Gh Actions Wisdom

GitHub Actions Wisdom

Reference best practices before writing or reviewing any GitHub Actions workflow. Load topic-specific references as needed from references/.

Runner Context Matters

Several rules below depend on whether your jobs run on ephemeral cloud runners (GitHub-hosted ubuntu-latest, RunsOn, BuildJet, Namespace, etc. — fresh VM per job, wiped between runs) or persistent self-hosted runners (long-lived machines with state that carries across runs). Advice that is correct in one context can be a hard-to-debug bug in the other.

Migration warning. When moving a workflow from self-hosted to ephemeral (or vice versa), audit every step and option that was added "for the runner". Leftover self-hosted-isms on a cloud runner produce mysterious failures: pnpm: command not found (no setup step because pnpm was preinstalled), Cache not found between jobs (cache backend differs), fatal: detected dubious ownership (because set-safe-directory: false is now actively wrong), etc. Specific rules below are gated by runner context where it matters.

Critical Rules (Always Apply)

1. Always Set timeout-minutes

The default timeout is 360 minutes (6 hours). A stuck job silently burns runner minutes.

jobs:
  build:
    runs-on: ubuntu-latest
    timeout-minutes: 15 # ALWAYS set this

Recommended values:

2. Always Set Concurrency Control

Prevent redundant runs and protect production deploys.

# PR checks: cancel previous runs on new push
concurrency:
  group: ${{ github.workflow }}-${{ github.ref }}
  cancel-in-progress: true

# Production deploy: never cancel in-progress
concurrency:
  group: deploy-production
  cancel-in-progress: false

3. Always Declare Permissions

Never rely on default permissions. Declare explicitly per workflow or per job.

permissions:
  contents: read

jobs:
  deploy:
    permissions:
      contents: read
      deployments: write

4. Pin Actions to Full SHA

Tags are mutable. The March 2025 tj-actions/changed-files supply chain attack (CVE-2025-30066) compromised 23,000+ repos via rewritten tags.

# Bad - tag can be rewritten
- uses: actions/checkout@v4

# Good - immutable SHA
- uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2

Caveat: Some repos (e.g., pnpm/action-setup) have force-pushed, invalidating previously pinned SHAs. If CI fails with Unable to resolve action ... unable to find version, look up the current SHA via gh api repos/OWNER/REPO/git/ref/tags/vX.Y.Z. See references/security.md for the full diagnostic procedure.

5. Do NOT Cache Package Managers (pnpm/npm/yarn)

Do not use cache: 'pnpm' (or cache: 'npm', cache: 'yarn') in actions/setup-node. GitHub Actions cache restore is often slower than a fresh pnpm install from npm's CDN. npm's CDN is highly optimized for package downloads, while GitHub's cache API has significant overhead for large stores (especially 1GB+). Benchmarking confirmed: direct install from CDN consistently beats cache restore + install.

# BAD - cache restore adds overhead, slower than fresh install
- uses: actions/setup-node@v4
  with:
    node-version-file: .node-version
    cache: pnpm  # REMOVE THIS

# GOOD - just install directly
- uses: actions/setup-node@v4
  with:
    node-version-file: .node-version
- run: pnpm install

This is especially true for self-hosted runners where the pnpm store is already local — caching to GitHub's remote cache and restoring it is pointless overhead.

6. set-safe-directory: leave default on ephemeral runners, set false on self-hosted

actions/checkout defaults set-safe-directory to true, which runs git config --global --add safe.directory on every run.

Ephemeral cloud runners — leave the default (true). Each run is a fresh VM, so there is no gitconfig to pollute. The default is also required for container jobs whose UID differs from the host runner user; without it, git inside the container errors with fatal: detected dubious ownership when it tries to operate on the mounted workspace.

# GOOD on ephemeral runners — let checkout do its default thing
- uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2

Persistent self-hosted runners — set it to false. Otherwise ~/.gitconfig accumulates a duplicate safe.directory entry on every run, polluting the shared gitconfig across every repo on that machine.

