InterLAB tools · OpenAPI · console · mock server and MSW

API Mock Studio: a working mock API

A working mock API for frontend development and integration checks before the backend is ready. Import OpenAPI or define endpoints, set responses, delays and errors, check the contract and download a mock server.

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Project and schemas

API name

Version

Generator seed

The same seed gives the same test data, delays and errors.

CORS for mock-server.mjs

One origin per line: http://localhost:3000. * — any. Empty — no CORS headers.

*

components.schemas (JSON)

Named schemas for $ref: {"User": {"type": "object", …}}.

{}

Endpoints

No endpoints yet. Add one manually, import OpenAPI or load the example.

How the tool works

Endpoints
Import OpenAPI 3.0/3.1 as JSON or YAML, or define endpoints by hand: method, path template /users/{id}, query parameters, request body schema and responses per status — a JSON template with tokens or data generated from a schema.
Test data
The generator is deterministic: one seed, the same data. It honours type and format (email, uuid, date-time, date, uri, ipv4), enum, bounds and lengths, simple patterns, arrays, nested objects, oneOf/anyOf/allOf and nullable; example and examples win over generation. Property names hint at plausible values: name, email, phone, city, price, createdAt, avatar.
Console and log
A request is handled in the browser: routing, delay, injected error, body and parameter checks, tokens and pagination. The log shows time, method, path, endpoint, status, duration, bodies and contract violations, with filters.
Contract and export
Each response is checked against its own schema, and you can paste a real backend response to check it. Downloads: a zero-dependency mock-server.mjs, MSW v2 handlers, OpenAPI 3.1, a Markdown summary and the project JSON.

Behaviour rules

The browser console and the downloaded mock-server.mjs follow one table. MSW handlers repeat the response behaviour; request schema checks run in the console and in mock-server.mjs.

ID	Rule
AMS-B01	Routing: templates match segment by segment; the most specific wins — a static segment beats a parameter, left to right (/products/featured before /products/{id}).
AMS-B02	Path found but not the method → 405 with an Allow header; no path → 404 listing the routes.
AMS-B03	The request number n for an endpoint (from zero) and the project seed give deterministic numbers: the same request order repeats the same delays and errors.
AMS-B04	Injected error: rand(seed, endpoint\|err, n) × 100 < error rate → the error status and an X-Mock-Injected-Error header.
AMS-B05	Delay: fixed, or min + ⌊rand(seed, endpoint\|delay, n) × (max − min + 1)⌋ ms.
AMS-B06	The request is checked against the body schema and query parameters; in “reject” mode a violation returns 422 with the list.
AMS-B07	Template tokens: {{params.id}}, {{query.page}}, {{body.email}}, {{now}}, {{seq}}; a string that is just a token keeps the value type, :number converts to a number.
AMS-B08	Echo: path parameter values are written into same-named fields of the response object (as numbers when the field is numeric).
AMS-B09	Pagination: the array is sliced by ?page= (from 1) and ?limit= (1–100, default 10); the total is in X-Total-Count.
AMS-B10	The X-Mock-Status: 404 header returns that status’s response for one request — handy for error screens.

AMS-B01

Rule: Routing: templates match segment by segment; the most specific wins — a static segment beats a parameter, left to right (/products/featured before /products/{id}).

AMS-B02

Rule: Path found but not the method → 405 with an Allow header; no path → 404 listing the routes.

AMS-B03

Rule: The request number n for an endpoint (from zero) and the project seed give deterministic numbers: the same request order repeats the same delays and errors.

AMS-B04

Rule: Injected error: rand(seed, endpoint|err, n) × 100 < error rate → the error status and an X-Mock-Injected-Error header.

AMS-B05

Rule: Delay: fixed, or min + ⌊rand(seed, endpoint|delay, n) × (max − min + 1)⌋ ms.

AMS-B06

Rule: The request is checked against the body schema and query parameters; in “reject” mode a violation returns 422 with the list.

AMS-B07

Rule: Template tokens: {{params.id}}, {{query.page}}, {{body.email}}, {{now}}, {{seq}}; a string that is just a token keeps the value type, :number converts to a number.

