Revision 1 — the current text. The evidence on this page is this revision’s own — it has not been carried forward from, or to, any other revision.
Covers finding and fixing the underlying JavaScript exception behind Error 1101, including the "script will never generate a response" variant caused by an unresolved promise. Does not cover Error 1102 (CPU time limit exceeded — a resource limit, not a thrown exception) or Error 1015 (rate limiting) — those have separate, more specific causes and remedies.
Unverifiedno reproductions yetWhy this confidence?8 steps, exactly as this revision published them.
Cloudflare returns Error 1101 whenever a Worker's own JavaScript throws an uncaught exception. The 1101 page itself never says what threw — the real cause is always in your own logs.
What happens next
Tail the Worker's logs while reproducing the failing request and look for the exception that immediately precedes the 1101 outcome.
npx wrangler tail --format prettyExpected result
A line such as "Uncaught TypeError: Cannot read properties of undefined (reading 'id')" immediately before the request completes with a 500.What happens next
Address whatever the trace points at — commonly an undefined/null property access, JSON.parse() on an empty or malformed body, or an unhandled rejection from a downstream call. Add a guard or a try/catch with a deliberate error response instead of letting it throw uncaught.
let body;
try {
body = await request.json();
} catch {
return new Response("Invalid JSON body", { status: 400 });
}What happens next
Some 1101s show no normal stack trace because nothing actually threw — the Workers runtime detected that all code finished running but no Response was ever returned. This happens when a Promise you're awaiting never resolves or rejects, or a WebSocket is left open with nothing to close it.
npx wrangler tail --format prettyExpected result
A message containing "The script will never generate a response" rather than a specific exception.What happens next
Deploy the fix and re-send the request (or input) that previously triggered the 1101.
Changes system or service state. Review before running.
Deploys the corrected Worker code to production, replacing the currently live version.
npx wrangler deployExpected result
wrangler tail shows the expected status code with no exception, and no 1101.What happens next
Find the Promise your handler depends on that can be left unresolved (a setTimeout-based promise with no fallback, an event listener that's never fired) and guarantee it resolves or rejects. Enabling the no-floating-promises ESLint rule catches most of these at review time.
let { promise, resolve } = Promise.withResolvers();
setTimeout(resolve, 0); // ensure this always fires
return promise.then(() => new Response("ok"));What happens next
If wrangler tail shows nothing useful in either form, this may be a Workers runtime issue rather than application code. Contact Cloudflare Support with the Ray ID from the error page, the Worker name, recent code changes, and steps to reproduce.
The request path handles the previously-unhandled case and no longer throws an uncaught exception.
What would strengthen it: 6 more independent reproductions. Reproductions from 3 more distinct environments.
This counts only what was recorded against revision 1 itself. Nothing reported against another revision is included here — see the revision history for why.