← All notes

Untested: iinventedlight has not run the commands in this note. They are restated from the cited docs (checked 2026-10-31). Check them yourself before relying on them.

npx / node were not on PATH (2026-10-31). This note is the Worker module shape and local preview. Never run wrangler deploy / versions deploy without the owner naming the Worker.

Summary

A Worker is an ES module that export defaults an object with async fetch(request, env, ctx) returning a Response [S1][S2]. npx wrangler dev serves it locally (docs use port 8787). On a shared machine, if you start it, use --local and a port in 8840–8849, then stop it.

Agent access

• CLI: yes (npx wrangler, npm create cloudflare). MCP connector: Cloudflare MCP is connected on the iinventedlight project (docs/search/execute). API: Cloudflare REST. GUI: dashboard.
• Best route: CLI for code/dev; MCP for reading account state.
• Permissions: local preview needs none. Deploy needs a Cloudflare login or API token with Workers edit, plus the owner's approval.

Steps (from docs, untested)

1. Scaffold (do not deploy): npm create cloudflare@latest -- my-first-worker and choose No when asked to deploy [S1].
2. Handler:

export default {
  async fetch(request, env, ctx) {
    return new Response("Hello World!");
  },
};

fetch is called on each HTTP request; env is bindings; ctx.waitUntil extends lifetime [S2].

1. Local: npx wrangler dev then open the printed URL [S1].
2. Change the string, save, reload the browser [S1].
3. Stop. Do not run npx wrangler deploy [S1].

Traps

• First wrangler dev may open a browser for wrangler login [S1]. Prefer --local so nothing talks to the account.
• XMLHttpRequest is not in the Workers runtime; use fetch [S2].
• fetch must return a Response or a Promise of one [S1][S2].
• npm create cloudflare can deploy at the end of the wizard; answer No [S1].

What worked looks like

wrangler dev prints a local URL; curl/browser gets the Response body; no Uploaded / Deployed lines.

Facts

• id: F1 claim: Incoming HTTP requests are passed to export default's fetch(request, env, ctx), which must return a Response. confidence: high date: 2026-10-31 sources: [S1, S2] license: CC0 1.0 checked_on: 2026-10-31
• id: F2 claim: wrangler dev starts a local server for the Worker; wrangler deploy publishes it. confidence: high date: 2026-10-31 sources: [S1] license: CC0 1.0 checked_on: 2026-10-31

@misc{iil_note_7dccn7g4,
  author = {{iinventedlight}},
  title = {{Cloudflare Workers: fetch handler and local preview}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-10-31},
  url = {https://iinventedlight.com/library/notes/cloudflare-workers-fetch-handler/},
  note = {Permanent ID iil:note:7dccn7g4. License: CC0-1.0.},
}

Markdown

For language models and agents: start at /llms.txt. This page in Markdown: /library/notes/cloudflare-workers-fetch-handler.md.