diff --git a/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/SiteRuntimeOptions.cs b/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/SiteRuntimeOptions.cs
index 598a58b8..391a961a 100644
--- a/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/SiteRuntimeOptions.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/SiteRuntimeOptions.cs
@@ -112,13 +112,34 @@ public class SiteRuntimeOptions
     public int ConfigFetchTimeoutSeconds { get; set; } = 30;
 
     /// <summary>
-    /// Fixed interval (ms) at which an Instance Actor re-sends a tag-subscribe
-    /// request that either failed or whose response was lost (S4/UA6). The retry is
-    /// armed on every send and cancelled on the first Success reply, so it closes
-    /// both the failed-response and the lost-response gaps. Default: 5000ms.
+    /// Floor interval (ms) at which an Instance Actor re-sends a tag-subscribe request
+    /// that either failed or whose response was lost (S4/UA6). The retry is armed on
+    /// every send and cancelled on the first Success reply. As of Lever 1 (2026-08-28,
+    /// MxGateway subscribe-storm prevention) it is the FLOOR of an exponential backoff:
+    /// each failed/lost attempt doubles the interval (±20 % jitter) up to
+    /// <see cref="TagSubscribeRetryMaxIntervalMs"/>, and a Success or a connection
+    /// returning to Good quality resets it here. Default: 5000ms.
     /// </summary>
     public int TagSubscribeRetryIntervalMs { get; set; } = 5000;
 
+    /// <summary>
+    /// Ceiling (ms) of the tag-subscribe retry backoff. Must be >=
+    /// <see cref="TagSubscribeRetryIntervalMs"/>; equal makes the interval fixed
+    /// (pre-Lever-1 behaviour). Default: 60000ms.
+    /// </summary>
+    public int TagSubscribeRetryMaxIntervalMs { get; set; } = 60000;
+
+    /// <summary>
+    /// How long (ms) an outstanding tag-subscribe request blocks a new one for the
+    /// same connection. While a request is in flight and younger than this, a retry
+    /// tick only re-arms the watchdog instead of issuing a duplicate subscribe — this
+    /// is what stops a slow gateway from receiving a fresh N-tag subscribe every
+    /// interval on top of the one it is still processing. Once older than this the
+    /// reply is presumed lost and a new request is allowed. Size it above the
+    /// gateway's per-command deadline (MxGateway default 30 s). Default: 45000ms.
+    /// </summary>
+    public int TagSubscribeInFlightCeilingMs { get; set; } = 45000;
+
     /// <summary>
     /// Grace period (ms) after a script's execution timeout elapses before the
     /// stuck-script watchdog declares the script's dedicated thread blocked and
diff --git a/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/SiteRuntimeOptionsValidator.cs b/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/SiteRuntimeOptionsValidator.cs
index 53e81c69..fac982c8 100644
--- a/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/SiteRuntimeOptionsValidator.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/SiteRuntimeOptionsValidator.cs
@@ -79,6 +79,16 @@ public sealed class SiteRuntimeOptionsValidator : OptionsValidatorBase<SiteRunti
             $"(was {options.TagSubscribeRetryIntervalMs}); a zero interval hot-loops the tag-subscribe " +
             "retry and a negative one throws inside the Instance Actor's retry scheduling.");
 
+        builder.RequireThat(options.TagSubscribeRetryMaxIntervalMs >= options.TagSubscribeRetryIntervalMs,
+            $"ScadaBridge:SiteRuntime:TagSubscribeRetryMaxIntervalMs must be >= TagSubscribeRetryIntervalMs " +
+            $"(was {options.TagSubscribeRetryMaxIntervalMs} vs {options.TagSubscribeRetryIntervalMs}); " +
+            "it is the ceiling of the tag-subscribe retry backoff.");
+
+        builder.RequireThat(options.TagSubscribeInFlightCeilingMs > 0,
+            $"ScadaBridge:SiteRuntime:TagSubscribeInFlightCeilingMs must be greater than 0 " +
+            $"(was {options.TagSubscribeInFlightCeilingMs}); it bounds how long an unanswered tag-subscribe " +
+            "blocks a duplicate for the same connection.");
+
         builder.RequireThat(options.StuckScriptGraceMs >= 0,
             $"ScadaBridge:SiteRuntime:StuckScriptGraceMs must be >= 0 " +
             $"(was {options.StuckScriptGraceMs}); a negative grace throws inside the stuck-script watchdog's delay.");
diff --git a/tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/SiteRuntimeOptionsValidatorTests.cs b/tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/SiteRuntimeOptionsValidatorTests.cs
index cb6b0356..e9f130eb 100644
--- a/tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/SiteRuntimeOptionsValidatorTests.cs
+++ b/tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/SiteRuntimeOptionsValidatorTests.cs
@@ -144,4 +144,38 @@ public class SiteRuntimeOptionsValidatorTests
         Assert.True(result.Failed);
         Assert.Contains("MaxConcurrentRunsPerScript", result.FailureMessage);
     }
+    // --- Lever 1 (2026-08-28): tag-subscribe backoff + in-flight ceiling ---
+
+    [Fact]
+    public void TagSubscribeRetryMaxIntervalMs_BelowFloor_IsRejected()
+    {
+        var result = Validate(new SiteRuntimeOptions
+        {
+            TagSubscribeRetryIntervalMs = 5000,
+            TagSubscribeRetryMaxIntervalMs = 4999,
+        });
+
+        Assert.True(result.Failed);
+        Assert.Contains("TagSubscribeRetryMaxIntervalMs", result.FailureMessage);
+    }
+
+    [Fact]
+    public void TagSubscribeRetryMaxIntervalMs_EqualToFloor_IsValid()
+    {
+        var result = Validate(new SiteRuntimeOptions
+        {
+            TagSubscribeRetryIntervalMs = 5000,
+            TagSubscribeRetryMaxIntervalMs = 5000,
+        });
+        Assert.True(result.Succeeded, result.FailureMessage);
+    }
+
+    [Fact]
+    public void ZeroTagSubscribeInFlightCeilingMs_IsRejected()
+    {
+        var result = Validate(new SiteRuntimeOptions { TagSubscribeInFlightCeilingMs = 0 });
+
+        Assert.True(result.Failed);
+        Assert.Contains("TagSubscribeInFlightCeilingMs", result.FailureMessage);
+    }
 }