Not thread local

https://github.com/sidekiq/sidekiq/blob/0226e311d02de8ea77563c17c96a3d6527d0ec7f/lib/sidekiq/processor.rb#L74

This variable is not thread local.

I suggest you use a thread attribute, e.g. see https://github.com/rspec/rspec-support/pull/610 for what I suggest as the recommended approach: Thread.attr_accessor :sidekiq_capsule.