HTTP Mock Response Builder
Build mock HTTP response payloads with status code, headers, body, and optional simulated delay.
How it works
- 1Configure your options above
- 2Click "Build Mock" — processing happens in your browser
- 3Copy or download the result
What to do next
About HTTP Mock Response Builder
HTTP Mock Response Builder is built for developer utility jobs that fit cleanly into a browser tab. Build mock HTTP response payloads with status code, headers, body, and optional simulated delay. The processing runs in the page itself, which is why the controls update instantly when you change settings and why a freshly loaded page is ready to do real work the moment it becomes interactive.
Under the hood, HTTP Mock Response Builder uses standard browser APIs to do the actual work. Input runs through the same engine, with a per-file ceiling of 0 MB so memory usage stays predictable on lower-end laptops and tablets. The engine ships as part of the page bundle, so once the page is loaded the tool keeps working even if your network connection drops.
HTTP Mock Response Builder sees the most use from site reliability engineers triaging logs and frontend developers prepping fixtures, but the design is intentionally generic enough that you do not need a specialist background to get a good result. The defaults aim at the most common case so a first-time user can get the right output without changing any settings.
The execution model is straightforward: your file is bytes in the tab's memory, the engine reads those bytes, computes the result, and hands the result back to the browser. The transformation happens locally, which is why the tool keeps working when your network connection drops mid-job and why it produces the same result every run for the same input.
HTTP Mock Response Builder works well as a bookmarked utility you reach for when you need it. The first visit shows you what the tool does; the second is when you realise it is a low-friction option for the task and worth keeping in your tab list.
Output handling is intentionally boring: HTTP Mock Response Builder produces a single output file and triggers your browser's standard "save" behaviour. If you have a default download folder configured, that is where it will land. There is no Favtoo-side history of jobs you have run.
The hard constraints are easy to remember. Maximum input: 0 MB. Multiple files per run: no — one input at a time, by design, to keep results predictable. The same controls apply on every run.
For multi-step jobs, HTTP Mock Response Builder sits next to cURL Command Builder, HTTP Request Builder, and API Payload Validator. None of them depend on each other — you can use HTTP Mock Response Builder on its own — but together they cover the common variations of the task this page exists to handle.
HTTP Mock Response Builder keeps the control set focused. Every option on the page is there because a real workflow needs it, and the defaults aim at the most common case so a first-time user can get the right output without changing any settings.
HTTP Mock Response Builder is structured around the idea that a useful tool should be its own page. Open the page, do the work, close the tab — the page is the entire product. There is no onboarding flow because there is nothing to onboard into.
HTTP Mock Response Builder is built around the moment of need: a focused page you open when you have a specific task, complete the task, and close. The catalog contains many adjacent tools so the same model serves the surrounding parts of a typical developer utility workflow.
Pro tip: HTTP Mock Response Builder works just as well in a private/incognito window as in a normal one, which is occasionally useful when you want zero browser-history footprint of the job. Another tip: if the tool ever feels slow, it is almost always because the browser tab is competing for CPU with another tab — pausing or closing the heavy ones gives the engine room to work.
Common gotchas worth flagging: the supported formats are listed in the upload area. The 0 MB ceiling is per-file, not per-session; you can run as many separate jobs as you like, but a single oversized input will be rejected on load.
If HTTP Mock Response Builder solved your problem, sharing the page link with someone who has the same problem is the most useful thing you can do. The catalog grows mostly through word of mouth; visitors arriving through a recommendation tend to be the ones the tool serves best.
How it works
- 1Open HTTP Mock Response Builder in your browser. The page loads quickly and the tool is ready to use the moment it becomes interactive.
- 2Drop a developer file onto the upload area, or click to pick one from your device.
- 3Pick any non-default settings you need. Most users leave the defaults alone for the first run and only revisit if the result needs tuning.
- 4Hit the run button. standard browser APIs does the work in your browser tab.
- 5Save the output when it is ready.
