Surface AG-UI REASONING_MESSAGE_CHUNK events as reasoning content

provider/aguiprovider/agui.go

@@ -446,6 +446,22 @@ func (a *toolCallAccumulator) onEvent(evt aguiEvents.Event) ([]*agent.ResponseUp
 			CreatedAt: eventTime(evt),
 			Contents:  message.Contents{&message.TextContent{Text: delta}},
 		}}, nil
+	case *aguiEvents.ReasoningMessageChunkEvent:
+		delta := deref(e.Delta)
+		if delta == "" {
+			return nil, nil
+		}
+		// A chunk may omit MessageID to continue the current reasoning message;
+		// reuse the last seen chunk MessageID, mirroring the text-chunk handling.
+		if id := deref(e.MessageID); id != "" {
+			a.lastChunkMessageID = id
+		}
+		return []*agent.ResponseUpdate{{
+			Role:      message.RoleAssistant,
+			MessageID: a.lastChunkMessageID,
+			CreatedAt: eventTime(evt),
+			Contents:  message.Contents{&message.TextReasoningContent{Text: delta}},
+		}}, nil
 	case *aguiEvents.ReasoningMessageContentEvent:
 		return []*agent.ResponseUpdate{{
 			Role:      message.RoleAssistant,

provider/aguiprovider/agui_test.go

@@ -713,3 +713,30 @@ func writeSSE(t *testing.T, w http.ResponseWriter, evt aguiEvents.Event) {
 func newTestClient(endpoint string) *aguiSSEClient.Client {
 	return aguiSSEClient.NewClient(aguiSSEClient.Config{Endpoint: endpoint})
 }
+
+func TestAGUIAgentRun_SurfacesReasoningMessageChunkEvents(t *testing.T) {
+	strPtr := func(s string) *string { return &s }
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "text/event-stream")
+		writeSSE(t, w, aguiEvents.NewRunStartedEvent("thread-1", "run-1"))
+		writeSSE(t, w, aguiEvents.NewReasoningMessageChunkEvent(strPtr("r1"), strPtr("think")))
+		writeSSE(t, w, aguiEvents.NewReasoningMessageChunkEvent(strPtr("r1"), strPtr("ing")))
+		writeSSE(t, w, aguiEvents.NewRunFinishedEvent("thread-1", "run-1"))
+	}))
+	defer server.Close()
+
+	a := aguiprovider.NewAgent(newTestClient(server.URL), aguiprovider.AgentConfig{})
+	resp, err := a.RunText(context.Background(), "hi").Collect()
+	if err != nil {
+		t.Fatalf("run error: %v", err)
+	}
+	var reasoning string
+	for content := range resp.Contents() {
+		if rc, ok := content.(*message.TextReasoningContent); ok {
+			reasoning += rc.Text
+		}
+	}
+	if reasoning != "thinking" {
+		t.Fatalf("reasoning text = %q, want %q", reasoning, "thinking")
+	}
+}