Missing PSR Container Interface causing Fatal Error

[Njengah]

Please give us a description of what happened
Yoast SEO plugin fails to load due to a missing ContainerInterface file in the vendor_prefixed directory, resulting in a fatal error that breaks the WordPress site. The plugin cannot initialize its dependency injection container because the PSR Container interface is missing.

To Reproduce

Step-by-step reproduction instructions

Install/update Yoast SEO plugin
Access WordPress site (front-end or admin)
Fatal error occurs preventing site from loading

Expected results

Plugin should load without errors
WordPress site should function normally
All vendor dependencies should be present and properly autoloaded

Actual results

PHP fatal error during plugin initialization
Entire WordPress site becomes inaccessible
Error indicates missing file: vendor_prefixed/psr/container/src/ContainerInterface.php

Screenshots, screen recording, code snippet
Error Log:
PHP Warning: include(vendor/composer/../../vendor_prefixed/psr/container/src/ContainerInterface.php): Failed to open stream: No such file or directory in vendor/composer/ClassLoader.php on line 571

PHP Warning: include(): Failed opening 'vendor_prefixed/psr/container/src/ContainerInterface.php' for inclusion in vendor/composer/ClassLoader.php on line 571

PHP Fatal error: Uncaught Error: Interface "YoastSEO_Vendor\Psr\Container\ContainerInterface" not found in vendor_prefixed/symfony/dependency-injection/ContainerInterface.php:23

Stack trace:

#0 vendor/composer/ClassLoader.php(571): include()
#1 vendor/composer/ClassLoader.php(428): Composer\Autoload\includeFile()
#2 vendor_prefixed/symfony/dependency-injection/ResettableContainerInterface.php(20): Composer\Autoload\ClassLoader->loadClass()
#3 vendor/composer/ClassLoader.php(571): include()
#4 vendor/composer/ClassLoader.php(428): Composer\Autoload\includeFile()
#5 vendor_prefixed/symfony/dependency-injection/Container.php(38): Composer\Autoload\ClassLoader->loadClass()
#6 vendor/composer/ClassLoader.php(571): include()
#7 vendor/composer/ClassLoader.php(428): Composer\Autoload\includeFile()
#8 src/generated/container.php(19): Composer\Autoload\ClassLoader->loadClass()
#9 src/main.php(57): require_once()
#10 lib/abstract-main.php(44): Yoast\WP\SEO\Main->get_container()
#11 src/functions.php(34): Yoast\WP\Lib\Abstract_Main->load()
#12 wp-seo-main.php(459): YoastSEO()
#13 wp-seo.php(50): require_once()

Analysis:

The autoloader cannot find the PSR Container interface that Symfony's DependencyInjection Container depends on. This appears to be an incomplete plugin installation where vendor dependencies were not properly packaged or the autoloader is misconfigured.
Technical info

If relevant, which editor is affected (or editors):

All editors (server-side error prevents site from loading)

Which browser is affected (or browsers):

All browsers (PHP fatal error occurs before any browser rendering)

Used versions

Operating system: Linux (Namecheap Shared Hosting - LiteSpeed)
PHP version: 8.3.26
WordPress version: 6.8.3
WordPress Theme: GeneratePress Child
Yoast SEO version: 26.3

Additional Context

Severity: Critical - Site completely broken
Missing file: vendor_prefixed/psr/container/src/ContainerInterface.php
Impact: Entire WordPress site inaccessible until plugin is deactivated
Possible cause: Incomplete vendor dependencies in plugin package or build process issue
Temporary workaround: Deactivate plugin via database or file system to restore site functionality

[josevarghese]

Hi @Njengah

Thanks for creating the issue. I am not able to replicate this at my end. From the error you have shared, it appears to be a case of missing files, likely due to an incomplete plugin update. Would it be possible for you to manually update (via FTP) with a fresh copy of Yoast SEO Free v26.3?

Before we discuss manual updates, let's cover a few important points to be aware of beforehand. First, Yoast SEO Premium stores SEO data and settings in the database, so manually updating is not expected to cause any data loss.

Second, we recommend creating regular backups of your site and database for your site's health and safety. This is especially important before installing, updating, or removing plugins. It provides you with a safety net in case something were to go wrong. Learn more about the benefits of regular backups.

Thirdly, updating a severely outdated plugin may cause unexpected issues. Thus, we highly recommend using a staging or testing site to reduce the impact on your live site. Some popular web hosts offer quick setups for a staging site, so please contact your web host for assistance. They will probably be able to help you out. If your web host does not offer staging sites, the WordPress plugin repository offers staging plugins or you can create your own staging site.

Manually updating via FTP entails deleting the old plugin files and uploading the new plugin files to the same folder path. We have a step-by-step guide to help you through the process:

• https://yoast.com/help/how-can-i-update-my-free-plugin/#ftp
• https://yoast.com/help/how-can-i-update-my-premium-plugin/#ftp

After manually updating/installing the free version, are you then able to successfully activate both plugins without any further issues?

[seogpa]

Thanks for sharing the details! 👏 This looks like a packaging issue with missing vendor dependencies in the Yoast SEO build. Try reinstalling the plugin or manually clearing the /vendor_prefixed/ folder and reuploading a fresh copy. If the error persists, it’s best to report it directly to Yoast support looks like a build integrity issue in version 26.3.