fix(gemini): harden dedicated provider compatibility

web/backend/api/session.go

@@ -281,6 +281,12 @@ func visibleSessionMessages(messages []providers.Message, toolFeedbackMaxArgsLen
 			}
 
 		case "assistant":
+			// Reasoning-only assistant messages are transient display artifacts and
+			// should not be restored from session history.
+			if assistantMessageTransientThought(msg) {
+				continue
+			}
+
 			toolSummaryMessages := visibleAssistantToolSummaryMessages(msg.ToolCalls, toolFeedbackMaxArgsLength)
 			if len(toolSummaryMessages) > 0 {
 				transcript = append(transcript, toolSummaryMessages...)
@@ -309,6 +315,13 @@ func visibleSessionMessages(messages []providers.Message, toolFeedbackMaxArgsLen
 	return transcript
 }
 
+func assistantMessageTransientThought(msg providers.Message) bool {
+	return strings.TrimSpace(msg.Content) == "" &&
+		strings.TrimSpace(msg.ReasoningContent) != "" &&
+		len(msg.ToolCalls) == 0 &&
+		len(msg.Media) == 0
+}
+
 func assistantMessageInternalOnly(msg providers.Message) bool {
 	return strings.TrimSpace(msg.Content) == handledToolResponseSummaryText
 }

web/backend/api/session_test.go

@@ -218,6 +218,59 @@ func TestHandleGetSession_JSONLStorage(t *testing.T) {
 	}
 }
 
+func TestHandleGetSession_OmitsTransientThoughtMessages(t *testing.T) {
+	configPath, cleanup := setupOAuthTestEnv(t)
+	defer cleanup()
+
+	dir := sessionsTestDir(t, configPath)
+	store, err := memory.NewJSONLStore(dir)
+	if err != nil {
+		t.Fatalf("NewJSONLStore() error = %v", err)
+	}
+
+	sessionKey := picoSessionPrefix + "detail-transient-thought"
+	for _, msg := range []providers.Message{
+		{Role: "user", Content: "hello"},
+		{Role: "assistant", ReasoningContent: "internal chain of thought"},
+		{Role: "assistant", Content: "final visible answer"},
+	} {
+		if err := store.AddFullMessage(nil, sessionKey, msg); err != nil {
+			t.Fatalf("AddFullMessage() error = %v", err)
+		}
+	}
+
+	h := NewHandler(configPath)
+	mux := http.NewServeMux()
+	h.RegisterRoutes(mux)
+
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequest(http.MethodGet, "/api/sessions/detail-transient-thought", nil)
+	mux.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusOK {
+		t.Fatalf("status = %d, want %d, body=%s", rec.Code, http.StatusOK, rec.Body.String())
+	}
+
+	var resp struct {
+		Messages []struct {
+			Role    string `json:"role"`
+			Content string `json:"content"`
+		} `json:"messages"`
+	}
+	if err := json.Unmarshal(rec.Body.Bytes(), &resp); err != nil {
+		t.Fatalf("Unmarshal() error = %v", err)
+	}
+	if len(resp.Messages) != 2 {
+		t.Fatalf("len(resp.Messages) = %d, want 2", len(resp.Messages))
+	}
+	if resp.Messages[0].Role != "user" || resp.Messages[0].Content != "hello" {
+		t.Fatalf("first message = %#v, want user/hello", resp.Messages[0])
+	}
+	if resp.Messages[1].Role != "assistant" || resp.Messages[1].Content != "final visible answer" {
+		t.Fatalf("second message = %#v, want assistant/final visible answer", resp.Messages[1])
+	}
+}
+
 func TestHandleGetSession_ReconstructsVisibleMessageToolOutput(t *testing.T) {
 	configPath, cleanup := setupOAuthTestEnv(t)
 	defer cleanup()