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Mar 7, 2026
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docs: document body validation with custom content type parsers #6556

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docs: document body validation with custom content type parsers
mcollina Mar 5, 2026
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mcollina Mar 6, 2026
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7 changes: 7 additions & 0 deletions docs/Reference/ContentTypeParser.md
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Original file line number Diff line number Diff line change
Expand Up @@ -17,6 +17,13 @@ declared in a plugin, it is available only in that scope and its children.
Fastify automatically adds the parsed request payload to the [Fastify
request](./Request.md) object, accessible via `request.body`.

> **Important:** When using a body schema with the
> [`content`](./Validation-and-Serialization.md#body-content-type-validation)
> property to validate per content type, only content types listed in the schema
> will be validated. If you add a custom content type parser but do not include
> its content type in the body schema's `content` property, the incoming data
> will be parsed but **not validated**.

Note that for `GET` and `HEAD` requests, the payload is never parsed. For
`OPTIONS` and `DELETE` requests, the payload is parsed only if a valid
`content-type` header is provided. Unlike `POST`, `PUT`, and `PATCH`, the
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39 changes: 38 additions & 1 deletion docs/Reference/Validation-and-Serialization.md
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Expand Up @@ -5,7 +5,11 @@ Fastify uses a schema-based approach. We recommend using
[JSON Schema](https://json-schema.org/) to validate routes and serialize outputs.
Fastify compiles the schema into a highly performant function.

Validation is only attempted if the content type is `application/json`.
Validation is only attempted if the content type is `application/json`,
unless the body schema uses the [`content`](#body-content-type-validation)
property to specify validation per content type. When the body schema defines
a `content` field, it must enumerate all possible content types the
application expects to handle with the associated handler.

All examples use the
[JSON Schema Draft 7](https://json-schema.org/specification-links.html#draft-7)
Expand Down Expand Up @@ -228,6 +232,9 @@ const schema = {
fastify.post('/the/url', { schema }, handler)
```

#### Body Content-Type Validation
<a id="body-content-type-validation"></a>

For `body` schema, it is further possible to differentiate the schema per content
type by nesting the schemas inside `content` property. The schema validation
will be applied based on the `Content-Type` header in the request.
Expand All @@ -250,6 +257,36 @@ fastify.post('/the/url', {
}, handler)
```

> **Important:** When using [custom content type
> parsers](./ContentTypeParser.md), the parsed body will **only** be validated
> if the request's content type is listed in the `content` object above. If
> a parser for a content type (e.g., `application/yaml`) is defined,
> but it is not not included in the body schema's `content` property,
> the incoming data will be parsed but **not validated**.
>
> ```js
> // Add a custom parser for YAML
> fastify.addContentTypeParser('application/yaml', { parseAs: 'string' }, (req, body, done) => {
> done(null, YAML.parse(body))
> })
>
> fastify.post('/the/url', {
> schema: {
> body: {
> content: {
> 'application/json': {
> schema: { type: 'object', properties: { name: { type: 'string' } }, required: ['name'] }
> },
> // Without this entry, application/yaml requests will NOT be validated
> 'application/yaml': {
> schema: { type: 'object', properties: { name: { type: 'string' } }, required: ['name'] }
> }
> }
> }
> }
> }, handler)
> ```

Note that Ajv will try to [coerce](https://ajv.js.org/coercion.html) values to
the types specified in the schema `type` keywords, both to pass validation and
to use the correctly typed data afterwards.
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