# API Designer

> Say what your API should do. Get a working OpenAPI spec back.

API Designer turns a plain-English description of an API into a working OpenAPI 3.1 specification, with paths, operations, schemas and examples already filled in.

Also: api designer, openapi generator, generate openapi spec, openapi from description, design api from english, openapi 3.1 generator, natural language to openapi, ai openapi generator, text to openapi.

## What it does

Writing an OpenAPI document by hand means learning the format before you can describe the API you already have in your head. This goes the other way round: describe what the API should do in a sentence or two, and get back a complete specification you can read, edit and download. The result is shown in an editor so you can check it rather than trust it, and you can ask for changes — adding authentication and pagination, or regenerating a different design — without starting again.

## When to use it

- You are sketching a new API and want a specification to react to rather than a blank file.
- You know the OpenAPI format exists but do not want to learn its syntax to get started.
- You want a realistic specification to prototype against before the real API is built.
- You need an example document to test tooling, mocks or documentation against.

## What you give it

| Input | Meaning |
| --- | --- |
| Description of the API | A sentence or two of plain English — the resources, what callers do with them, and anything that matters like due dates or statuses. |

## What you get back

| Output | Meaning |
| --- | --- |
| OpenAPI 3.1 specification | A complete document with paths, operations, request and response schemas, and examples. Viewable as YAML or JSON. |
| Endpoint summary | The methods and paths the specification defines, listed so you can check the shape at a glance. |

## Key terms

- **OpenAPI** — a standard, machine-readable description of an HTTP API — its endpoints, parameters, request and response shapes, and authentication. Tooling for documentation, mocking and client generation all read this one format.
- **Design-first** — agreeing the API contract before writing the implementation, so the shape is reviewable while it is still cheap to change. Generating a specification from a description is a way to reach a reviewable draft in minutes rather than hours.
- **Schema** — the description of a request or response body — which fields exist, their types, and which are required. Most of the length of an OpenAPI document is schemas.

## Questions

### What does this tool give you?

Describe your API in plain English and get an AI-generated starter OpenAPI spec, with 3 free AI prompts to iterate on it.

### What does Postman Agent Mode give you?

Agent Mode creates specs in any protocol — OpenAPI, AsyncAPI, GraphQL, Proto, or Smithy — backed by visual and code editors, governance rules enforced automatically via CI/CD, spec-to-collection generation with two-way sync, Git-based team collaboration and version control, and 50 to 800 monthly AI credits depending on your plan.

### How do I create an OpenAPI spec without writing YAML by hand?

Describe the API in plain English and the AI-generated starter spec gives you a working OpenAPI draft to refine, rather than writing YAML from scratch.

### Can I generate specs in AsyncAPI, GraphQL, or Proto too, not just OpenAPI?

Yes — Agent Mode creates specs in OpenAPI, AsyncAPI, GraphQL, Proto, or Smithy, not just OpenAPI.

### Is there a free way to generate an OpenAPI spec online?

Yes — this tool gives 3 free AI prompts to generate and iterate on a starter spec with no signup.


## Privacy

Unlike the browser-only tools here, this one sends your description to an AI model to generate the specification. Nothing is stored: the page clears when you refresh, and no account is needed to use it.

## Links

- [Use the tool](https://www.postman.com/tools/api-designer) — the interactive version of this page.
- [Postman Tools](https://www.postman.com/tools) — every free tool in this set.
- [API Designer docs](https://learning.postman.com/docs/design-apis/specifications/overview/) — Postman documentation.