diff --git a/spec/logtail-rails/json_encoding_spec.rb b/spec/logtail-rails/json_encoding_spec.rb
new file mode 100644
index 0000000..fb099f5
--- /dev/null
+++ b/spec/logtail-rails/json_encoding_spec.rb
@@ -0,0 +1,96 @@
+require "spec_helper"
+
+# With json 3, ActiveSupport 8.0 and older can't encode anything through `to_json` called directly:
+# ActiveSupport::JSON.encode passes `quirks_mode:` to JSON.generate, which json 3 rejects. Calls made
+# by JSON.generate itself pass a JSON::State and keep working. The stub raises where json 3 does.
+RSpec.describe "JSON encoding when to_json raises like json 3 under ActiveSupport 8.0" do
+  let(:time) { Time.utc(2016, 9, 1, 12, 0, 0) }
+  let(:io) { StringIO.new }
+  let(:logger) do
+    logger = Logtail::Logger.new(io)
+    logger.level = ::Logger::INFO
+    logger
+  end
+
+  around(:each) do |example|
+    class JsonEncodingController < ActionController::Base
+      layout nil
+
+      def index
+        render template: "template"
+      end
+
+      def error
+        raise "Boom!"
+      end
+
+      def method_for_action(action_name)
+        action_name
+      end
+    end
+
+    ::RailsApp.routes.draw do
+      get '/json_encoding' => 'json_encoding#index'
+      get '/json_encoding_error' => 'json_encoding#error'
+    end
+
+    # Raise exceptions instead of rendering them. Since Rails 7.2 the spec app's false means :all.
+    env_config = ::Rails.application.env_config
+    show_exceptions = env_config["action_dispatch.show_exceptions"]
+    env_config["action_dispatch.show_exceptions"] = :none if ::Rails::VERSION::MAJOR > 7 || ::Rails::VERSION::MAJOR == 7 && ::Rails::VERSION::MINOR >= 1
+
+    begin
+      with_rails_logger(logger) do
+        Timecop.freeze(time) { example.run }
+      end
+    ensure
+      env_config["action_dispatch.show_exceptions"] = show_exceptions
+    end
+
+    Object.send(:remove_const, :JsonEncodingController)
+  end
+
+  before(:each) do
+    allow(::ActiveSupport::JSON).to receive(:encode).and_raise(ArgumentError, "unknown keyword: :quirks_mode")
+  end
+
+  it "should log a request" do
+    response = dispatch_rails_request("/json_encoding?page=2")
+
+    expect(response.status).to eq(200)
+
+    request = logged_event("http_request_received")
+    expect(request).to include("headers_json")
+    expect(JSON.parse(request["headers_json"])).to include("X_Request_Id" => "unique-request-id-1234")
+
+    controller_call = logged_event("controller_called")
+    expect(controller_call).to include("controller" => "JsonEncodingController", "action" => "index")
+    expect(JSON.parse(controller_call["params_json"])).to eq("page" => "2")
+
+    expect(logged_event("template_rendered")).to include("name" => "spec/support/rails/templates/template.html")
+
+    response_sent = logged_event("http_response_sent")
+    expect(response_sent).to include("status" => 200)
+    expect(JSON.parse(response_sent["headers_json"])).to be_a(Hash)
+  end
+
+  it "should log an exception and let it through" do
+    expect { dispatch_rails_request("/json_encoding_error") }.to raise_error(RuntimeError, "Boom!")
+
+    error = logged_event("error")
+    expect(error).to include("name" => "RuntimeError", "message" => "Boom!")
+    expect(JSON.parse(error["backtrace_json"])).to be_an(Array)
+  end
+
+  it "should log a Time and NaN as ActiveSupport encoded them" do
+    ::Rails.logger.info("JSON values", custom: {at: time, ratio: Float::NAN})
+
+    expect(JSON.parse(io.string)["custom"]).to eq("at" => "2016-09-01T12:00:00.000Z", "ratio" => nil)
+  end
+
+  # The first logged event with this name. The Rack middlewares nest theirs under "event".
+  def logged_event(name)
+    lines = io.string.split("\n").map { |line| JSON.parse(line) }
+    lines.map { |line| line[name] || (line["event"] || {})[name] }.compact.first
+  end
+end