I honestly thought we’d moved past the days of minor updates breaking core admin functionality, but WordPress 6.9.1 RC1 proves that even the best release cycles can have a messy fallout. If you’ve been pulling your hair out over broken checkboxes in the admin panel or sporadic mail delivery failures since upgrading to 6.9, this release candidate is exactly what you’ve been waiting for.
Maintenance releases like WordPress 6.9.1 RC1 aren’t about flashy new features; they are about stability. Specifically, this RC tackles 23 core bugs and 25 Block Editor issues. For those of us running high-traffic WooCommerce sites or complex multisite environments, these “minor” bugs are often the ones that trigger the most expensive support tickets.
The Technical “Gotchas” in WordPress 6.9.1 RC1
One of the most irritating bugs being addressed is the HTML API double-escaping class names. If you’re building custom blocks or using the new HTML API to manipulate tags, you might have noticed classes being duplicated or mangled. Furthermore, the wp_mail_succeeded filter was missing the $embeds element, which is a classic race condition or oversight that breaks sophisticated transactional email workflows.
Then there’s the Could not instantiate mail function error. In my 14 years of debugging WordPress, mail issues are the hardest to explain to a client. It’s rarely the server; usually, it’s a dependency conflict or a misconfigured transient. This RC fixes that specific regression, along with broken styling on the “Add Plugins” screen that made the dashboard look like a CSS experiment gone wrong.
If you’re already feeling the heat, you might want to check out my previous guide on dealing with WordPress 6.9 hotfixes to see how we managed until this RC arrived.
How to Test the RC Without Breaking Production
Stop testing on live sites. Use a staging environment and WP-CLI. It’s the fastest way to verify if your specific plugin stack survives the update. Run the following command to pull the latest build:
# Update to the latest WordPress 6.9.1 RC1 build via WP-CLI
wp core update https://wordpress.org/wordpress-6.9.1-RC1.zipSpecifically, keep an eye on your XDebug configuration. Ticket #63979 identifies that core setup wasn’t configuring XDebug properly, which can be a nightmare for local development performance. If your local environment feels sluggish, this fix alone is worth the update.
Furthermore, developers should note that WordPress 6.9.1 RC1 now warns when enqueuing scripts or modules with missing dependencies. No more silent failures that leave you digging through the browser console for hours.
Gutenberg Refinement: No More Infinite Loops
On the Block Editor side, we’re seeing some critical refactoring. PR #73955 fixes an infinite loop in the Font Style UI within Global Styles. I’ve seen these types of loops crash a browser tab and lead to massive memory leaks. Consequently, getting this into your workflow before the final release on February 3, 2026, is vital for a smooth transition. For more on the roadmap, see the 6.9.1 release schedule.
Look, if this WordPress 6.9.1 RC1 stuff is eating up your dev hours, let me handle it. I’ve been wrestling with WordPress since the 4.x days.
The Takeaway for Developers
Don’t ignore this maintenance release. While the community focuses on the upcoming WordPress 7.0, the 6.9.1 update is what keeps your current clients’ sites alive. Test the mail functions, verify your block attributes, and ship with confidence. If you find a bug, report it in the #6-9-release-leads Slack channel. Stability is a team sport.
