Wp REST API

WordPress REST API: scaffold, review, secure

For WordPress 4.7+ REST endpoints registered via register_rest_route(). This skill covers the full lifecycle of a custom endpoint — registration, authorization, input validation, response shaping, error handling — and the patterns that distinguish a clean endpoint from one that ships a vulnerability.

This is the default path for new server endpoints in modern WordPress. Use REST instead of admin-ajax unless you have a concrete reason (legacy interop, Heartbeat, etc.).

When to use this skill

Trigger when ANY of the following is true:

• Scaffolding a new REST endpoint or reviewing one in a PR.
• The diff or file contains: register_rest_route, rest_api_init, WP_REST_Request, WP_REST_Response, WP_Error, permission_callback, register_rest_field, rest_ensure_response, X-WP-Nonce.
• The user is migrating an admin-ajax handler to REST, or asking which to use.
• The user is debugging a 401, 403, or "Sorry, you are not allowed to do that" response.
• The plugin is going headless / mobile / external-integration heavy and needs a stable API contract.

Architecture in one paragraph

Every REST endpoint is a (namespace, route, method) triple registered on rest_api_init. WP routes the request through WP_REST_Server::dispatch(), which runs WP_REST_Request::has_valid_params() (validate) and WP_REST_Request::sanitize_params() (sanitize) FIRST, then calls your permission_callback (returning true / false / WP_Error), then your callback. The callback returns a WP_REST_Response for success or a WP_Error for failure. Cookie-authenticated requests must include a _wpnonce query param or X-WP-Nonce header (the wp-api nonce, generated via wp_create_nonce('wp_rest')); other auth schemes — application passwords, OAuth, JWT — bypass the nonce.

Order matters for security. Because validation and sanitization run BEFORE the permission check, never put expensive lookups, DB writes, or other side effects inside a validate_callback / sanitize_callback. They run for unauthenticated requests too. Reserve those for the main callback, which only runs after permission has been granted.

Workflow — minimal scaffold

add_action( 'rest_api_init', static function (): void {
    register_rest_route(
        'myplugin/v1',
        '/items/(?P<id>\d+)',
        array(
            'methods'             => WP_REST_Server::READABLE, // 'GET'
            'callback'            => 'myplugin_get_item',
            'permission_callback' => static function ( WP_REST_Request $request ) {
                return current_user_can( 'read_post', (int) $request['id'] );
            },
            'args'                => array(
                'id' => array(
                    'required'          => true,
                    'type'              => 'integer',
                    'validate_callback' => static fn( $v ) => is_numeric( $v ) && (int) $v > 0,
                    'sanitize_callback' => 'absint',
                ),
            ),
        )
    );
} );

/**
 * @return WP_REST_Response|WP_Error
 */
function myplugin_get_item( WP_REST_Request $request ) {
    $id   = (int) $request['id'];
    $post = get_post( $id );

    if ( ! $post ) {
        return new WP_Error(
            'myplugin_not_found',
            __( 'Item not found.', 'myplugin' ),
            array( 'status' => 404 )
        );
    }

    return rest_ensure_response( array(
        'id'    => $post->ID,
        'title' => get_the_title( $post ),
    ) );
}

That snippet contains every required moving part — namespace + version, route with named param, method constant, permission_callback, args schema, response via rest_ensure_response, error via WP_Error with status data.

Critical rules

1. permission_callback is REQUIRED, and __return_true is rarely correct

The single most common plugin-side vulnerability on wp.org. Rules:

• NEVER use 'permission_callback' => '__return_true' on a route that writes (POST / PUT / PATCH / DELETE). It means "any unauthenticated visitor can call this".
• Only acceptable for genuinely public read-only routes (e.g. site status, public catalog). Even then, document the choice with a comment and consider rate limiting.
• For state-changing routes, check at minimum a capability (current_user_can('edit_posts')) and ideally an object-level cap with the target ID:

  'permission_callback' => fn( $req ) => current_user_can( 'edit_post', (int) $req['id'] ),
  

• When permission_callback returns false / null, WP wraps it in a rest_forbidden WP_Error whose status comes from rest_authorization_required_code() — that returns 401 if the user is logged out, 403 if logged in but unauthorized. Returning a custom WP_Error with explicit status lets you control the code and the message; false is fine when the default is correct for your route.

2. Always declare an args schema

Each accepted parameter (URL, query string, body) needs an args entry. Otherwise input lands raw in $request->get_param() — no validation, no sanitization.

