Parallel/scripted npx wrangler r2 object put <bucket>/<key> --file <path> fails with '[ERROR] The file "..." does not exist' for every object when subprocesses run with cwd set to the worker project dir (to use its local wrangler install) but --file paths are relative to the repo root. wrangler resolves --file against its own cwd.
Pass an absolute path to --file (os.path.abspath(...)) whenever you set cwd for the wrangler subprocess. Symptom signature: every upload fails instantly with 'file does not exist' while the same command works from the repo root. Also note: ~8 concurrent wrangler r2 object put processes is a safe, effective way to bulk-upload ~1k objects (node startup dominates serial throughput; parallel cut a 70-min serial upload to 3.5 min).