"The Response Is Not a Valid JSON Response" in WordPress (2026 Fix)

"Updating failed. The response is not a valid JSON response" means Gutenberg's save request hit the REST API and got back garbage - an HTML error page, a redirect, a firewall block - instead of JSON. The API is being intercepted somewhere. Six checks, ordered by how often each is the culprit.

1. Re-Save Permalinks (The #1 Fix)

The REST API lives at /wp-json/ - which depends on rewrite rules. Go to Settings > Permalinks and click Save Changes without modifying anything. This regenerates .htaccess and fixes the large majority of cases. If it recurs after migration or a permalink change, this is almost always it.

2. Test the API Directly

Open yoursite.com/wp-json/ in a browser while logged in. You should see a wall of JSON. What you see instead names the layer:

• -403 page = security plugin or mod_security blocking it (fix 3)
• -404 page = rewrite rules broken (back to fix 1, or .htaccess permissions)
• -Redirect to http or www variant = URL mismatch (fix 4)

Tools > Site Health also reports REST API status under "The REST API encountered an error" - check it.

3. Check Security Plugins and WAF

Security plugins that "harden" WordPress often restrict REST API access - look for REST API toggles in Wordfence, iThemes, or whatever firewall you run, and allow the API for logged-in users at minimum. Host-level mod_security can also block POST bodies that look "suspicious" - if the error only fires on posts containing certain words or code snippets, that's a WAF signature, and support can whitelist it.

4. Fix the http/https and www Mismatch

If you recently enabled SSL or changed the domain, check Settings > General: WordPress Address and Site Address must be identical, both https, same www preference. When the editor runs on https but the API URL is http, the request gets redirected - and a redirect response isn't JSON. If wp-admin is inaccessible or the fields are greyed out, set them in wp-config.php:

Related: if the site loads pages over https but still requests http assets, see our mixed content fix.

5. Deactivate Plugins, Switch Theme

Any plugin can pollute the API response - a PHP notice printed before JSON corrupts it. Enable WP_DEBUG_LOG, reproduce the error, check wp-content/debug.log for "headers already sent" or notices naming a file. Then deactivate plugins one by one, starting with anything that touches headers, caching, or the editor. Still broken? Switch to a default theme - a theme echoing whitespace in functions.php can break JSON too.

6. Classic Editor as a Bridge (Not a Fix)

Need to publish while you debug? Install Classic Editor - it saves via the old path, bypassing the REST API. Treat it as a bridge only: the API also powers contact forms, WooCommerce blocks, and the mobile app. A broken API has blast radius beyond the editor.

Related: 403 forbidden error - redirect loop fix - 500 internal server error - Gutenberg editor guide.