Reading through the other issues, I'm sorry to break the news, but it looks like your favorite issue with #to_json and #as_json is back with the recent release of Rails 7.2.0. The issue doesn't show up with Rails 7.1.x, but looking through the Rails source code it isn't immediately apparent what changed.

The issue shows up upon calling Oj.mimic_JSON. Adding or removing Oj.optimize_rails, doesn't change the behavior.

require 'bundler/inline'

gemfile do
  source 'https://rubygems.org'

  gem 'rails', '7.2.0' # No issues with 7.1.0
  gem 'oj', '3.16.5'
end

Oj.mimic_JSON # This seems to cause the issue
Oj.optimize_rails # Does not have an effect on the issue

class TestObject
  def as_json
    { foo: :bar }
  end
end

puts TestObject.new.to_json

Expected output:

{"foo":"bar"}

Actual output:

"#<Test:0x000000012024e9d0>"

Thanks a lot for this gem! I've been using it without any problems for years now 💛