Decide which side branch to checkout from as base_branch when using github issues

[tomerr-ma]

I’m using Claude Code with GitHub Actions in “mention” mode — when I write @ claude in a GitHub issue, it will always takes the main/master default branch as the "base_branch".
Is there a way to tell Claude which side branch to checkout from as the "base_branch" from the "issues" tab in Github?

(I do not want to statically hardcode base_branch in the github action file as it will affect all other developers using this repo.

[mj-deving]

There's no built-in way to set base_branch dynamically per-issue. The parameter is a static string in your workflow file, so it applies to every run.

That said, you can work around this with a bit of GitHub Actions scripting. The idea: parse the branch name from the issue body or a label, then pass it to the action.

Option 1: Parse from issue body

Ask developers to include a base: branch-name line in the issue:

jobs:
  claude:
    runs-on: ubuntu-latest
    steps:
      - name: Extract base branch
        id: branch
        run: |
          BASE=$(echo "$ISSUE_BODY" | grep -oP '(?<=base:\s)\S+' || echo "main")
          echo "name=$BASE" >> $GITHUB_OUTPUT
        env:
          ISSUE_BODY: ${{ github.event.issue.body }}

      - uses: anthropics/claude-code-action@v1
        with:
          base_branch: ${{ steps.branch.outputs.name }}
          # ... rest of config

Option 2: Use labels as branch selectors

Create labels like base:staging, base:develop, then map them:

      - name: Branch from label
        id: branch
        run: |
          LABELS="${{ join(github.event.issue.labels.*.name, ',') }}"
          if echo "$LABELS" | grep -q 'base:staging'; then
            echo "name=staging" >> $GITHUB_OUTPUT
          elif echo "$LABELS" | grep -q 'base:develop'; then
            echo "name=develop" >> $GITHUB_OUTPUT
          else
            echo "name=main" >> $GITHUB_OUTPUT
          fi

Both approaches keep the workflow file generic — no hardcoding per developer. The label approach is cleaner since it's visible at a glance and doesn't require a specific format in the issue body.