Fluentd Blog

Fluentd v1.20.1 has been released

Hi users!

We released v1.20.1 on October 26, 2026. ChangeLog is here.

This release is a maintenance release for the v1.20 series. It removes yajl-ruby from the runtime dependencies of Fluentd, so that Fluentd can be installed on Ruby 4.1.

yajl-ruby is no longer a dependency

In the v1.20.0 announcement, we said that Fluentd keeps yajl-ruby as a dependency for a while. We kept it because some third-party plugins require 'yajl' without declaring it in their gemspec.

However, yajl-ruby cannot be built on Ruby 4.1, which is under development. The gemspec of Fluentd v1.20.0 adds the dependency only for Ruby 4.0 or earlier. This condition is evaluated only when the gem is built, so the released gem always depends on yajl-ruby. As a result, gem install fluentd fails on Ruby 4.1.

Fluentd itself has not used yajl-ruby since v1.20.0. So we removed it from the dependencies in this release. (#5574)

For plugin developers

If your plugin uses require 'yajl', please migrate to the json gem. If you keep using yajl-ruby, please add it to the dependencies of your gemspec.

spec.add_runtime_dependency("yajl-ruby", "~> 1.4")

Note that yajl-ruby cannot be installed on Ruby 4.1 at the moment.

For users

If you see the following error after upgrading Fluentd, one of your plugins uses yajl-ruby without declaring it.

LoadError: cannot load such file -- yajl

You can avoid it by adding yajl-ruby to your Gemfile, or by running gem install yajl-ruby. This works only on Ruby 4.0 or earlier.

fluent-package already bundles yajl-ruby, so its users are not affected.

Enjoy logging!

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