# Extending OpenAPI { #extending-openapi }

There are some cases where you might need to modify the generated OpenAPI schema.

In this section you will see how.

## The normal process { #the-normal-process }

The normal (default) process, is as follows.

A `FastAPI` application (instance) has an `.openapi()` method that is expected to return the OpenAPI schema.

## Conditional OpenAPI from settings and env vars { #conditional-openapi-from-settings-and-env-vars }

You can easily use the same Pydantic settings to configure your generated OpenAPI and the docs UIs.

For example:

{* ../../docs_src/conditional_openapi/tutorial001.py hl[6,11] *}

Here we declare the setting `openapi_url` with the same default of `"/openapi.json"`.

## Documenting webhooks with **FastAPI** and OpenAPI { #documenting-webhooks-with-fastapi-and-openapi }

With **FastAPI**, using OpenAPI, you can define the names of these webhooks, the types of HTTP operations that your app can send (e.g. `POST`, `PUT`, etc.) and the request **bodies** that your app would send.

# OpenAPI Callbacks { #openapi-callbacks }

You could create an API with a *path operation* that could trigger a request to an *external API* created by someone else (probably the same developer that would be *using* your API).

# Separate OpenAPI Schemas for Input and Output or Not { #separate-openapi-schemas-for-input-and-output-or-not }

When using **Pydantic v2**, the generated OpenAPI is a bit more exact and **correct** than before. 😎

In fact, in some cases, it will even have **two JSON Schemas** in OpenAPI for the same Pydantic model, for input and output, depending on if they have **default values**.

Let's see how that works and how to change it if you need to do that.

### OpenAPI-specific `examples` { #openapi-specific-examples }

Since before **JSON Schema** supported `examples` OpenAPI had support for a different field also called `examples`.

This **OpenAPI-specific** `examples` goes in another section in the OpenAPI specification. It goes in the **details for each *path operation***, not inside each JSON Schema.

      advanced/graphql.md: how-to/graphql.md
      advanced/custom-request-and-route.md: how-to/custom-request-and-route.md
      advanced/conditional-openapi.md: how-to/conditional-openapi.md
      advanced/extending-openapi.md: how-to/extending-openapi.md
      advanced/testing-database.md: how-to/testing-database.md
  mkdocstrings:
    handlers:
      python:
        options:
          extensions:

## OpenAPI Metadata - Docs { #openapi-metadata-docs }

To add metadata to your OpenAPI schema, including a license, version, contact, etc, read the docs for [Tutorial - Metadata and Docs URLs](../tutorial/metadata.md){.internal-link target=_blank}.

## OpenAPI Custom URL { #openapi-custom-url }

## OpenAPI URL { #openapi-url }

By default, the OpenAPI schema is served at `/openapi.json`.

But you can configure it with the parameter `openapi_url`.

For example, to set it to be served at `/api/v1/openapi.json`:

{* ../../docs_src/metadata/tutorial002.py hl[3] *}

### Generate a TypeScript Client with the Preprocessed OpenAPI { #generate-a-typescript-client-with-the-preprocessed-openapi }

Since the end result is now in an `openapi.json` file, you need to update your input location:

```sh
npx @hey-api/openapi-ts -i ./openapi.json -o src/client
```