diff --git a/main.go b/main.go
index a26b871..de4d563 100644
--- a/main.go
+++ b/main.go
@@ -181,7 +181,8 @@ func initConfig() {
 		retry.WithHTTPClient(baseHTTPClient),
 	)
 	if err != nil {
-		panic(fmt.Sprintf("failed to create retry client: %v", err))
+		fmt.Fprintf(os.Stderr, "Error: failed to create retry client: %v\n", err)
+		os.Exit(1)
 	}
 
 	// Initialize token store based on mode
@@ -387,7 +388,7 @@ func run(ctx context.Context, d tui.Displayer) error {
 	// Demonstrate automatic refresh on 401
 	if err := makeAPICallWithAutoRefresh(ctx, &storage, d); err != nil {
 		// Check if error is due to expired refresh token
-		if err == ErrRefreshTokenExpired {
+		if errors.Is(err, ErrRefreshTokenExpired) {
 			d.ReAuthRequired()
 			storage, err = performDeviceFlow(ctx, d)
 			if err != nil {
@@ -401,7 +402,6 @@ func run(ctx context.Context, d tui.Displayer) error {
 				d.Fatal(err)
 				return err
 			}
-			d.APICallOK()
 		} else {
 			d.APICallFailed(err)
 		}
@@ -477,13 +477,13 @@ func requestDeviceCode(ctx context.Context) (*oauth2.DeviceAuthResponse, error)
 
 // performDeviceFlow performs the OAuth device authorization flow
 func performDeviceFlow(ctx context.Context, d tui.Displayer) (credstore.Token, error) {
+	// Only TokenURL and ClientID are used downstream;
+	// requestDeviceCode() builds its own request directly.
 	config := &oauth2.Config{
 		ClientID: clientID,
 		Endpoint: oauth2.Endpoint{
-			DeviceAuthURL: serverURL + endpointDeviceCode,
-			TokenURL:      serverURL + endpointToken,
+			TokenURL: serverURL + endpointToken,
 		},
-		Scopes: []string{"read", "write"},
 	}
 
 	// Step 1: Request device code (with retry logic)
@@ -527,7 +527,7 @@ func performDeviceFlow(ctx context.Context, d tui.Displayer) (credstore.Token, e
 }
 
 // pollForTokenWithProgress polls for token while reporting progress via Displayer.
-// Implements exponential backoff for slow_down errors per RFC 8628.
+// Implements additive backoff (+5s) for slow_down errors per RFC 8628 §3.5.
 func pollForTokenWithProgress(
 	ctx context.Context,
 	config *oauth2.Config,
@@ -540,9 +540,8 @@ func pollForTokenWithProgress(
 		interval = 5 // Default to 5 seconds per RFC 8628
 	}
 
-	// Exponential backoff state
+	// Backoff state
 	pollInterval := time.Duration(interval) * time.Second
-	backoffMultiplier := 1.0
 
 	pollTicker := time.NewTicker(pollInterval)
 	defer pollTicker.Stop()
@@ -572,12 +571,8 @@ func pollForTokenWithProgress(
 							continue
 
 						case oauthErrSlowDown:
-							// Server requests slower polling - increase interval
-							backoffMultiplier *= 1.5
-							pollInterval = min(
-								time.Duration(float64(pollInterval)*backoffMultiplier),
-								60*time.Second,
-							)
+							// Server requests slower polling - add 5s per RFC 8628 §3.5
+							pollInterval = min(pollInterval+5*time.Second, 60*time.Second)
 							pollTicker.Reset(pollInterval)
 							d.PollSlowDown(pollInterval)
 							continue
@@ -829,16 +824,19 @@ func makeAPICallWithAutoRefresh(
 	if err != nil {
 		return fmt.Errorf("API request failed: %w", err)
 	}
-	defer resp.Body.Close()
 
-	// If 401, try to refresh and retry
+	// If 401, drain and close the first response body to allow connection reuse,
+	// then refresh the token and retry.
 	if resp.StatusCode == http.StatusUnauthorized {
+		_, _ = io.Copy(io.Discard, resp.Body)
+		resp.Body.Close()
+
 		d.AccessTokenRejected()
 
 		newStorage, err := refreshAccessToken(ctx, storage.RefreshToken, d)
 		if err != nil {
 			// If refresh token is expired, propagate the error to trigger device flow
-			if err == ErrRefreshTokenExpired {
+			if errors.Is(err, ErrRefreshTokenExpired) {
 				return ErrRefreshTokenExpired
 			}
 			return fmt.Errorf("refresh failed: %w", err)
@@ -846,9 +844,7 @@ func makeAPICallWithAutoRefresh(
 
 		// Update storage in memory
 		// Note: newStorage has already been saved to disk by refreshAccessToken()
-		storage.AccessToken = newStorage.AccessToken
-		storage.RefreshToken = newStorage.RefreshToken
-		storage.ExpiresAt = newStorage.ExpiresAt
+		*storage = newStorage
 
 		d.TokenRefreshedRetrying()
 
