workerd: uncaught exception in scheduled handler, cron not firing

Fix uncaught exception in scheduled handler, cron not firing

TL;DR

Wrap the entire scheduled() body in try/catch and log the error, because an uncaught exception in a cron trigger looks exactly like a cron that never fired. Then verify the cron pattern is registered under [triggers] crons in wrangler.toml and watch wrangler tail during the scheduled minute to confirm it fires.

Verbatim error

workerd: uncaught exception in scheduled handler, cron not firing

Steps

1. Open wrangler.toml and confirm a [triggers] section with your schedule, e.g. crons = ["*/5 * * * *"]. Expected: the pattern parses as a valid cron expression.
2. Run npx wrangler tail in a separate terminal. Expected: tail connects and starts streaming logs.
3. Wrap the scheduled handler: try { ...your work... } catch (e) { console.error("cron failed", e); }. Expected: the worker deploys cleanly.
4. Wait for the next scheduled minute and watch tail. Expected: you see the trigger fire and either your success logs or the caught error.
5. If nothing appears, check the worker's triggers tab in the dashboard. Expected: the cron shows as an active trigger on the deployed worker.
6. Test the same code path with a direct fetch to a debug route. Expected: the shared logic works, which isolates the problem to the trigger registration.

Use this when

• A Worker cron never seems to run
• The scheduled handler throws an uncaught exception
• Cron events fire but produce no visible effect

Not for this skill when

• The cron fires but the business logic is wrong (debug the handler code itself)
• You need sub-minute scheduling (not supported; use a different mechanism)
• You want one-shot future wakeups (use a Durable Object alarm instead)

Variant phrasings

• cloudflare workers cron trigger not working
• scheduled event not firing workers
• crons = [] ignored wrangler.toml
• worker scheduled handler never called

Why it happens

Cron events have no HTTP response, so failures are invisible unless you watch tail. An uncaught throw inside scheduled() gets logged server-side but never surfaces to you, so the cron looks dead. The other classic cause is a stale deploy: the pattern was added to the config but never deployed, so nothing is registered.

Edge cases

• Cron triggers only fire on the deployed worker, never in wrangler dev; trigger the logic manually when developing locally.
• Minimum granularity is one minute; tighter schedules are silently treated as one minute.
• Long-running crons can overlap with the next tick; make the handler idempotent.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_LPKWWonsou0KXJOVEdr78g