# GOOD on self-hosted runners — prevent gitconfig pollution
- uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
  with:
    set-safe-directory: false

When migrating self-hosted → ephemeral, forgetting to remove set-safe-directory: false is a common gotcha. Non-container jobs may still work (the runner user owns the workspace), but the moment a job runs in a container, git inside hits dubious-ownership and fails with confusing errors.

Container jobs need an extra manual step (regardless of runner type)

actions/checkout (a node action) writes safe.directory to the node-action HOME (/root/.gitconfig inside many containers). Shell run: steps inside the container have a different HOME (/github/home), so they read a different gitconfig and don't see the safe.directory entry. Lifecycle scripts (pnpm install calling prepare → lefthook install → git) then fail with fatal: detected dubious ownership.

For container jobs, add a manual step before checkout that writes safe.directory to the shell-side gitconfig:

test:
  runs-on: ubuntu-latest
  container:
    image: foo:bar
  steps:
    - name: Mark workspace as safe for git
      run: git config --global --add safe.directory "$GITHUB_WORKSPACE"

    - uses: actions/checkout@v4
    # ... rest of the job

This is orthogonal to the set-safe-directory option — it covers shell-step git invocations, which checkout's option doesn't reliably reach in container jobs. Plain (non-container) jobs do not need it.

7. actions/cache for build tools: yes on ephemeral, no on self-hosted

Persistent self-hosted runners — build tool caches (Cargo, Go modules, Gradle, etc.) already persist on the runner's local disk. Using actions/cache uploads them to GitHub's remote cache API on every run and creates duplicate entries, wasting storage.

# BAD on self-hosted — uploads local cache to remote on every run
- uses: actions/cache@v4
  with:
    path: ~/.cargo/registry
    key: cargo-${{ hashFiles('Cargo.lock') }}

# GOOD on self-hosted — just use the local disk cache directly
# (no actions/cache step needed)

Ephemeral cloud runners — disk is wiped between runs, so actions/cache is essential to avoid re-downloading the dependency tree from scratch every time. Use it for ~/.cargo/registry, ~/.gradle/caches, the Go module cache, etc.

# GOOD on ephemeral runners — survives across runs
- uses: actions/cache@v4
  with:
    path: ~/.cargo/registry
    key: cargo-${{ runner.os }}-${{ hashFiles('Cargo.lock') }}

Note: rule 5 ("Don't cache package managers in setup-node") still applies on both runner types — that rule is about npm package downloads where the CDN is faster than cache restore. Rule 7 is about general build-tool caches.

8. Avoid curl | sh Installers — Use Prebuilt-Binary Actions

Installer scripts like curl https://.../init.sh | sh (wasm-pack, rustup, many language toolchains) do one HTTP request with no retry. A single transient 5xx from the redirect target (e.g., a GitHub release asset) kills the entire workflow. Seen in the wild: rustwasm.github.io → github.com/rustwasm/wasm-pack/releases/... returning 504 mid-deploy.

# BAD — one curl, no retry, fails on any 5xx
- name: Install wasm-pack
  run: curl https://rustwasm.github.io/wasm-pack/installer/init.sh -sSf | sh

# GOOD — prebuilt binary from GitHub releases, with retries + runner caching
- uses: taiki-e/install-action@v2
  with:
    tool: wasm-pack

taiki-e/install-action covers most Rust/Go/Node tools (wasm-pack, cargo-nextest, just, mdbook, etc.). For tools it doesn't cover, use actions/cache on a pinned-version binary, or wrap the curl in a retry loop with curl --retry 5 --retry-all-errors --retry-delay 5.

9. Use Cache (Not Artifacts) for Inter-Job Data Sharing

upload-artifact/download-artifact counts toward shared org storage (often limited). For passing build output between jobs in the same workflow, use actions/cache instead — it has a separate 10 GB per-repo limit.

