diff --git a/server/context_window_navigation_test.go b/server/context_window_navigation_test.go
new file mode 100644
index 0000000000000000000000000000000000000000..685aefc60a2c15ca0732b139ec1c81fbfbdf83a4
--- /dev/null
+++ b/server/context_window_navigation_test.go
@@ -0,0 +1,84 @@
+package server
+
+import (
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+// TestContextWindowSizePreservedOnNavigation tests that context window size
+// is correctly returned when loading different conversations
+func TestContextWindowSizePreservedOnNavigation(t *testing.T) {
+	h := NewTestHarness(t)
+	defer h.Close()
+
+	// Create first conversation and get a response
+	h.NewConversation("echo: first message", "/tmp")
+	resp1 := h.WaitResponse()
+	t.Logf("First conversation first response: %q", resp1)
+
+	// Get context window size for first conversation
+	firstConvID := h.convID
+	firstConvSize := h.GetContextWindowSize()
+	t.Logf("First conversation context window size: %d", firstConvSize)
+	if firstConvSize == 0 {
+		t.Fatal("expected non-zero context window size for first conversation")
+	}
+
+	// Create second conversation and get a response
+	h.NewConversation("echo: second message with much more text to ensure different context size", "/tmp")
+	resp2 := h.WaitResponse()
+	t.Logf("Second conversation first response: %q", resp2)
+
+	secondConvID := h.convID
+	secondConvSize := h.GetContextWindowSize()
+	t.Logf("Second conversation context window size: %d", secondConvSize)
+	if secondConvSize == 0 {
+		t.Fatal("expected non-zero context window size for second conversation")
+	}
+
+	// Now simulate "navigating" back to the first conversation by fetching it via GET
+	// This is what the UI does when switching conversations
+	req := httptest.NewRequest("GET", "/api/conversation/"+firstConvID, nil)
+	w := httptest.NewRecorder()
+	h.server.handleGetConversation(w, req, firstConvID)
+
+	if w.Code != http.StatusOK {
+		t.Fatalf("GET first conversation returned %d: %s", w.Code, w.Body.String())
+	}
+
+	var resp StreamResponse
+	if err := json.NewDecoder(w.Body).Decode(&resp); err != nil {
+		t.Fatalf("Failed to decode response: %v", err)
+	}
+
+	t.Logf("First conversation on navigation: context_window_size=%d, messages=%d",
+		resp.ContextWindowSize, len(resp.Messages))
+
+	if resp.ContextWindowSize != firstConvSize {
+		t.Errorf("context_window_size mismatch on navigation: got %d, want %d",
+			resp.ContextWindowSize, firstConvSize)
+	}
+
+	// Now navigate to second conversation
+	req = httptest.NewRequest("GET", "/api/conversation/"+secondConvID, nil)
+	w = httptest.NewRecorder()
+	h.server.handleGetConversation(w, req, secondConvID)
+
+	if w.Code != http.StatusOK {
+		t.Fatalf("GET second conversation returned %d: %s", w.Code, w.Body.String())
+	}
+
+	if err := json.NewDecoder(w.Body).Decode(&resp); err != nil {
+		t.Fatalf("Failed to decode response: %v", err)
+	}
+
+	t.Logf("Second conversation on navigation: context_window_size=%d, messages=%d",
+		resp.ContextWindowSize, len(resp.Messages))
+
+	if resp.ContextWindowSize != secondConvSize {
+		t.Errorf("context_window_size mismatch on navigation: got %d, want %d",
+			resp.ContextWindowSize, secondConvSize)
+	}
+}
diff --git a/server/testharness_test.go b/server/testharness_test.go
index 2d1ece5409277c8450ce91a60446fb47e2b782fa..99e3306f57776fd23576177274860e97bd8482fe 100644
--- a/server/testharness_test.go
+++ b/server/testharness_test.go
@@ -85,6 +85,7 @@ func (h *TestHarness) NewConversation(msg, cwd string) *TestHarness {
 		h.t.Fatalf("NewConversation: failed to parse response: %v", err)
 	}
 	h.convID = resp.ConversationID
+	h.responsesCount = 0 // Reset for new conversation
 	return h
 }
 
diff --git a/ui/src/components/ChatInterface.tsx b/ui/src/components/ChatInterface.tsx
index bad79ffe8a1a216d31b059e75a6ce27fe755b570..4c38656c6ef6ffa0f2165e218f4e95ba88b82698 100644
--- a/ui/src/components/ChatInterface.tsx
+++ b/ui/src/components/ChatInterface.tsx
@@ -522,9 +522,9 @@ function ChatInterface({
       const response = await api.getConversation(conversationId);
       setMessages(response.messages ?? []);
       setAgentWorking(Boolean(response.agent_working));
-      if (typeof response.context_window_size === "number") {
-        setContextWindowSize(response.context_window_size);
-      }
+      // Always update context window size when loading a conversation.
+      // If omitted from response (due to omitempty when 0), default to 0.
+      setContextWindowSize(response.context_window_size ?? 0);
       if (onConversationUpdate) {
         onConversationUpdate(response.conversation);
       }