- 6Re-run with different settings as often as you want. Each run produces a fresh output and the original file on disk is never modified.
Common use cases
- Convert between data formats while wiring up an integration using HTTP Mock Response Builder.
- Compare two API responses to spot a regression.
- Generate a quick fixture without leaving the browser.
- Pretty-print a minified blob during incident triage.
- Inspect a payload during local development without writing a script.
- Generate boilerplate from a single specification line.
- Hash a string for a quick reproducibility check.
- Inspect a regex against a test string before committing it.
- Format a noisy log line into something a teammate can read.
FAQ
Does it create a mock server?
No — it generates the JSON definition. Use tools like json-server or Mockoon to serve it.
Custom status codes?
Select from common codes in the dropdown; edit the output for custom codes.
Multiple responses?
Build one response at a time and combine them into a mock collection manually.
Private?
Yes — built locally in your browser.
Delay field?
The delay value in milliseconds is included in the output for mock servers that support it.
Binary responses?
Only text-based response bodies are supported; binary data is not generated.
Will HTTP Mock Response Builder keep working in a year?
HTTP Mock Response Builder is updated whenever the underlying engine releases an improvement or a bug fix. Because the tool is delivered as a static page, every visit fetches the latest version automatically — there is no "version" to manage on your end. If a particular release ever changes default behaviour, the change is documented on Favtoo's changelog so you can confirm what shifted.
Are there any usage limits on HTTP Mock Response Builder?
Inputs are capped at 0 MB per file, which keeps memory usage stable across phones, tablets and older laptops. You can run HTTP Mock Response Builder as often as you need; every run produces a full-quality result.
Can I use HTTP Mock Response Builder on iOS or Android?
HTTP Mock Response Builder runs in any modern mobile browser — Safari, Chrome, Firefox and the in-app browsers in most messaging apps all support the underlying APIs. Performance depends on the device: a recent phone handles typical inputs nearly as fast as a laptop, while older devices may take a few seconds longer near the 0 MB ceiling. The interface lays out cleanly on small screens, so you do not need to pinch-zoom to see the controls.
Are there any hidden fees with HTTP Mock Response Builder?
HTTP Mock Response Builder is free to use. The processing runs in your browser, which keeps the per-user cost low enough that the tool can be offered openly. The download is the same file the engine produced — you can use it for as many runs as you need.
Does HTTP Mock Response Builder work in Safari, Firefox, Chrome and Edge?
HTTP Mock Response Builder works in any modern browser released in the last few years — Chrome, Edge, Firefox, Safari, Brave, Arc and the major Chromium derivatives are all supported. The underlying engine relies on widely-supported web APIs, so there is nothing exotic to install. If you are on a very old browser version and the tool fails to load, updating to the latest release of your preferred browser is the only fix needed.
Can I call HTTP Mock Response Builder from a script?
HTTP Mock Response Builder is a browser-only tool by design and does not expose a hosted API. The reason is the same as the privacy story: there is no Favtoo backend doing the work, so there is no service to call. If you need to script the same transformation, the underlying engine (standard browser APIs) is open-source and can be used directly from your own code.
What does HTTP Mock Response Builder do that command-line tools do not?
Desktop apps usually have more advanced features but require installation, maintenance and (often) a licence. Paid online tools are convenient but route your file through their servers and gate downloads behind accounts. HTTP Mock Response Builder sits in between: free, instant, and private, but intentionally narrow in scope. For one-off jobs and the common developer utility operations, it is usually the lowest-friction choice; for highly specialised work, a dedicated app is still the right answer.
What does the error message in HTTP Mock Response Builder mean?
Failures usually fall into one of three buckets: the input is in an unsupported format, the input is over the size cap, or the input is structurally malformed (a truncated download, a partial export, or a stream the engine does not recognise). The first two are easy to confirm — check that your file is in a supported format and that it is below 0 MB. For the third, opening the file in its native viewer first is the fastest way to confirm the source is intact.