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infoWordPress Core•apiREST APIerrorerrorcodejsoncodeJavaScriptextensionplugin•labelCommon•warningModerate

Failed to load block: Invalid block JSON

WordPress loads a block through the REST API. The API must return a JSON payload that describes the block. When the response is HTML, an error appears in the editor.

The error indicates the editor cannot parse the block definition. It usually happens after a migration, a plugin update, or a security rule that blocks the REST request.

Symptoms

[dashicons-warning]
Editor banner
An orange banner shows the message Failed to load block: Invalid block JSON.
[dashicons-admin-site]
Placeholder block
The block appears as a gray placeholder with a Retry button.
[dashicons-no]
Publish failure
Saving the post shows Updating failed. The response is not a valid JSON response.

Common Causes

Firewall or WAF
A ModSecurity rule blocks the REST endpoint and returns HTML.
Mixed HTTP/HTTPS URLs
Site URL uses a different protocol causing the request to redirect to a non‑JSON page.
Corrupt block.json
The block’s JSON file has syntax errors or is missing.
Plugin or theme conflict
A plugin or theme filters REST requests or breaks block registration.
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