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

“Not a valid JSON response” means the editor sent your changes to your site, but the reply that came back wasn't the data it expected.

• Your permalinks, the rules that turn a page address into a page, are out of date, so the save lands on the wrong address.
• Your site address in Settings › General doesn't match the one in your browser: http against https, or www against no www.
• A plugin or your theme prints extra text, such as a notice or a comment, in front of the reply.
• A security plugin or your host's firewall blocks the save, often because the page has code, a form or an embed in it.
• A plugin or your host switches off or blocks the REST API, the part of your site the editor saves through.

Find the cause

The block editor, where you write pages and posts, saves through your site's REST API. That's an address on your site, yoursite.com/wp-json/, that takes requests and answers in JSON. JSON is a plain data format programs use to pass information.

When you press Save, the editor sends your changes there and waits for JSON back. If anything else comes back, it shows this message. That can be an error page, a blocked request or a stray line of text.

Your changes may or may not have saved. Keep the editor open, and open the page in a new tab to see what's there.

Look at what your site sends back

Open yoursite.com/wp-json/ in a new browser tab. A working site shows a page of data that starts with {"name": and your site's name. An error page, a login screen or text before that first { is your clue.

For the exact reply, right-click the editor and pick Inspect. Open the Network tab, press Save again, and click the line shown in red. Its Response tab shows what came back.

Check Site Health

Go to Tools › Site Health. If it says “The REST API encountered an error” or “The REST API encountered an unexpected result”, open that line. It often names the problem, such as a blocked request.

Read the error log

Your error log lists problems PHP had, with the time. PHP is the language WP runs on; your host sets its version.

Look for lines from the minute you pressed Save. Many hosts show the log in their control panel, often under Logs.

Fix it by cause

Permalinks are out of date

If permalinks, the rules that turn a page address into a page, are out of date, the save lands on the wrong address. Go to Settings › Permalinks and press Save Changes without changing anything. That writes the rules again.

Some hosts keep these rules on the server, where WP can't write them. If saving permalinks doesn't help, ask your host to check them.

Your site address doesn't match

In Settings › General, WordPress Address (URL) and Site Address (URL) should match the address in your browser. Watch for http against https, and www against no www.

Change them only if you're sure. A wrong address can lock you out of your dashboard, the screens you log in to at yoursite.com/wp-admin. If the boxes are greyed out, they're set in wp-config.php, and your host can help.

A plugin or theme adds text to the reply

Some plugins print a line of text, a notice or a comment, in front of everything they touch. In a JSON reply, that line breaks the data. It's the cause we set up on our test site, a WP site we run to try every task before we write about it.

Switch off the plugin you added or updated last in Plugins › Installed Plugins, and try Save again. Still failing? Switch the others off one at a time, and try after each. Then try a default theme, such as Twenty Twenty-Five, in Appearance › Themes.

Switching a plugin off keeps its settings, and Activate brings it back as it was. Don't delete plugins to test them.

A firewall blocks the save

Security plugins and host firewalls sometimes block a save. Code, a form or an embed in the page can look like an attack to them. If it fails on one page only, this is a likely cause.

Check your security plugin's log for a blocked request at that time. If it's your host's firewall, ask your host to allow requests to /wp-json/ from your dashboard. Give them the time you pressed Save.

Get by while you look

The Classic Editor plugin, from wordpress.org, saves pages without the REST API, so the message goes away. It hides the problem rather than fixing it. Other parts of your site that use the REST API stay broken.

Stop it coming back

Add or update one plugin at a time, and save a page after each. If saving breaks, you know which one did it. Test a save again after you move hosts or switch to https.

With CMSHands on your site before the error, you can ask the chat what's causing it. It reads your error log, your plugins and settings to find the cause.

We tried it there. On Oct 14, 2026, we added a small test plugin that prints a comment in front of every REST API reply. Saving the About page then failed with this message.

We asked the chat what was causing it. It loaded a page the way a visitor would, read the error log, listed the plugins and read a file on the site. It named the plugin and showed the line it adds in front of every reply.

It said visitors and our pages weren't affected, only saving. It started to prepare the switch-off for our OK, but said a firewall on the site refused that request. Nothing changed, and it told us to switch the plugin off under Plugins.