Agents ship the code. You are the last gate.

Code got cheap. Your users still click, wait, retry and give up like humans do. Tweak is the browser toolkit for the last gate: mock any API response or change the real one, look inside the page, break it on purpose, and hand the evidence to your agent.

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★ 4.6 on Chrome Web Store 70,000+ usersFree for everyone

★★★★★This app works like a charm. It's the best api-mocker available.Harsh Jha ★★★★★Such a convenient simple way to overwrite requestsAija Avota ★★★★★Working perfectly. No complaints.Eliamar Tani ★★★★★first time to use it, amazing!Thuyên Trọng ★★★★★Frontend developer's best friendRu-Tso Luo ★★★★★… I don't know how I lived without it :)Ilarion Halushka ★★★★★Really handy plugin for exploratory testing! Love itDave Gordon ★★★★★… This is a time AND money saver!Felix ★★★★★it's very nice to use, tiny, simple and reliableLei Peng ★★★★★Must have for any developer!Coder Mau ★★★★★Best tweaker everAndrei Ramanouski

The last gate is you

Ask every what if before your users do.

Agents write the code now. People still use it. Somebody has to hold the software in their hands and push on it before it ships. Tweak gives that person every condition, one click away, on the real page.

00 · Baseline

This isthe happy path.

A booking screen, as the agent built it and as the demo showed it. Everything is fine. Scroll, or pick a condition above the app, and break it the way real life will.

01 · Mock

What ifthe API returns 500?

Mock any XHR or fetch response. Status, headers, body. The page never knows the server was not asked.

GET/api/flightsMock500

02 · Mock

What ifthe list is empty?

Return [] and see the empty state your designer drew. Or the one nobody did.

GET/api/passengersMock[]

03 · Delay

What ifit takes 5 seconds?

Give a rule a delay and the response arrives exactly when you say. See the skeletons, the spinners and the timeouts your app never shows on a fast laptop.

GET/api/quoteMockDelay by 5000 ms

04 · Modify

What ifthe price is 1,000,000?

Modify lets the real request go out, then patches one field of the real answer. Your layout meets a number it was never designed for.

GET/api/quoteModifytotal: 1000000Basic

05 · Toolbox

What ifthe user is in Tokyo?

Override the page's time zone before its first script runs. Every new Date() answers from Asia/Tokyo.

Signed at the gate

4.6★Average ratingOver 200 reviews on Chrome Web Store 70,000+Users

Thousands of developers and testers already build and test with tweak. They came for mocking. The toolbox is what comes next.

Developer Toolbox · Ctrl Shift Y

Stop juggling ten extensions.

One bar on every page you test, instead of ten different UIs to learn. It docks on the page itself, and everything is undone when you close it.

Meet the tools

Design modeFreeEdit any text on the page in place.Layout outlinesFreeEvery box, every margin, at once.EraserRemove an element to see what moves.Marker & labelsFreeDraw and write straight on the page.RedactorBlack bars over personal data, in place.MagnifierFreePixel loupe with colour readout.ScreenshotFreeVisible area, full page or a dragged area.Offline & throttlingFreeNo network, 3G and friends.Time zoneThe page believes it is somewhere else.Cookies & storageRead and edit what the app remembers.Tech stackFreeWhat the page is built with.MarkdownThe page's content as a .md file.Responsive gridsSeveral widths, one input broadcast to all.World clocks & time utilsEpochs, ISO, durations, cron.Edge casesFill a form with the inputs nobody tries.Skip validationLift the form's limits. Put them back.

Agent skills

Your agent can use tweak too.

The open skill tweak-creating-rules teaches Claude Code and other coding agents to write tweak rules. In the browser they can talk to tweak. You decide whether they can.

Off by default. Switch it on in Settings, Preferences, AI Agents.
Reuse before create. The agent searches your rules first and proposes the one you have.
Always labelled. Agent rules land in "AI Generated", with notes signed by the agent.

tweak-extension/skills on GitHub →

// what the agent runs on your tab
await tweak.searchRules({ url: '/api/cart' })
// → []  nothing to reuse

await tweak.createRules([{
  interceptionPolicy: 'MOCK',
  method: 'GET',
  url: '/api/cart',
  status: 500,
  delay: 0,
  responsePayload: '{"error":"cart unavailable"}',
  notes: 'Reproduces the blank cart on a 500.',
}])
// → filed in AI Generated · you review it

Humans stay accountable.
You deserve better tools.

Give it a try. It's free.