Yoast SEO: “Error parsing the response to JSON” – How to Fix It

This error means the optimization request returned something that wasn’t valid JSON. The most common causes are PHP notices breaking the output, a blocked REST API/WAF, or stale Yoast “indexables” after a migration. Fix it in this order:

1) Eliminate noisy PHP output (quickest win)

1. Turn off debugging in production: In wp-config.php, make sure:

   define('WP_DEBUG', false);
   define('WP_DEBUG_DISPLAY', false); // or remove if present
   

2. Retry Yoast → Tools → Start SEO data optimization.

2) Reset Yoast’s indexables, then rerun

1. Install the free Yoast Test Helper plugin.
2. Go to Tools → Yoast Test and click:
   • Reset indexables tables & migrations
   • Reset Internal link counter
   • Reset Prominent words calculation (optional)
3. Return to Yoast → Tools and run SEO data optimization again.

3) Make sure the REST API isn’t blocked

Yoast’s optimizer calls WordPress REST endpoints. If a security plugin, WAF (e.g., Comodo), or server rule blocks /wp-json/ you’ll see 401/403 or JSON parse errors.

• Visit https://your-site.com/wp-json/. You should see a JSON payload, not a 401/403/HTML page.
• Temporarily disable security/WAF rules or whitelist REST requests; disable redirect plugins that hijack API calls.

4) Check console/network for status codes

Open your browser DevTools → Network tab, run the optimization again, and look for 301/302 (redirects), 401/403 (blocked), or 500/504 (server errors). Fix the cause you see:

• 301/302: a redirect plugin or server rule is catching API calls—disable or adjust it.
• 401/403: REST/API blocked—allow it in the WAF or security plugin.
• 500/504: check PHP error logs, raise memory/time limits, and try WP‑CLI below.

5) Reindex via WP‑CLI (reliable on busy sites)

Shell into the server and run:

wp yoast index --reindex

If WordPress isn’t in the web root, include --path:

wp --path=/home/account/public_html yoast index --reindex

This bypasses the browser and fills Yoast’s indexable tables directly.

6) Still failing? Quick checklist

• Environment type: On staging/dev (wp_get_environment_type() ≠ production) Yoast may prevent indexation. Switch to production or temporarily override to test.
• Server pages caching: Purge any reverse proxy/CDN caching (Cloudflare, Nginx microcache) after changes.
• Admin‑AJAX or cron throttling: If background tasks are blocked, the run can stall—allow admin-ajax.php and ensure WP‑Cron triggers.

Tip: Even if the UI run fails, Yoast still indexes new/updated content over time. Completing a full run improves consistency and speed, so it’s worth finishing.

After a migration, several subtle issues can break the optimizer’s JSON response. Work through these migration checks:

A) URLs and serialization

• Confirm Settings → General → WordPress Address (URL) and Site Address (URL) are correct and match your final domain (no mixed HTTP/HTTPS, no stray subdirectory).
• If you moved domains/paths, run a proper serialized URL replacement (e.g., WP-CLI wp search-replace or a safe migration tool) across the DB so embedded URLs and serialized arrays aren’t half‑migrated.

B) Authentication keys/salts and cookies

Regenerate auth keys/salts in wp-config.php post‑migration to avoid odd “failed to fetch / cookie check” behaviors:

// Use the WordPress.org secret-key service to generate fresh values
define('AUTH_KEY',         '...');
define('SECURE_AUTH_KEY',  '...');
define('LOGGED_IN_KEY',    '...');
define('NONCE_KEY',        '...');
define('AUTH_SALT',        '...');
define('SECURE_AUTH_SALT', '...');
define('LOGGED_IN_SALT',   '...');
define('NONCE_SALT',       '...');

C) Security layers

• Temporarily relax WAF/security plugin rules that block /wp-json/ or /wp-admin/admin-ajax.php until the run completes.
• Ensure basic auth on staging isn’t blocking the optimizer (401/403).

D) Finalize with a clean reindex

1. Install Yoast Test Helper → Reset indexables tables & migrations.
2. Run the optimizer again, or use:

   wp yoast index --reindex

3. Purge caches/CDN and recheck Yoast notifications.