OpenAPI Validator — Check API Specifications
Validate OpenAPI 3.x and Swagger 2.0 JSON documents for required fields, structure, and common issues.
How it works
- 1Paste or type your text in the input field
- 2Click "Validate" — processing happens in your browser
- 3Copy the result or download as a text file
What to do next
About OpenAPI Validator
OpenAPI Validator is a single-page tool for the common developer utility task it is named after. Validate OpenAPI 3.x and Swagger 2.0 JSON documents for required fields, structure, and common issues. The interface keeps the input on one side, the configurable options in the middle, and the result on the other side. Most jobs start and finish without any scrolling.
From a technical standpoint, OpenAPI Validator is JavaScript and standard browser APIs running in your tab. The browser is the runtime; the page is the interface. Maximum input size: 0 MB per run.
OpenAPI Validator parses your file with standard browser APIs entirely inside the browser, applies the options you selected, and returns a download. The processing has no network step, which means a slow or intermittent connection does not slow down the work — once the page is loaded, only your CPU and RAM are involved.
Anyone who works with developer utility on a casual basis — devops engineers crafting one-liners, QA engineers writing repro cases, backend developers inspecting requests — finds OpenAPI Validator a quick way to get the result. The page loads in under a second, the controls are visible from a single screen, and the result downloads or copies in one click.
Reach for OpenAPI Validator when you need a predictable result on a single file. The page works on the first visit, the controls are visible without a menu, and the output is delivered the moment the engine finishes.
The 0 MB ceiling on input size is the only fixed limit. Output files are produced in standard formats that every common viewer recognises, and the tool runs the same way regardless of how many times you have used it during the session.
OpenAPI Validator sits in a small group of related tools. Useful neighbours include Swagger to Markdown, API Documentation Generator, JSON Validator, and JSON Viewer / Formatter. They are designed to compose: the output of one is a sensible input to the next, so a multi-step task is usually a sequence of single-click operations.
OpenAPI Validator is honest about scope: it handles a single, well-defined developer utility step. Specialist edge-case work — uncommon formats, very large inputs, or pipelines that need scripting — is what dedicated desktop apps are for. This page handles the common case quickly.
The download is delivered as a clearly named file the moment processing completes — no email link, no "your result will be ready in 5 minutes" queue, no expiry timer. The file is generated in your browser and saved by your browser's normal download flow.
A short note on how OpenAPI Validator came to look the way it does: every iteration started by watching how someone unfamiliar with the tool actually used it, then removing whatever got in their way. That is why the upload area dominates the screen, the run button is bigger than the secondary controls, and the result panel is unmissable when the job finishes.
If you also use a command-line tool for openapi validator, OpenAPI Validator is a convenient alternative for the times you are on a different machine or helping someone who is not comfortable in a terminal. The output is a standard file in the format documented above.
A few practical tips that experienced users of OpenAPI Validator pick up over time. First, keep your default browser updated — the engine relies on standard web APIs and newer browser versions are noticeably faster than ones from a few years ago. Second, close other heavy tabs before processing a large input; the engine shares CPU and memory with whatever else is open. Third, if you re-run the same kind of job often, your last-used settings are remembered for the rest of the tab session, so subsequent runs are essentially one click.
When something goes wrong, the cause is usually one of three things: a malformed input, a browser that is out of memory, or a corporate proxy that is interfering with the page's static assets. The first two are easy to diagnose; the third typically requires asking your IT team to allow standard browser APIs to load.
Open the workspace above to start using OpenAPI Validator. The engine loads on the first interaction so the page itself stays light, and once the tool is warm it processes subsequent jobs quickly. The moment the page is interactive, the tool is ready to do real work on your file.
How it works
- 1Open the OpenAPI Validator workspace above. The interface is a single page, so there is nothing to navigate.
- 2Select the developer file you want to process — drag-and-drop and the file picker both work.
- 3Tweak the controls if the defaults are not quite right for your input. The options are kept short and labelled in plain language.
- 4Hit the run button. standard browser APIs does the work in your browser tab.
- 5Save the output when it is ready.
- 6Repeat the process for additional inputs whenever you need to. The page stays loaded, so subsequent runs are quick.
Common use cases
- Compare two API responses to spot a regression using OpenAPI Validator.
- Decode a token to confirm its claims during a debugging session.
- Inspect a regex against a test string before committing it.
- Pretty-print a minified blob during incident triage.
- Encode binary content for transport in a JSON body.
- Convert between data formats while wiring up an integration.
- Format a noisy log line into something a teammate can read.
- Generate a quick fixture without leaving the browser.
- Generate boilerplate from a single specification line.
- Inspect a payload during local development without writing a script.
FAQ
What versions are supported?
OpenAPI 3.x (openapi field) and Swagger 2.0 (swagger field) JSON documents.
What is checked?
Required fields (openapi/swagger, info.title, info.version, paths), path format (must start with /), and basic structure.
Does it validate YAML?
Only JSON is supported. Convert YAML to JSON first using a YAML-to-JSON converter.
Is it a full spec validator?
This performs basic structural validation. For full schema validation, use the official OpenAPI tools.
Is my API spec private?
Yes — validation runs entirely in your browser. Nothing is uploaded.
Does it show path details?
It reports the number of paths and validates that each path key starts with /.
How long does OpenAPI Validator take to process a file?
Most jobs finish in seconds. Speed scales with input size and with how many CPU cycles your browser tab has available — the engine runs in your browser, so it shares resources with whatever else you have open. For inputs near the 0 MB ceiling, expect anywhere from a few seconds to roughly a minute on a typical laptop. Closing other heavy tabs noticeably speeds things up.
Are there any hidden fees with OpenAPI Validator?
OpenAPI Validator 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.
What input formats are supported by OpenAPI Validator?
The accepted formats are listed in the upload area on the tool itself. If your input is in a format that is not directly supported, convert it first using one of Favtoo's converter tools — every Favtoo converter outputs a file that is a clean input to the next tool in the chain.
How many times per day can I use OpenAPI Validator?
Inputs are capped at 0 MB per file, which keeps memory usage stable across phones, tablets and older laptops. You can run OpenAPI Validator as often as you need; every run produces a full-quality result.
How accurate is OpenAPI Validator?
OpenAPI Validator is built on standard browser APIs, which is the same class of engine used by professional developer utility pipelines. For deterministic operations, the output is byte-identical to what an equivalent CLI run would produce; for operations involving a codec or a model, the result is well within the range of what comparable tools generate. If you have a specific reference output you need to match, run a small test job first to confirm the configuration produces what you expect.
Will OpenAPI Validator keep working in a year?
OpenAPI Validator 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.
Does OpenAPI Validator upload my file to a server?
Your file is processed inside your browser by standard browser APIs. The engine reads the file's bytes from your tab's memory, computes the result, and writes the result back into the tab. You can confirm what the page does by opening developer tools and watching the Network tab during a run — the requests you see are for the tool's static assets only.
Is the source for OpenAPI Validator available?
OpenAPI Validator is a static page running an open-source engine in your browser, so a typical corporate firewall does not get in the way as long as it allows JavaScript to load from Favtoo. For teams that need to host it themselves on an internal network, the underlying engine (standard browser APIs) is open-source and can be packaged into a private build with the same behaviour. Reach out via the Contact page if that is something you are exploring.