GitHub Actions 'No space left on device': three causes and how to tell them apart

The 'No space left on device' error in GitHub Actions means a runner's disk filled during a job, so the next write failed. The same error can come from three separate problems: running out of bytes, running out of inodes, or exhausting the inotify watcher limit, and the df -h and df -i commands distinguish them quickly. Common triggers include jobs that need more space than the runner provides, Docker layers and build cache that build up across the job, and restored or saved caches that share the same disk as the work. Inode and watcher exhaustion can occur while df -h still reports free space, so deleting large files will not help in those cases. The recommended approach is to measure disk state with df and du first, add diagnostic steps that run on failure, and free space early in the job while cleanup is cheap.