Add a hook before creating image sub-sizes

[GregLone]

Since things are moving for media thumbnails lately, it could be a good time to add a hook that fires right before thumbnails are created.
It would allow plugins to perform operations on the main file for example.

[SergeyBiryukov]

Moving to the milestone for visibility.

[jorbin]

I chatted with @mikeschroder and we discussed that we want to spend some time ensuring that we get the parameters and placement correct for this, so bumping to the .1 release to allow time for the exploration.

[GregLone]

I understand, I'm late and we're close to the release. Thanks for considering it :)

[marybaum]

Ooh I like this!

[azaozz]

It would allow plugins to perform operations on the main file for example.

What would be the intended use there? Currently there are (at least) three hooks fired right after a file was uploaded:

• ​https://developer.wordpress.org/reference/hooks/action_prefilter/
• ​https://developer.wordpress.org/reference/hooks/pre_move_uploaded_file/
• ​https://developer.wordpress.org/reference/hooks/wp_handle_upload/

These give full access to all aspects of handling an uploaded file. Do we really need a forth hook specific for images? :)

[GregLone]

Replying to azaozz:

Currently there are (at least) three hooks fired right after a file was uploaded:

The idea is not to have a hook that triggers after a file is uploaded but, as the hook’s name suggests, before sub-sizes are created. The difference is that it can happen even if the file wasn’t just uploaded: when a plugin regenerates thumbnails for example.
Currently I’m also thinking adding one after the sub-sizes are created and recorded in the database, maybe at the end of wp_update_attachment_metadata(), with an additional context argument (create/update), like you suggested recently if I remember well.

[azaozz]

Replying to GregLone:

Currently I’m also thinking adding one after the sub-sizes are created and recorded in the database, maybe at the end of wp_update_attachment_metadata(), with an additional context argument (create/update), like you suggested recently if I remember well.

Yeah, this can potentially be useful. At the same time I'd prefer not to "rush it" in 5.3.1. Perhaps better to confirm the use cases before adding the hooks. Then a good decisions can be made whether to pass context, etc.

Moving this to 5.4 perhaps pending a revised/updated patch.

[GregLone]

I agree, it will give us time to think a bit more about it.

[davidbaumwald]

This will at least need a small call out in the misc Dev Notes once it lands in a cycle. Tagging as such.

[davidbaumwald]

This ticket still needs a final decision and a refreshed patch, and with 5.4 Beta 1 landing tomorrow, this is being moved to Future Release. If any maintainer or committer feels this can be included in 5.4 or wishes to assume ownership during a specific cycle, feel free to update the milestone accordingly.

Trac ticket: #48477

[ozgursar]

Patch Testing Report

Patch Tested: ​https://github.com/WordPress/wordpress-develop/pull/9453

Environment

• WordPress: 7.0-alpha-61215-src
• PHP: 8.2.29
• Server: nginx/1.29.4
• Database: mysqli (Server: 8.4.7 / Client: mysqlnd 8.2.29)
• Browser: Chrome 144.0.0.0
• OS: macOS
• Theme: Twenty Twenty-One 2.7
• MU Plugins: None activated
• Plugins:
  • Code Snippets 3.9.4
  • Test Reports 1.2.1

Steps taken

1. Add the following snippet to functions.php or via Code Snippets plugin

add_action('wp_before_make_subsizes', function($new_sizes, $file, $image_meta, $attachment_id) {
    error_log("=== wp_before_make_subsizes HOOK FIRED ===");
    error_log("Attachment ID: " . $attachment_id);
    error_log("File: " . $file);
    error_log("Sizes to create: " . implode(', ', array_keys($new_sizes)));
    error_log("==========================================");
}, 10, 4);

2. Add an image to Media
3. Ensure WP_DEBUG and WP_DEBUG_LOG are enabled in wp-config.php
4. Check /wp-content/debug.log to confirm the hook is running as expected
5. When I uploaded a random image, the following lines are logged:

[01-Feb-2026 12:30:30 UTC] === wp_before_make_subsizes HOOK FIRED ===
[01-Feb-2026 12:30:30 UTC] Attachment ID: 98
[01-Feb-2026 12:30:30 UTC] File: /var/www/src/wp-content/uploads/2026/02/images.jpg
[01-Feb-2026 12:30:30 UTC] Sizes to create: medium, large, thumbnail, medium_large, 1536x1536, 2048x2048, post-thumbnail
[01-Feb-2026 12:30:30 UTC] ==========================================

6. ✅ Patch is solving the problem

Expected result

• We are expecting wp_before_make_subsizes hook to run before creating image sub-sizes when an image is uploaded.