fix(analytics): surface dropped exposure when distinct_id missing from context (SDK-84)

lib/mixpanel-ruby/flags/flags_provider.rb

@@ -88,6 +88,14 @@ def track_exposure_event(flag_key, selected_variant, context, latency_ms = nil)
         distinct_id = context['distinct_id'] || context[:distinct_id]
 
         unless distinct_id
+          # Local eval succeeds when the flag's Variant Assignment Key is
+          # something other than distinct_id (e.g., device_id), but the
+          # exposure event still needs distinct_id to attribute the user.
+          # Surface the drop instead of silently returning so callers can
+          # see they need to include distinct_id in the context.
+          @error_handler&.handle(MixpanelError.new(
+            "Cannot track exposure event for flag '#{flag_key}' without a distinct_id in the context"
+          ))
           return
         end
 

spec/mixpanel-ruby/flags/local_flags_spec.rb

@@ -718,6 +718,29 @@ def user_context_with_properties(properties)
 
       provider.send(:track_exposure_event, 'test_flag', variant, test_context)
     end
+
+    # SDK-84: when local eval succeeds via a non-distinct_id Variant Assignment
+    # Key but the context lacks distinct_id, the exposure can't fire. Surface
+    # via error_handler instead of silently returning.
+    it 'reports through error_handler when distinct_id is missing' do
+      variant = Mixpanel::Flags::SelectedVariant.new(
+        variant_key: 'treatment', variant_value: 'treatment'
+      )
+
+      expect(mock_tracker).not_to receive(:call)
+      expect(mock_error_handler).to receive(:handle) do |err|
+        expect(err).to be_a(Mixpanel::MixpanelError)
+        expect(err.message).to include('test_flag')
+        expect(err.message).to include('distinct_id')
+      end
+
+      provider.send(
+        :track_exposure_event,
+        'test_flag',
+        variant,
+        { 'device_id' => 'abc-123' }
+      )
+    end
   end
 
   describe 'polling' do