High Priority (Fix These First):

Fix MFA Response Handling:

gofunc (a *GarthAuthenticator) authenticate(/* params */) (*Token, error) {
    // ... existing code ...
    
    if resp.StatusCode == http.StatusPreconditionFailed {
        body, err := io.ReadAll(resp.Body)
        if err != nil {
            return nil, fmt.Errorf("failed to read MFA challenge: %w", err)
        }
        return a.handleMFA(ctx, username, password, mfaToken, string(body))
    }
    
    // ... rest of method
}

Create Complete Client Factory:

go// Add to garth.go
func NewGarminClient(ctx context.Context, username, password, mfaToken string) (*GarminClient, error) {
    storage := NewMemoryStorage()
    auth := NewAuthenticator(ClientOptions{
        Storage:  storage,
        TokenURL: "https://connectapi.garmin.com/oauth-service/oauth/token",
        Timeout:  30 * time.Second,
    })
    
    token, err := auth.Login(ctx, username, password, mfaToken)
    if err != nil {
        return nil, err
    }
    
    transport := NewAuthTransport(auth.(*GarthAuthenticator), storage, nil)
    httpClient := &http.Client{Transport: transport}
    apiClient := NewAPIClient("https://connectapi.garmin.com", httpClient)
    
    return &GarminClient{
        Activities: NewActivityService(apiClient),
        Profile:    NewProfileService(apiClient),
        Workouts:   NewWorkoutService(apiClient),
    }, nil
}

Add Integration Test:

go//go:build integration
// +build integration

func TestRealGarminFlow(t *testing.T) {
    client, err := NewGarminClient(ctx, username, password, "")
    require.NoError(t, err)
    
    // Test actual API calls
    profile, err := client.Profile.Get(ctx)
    require.NoError(t, err)
    require.NotEmpty(t, profile.UserID)
    
    activities, err := client.Activities.List(ctx, ActivityListOptions{Limit: 5})
    require.NoError(t, err)
}