GitHub Actions actions/cache keeps missing ('Cache not found for input keys'). How should I design cache keys for npm and pip so restores hit?

Cause: A cache hit needs an exact key match, or a prefix match from restore-keys. Existing caches are immutable, so a key that never changes never updates, and a key that changes every run (a timestamp, github.sha, run id) never hits. Caches are also scoped by branch: a run can restore from its own branch and the default branch (PR runs also from the base branch), but not from sibling branches, and PR-run caches are scoped to the merge ref. Fix: 1. Key on the lockfile content, not the commit: for npm use key: ${{ runner.os }}-npm-${{ hashFiles('**/package-lock.json') }}; for pip use key: ${{ runner.os }}-pip-${{ hashFiles('**/requirements*.txt') }}. The key then changes only when dependencies change. 2. Add restore-keys with a shorter prefix, ordered most to least specific, e.g. ${{ runner.os }}-npm- . On an exact miss this restores the newest cache with that prefix, and the job saves a fresh cache under the new key afterwards. 3. Cache the package manager's download cache (npm's ~/.npm, pip's cache directory), not node_modules, and still run npm ci / pip install. Alternatively use the cache option of actions/setup-node or actions/setup-python, which builds this key for you. 4. Make sure the hashed file path exists at the checked-out location; a hashFiles pattern matching nothing produces a constant key. Run actions/checkout first. 5. Seed caches on the default branch (a push workflow), since feature branches can read default-branch caches but not each other's. 6. Watch limits: keys may be at most 512 characters, caches unused for 7 days are removed, and when the repository total (10 GB by default) is exceeded the least recently used caches are evicted, which can look like random misses. 7. Include anything that changes the content in the key (OS, architecture, Node or Python version). Verify: the second run on the same branch logs "Cache restored from key" for the exact key, and the install step time drops. Sources: https://docs.github.com/en/actions/using-workflows/caching-dependencies-to-speed-up-workflows https://github.com/actions/cache