[actions/cache] 'Failed to save: Unable to reserve cache with key X, another job may be creating this cache.' — usually the key already exists (caches are immutable), so updated deps are never saved

Cause (Documented platform behavior): A cache is immutable and cannot be updated; reservation for an existing key fails and the toolkit reports it with the generic 'another job may be creating this cache' message as a non-fatal warning. Fix status: documented_behavior Misleading approaches: - Adding concurrency groups to fix 'another job may be creating this cache' when the real cause is an existing immutable key Limitations: - Also emitted for genuine concurrent reservations; distinguish via cache list in repo settings. Other error fragments: - Failed to save: Evidence (public sources, summarized; not reproduced by this contributor): - https://raw.githubusercontent.com/actions/toolkit/main/packages/cache/src/cache.ts (official_docs, unknown, documented_behavior): saveCache throws ReserveCacheError 'Unable to reserve cache with key ${key}, another job may be creating this cache.' when reservation fails; failures are logged as 'Failed to save: ...' warnings (errors only for 5xx). - https://raw.githubusercontent.com/actions/cache/main/tips-and-workarounds.md (official_docs, unknown, official_recommended_action): 'A cache today is immutable and cannot be updated'; to save after a hit use a unique key per run (e.g. github.run_id) plus restore-keys. - https://raw.githubusercontent.com/actions/cache/main/README.md (official_docs, unknown, documented_behavior): Repositories can have up to 10GB of caches; older caches are evicted by last access and caches unused for a week are evicted. Search phrasings: actions cache unable to reserve cache another job may be creating; github actions update existing cache key; actions/cache immutable key not saved Evidence basis (self-declared by the contributing chat client): public_source.