diff --git a/docs/release-control/v6/internal/subsystems/notifications.md b/docs/release-control/v6/internal/subsystems/notifications.md
index 290aa0369..a8ac05bad 100644
--- a/docs/release-control/v6/internal/subsystems/notifications.md
+++ b/docs/release-control/v6/internal/subsystems/notifications.md
@@ -139,6 +139,21 @@ negative overflow cases alongside ordinary seconds, dates, whitespace and
 invalid inputs. This is pure parser proof: it does not establish elapsed HTTP
 retry timing, queue persistence, provider acceptance or recipient receipt.
 
+For HTTP 429 transport retries, a valid `Retry-After` overrides only the wait
+following that response. It must not replace the independent exponential
+backoff schedule. In particular, a zero-delay response cannot cause later
+headerless 429 or 503 failures to exhaust their remaining retries immediately.
+The schedule continues doubling up to `WebhookMaxBackoff`; retry budgets,
+classification, parsing and the existing header-delay cap remain unchanged.
+This does not extend header handling to 503 responses.
+
+`TestSendWebhookWithRetry_ZeroRetryAfterPreservesLaterBackoff` in
+`internal/notifications/webhook_enhanced_test.go` sends a zero-delay 429,
+then a headerless 429 or 503, then success through a local HTTP fixture. It
+verifies three requests and the two-second exponential wait before the final
+request. This is transport timing proof, not durable queue retry, installed
+provider acceptance or recipient receipt.
+
 Notification-management HTTP production and its unit/contract proof now live
 together under `internal/api/alerting/`. Router-level scope and integration
 tests remain in `internal/api`, while the compatibility aliases there keep the
diff --git a/internal/notifications/webhook_enhanced.go b/internal/notifications/webhook_enhanced.go
index 48d346eab..0917468ed 100644
--- a/internal/notifications/webhook_enhanced.go
+++ b/internal/notifications/webhook_enhanced.go
@@ -330,7 +330,8 @@ func (n *NotificationManager) sendWebhookWithRetry(webhook EnhancedWebhookConfig
 							Dur("retryAfter", customBackoff).
 							Msg("using Retry-After header for backoff")
 						time.Sleep(customBackoff)
-						backoff = customBackoff // Use this for next iteration
+						// The header overrides this wait only. Preserve the exponential
+						// schedule so a zero delay cannot erase later failure backoff.
 						usedBackoff = true
 					} else {
 						log.Debug().
diff --git a/internal/notifications/webhook_enhanced_test.go b/internal/notifications/webhook_enhanced_test.go
index d47f4f482..68a2ce0c9 100644
--- a/internal/notifications/webhook_enhanced_test.go
+++ b/internal/notifications/webhook_enhanced_test.go
@@ -844,3 +844,41 @@ func TestIsRetryableWebhookErrorEnhanced(t *testing.T) {
 		})
 	}
 }
+
+// A response-specific zero delay must not erase backoff for later failures.
+func TestSendWebhookWithRetry_ZeroRetryAfterPreservesLaterBackoff(t *testing.T) {
+	for _, status := range []int{http.StatusServiceUnavailable, http.StatusTooManyRequests} {
+		t.Run(fmt.Sprint(status), func(t *testing.T) {
+			nm := NewNotificationManager("http://pulse.local")
+			t.Cleanup(nm.Stop)
+			require.NoError(t, nm.UpdateAllowedPrivateCIDRs("127.0.0.1"))
+			attemptTimes := make(chan time.Time, 3)
+			attempts := 0
+			server := newIPv4HTTPServer(t, http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				attempts++
+				attemptTimes <- time.Now()
+				switch attempts {
+				case 1:
+					w.Header().Set("Retry-After", "0")
+					w.WriteHeader(http.StatusTooManyRequests)
+				case 2:
+					w.WriteHeader(status)
+				default:
+					w.WriteHeader(http.StatusOK)
+				}
+			}))
+			defer server.Close()
+			webhook := EnhancedWebhookConfig{
+				WebhookConfig: WebhookConfig{Name: "Backoff regression", URL: server.URL},
+				RetryEnabled:  true,
+				RetryCount:    2,
+			}
+			require.NoError(t, nm.sendWebhookWithRetry(webhook, []byte("{}"), "backoff:alert"))
+			require.Len(t, attemptTimes, 3)
+			<-attemptTimes
+			second, third := <-attemptTimes, <-attemptTimes
+			assert.GreaterOrEqual(t, third.Sub(second), 2*WebhookInitialBackoff,
+				"later failure must retain exponential backoff after a zero Retry-After")
+		})
+	}
+}