'args' => array(
    'email' => array(
        'required'          => true,
        'type'              => 'string',
        'format'            => 'email',
        'validate_callback' => 'is_email',
        'sanitize_callback' => 'sanitize_email',
    ),
    'role' => array(
        'required'          => false,
        'type'              => 'string',
        'enum'              => array( 'subscriber', 'contributor', 'author' ),
        'default'           => 'subscriber',
    ),
    'count' => array(
        'type'              => 'integer',
        'minimum'           => 1,
        'maximum'           => 100,
        // No custom sanitize_callback — when 'type' is set and no callback
        // is given, WP defaults to rest_parse_request_arg, which runs both
        // schema validation (minimum/maximum/enum/type) AND sanitization.
        // The moment you set your own sanitize_callback, that default is
        // replaced and the schema constraints become documentation only —
        // unless you also set 'validate_callback' => 'rest_validate_request_arg'.
    ),
),

validate_callback returns true / false / WP_Error. sanitize_callback runs after validation. Use built-ins (absint, sanitize_text_field, sanitize_email, sanitize_key, rest_sanitize_boolean) where possible — but if you set a custom sanitize_callback, ALSO set 'validate_callback' => 'rest_validate_request_arg' (or your own validator) so schema constraints actually run. Otherwise 'minimum' => 1, 'maximum' => 100 is silently ignored.

3. Read input through $request, not $_POST / $_GET

$request->get_param('foo') returns the parameter from URL, query, or body, already unslashed and run through your sanitize_callback. Don't read superglobals inside REST callbacks — you bypass the schema.

For JSON bodies specifically: $request->get_json_params() returns the decoded array. WP also reads multipart and form-encoded automatically.

4. Return WP_REST_Response or WP_Error, never wp_send_json_*

wp_send_json_* is for admin-ajax. In REST, return objects:

return rest_ensure_response( $data );        // 200 OK with $data as JSON
return new WP_REST_Response( $data, 201 );   // explicit status
return new WP_Error( 'code', 'message', array( 'status' => 422 ) ); // error

WP_Error codes should be namespaced (myplugin_validation_failed, not validation_failed). Status codes follow HTTP semantics: 400 validation, 401 unauthenticated, 403 forbidden, 404 not found, 409 conflict, 422 semantic validation, 500 server error.

5. Don't leak sensitive columns

Never return raw $wpdb->get_results() rows — they contain user_pass, user_activation_key, internal meta. Build response objects explicitly:

// WRONG — leaks user_pass and other private columns
return rest_ensure_response( $wpdb->get_row( ... ) );

// RIGHT — explicit allowlist
return rest_ensure_response( array(
    'id'    => (int) $user->ID,
    'name'  => $user->display_name,
    'email' => $user->user_email,
) );

6. Meta capabilities need the object ID

// WRONG — meta cap without object: result is unreliable / not what you think
current_user_can( 'edit_post' )

// RIGHT — meta cap mapped to a specific object via map_meta_cap()
current_user_can( 'edit_post', $post_id )

edit_post / delete_post / read_post / edit_user etc. are meta capabilities — WP's map_meta_cap() resolves them to primitive caps PLUS object-ownership rules using the ID. Without the ID, the resolution is unreliable (it may pass for users who shouldn't have access to that specific object, or fail for ones who should). For any object-level route — single post, single user, single order, single subscription — always pass the relevant ID as the second argument.

7. Cookie-authenticated requests need a nonce

Browser-side requests using cookie auth need _wpnonce (query) or X-WP-Nonce (header) with the value from wp_create_nonce('wp_rest'). The official @wordpress/api-fetch package adds this automatically; manual fetch() calls must add it themselves.

fetch( '/wp-json/myplugin/v1/items/42', {
    method: 'POST',
    credentials: 'include',
    headers: {
        'Content-Type': 'application/json',
        'X-WP-Nonce': wpApiSettings.nonce, // localized via wp_localize_script
    },
    body: JSON.stringify( { ... } ),
} );

The official @wordpress/api-fetch package adds the nonce automatically only inside the WordPress admin / block editor, where WP localizes wpApiSettings.nonce and api-fetch reads it. In a decoupled frontend (headless app, public-site SPA, mobile client), you must set the nonce yourself — or use a different auth scheme entirely (Application Passwords, OAuth, JWT). Application passwords, OAuth, JWT don't need the nonce at all; the Authorization header itself carries the proof.

