Keep AdditionalProperties on every Responses output message

provider/openaiprovider/responses.go

@@ -851,7 +851,12 @@ func responsesProcessResponse(resp *responses.Response, seqNum int64, yield func
 				if !yield(currentUpdate, nil) {
 					return
 				}
-				currentUpdate = &agent.ResponseUpdate{}
+				// Reset for the next message, carrying the response-level
+				// properties forward so the second and later messages keep
+				// AdditionalProperties (e.g. EndUserId).
+				currentUpdate = &agent.ResponseUpdate{
+					AdditionalProperties: responsesPopulateAdditionalProperties(resp),
+				}
 			}
 			currentUpdate.MessageID = out.ID
 			currentUpdate.ResponseID = resp.ID

provider/openaiprovider/responses_test.go

@@ -6818,6 +6818,58 @@ func responsesBodyEqual(t *testing.T, got string, want string) {
 	}
 }
 
+// A non-streaming Responses result with more than one output message must carry
+// response-level AdditionalProperties (e.g. EndUserId) on every message, not
+// only the first. currentUpdate is reset to a fresh value for each message
+// after the first, so the properties must be repopulated per message.
+func TestResponses_NonStreaming_AllMessagesKeepAdditionalProperties(t *testing.T) {
+	const input = `
+		{
+			"model":"gpt-4o-mini",
+			"input": [{
+				"type":"message",
+				"role":"user",
+				"content":[{"type":"input_text","text":"hello"}]
+			}]
+		}`
+	const output = `
+		{
+			"id":"resp_multi",
+			"object":"response",
+			"created_at":1741891428,
+			"status":"completed",
+			"error":null,
+			"incomplete_details":null,
+			"model":"gpt-4o-mini",
+			"user":"end-user-42",
+			"output":[
+				{"type":"message","id":"msg_1","status":"completed","role":"assistant","content":[{"type":"output_text","text":"first","annotations":[]}]},
+				{"type":"message","id":"msg_2","status":"completed","role":"assistant","content":[{"type":"output_text","text":"second","annotations":[]}]}
+			]
+		}`
+
+	server := newTestResponsesServer(t, input, output)
+	defer server.Close()
+	a := newTestResponsesClient(server, "gpt-4o-mini")
+
+	var msgUpdates int
+	for u, err := range a.RunText(t.Context(), "hello") {
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if u.MessageID == "" {
+			continue
+		}
+		msgUpdates++
+		if got, _ := u.AdditionalProperties["EndUserId"].(string); got != "end-user-42" {
+			t.Errorf("message %q: EndUserId = %q, want %q", u.MessageID, got, "end-user-42")
+		}
+	}
+	if msgUpdates != 2 {
+		t.Fatalf("expected 2 message-bearing updates, got %d", msgUpdates)
+	}
+}
+
 // TestResponsesStreamingFailedResponseSurfacesError verifies that a streamed
 // response.failed event surfaces its error as ErrorContent rather than an
 // empty update.