tweak blog: mock APIs, debug HTTP and test frontends

You are building a screen against an API that is not ready, or you need to see what the page does on a 500, an empty list or a slow response. The fastest way to get there is a browser extension that answers the request for you, so the page receives your response as if the server had sent it.

There are dozens of extensions that claim to do this. This post covers the ones worth installing in 2026: what each does, what it is best for, what the free tier gets you and where it stops. I build tweak, so it comes first, but I will tell you when another tool is the better fit.

Your computer is in one time zone, your CI runner is in UTC, and your users are everywhere else. A date that renders fine on your screen can be a day off in Sydney, an hour off the week clocks change, and "today" can mean yesterday for a customer in Los Angeles. Most of these bugs are invisible until someone in the wrong place opens the page.

The fix is to look at the page from another time zone on purpose. This guide covers four ways to change the timezone in Chrome for testing, what each one actually changes, and a checklist of the cases worth checking once you can.

ModHeader was the default way to change HTTP headers in the browser for years. In July 2026 it was removed from the Chrome Web Store and Edge Add-ons, so many developers who relied on it now need something else for the same job: an Authorization header on a staging API, a feature flag header, a User-Agent override, a CORS header on a local server.

This post covers why people are moving, and the options that cover the job, from a free extension to the browser's own tools. I build tweak, so read with that in mind; each section says what the tool is best for.

The bug only happens on production. The fix is in your editor. Deploying a branch to find out whether it works takes twenty minutes, and you will need three tries. What you want is the production page, with its real data and its real login, talking to the code running on your machine.

That is a redirect: when the page asks for https://api.example.com/... or https://cdn.example.com/app.js, the browser goes to http://localhost instead. This guide shows how to do it in Chrome with a free redirect rule, and the four or five things that will get in your way the first time.

Requestly does a lot: an HTTP interceptor as a browser extension and a desktop app, a long list of rule types, an API client and team workspaces. That breadth is exactly why some developers look elsewhere. Some only need a mock in one tab and want a smaller tool. Others need more than a browser can see, or want their mocks in code.

This post lists seven alternatives, grouped by the job they do best. I build tweak, so it comes first; every section also says when another tool is the better pick.

The Network tab is where most frontend bugs are actually solved. A blank screen, a spinner that never ends, a form that "does nothing": open the Network tab and the answer is usually one row away. Most developers use about a fifth of it. This guide goes through the rest: the filters, the details panes, the right-click menu, and the features hidden under More tools.

Everything here is Chrome's DevTools, and the same in Edge and other Chromium browsers. Firefox's Network Monitor has most of the same features under slightly different names.

You call an API from your frontend, the request shows up in the Network tab with a perfectly good response, and your code still gets nothing but this in the console:

Access to fetch at 'https://api.example.com/orders' from origin 'http://localhost:5173'
has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on
the requested resource.

This post explains what the browser is doing, how to read the variations of that message, how to fix it properly, and how to get unblocked right now while you develop, with one click.

Every frontend developer knows the feeling. The design is approved, the ticket is yours, and the API you need is "almost ready". Maybe the endpoint exists but returns an empty list. Maybe it exists only as a line in a spec document. Either way, you can't see your screen with real data, and you can't test what happens when that data is missing, slow or broken. This guide covers every practical way to build and test a frontend without a working backend: what each approach costs, where it shines, and how to pick one without turning your mocks into a second codebase.

Resource Override has been the go-to extension for one simple job for years: take a file a website loads and serve a different one instead. Point a production script at your local build, swap a stylesheet, send an API call to localhost. If that is why you installed it, this post shows how tweak's redirect rule does the same job, and why it holds up better today.

Simple Modify Headers does what its name says: it adds, changes or removes HTTP headers on the requests your browser makes. If that is why you installed it, this post shows how tweak's headers only rule does the same job, for free, with a url per rule, real regex, one click templates, and a team that still ships updates. It is a replacement guide for that one extension: what maps to what, and how to move your rules over.