Build and test only what changed in GitHub Actions

In a workspace whose tooling knows the dependency graph (Turborepo, Nx, Bazel, Gradle, or a --changed-capable test runner), the long-pole job runs only the targets affected by the diff against the merge base. Crucially, it fails safe: if the tool can't resolve the graph or returns an empty set on a real diff, it runs the full build/suite instead of passing green having run nothing.123

- uses: actions/checkout@v4
  with:
    fetch-depth: 0                     # full history so the base commit is reachable
- uses: nrwl/nx-set-shas@v4            # resolves NX_BASE / NX_HEAD robustly
- run: |
    # Fail safe: if the base didn't resolve, run the FULL build/test
    # instead of silently running nothing.
    if [ -z "$NX_BASE" ]; then
      npx nx run-many -t build test
    else
      npx nx affected -t build test --base=$NX_BASE --head=$NX_HEAD
    fi

Every PR rebuilds and retests the whole tree, even a one-line change to one package.

- run: turbo run build test   # every package, every PR, even a
                                #   one-line change to one app

In a large monorepo, rebuilding and retesting everything on every PR is the biggest source of redundant CI work. But this is the most dangerous lever in the catalog: scoping trades correctness headroom for speed. An incomplete dependency graph can mark a truly-affected target 'unaffected' and skip it, and a green check that ran fewer tests is indistinguishable from a real speedup, until a bug ships. Adopt it only with the guardrails below.

Hand this prompt to your coding agent (Claude Code, Cursor, and the like) to audit and fix this practice in your own repo.

Inspect this repo's .github/workflows for a job that rebuilds and retests the whole tree on every PR. Confirm the repo has a real dependency graph to scope against (`turbo.json`, `nx.json`, `WORKSPACE`, `settings.gradle`, or a `--changed`-capable runner), then scope the long-pole job to only the targets affected by the diff against the merge base (use `nrwl/nx-set-shas` or `git merge-base` to resolve the base). This is a correctness-sensitive change: it MUST fail safe, so if the base does not resolve or the scoper returns an empty set on a real diff, run the full build and test instead of passing green having run nothing, and keep the full run on the merge queue and `main`. Show me the diff and open a PR rather than applying it blindly, and do not cut over without a scoped-vs-full shadow period.