The inbound cron-fire verifier constructed a fresh PyJWKClient on every
fire, discarding the client's key cache and forcing a synchronous JWKS
HTTP GET to the portal on each fire. Under a burst of concurrent fires
(a hosted instance with several cron jobs firing in the same window) this
fanned out into N simultaneous JWKS fetches that the portal rate-limited
(HTTP 403 -> verification fails -> agent 401), or that blocked the event
loop long enough that the fire webhook could not return its 202 before
the relay's 30s timeout (observed in prod as relay 504s concentrated on
high-job-count instances).
Cache one PyJWKClient per JWKS URL at module scope (double-checked lock)
so the signing keys are reused across fires; NAS keys rotate rarely, so
the steady state is zero JWKS fetches per fire.
Regression test proves 5 fires -> 1 client construction (was 5).
Phase 4E (E.1 + E.2). The inbound side of Chronos: NAS POSTs the agent when a
one-shot fires; the agent verifies a NAS-minted JWT and runs the job.
E.1 — plugins/cron/chronos/verify.py:
- verify_nas_fire_token(token, expected_audience, jwks_or_key, issuer): verifies
signature against the NAS JWKS (RS/ES family; symmetric rejected), aud == this
agent, exp/nbf, iss, and purpose == "cron_fire" (so a general agent JWT can't
be replayed against the fire endpoint). Returns claims or None; never raises.
Crypto delegated to PyJWT[crypto] (already a declared dep) — no hand-rolled
JWT, no new dependency. No key configured → refuse (never unsigned-decode a
security boundary).
- get_fire_verifier(): pluggable indirection so the DQ-4 escape hatch
(direct per-job cron-key) can swap in with no handler change.
E.2 — gateway/platforms/api_server.py:
- POST /api/cron/fire (registered only when _CRON_AVAILABLE). Authenticated by
the NAS-JWT via get_fire_verifier() — NOT API_SERVER_KEY (NAS holds no API
key; this is the only inbound that triggers remote job execution, so it gets
its own purpose-scoped check). Verifier args come from cron.chronos.* config.
401 on bad/missing/forged token. 400 on missing job_id. On success: 202 +
fire_due runs in the background (so a long agent turn never trips NAS's HTTP
timeout); the store CAS claim inside fire_due de-dupes a scheduler retry.
Tests:
- test_chronos_verify (11): REAL RS256 signing — valid→claims, wrong-aud,
missing/wrong purpose, expired, wrong-iss, tampered-signature (attacker key),
no-key-refuse, empty-token, JWKS-URL key resolution, get_fire_verifier.
- test_cron_fire_webhook (5): valid→202+fire, invalid→401+no-fire, missing
token→401, missing job_id→400, and fire path does NOT require API_SERVER_KEY.
api_server regression suites (214) green.
E.3 (NAS endpoints) is a separate cross-repo PR; the wire contract lands next
(docs/chronos-managed-cron-contract.md).