@@ -869,6 +865,8 @@ func makeAPICallWithAutoRefresh(
 			return fmt.Errorf("retry failed: %w", err)
 		}
 		defer resp.Body.Close()
+	} else {
+		defer resp.Body.Close()
 	}
 
 	body, err := io.ReadAll(resp.Body)
diff --git a/main_test.go b/main_test.go
index fe888ae..2221e00 100644
--- a/main_test.go
+++ b/main_test.go
@@ -8,6 +8,7 @@ import (
 	"net/http/httptest"
 	"os"
 	"path/filepath"
+	"strings"
 	"sync"
 	"sync/atomic"
 	"testing"
@@ -271,7 +272,7 @@ func TestValidateTokenResponse(t *testing.T) {
 					t.Errorf("validateTokenResponse() expected error but got nil")
 					return
 				}
-				if tt.errContains != "" && !contains(err.Error(), tt.errContains) {
+				if tt.errContains != "" && !strings.Contains(err.Error(), tt.errContains) {
 					t.Errorf(
 						"validateTokenResponse() error = %v, want error containing %q",
 						err,
@@ -285,21 +286,6 @@ func TestValidateTokenResponse(t *testing.T) {
 	}
 }
 
-// contains checks if string s contains substr
-func contains(s, substr string) bool {
-	return len(s) >= len(substr) && (s == substr || len(substr) == 0 ||
-		(len(s) > 0 && len(substr) > 0 && stringContains(s, substr)))
-}
-
-func stringContains(s, substr string) bool {
-	for i := 0; i <= len(s)-len(substr); i++ {
-		if s[i:i+len(substr)] == substr {
-			return true
-		}
-	}
-	return false
-}
-
 func TestRefreshAccessToken_RotationMode(t *testing.T) {
 	// Save original values
 	origServerURL := serverURL
@@ -526,7 +512,7 @@ func TestRefreshAccessToken_ValidationErrors(t *testing.T) {
 					t.Errorf("refreshAccessToken() expected error but got nil")
 					return
 				}
-				if tt.errContains != "" && !contains(err.Error(), tt.errContains) {
+				if tt.errContains != "" && !strings.Contains(err.Error(), tt.errContains) {
 					t.Errorf(
 						"refreshAccessToken() error = %v, want error containing %q",
 						err,
diff --git a/polling_test.go b/polling_test.go
index 02e9d28..71a12db 100644
--- a/polling_test.go
+++ b/polling_test.go
@@ -77,10 +77,10 @@ func TestPollForToken_SlowDown(t *testing.T) {
 	slowDownCount := atomic.Int32{}
 
 	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-		attempts.Add(1)
+		n := attempts.Add(1)
 
-		// Return slow_down for first 2 attempts
-		if attempts.Load() <= 2 {
+		// Return slow_down on the first attempt
+		if n == 1 {
 			slowDownCount.Add(1)
 			w.Header().Set("Content-Type", "application/json")
 			w.WriteHeader(http.StatusBadRequest)
@@ -91,8 +91,8 @@ func TestPollForToken_SlowDown(t *testing.T) {
 			return
 		}
 
-		// Return authorization_pending after slow_down
-		if attempts.Load() < 5 {
+		// Return authorization_pending on second attempt
+		if n == 2 {
 			w.Header().Set("Content-Type", "application/json")
 			w.WriteHeader(http.StatusBadRequest)
 			_ = json.NewEncoder(w).Encode(map[string]string{
@@ -102,7 +102,7 @@ func TestPollForToken_SlowDown(t *testing.T) {
 			return
 		}
 
-		// Success
+		// Success on third attempt
 		w.Header().Set("Content-Type", "application/json")
 		_ = json.NewEncoder(w).Encode(map[string]any{
 			"access_token":  testAccessToken,
@@ -125,7 +125,9 @@ func TestPollForToken_SlowDown(t *testing.T) {
 		Interval:   1, // 1 second for testing
 	}
 
-	ctx, cancel := context.WithTimeout(context.Background(), 15*time.Second)
+	// After 1 slow_down the interval becomes 1+5=6s; with an additional authorization_pending
+	// before success, the third attempt occurs after ~1s + 6s + 6s ≈ 13s, so use a generous timeout.
+	ctx, cancel := context.WithTimeout(context.Background(), 25*time.Second)
 	defer cancel()
 
 	token, err := pollForTokenWithProgress(ctx, config, deviceAuth, tui.NoopDisplayer{})
@@ -137,14 +139,14 @@ func TestPollForToken_SlowDown(t *testing.T) {
 		t.Errorf("Expected access token 'test-access-token', got '%s'", token.AccessToken)
 	}
 
-	if slowDownCount.Load() < 2 {
-		t.Errorf("Expected at least 2 slow_down responses, got %d", slowDownCount.Load())
+	if slowDownCount.Load() < 1 {
+		t.Errorf("Expected at least 1 slow_down response, got %d", slowDownCount.Load())
 	}
 
 	// Verify that polling continued after slow_down
-	if attempts.Load() < 5 {
+	if attempts.Load() < 3 {
 		t.Errorf(
-			"Expected at least 5 attempts (2 slow_down + 2 pending + 1 success), got %d",
+			"Expected at least 3 attempts (1 slow_down + 1 pending + 1 success), got %d",
 			attempts.Load(),
 		)
 	}