8. Treat the version segment as mandatory

/myplugin/v1/items — the v1 is not enforced by WP at runtime, but treat it as a project policy. When you need a breaking change, ship v2 alongside, deprecate v1, and remove it after a long migration window. NEVER change the contract of an existing versioned route — clients in the wild won't know.

Common mistakes

// WRONG — public write endpoint
register_rest_route( 'myplugin/v1', '/save', array(
    'methods'             => 'POST',
    'callback'            => 'save_thing',
    'permission_callback' => '__return_true', // 🔥 anyone can write
) );

// WRONG — no args schema, raw input
function save_thing( WP_REST_Request $req ) {
    $title = $req['title'];                    // not validated, not sanitized
    $body  = $_POST['body'];                   // bypasses REST entirely
    update_post_meta( $req['id'], 'thing', $req['payload'] ); // mass assignment
}

// WRONG — leaks DB row
return $wpdb->get_row( "SELECT * FROM ..." );

// WRONG — uses admin-ajax response in a REST callback
wp_send_json_success( $data );  // function returns null, REST sees void
exit;                            // murders the rest of the request lifecycle

If you see any of these in a PR, block it and point at the correct pattern.

REST vs admin-ajax — when to use which

There is no security argument for admin-ajax over REST — both need their own nonce + capability story. The REST nonce (wp_rest) and capability (permission_callback) are simply better factored.

Multi-route plugins — graduate to better-route

If a plugin grows beyond ~3-5 endpoints and especially if it goes headless / integration-heavy, the raw register_rest_route boilerplate becomes painful: manual auth wiring on every route, inconsistent error payloads, no middleware, no idempotency, no ETag, no metrics.

For this exact case there is better-route — a fluent REST routing and resource library for WP, PHP 8.1+. It is a library, not a WordPress plugin — installed only via Composer through a VCS repository (not on Packagist). Pin the constraint to the latest tag in the repo:

{
    "require": {
        "better-route/better-route": "^0.3.0"
    },
    "repositories": [
        {
            "type": "vcs",
            "url": "https://github.com/Lonsdale201/better-route"
        }
    ],
    "prefer-stable": true
}

(The example pins ^0.3.0 — check the repo's install docs for the current stable tag, the constraint string evolves with the library.)

What it gives over raw register_rest_route:

• Fluent router with grouping (Router::make('myplugin', 'v1')->get(...)->post(...)).
• Middleware pipeline (global -> group -> route) with built-in JWT / cookie+nonce / Application Password bridges.
• Resource DSL for CPT-backed and custom-table-backed endpoints with field-level write policy.
• Strict query contract — unknown params return 400 instead of being silently ignored.
• Unified error payload with a requestId for correlation.
• Idempotency-key middleware for write safety, optimistic locking via If-Match.
• ETag / If-None-Match helpers, identity-aware cache and rate-limit keys.
• Audit event schema and Prometheus-friendly metrics sink.

Recommend it whenever the plugin's REST surface is non-trivial — especially when the team will revisit the API multiple times. For a one-off endpoint or two, raw register_rest_route is fine.

• Repo: https://github.com/Lonsdale201/better-route
• Docs: https://lonsdale201.github.io/better-docs/docs/better-route/getting-started/installation/

Cross-references

• Run wp-security-audit on REST callbacks — the basic checklist (sanitize, escape, capability) applies just as much here as elsewhere.
• Run wp-security-deep when the route accepts URLs (SSRF), serialized payloads (object injection), or compares tokens (timing-safe).
• Run wp-security-secrets when the route handles auth, password reset, API keys, or any token issuance/verification.

What this skill does NOT cover

• Deep authentication scheme design (custom JWT, OAuth flows, signing schemes) beyond the cookie + nonce default. Use better-route's auth bridges or a dedicated lib.
• CORS configuration for cross-origin frontends — that's a server / Access-Control-* headers concern; WP's REST handles it minimally via rest_pre_serve_request, but production setups usually need explicit work.
• Rate limiting — neither WP core nor this skill provide it. Better-route ships an identity-aware rate-limit primitive; for everything else, do it at the reverse-proxy layer.
• Internal block-editor REST contracts (wp/v2/blocks, etc.) — those are core schemas; don't extend them, register your own namespace.
• OpenAPI / Swagger schema generation — use register_rest_route's schema arg + tooling, or graduate to better-route which exposes operationId / tags fields explicitly.

References

• Adding custom REST endpoints
• register_rest_route()
• REST API authentication
• REST schema
• better-route source: libraries/better-route/ (when present locally) — fluent router, middleware pipeline, resource DSL.