diff --git a/internal/tui/model_test.go b/internal/tui/model_test.go
new file mode 100644
index 0000000..cf7e218
--- /dev/null
+++ b/internal/tui/model_test.go
@@ -0,0 +1,76 @@
+package tui
+
+import (
+	"testing"
+	"time"
+)
+
+func TestEventWriter(t *testing.T) {
+	ch := make(chan Event, 10)
+	w := NewEventWriter(ch)
+
+	w.Write([]byte("hello"))
+	w.Write([]byte(" world\n"))
+
+	select {
+	case ev := <-ch:
+		if ev.Type != "stream" {
+			t.Errorf("expected stream event, got %s", ev.Type)
+		}
+		if ev.Content != "hello world\n" {
+			t.Errorf("expected 'hello world\\n', got %s", ev.Content)
+		}
+	case <-time.After(time.Second):
+		t.Error("timeout waiting for event")
+	}
+}
+
+func TestModelHandleEvent(t *testing.T) {
+	m := Model{
+		messages:  []ChatMessage{},
+		events:    make(chan Event, 10),
+		agentRuns: make(map[string]bool),
+	}
+
+	m.handleEvent(Event{Type: "stream", Content: "Hello"})
+	if len(m.messages) != 1 {
+		t.Fatalf("expected 1 message, got %d", len(m.messages))
+	}
+	if m.messages[0].Content != "Hello" {
+		t.Errorf("expected 'Hello', got %s", m.messages[0].Content)
+	}
+
+	m.handleEvent(Event{Type: "stream", Content: " World"})
+	if m.messages[0].Content != "Hello World" {
+		t.Errorf("expected 'Hello World', got %s", m.messages[0].Content)
+	}
+
+	m.handleEvent(Event{Type: "agent_start", Agent: "calculator"})
+	if !m.agentRuns["calculator"] {
+		t.Error("expected calculator to be running")
+	}
+
+	m.handleEvent(Event{Type: "agent_end", Agent: "calculator"})
+	if m.agentRuns["calculator"] {
+		t.Error("expected calculator to not be running")
+	}
+}
+
+func TestModelFormatMessage(t *testing.T) {
+	m := Model{}
+
+	user := m.formatMessage(ChatMessage{Role: "user", Content: "hi"})
+	if user == "" {
+		t.Error("user message should not be empty")
+	}
+
+	assistant := m.formatMessage(ChatMessage{Role: "assistant", Content: "hello"})
+	if assistant == "" {
+		t.Error("assistant message should not be empty")
+	}
+
+	system := m.formatMessage(ChatMessage{Role: "system", Content: "error"})
+	if system == "" {
+		t.Error("system message should not be empty")
+	}
+}