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WP REST API Schema Validator

Validate WP REST API requests using a complete JSON Schema validator.

WordPress ships with a validator, rest_validate_request_arg(), that supports a limited subset of the JSON Schema spec. This library allows the full JSON Schema spec to be used when writing endpoint schemas with minimal configuration.

This library relies upon the justinrainbow/json-schema package to do the actual schema validation. This simply bridges the gap between the two.


  • PHP 5.3+
  • WordPress 4.5+


composer require ironbound/wp-rest-api-schema-validator


Initialize a Middleware instance with your REST route namespace and an array of localized strings. This middleware should be initialized before the rest_api_init hook is fired. For example, plugins_loaded.

Additionally, schemas must be created with a title attribute on the top level. This title should be unique within the versioned namespace.

$middleware = new \IronBound\WP_REST_API\SchemaValidator\Middleware( 'namespace/v1', [
  'methodParamDescription' => __( 'HTTP method to get the schema for. If not provided, will use the base schema.', 'text-domain' ),
  'schemaNotFound'         => __( 'Schema not found.', 'text-domain' ),
] );

That's it!


GET and DELETE Requests

Query parameters passed with GET or DELETE requests are validated against the args option that is passed when registering the route.

Technical Details

On rest_api_init#100, the middleware will iterate over the registered routes in the provided namespace. The default WordPress core validation and sanitization functions will be disabled.

Schema validation will be performed on the rest_dispatch_request#10 hook.

WP_Error objects will be returned that match the format in WP_REST_Request. Mainly, an error code of rest_missing_callback_param or rest_invalid_param, a 400 response status code, and detailed error information in data.params.

For missing parameters, data.params will contain a list of the missing parameter names. For invalid parameters, a map of parameter names to a specific validation error message.

Procedural Validation

In the vast majority of cases, validation should be configured using JSON Schema definitions. However, this is not always the case. For example, verifying that a username is not taken requires making calls to the database that would be impossible to replicate in the schema definition. In these cases, a validate_callback can still be provided and will be executed before JSON Schema validation takes place.

return [
    '$schema'    => '',
    'title'      => 'users',
    'type'       => 'object',
    'properties' => [
        'username' => [
            'description' => __( 'Login name for the user.', 'text-domain' ),
            'type'        => 'string',
            'context'     => [ 'view', 'edit', 'embed' ],
            'arg_options' => [
                'validate_callback' => function( $value ) {
                    return ! username_exists( $value );

Variable Schemas

In most cases, the schema document should be the same for all HTTP methods on a given endpoint. In the rare case that a separate schema document is provided, a schema option can be provided to the route args for that HTTP method. The title for the separate schema document MUST be the same as the base schema.

register_rest_route( 'namespace/v1', 'route', [
        'methods'  => 'GET',
        'callback' => [ $this, 'get_item' ],
        'args'     => $this->get_endpoint_args_for_item_schema( 'GET' ),
        'methods'  => 'POST',
        'callback' => array( $this, 'create_item' ),
         // See WP_REST_Controller::get_endpoint_args_for_item_schema() for reference.
        'args'     => $this->get_endpoint_args_for_post_schema(),
        'schema'   => [ $this, 'get_public_item_post_schema' ],
        'methods'  => 'PUT',
        'callback' => [ $this, 'update_item' ],
        'args'     => $this->get_endpoint_args_for_item_schema( 'PUT' ),
    'schema' => [ $this, 'get_public_item_schema' ],
] );

Reusing Schemas

JSON Schema provides a mechanism to utilize a referenced Schema document for validation. This package allows you to accomplish this by using the Middleware::get_url_for_schema( $title ) method.

For example, this Schema will validate the card property according to the Schema document with the title card.

    '$schema'    => '',
    'title'      => 'transaction',
    'type'       => 'object',
    'properties' => [
        'card' => [
            '$ref' => $middleware->get_url_for_schema( 'card' )   

But what if there is no /cards route? Or a more general schema is required? In this case, a shared schema can be used.

$middleware->add_shared_schema( [
    '$schema'    => '',
    'title'      => 'card',
    'type'       => 'object',
    'properties' => [
        'card_number' => [
            'type'    => 'string',
            'pattern' => '^[0-9]{11,19}$',
        'exp_year'  => [ 'type' => 'integer' ],
        'exp_month' => [ 
            'type' => 'integer',
            'minimum' => 1,
            'maximum' => 12,
] );

Schema Routes

After all routes have been registered, the middleware will register its own route.


This route returns the plain schema document for the given title. To retrieve a schema for a given HTTP method, pass the desired upper-cased HTTP method to the method query param.



Validate WP REST API requests using a complete JSON Schema validator.








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