# BAD — artifacts accumulate in shared org storage
- uses: actions/upload-artifact@v4
  with:
    name: build-output
    path: dist/
    retention-days: 1

# GOOD — cache uses separate per-repo quota
- uses: actions/cache/save@v4
  with:
    path: dist/
    key: build-${{ github.run_id }}

# In the downstream job:
- uses: actions/cache/restore@v4
  with:
    path: dist/
    key: build-${{ github.run_id }}

If the build and deploy steps can run on the same runner, merging them into a single job is even simpler.

Caveat for cloud runners that proxy the cache layer (e.g., RunsOn with extras=s3-cache+magic-cache) — the runner injects a sidecar at ACTIONS_RESULTS_URL that intercepts both v2 cache and v4 artifact API calls. The sidecar speaks the cache protocol but does not always speak the v4 artifact protocol. With magic-cache enabled, actions/upload-artifact@v4 may fail with Unexpected token '...' is not valid JSON because the sidecar returns plain-text errors for artifact endpoints.

If you hit this, the symptoms vary by transport:

• Cache-based passing across instances: works only if the sidecar is reachable. From inside a container job whose docker network is isolated from the runner host, the sidecar's host IP is unreachable → Cache not found even when the upstream job successfully saved.
• Artifact-based passing: works if you remove the proxy interception (drop magic-cache from the runs-on label) so v4 artifact calls reach api.github.com directly.

When in doubt on a cloud runner that proxies caching, prefer upload-artifact/download-artifact over actions/cache and disable any cache-proxy extras. Artifacts go straight to the GitHub API which is reachable from any container or instance.

Quick Reference by Topic

For detailed guidance, read the appropriate reference file:

• Timeouts and resource limits: See references/timeouts.md
• Security: See references/security.md - action pinning, pull_request_target, script injection, secrets, OIDC
• Performance: See references/performance.md - caching, path filters, matrix, parallelization
• Reliability: See references/reliability.md - retries, error handling, conditional execution
• Anti-patterns: See references/anti-patterns.md - common foot guns and how to avoid them
• Workflow organization: See references/organization.md - reusable workflows, composite actions, splitting strategies

Debugging: Local First, Push Second

Never debug CI issues by pushing and waiting. CI runs consume time (10-15 min per cycle) and runner minutes. Always verify locally first:

# Run the same checks CI runs, locally
pnpm check          # typecheck + lint + format
pnpm build          # production build
pnpm test           # unit tests

# Only after ALL pass locally:
git push
# Then monitor:
/watch-ci

The workflow: fix locally → verify locally → push once → /watch-ci. If CI fails after local verification, it's either an environment difference (Node version, missing env vars) or a path/dependency issue specific to CI — much easier to diagnose than a code bug.

Workflow Review Checklist

When reviewing or writing a workflow, verify:

1. Every job has timeout-minutes
2. concurrency group is set with appropriate cancel-in-progress
3. permissions are declared (least privilege)
4. Third-party actions pinned to SHA with version comment
5. pull_request_target is NOT used with PR code checkout
6. No string interpolation of user-controlled values in run: blocks
7. Secrets passed individually, not via secrets: inherit
8. No cache: parameter in setup-node (fresh install from CDN is faster — see rule 5)
9. Path filters used where possible to skip irrelevant runs
10. Deploy steps have retry logic for network operations
11. actions/checkout matches the runner type — default on ephemeral, set-safe-directory: false on self-hosted only (see rule 6)
12. actions/cache for build tools matches the runner type — used on ephemeral, NOT on self-hosted (see rule 7)
13. No curl | sh installers — use taiki-e/install-action or similar with retries (see rule 8)
14. Inter-job data sharing uses actions/cache not upload-artifact to avoid org storage limits — but switch to artifacts when a cloud runner's cache-proxy sidecar (e.g. RunsOn magic-cache) breaks v4 caching from container jobs (see rule 9)
15. When migrating between self-hosted and ephemeral runners, audit every step for runner-type-specific options that may now be wrong (see "Runner Context Matters")