Kotlin 2.4.21

Kotlin 2.4.21 is out. Released October 8, 2026. A bug-fix release for the 2.4 line.

What it is: a patch on top of 2.4.20, which shipped September 1, 2026. Nothing new to learn. No language changes. Fixes only.

Key Facts

• Type: bug-fix release. Maintains binary compatibility with 2.4.0 and 2.4.20.
• Upstream: tagged and published on GitHub, with the full changelog on the Kotlin release page.
• Backdrop: Kotlin 2.4.0 landed June 3, 2026. The 2.4 series brought new language features, Swift package support as dependencies, and an 18-month security support policy for the standard library.

Do You Need It

If you are on 2.4.x, yes. Patch releases carry the compiler, IDE-plugin, and stdlib fixes that come out of reported regressions. Update the Kotlin version in your build file and sync. Remember to bump the Kotlin Gradle plugin and any Compose compiler dependency so the toolchain stays in step.

If you are on 2.3.x or older, jump straight to 2.4.21 and read the 2.4.0 release notes for the migration items.

What Comes Next

• 2.5.0 is planned for December 2026.
• 2.5.20 is planned for March 2027.
• 2.5.0-Beta1 is already available, dated September 23, 2026. It stabilizes two previously experimental language features and introduces experimental companion extensions.

Bottom line: 2.4.21 is a maintenance upgrade. Fast to install, low risk, worth doing this week.