diff --git a/chat/chat.go b/chat/chat.go
index be7daf1..f051604 100644
--- a/chat/chat.go
+++ b/chat/chat.go
@@ -3,6 +3,8 @@ package chat
 import (
 	"context"
 	"strings"
+
+	"github.com/x2d7/interlude/chat/tools"
 )
 
 func (c *Chat) Complete(ctx context.Context, client Client) <-chan StreamEvent {
@@ -69,7 +71,20 @@ func (s *sessionState) flushLastToolCall() bool {
 	return ok
 }
 
+func (c *Chat) ensureDefaults() {
+	if c.Messages == nil {
+		c.Messages = NewMessages()
+	}
+	if c.Tools == nil {
+		t := tools.NewTools()
+		c.Tools = &t
+	}
+}
+
 func (c *Chat) Session(ctx context.Context, client Client) <-chan StreamEvent {
+	// ensuring default values
+	c.ensureDefaults()
+
 	// insert chat context into client input configuration
 	client = client.SyncInput(c)
 
diff --git a/chat/chat_test.go b/chat/chat_test.go
index f569de8..4e3a04d 100644
--- a/chat/chat_test.go
+++ b/chat/chat_test.go
@@ -1416,3 +1416,112 @@ func TestSession_MixedTokensAndToolCalls(t *testing.T) {
 		t.Errorf("Expected 2 tool calls in history, got %d", toolCallsInHistory)
 	}
 }
+
+// ==================== ensureDefaults Tests ====================
+
+// TestEnsureDefaults_MessagesNil tests that Messages is initialized when nil
+func TestEnsureDefaults_MessagesNil(t *testing.T) {
+	tools := tools.NewTools()
+	chat := &Chat{
+		Messages: nil,
+		Tools:    &tools,
+	}
+
+	chat.ensureDefaults()
+
+	if chat.Messages == nil {
+		t.Error("Messages should not be nil after ensureDefaults")
+	}
+
+	// Verify Messages is usable
+	messages := chat.Messages.Snapshot()
+	if len(messages) != 0 {
+		t.Errorf("Expected empty messages, got %d", len(messages))
+	}
+}
+
+// TestEnsureDefaults_ToolsNil tests that Tools is initialized when nil
+func TestEnsureDefaults_ToolsNil(t *testing.T) {
+	chat := &Chat{
+		Messages: NewMessages(),
+		Tools:    nil,
+	}
+
+	chat.ensureDefaults()
+
+	if chat.Tools == nil {
+		t.Error("Tools should not be nil after ensureDefaults")
+	}
+
+	// Verify Tools is usable (empty slice, not nil)
+	toolList := chat.Tools.Snapshot()
+	if len(toolList) != 0 {
+		t.Errorf("Expected empty tools, got %d", len(toolList))
+	}
+}
+
+// TestEnsureDefaults_BothNil tests that both Messages and Tools are initialized when both are nil
+func TestEnsureDefaults_BothNil(t *testing.T) {
+	chat := &Chat{
+		Messages: nil,
+		Tools:    nil,
+	}
+
+	chat.ensureDefaults()
+
+	if chat.Messages == nil {
+		t.Error("Messages should not be nil after ensureDefaults")
+	}
+
+	if chat.Tools == nil {
+		t.Error("Tools should not be nil after ensureDefaults")
+	}
+}
+
+// TestEnsureDefaults_AlreadySet tests that existing values are not overwritten
+func TestEnsureDefaults_AlreadySet(t *testing.T) {
+	// Setup chat with existing messages and tools
+	ts := tools.NewTools()
+	chat := &Chat{
+		Messages: NewMessages(),
+		Tools:    &ts, // Use NewTools() to properly initialize
+	}
+
+	// Add some data
+	chat.AddMessage(SenderUser{}, "test message")
+	tool, err := tools.NewTool("test-tool", "Test tool",
+		func(input map[string]string) (string, error) {
+			return "result", nil
+		})
+	if err != nil {
+		t.Fatalf("Failed to create tool: %v", err)
+	}
+	chat.Tools.Add(tool)
+
+	// Get pointers to verify they're not replaced
+	messagesBefore := chat.Messages
+	toolsBefore := chat.Tools
+
+	// Call ensureDefaults
+	chat.ensureDefaults()
+
+	// Verify same instances are preserved
+	if chat.Messages != messagesBefore {
+		t.Error("Messages should not be replaced when already set")
+	}
+
+	if chat.Tools != toolsBefore {
+		t.Error("Tools should not be replaced when already set")
+	}
+
+	// Verify data is still intact
+	messages := chat.Messages.Snapshot()
+	if len(messages) != 1 {
+		t.Errorf("Expected 1 message, got %d", len(messages))
+	}
+
+	toolList := chat.Tools.Snapshot()
+	if len(toolList) != 1 {
+		t.Errorf("Expected 1 tool, got %d", len(toolList))
+	}
+}