Troubleshooting WordPress REST API and development issues requires a systematic approach to identify and resolve common errors that disrupt API functionality. The WordPress REST API enables developers to interact with WordPress data using HTTP requests, but misconfigurations, plugin conflicts, and server limitations often lead to errors like 401 Unauthorized, 403 Forbidden, 404 Not Found, and 500 Internal Server Error. These issues typically stem from incorrect permalink settings, disabled API access, outdated software, or insufficient server resources. Addressing these problems involves checking permalink structures, deactivating conflicting plugins, updating WordPress and plugins, and verifying server configurations.

• Common error types: 401 Unauthorized, 403 Forbidden, 404 Not Found, 500 Internal Server Error, and 503 Service Unavailable are frequently encountered ^[3].
• Primary causes: Incorrect permalink settings, plugin conflicts, outdated WordPress or plugins, and insufficient PHP memory limits ^[2].
• Key solutions: Reset permalinks, deactivate plugins to identify conflicts, update software, and increase PHP memory limits ^[8].
• Advanced issues: Domain changes, content stripping due to sanitization, and custom endpoint misconfigurations require targeted fixes ^[7][10].

Troubleshooting WordPress REST API Issues

Common Errors and Immediate Fixes

WordPress REST API errors often manifest as HTTP status codes, each indicating a specific type of failure. The most common errors include 401 Unauthorized, 403 Forbidden, 404 Not Found, and 500 Internal Server Error, each requiring distinct troubleshooting steps. For example, a 401 error typically points to authentication issues, while a 404 suggests incorrect endpoint URLs or disabled API access. Immediate fixes involve verifying permalink settings, ensuring the REST API is enabled, and checking for plugin conflicts.

• 401 Unauthorized: Verify authentication credentials, ensure the REST API is enabled, and check for plugin conflicts that may block access ^[3].
• 403 Forbidden: Review server permissions, deactivate security plugins, and check for IP blocking or incorrect user roles ^[3].
• 404 Not Found: Reset permalink settings to default and back to the desired structure, ensuring the API endpoint URL is correct ^[1][9].
• 500 Internal Server Error: Increase PHP memory limits by editing wp-config.php (e.g., define('WPMEMORYLIMIT', '256M');) and check server logs for specific errors ^[2][8].
• Invalid URI errors: Verify the endpoint structure includes wp-json/ and check for SSL certificate issues, especially after domain changes ^[4][9].

For persistent issues, enable WordPress debugging by adding define('WP_DEBUG', true); to wp-config.php to log detailed errors ^[6]. This helps identify PHP notices or warnings that may not be immediately visible but could disrupt API functionality.

Advanced Troubleshooting for Developers

Developers often encounter more complex issues, such as content stripping in custom posts, custom endpoint failures, or API disruptions after domain migrations. These problems require deeper technical interventions, including modifying WordPress hooks, registering custom endpoints, or updating database references. For instance, content stripping occurs due to WordPress's sanitization filters, which remove unsupported HTML or block attributes during API requests. Custom solutions involve leveraging filters like restpreinsert_post or creating dedicated endpoints to bypass default sanitization.

• Content stripping issues: Use the restpreinsert_post filter to modify post data before saving or register a custom endpoint with registerrestroute to avoid default sanitization:

addfilter('restpreinsertpost', 'customrestcontentfilter', 10, 2);
function customrestcontentfilter($preparedpost, $request) { $preparedpost->post_content = $request->getparam('content'); return $preparedpost; }

• Custom endpoint failures: Ensure the endpoint URL includes wp-json/ and the namespace/route is correctly registered. For example:

addaction('restapiinit', function () {
registerrest_route('custom/v1', '/endpoint/', [ 'methods' => 'GET', 'callback' => 'customendpointcallback', ]); });

• Domain migration issues: After changing domains, update the siteurl and home values in the wp_options table via phpMyAdmin or WP-CLI. Also, search the database for old domain references using tools like "Better Search Replace" ^[10].
• Debugging array offset errors: Log problematic post IDs or values to identify boolean-to-array conversion issues, common in media handling. For example:

errorlog('Problematic post ID: ' . $postid);

• Memory and server limits: For 503 Service Unavailable errors, check server logs for resource exhaustion. Increase PHP memory limits in wp-config.php or contact hosting support to adjust server configurations ^[2].

For plugins or themes causing conflicts, use a binary search method: deactivate half the plugins at a time and test the API until the culprit is isolated. Similarly, switch to a default theme like Twenty Twenty-Four to rule out theme-related issues ^[8]. If the problem persists, review the plugin or theme’s code for hooks that modify REST API behavior, such as restauthenticationerrors or restpredispatch.