Keep the sub-second precision Rails renders

lib/openapi_kit/codec/date_time.rb

@@ -26,8 +26,13 @@ def self.from_wire(value)
         end
       end
 
+      # Rails renders a Time through as_json at three decimal places
+      # (ActiveSupport::JSON::Encoding.time_precision), so an application moving onto
+      # openapi_kit keeps emitting the timestamps its clients already parse.
+      PRECISION = 3
+
       sig { override.params(value: ::Time).returns(OpenAPIKit::Wire) }
-      def self.to_wire(value) = value.utc.iso8601
+      def self.to_wire(value) = value.utc.iso8601(PRECISION)
     end
   end
 end

spec/openapi_kit/codec_spec.rb

@@ -34,7 +34,13 @@
   describe "round trips" do
     it "normalises a date-time to UTC" do
       value = described_class::DateTime.from_wire("2026-09-02T12:00:00+02:00")
-      expect(described_class::DateTime.to_wire(value)).to eq("2026-09-02T10:00:00Z")
+      expect(described_class::DateTime.to_wire(value)).to eq("2026-09-02T10:00:00.000Z")
+    end
+
+    it "keeps the sub-second precision Rails renders, so a migrating client sees no change" do
+      value = described_class::DateTime.from_wire("2026-09-02T12:00:00.250Z")
+
+      expect(described_class::DateTime.to_wire(value)).to eq("2026-09-02T12:00:00.250Z")
     end
 
     it "keeps decimal precision as a string" do

spec/scripts/round_trip.rb

@@ -48,7 +48,7 @@ def check(label)
 check("differing values are unequal") { KitchenSink::Types::Mod::Codec.from_wire(WIRE.merge("id" => 8)) != mod }
 
 wire = KitchenSink::Types::Mod::Codec.to_wire(mod)
-check("dumped date-time") { wire["updatedAt"] == "2026-09-02T10:00:00Z" }
+check("dumped date-time") { wire["updatedAt"] == "2026-09-02T10:00:00.000Z" }
 check("dumped enum") { wire["status"] == "under-moderation" }
 check("required nullable stays null") { wire.key?("deletedAt") && wire["deletedAt"].nil? }
 check("absent optional omitted") { !wire.key?("summary") }