AMS-B08

Rule: Echo: path parameter values are written into same-named fields of the response object (as numbers when the field is numeric).

AMS-B09

Rule: Pagination: the array is sliced by ?page= (from 1) and ?limit= (1–100, default 10); the total is in X-Total-Count.

AMS-B10

Rule: The X-Mock-Status: 404 header returns that status’s response for one request — handy for error screens.

Behaviour rules v1.0 (2026-10-13)

Project checks

The table is generated from the same code as the checks in the tool.

ID	Level	Check	What to do
AMS-E01	error	Invalid path template	Start with /, make each parameter a whole segment like {id}, do not repeat names.
AMS-E02	error	Two endpoints with the same method and path	Keep one: the second never receives a request (parameter names do not make paths different).
AMS-E03	error	The response body is not JSON	Fix the template JSON — until then the endpoint answers 500 mock_template_invalid.
AMS-E04	error	The schema is not JSON	Write the schema as a JSON object: {"type": "object", …}.
AMS-E05	warning	No response is defined for the active status	A default {error, message} body is returned. Add a response or pick another status.
AMS-E06	warning	The response does not match its schema	Fix the example or the schema: the frontend would be built against a wrong contract.
AMS-E07	warning	A $ref points nowhere	Add the schema to components.schemas or fix the name.
AMS-E08	tip	A body schema on GET/DELETE is not used	Bodies are checked only for POST, PUT and PATCH.
AMS-E09	warning	A 204/304 response has a body	Per HTTP these responses have no body — the text is ignored.

AMS-E01

Level: error
Check: Invalid path template
What to do: Start with /, make each parameter a whole segment like {id}, do not repeat names.

AMS-E02

Level: error
Check: Two endpoints with the same method and path
What to do: Keep one: the second never receives a request (parameter names do not make paths different).

AMS-E03

Level: error
Check: The response body is not JSON
What to do: Fix the template JSON — until then the endpoint answers 500 mock_template_invalid.

AMS-E04

Level: error
Check: The schema is not JSON
What to do: Write the schema as a JSON object: {"type": "object", …}.

AMS-E05

Level: warning
Check: No response is defined for the active status
What to do: A default {error, message} body is returned. Add a response or pick another status.

AMS-E06

Level: warning
Check: The response does not match its schema
What to do: Fix the example or the schema: the frontend would be built against a wrong contract.

AMS-E07

Level: warning
Check: A $ref points nowhere
What to do: Add the schema to components.schemas or fix the name.

AMS-E08

Level: tip
Check: A body schema on GET/DELETE is not used
What to do: Bodies are checked only for POST, PUT and PATCH.

AMS-E09

Level: warning
Check: A 204/304 response has a body
What to do: Per HTTP these responses have no body — the text is ignored.

Project checks

Limitations

YAML is read by a built-in parser for the common subset: anchors (&, *), tags (!), complex keys and multiple documents are not supported — save the spec as JSON.
External $refs (other files and URLs) are not loaded; responses are JSON only, and methods are GET, POST, PUT, PATCH and DELETE.
Schema checks cover a JSON Schema subset: type, required, properties, additionalProperties, items, enum, const, format (email, uuid, date-time, date, uri, ipv4), bounds, lengths, pattern and allOf/anyOf/oneOf. Keywords outside it are not checked.
The generator picks plausible but fictional values; complex regular expressions are not reproduced — such an example will not match its pattern, and the check will show it.
mock-server.mjs is a development test server, not production: no auth, no rate limits and no data kept between restarts.

Privacy

Specs, request and response bodies are processed in your browser; the console makes no network calls and the page address does not change.
The project and versions are stored only in this browser (localStorage); the request log lives only in the tab’s memory. “Clear all” deletes the data.
Analytics receives only the fact of a request, check or export and a category (status class, file format) — no paths, bodies or schemas.
The result reaches us only if you send a request yourself and keep the “Attach the result” box ticked — the form shows exactly what will be sent: the number of endpoints, a “method · path · statuses” list and contract violations. Response bodies, schemas and headers are never included.

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