Initial assessment
WordPress Error Cases

Updating failed. The response is not a valid JSON response.

“Updating failed. The response is not a valid JSON response.” in WordPress: causes and what to check

Updated 2026-10-30

The block editor saves through the WordPress REST API. When the REST API request is blocked or its response contains extra output, the editor cannot read the response and shows this message.

Which of these matches what you see?

The REST API path is blocked or redirected. Open /wp-json/ on the site in a browser; it should return JSON text, not an error page or the home page.

Content in those posts, or a plugin that runs only on them, adds output to the response. Note which posts fail and what they contain, such as embeds or shortcodes.

The change can alter the REST API URL or block it. Check what was changed shortly before the message appeared.

Common causes

REST API blockedA security plugin, firewall rule, or host setting blocks requests to /wp-json/.
Permalink or rewrite rulesThe REST API URL does not resolve after a permalink or .htaccess change.
Extra output in the responseA PHP notice or plugin output is printed before the JSON.
Site URL mismatchThe WordPress Address and Site Address use different protocols or domains.

What the system does

When the REST API path is blocked or its response is polluted, the editor cannot save. This is handled automatically: the system fixes the REST path and re-verifies that the editor can save.

  1. Initial assessmentEnter the site URL
  2. Copy to the recovery stationWork runs on the copy
  3. Find the causeItem-by-item checks
  4. Recover and re-checkHome and inner pages
  5. Write back or deliverOnly after re-checks pass

Use WP119 when you prefer not to change security, permalink, or server settings yourself.

Assess my site →
Initial assessment →