A Python worker crashes with JSONDecodeError on an empty body while syncing Firepower FDM objects. Earlier in the same request log, a proxy hop returned 502 and the upstream call completed with 204. What is the sound diagnostic move?

The sound diagnostic move is to walk backward from the JSONDecodeError to the preceding hop statuses that left an empty body—here a proxy 502 and an upstream 204. The decode exception is a local symptom: the Python worker tried to parse nothing. Empty content often follows no-content success semantics (204) or a proxy failure (502) that never delivered a JSON document. Diagnosis follows the trail up the request path rather than treating the stack trace as the origin of the outage while syncing Firepower FDM objects.

Treating the stack trace as the root cause and rewriting the JSON schema library attacks the parser for correctly rejecting emptiness; the library is not why the body was empty. Disabling TLS on the FDM client because decode errors supposedly mean certificate mismatch confuses cryptographic failures with content failures; certificate problems typically surface as handshake or verification errors, not empty-body decode. Increasing the JSON recursion limit so empty bodies parse is nonsensical—empty input is not a deep tree, and forcing a fake parse hides the real hop failure.

Exam caveat: some APIs legitimately return 204 on success; clients must branch on status before calling JSON loads, and proxies can turn upstream issues into 502 with empty bodies. Operational check: log status code and content-length or body presence immediately before parse; in a lab, reproduce 204 and 502 paths and confirm the worker raises a clear empty-body-after-status error instead of only a raw JSONDecodeError.