying to setup and use the coding agent. We have what appears to be a working copilot-setup-steps.yml and can start the agent sessions. However they are failing during the agent workflow run when it attempts to report the plan and continue. See log examples below:

Now let me report my plan and then run the app to take a screenshot.
git ls-files --exclude-standard --others --modified
  Error: Command failed with exit code null: git ls-files --exclude-standard --others --modified
      at ChildProcess.<anonymous> (file:///home/runner/work/_temp/***-action-main/dist/index.js:8:3419406)
      at ChildProcess.emit (node:events:531:35)
      at maybeClose (node:internal/child_process:1101:16)
      at ChildProcess._handle.onexit (node:internal/child_process:304:5) {
    cmd: 'git ls-files --exclude-standard --others --modified',
    code: null,
    stdout: 'vendor/bundle/ruby/3.4.0/bin/bootsnap\n' +
      'vendor/bundle/ruby/3.4.0/bin/brakeman\n' +
      ...

git diff REDACTED REDACTED
  Error: Command failed with exit code 128: git diff REDACTED REDACTED
      at ChildProcess.<anonymous> (file:///home/runner/work/_temp/***-action-main/dist/index.js:8:3419406)
      at ChildProcess.emit (node:events:531:35)
      at maybeClose (node:internal/child_process:1101:16)
      at ChildProcess._handle.onexit (node:internal/child_process:304:5) {
    cmd: 'git diff REDACTED REDACTED',
    code: 128,
    stdout: '',
    stderr: "fatal: ambiguous argument 'refs/heads/master': unknown revision or path not in the working tree.\n" +
      "Use '--' to separate paths from revisions, like this:\n" +
      "'git <command> [<revision>...] -- [<file>...]'\n",
    signal: null
  }
GitError unknown git error: Command failed with exit code null: git ls-files --exclude-standard --others --modified git ls-files --exclude-standard --others --modified
vendor/bundle/ruby/3.4.0/bin/bootsnap
....

I truncated many files these git commands were trying to use and I assume the cause issues we are having. Any idea what we're doing wrong here?

[DavitEgoian]

What to try (in order):

1. Make sure you checkout the repo with full refs/history. Add or update the checkout step:
   uses: actions/checkout@v3
   with:
   fetch-depth: 0
   This fetches all branches and history so refs/heads/master or origin/main exist.

2. Make the agent use the repo’s actual default branch instead of assuming master. If your repo uses main, point the agent at main or use the event base ref (github.event.pull_request.base.ref) as the base for diffs. If the agent is hardcoded to master, set an env fallback like DEFAULT_BASE=main.

3. Add a short debug step before the agent runs so you can see what the runner actually has:
   git rev-parse --abbrev-ref HEAD || true
   git branch -a || true
   git log -n 3 --oneline || true
   This will confirm whether the expected branch/ref is available.

4. Handle initial commits or missing refs gracefully. If there’s no base ref (first commit or orphan branch), fall back to HEAD~1 or skip the diff step — otherwise git diff will fail with code 128.

5. If you can’t change the agent’s code, add a small step that fetches the missing ref manually:
   git fetch origin 'refs/heads/:refs/remotes/origin/' --prune

[mcg]

Any value that is set for the fetch-depth option of the actions/checkout action will be overridden to allow the agent to rollback commits upon request, while mitigating security risks. For more information, see actions/checkout/README.md.

Do I need to do another checkout somewhere? It makes no sense that I would have to do a git fetch origin.... as you suggest, when this seem like base stuff the coding agent should be doing? Or I am missing something basic here.

[dbuzatto]

The errors you’re seeing point to two separate (but related) Git issues in the agent environment, not to the number of files being listed.

1. git diff failing with refs/heads/master

fatal: ambiguous argument 'refs/heads/master'

This indicates that the repository does not have a master branch. Many newer repositories use main instead.
The coding agent workflow appears to be assuming master exists when it runs git diff.

Fix: update the workflow (or agent configuration) to reference the correct default branch (for example, main), or explicitly pass the base ref instead of assuming master.

---

2. git ls-files exiting with code: null

This usually happens when:

• the working directory is not a fully initialized Git repository at that step, or
• Git is invoked before actions/checkout (or equivalent) has completed, or
• the repo is checked out in a detached or partial state.

The large number of files is not the root cause — git ls-files can handle that.

Things to verify:

• actions/checkout runs before the coding agent starts
• fetch-depth: 0 is used (no shallow clone)
• the agent is running from the repository root

---

Summary

• The git diff error is caused by a hard-coded assumption that master exists
• The git ls-files error indicates an incomplete or invalid Git state during the agent step
• Truncating or excluding files will not fix the issue

Once the correct base branch is used and the repository is fully checked out, the agent should proceed normally.