Deno Deploy: 'Unexpected non-whitespace character after JSON at position 3' when fetching own API (508 loop)

TL;DR: if a Fresh app works locally but on Deno Deploy a server-side fetch to the app's own API route fails with SyntaxError: Unexpected non-whitespace character after JSON at position 3 and the logs show Response status: 508, the JSON error is a red herring. Deno Deploy returns 508 when it detects a request loop: a deployment making an HTTP request to itself. Do not fetch your own deployment over HTTP from server-side code; import the route handler and call it directly (the reporter invoked trialRequest.POST(fakeRequest) with a stubbed request). Alternatively, move the fetch to a frontend island so the request comes from the browser.

SyntaxError: Unexpected non-whitespace character after JSON at position 3
Response status: 508

Steps

1. Replace the self-fetch with a direct handler call:

import { trialRequest } from './routes/api/members/securedApi/trial.js';
const res = await trialRequest.POST(fakeRequest);

2. Redeploy.

   Expected: no more 508. Success check: the page renders with the API data.

When to use this

• Fresh on Deno Deploy, server code fetches the deployment's own routes.
• Local dev works fine.

When NOT to use this

• The JSON is genuinely malformed; validate the payload.
• No self-fetch involved; the 508 has another cause.

Compatibility

• Deno Deploy with Fresh.

Variant phrasings

• "deno deploy 508 request loop"
• "deno deploy fetch own api fails"

Root cause

Deno Deploy blocks recursive requests where a deployment fetches itself, returning 508; the error body is not JSON, hence the parse error.

Edge cases

• Frontend islands are the other sanctioned path since the request then comes from the browser.

In the thread Deno Deploy: "Unexpected non-whitespace character after JSON at position 3" when fetching own API (508 loop), the question was: A Fresh app worked fine in local development but on Deno Deploy a server-side fetch to the app's own API route failed with: SyntaxError: Unexpected non-whitespace character after JSON at position 3. The deploy logs also showed Response status: 508. The reporter was fetching ${baseurl}/api/members/securedApi/trial from a server-rendered page — i.e. the deployment was making an HTTP request to itself. The verified resolution: The JSON parse error was a red herring: the real problem was the 508 status, which Deno Deploy returns when it detects a request loop — a deployment making an HTTP request to itself. Deno Deploy blocks recursive requests where deployment A fetches deployment A. The maintainer's guidance: don't fetch your own deployment over HTTP from server-side code; call the handler function directly instead. The reporter confirmed this fixed it by importing the route handler and invoking trialRequest.POST(fakeRequest) with a stubbed request object. Alternatively, move the fetch to a frontend island so the request comes from the browser, not the Deploy server. Original discussion: https://vectle.com/threads/thrR7ZLIveNx4JZyf9az8BT9A