diff --git a/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Actors/ScriptActor.cs b/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Actors/ScriptActor.cs
index 5405bc19..32e6d0c1 100644
--- a/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Actors/ScriptActor.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Actors/ScriptActor.cs
@@ -59,6 +59,12 @@ public class ScriptActor : ReceiveActor, IWithTimers
     private readonly Script<object?>? _compiledTriggerExpression;
     private bool _lastExpressionResult;
     private readonly Dictionary<string, object?> _attributeSnapshot = new();
+    // Coalescing guard for off-dispatcher expression evaluation (P1): at most one
+    // evaluation runs on the script scheduler at a time; a change arriving while
+    // one is in flight sets _evalPending so exactly one more runs afterwards
+    // against the latest snapshot. Both flags are touched only on the actor thread.
+    private bool _evalInFlight;
+    private bool _evalPending;
 
     // WhileTrue trigger state: the most recent truth value of a Conditional
     // trigger's comparison, used to detect false->true / true->false edges.
@@ -150,6 +156,9 @@ public class ScriptActor : ReceiveActor, IWithTimers
 
         // Handle execution completion (for logging/metrics)
         Receive<ScriptExecutionCompleted>(HandleExecutionCompleted);
+
+        // Handle the off-dispatcher trigger-expression evaluation result (P1).
+        Receive<ExpressionEvalResult>(HandleExpressionEvalResult);
     }
 
     /// <inheritdoc />
@@ -254,56 +263,74 @@ public class ScriptActor : ReceiveActor, IWithTimers
         }
         else if (_triggerConfig is ExpressionTriggerConfig)
         {
-            EvaluateExpressionTrigger();
+            StartExpressionEvaluation();
         }
     }
 
     /// <summary>
-    /// Evaluates the compiled trigger expression against the current attribute
-    /// snapshot. In <see cref="TriggerMode.OnTrue"/> mode the script runs once
-    /// per false→true transition; in <see cref="TriggerMode.WhileTrue"/> mode it
-    /// fires on the edge and the re-fire timer is started/stopped with the
-    /// expression's truth value. A throwing or non-bool expression is treated as
-    /// false and logged as a script error; the actor never crashes.
+    /// Starts an off-dispatcher evaluation of the compiled trigger expression (P1).
+    /// The expression previously ran synchronously on the actor's dispatcher thread
+    /// via <c>RunAsync(...).GetAwaiter().GetResult()</c>, blocking the dispatcher on
+    /// every attribute change. It now runs on the dedicated script-execution
+    /// scheduler against a point-in-time snapshot; the boolean result is piped back
+    /// to this actor as an <see cref="ExpressionEvalResult"/> so all edge state is
+    /// applied on the actor thread. Bursts coalesce: at most one evaluation is in
+    /// flight, at most one pending, so a storm of changes collapses to the latest
+    /// snapshot without unbounded task fan-out.
     /// </summary>
-    private void EvaluateExpressionTrigger()
+    private void StartExpressionEvaluation()
     {
-        if (_compiledTriggerExpression == null) return;
-        if (_triggerConfig is not ExpressionTriggerConfig exprConfig) return;
+        if (_compiledTriggerExpression == null || _triggerConfig is not ExpressionTriggerConfig) return;
+        if (_evalInFlight) { _evalPending = true; return; } // coalesce bursts: one in flight, one pending
+        _evalInFlight = true;
 
-        bool result;
-        try
+        var snapshot = new Dictionary<string, object?>(_attributeSnapshot); // point-in-time copy, actor thread
+        var expression = _compiledTriggerExpression;
+        var self = Self;
+        Task.Factory.StartNew(async () =>
         {
-            var globals = new TriggerExpressionGlobals(_attributeSnapshot);
-            // Bound evaluation with a short timeout. The CancellationToken
-            // covers cooperative/async cases; a pathological CPU-bound
-            // expression is not fully interruptible. Acceptable because
-            // trigger expressions are authored by trusted Design-role users
-            // and are compile-checked pre-deployment.
-            using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(2));
-            var state = _compiledTriggerExpression
-                .RunAsync(globals, cancellationToken: cts.Token)
-                .GetAwaiter().GetResult();
-            result = state.ReturnValue is bool b && b;
-        }
-        catch (Exception ex)
-        {
-            // OperationCanceledException (timeout) falls through here too,
-            // and is correctly treated as false.
-            LogExpressionError(ex);
-            result = false;
-        }
+            try
+            {
+                // Bound evaluation with a short timeout. The CancellationToken covers
+                // cooperative/async cases; a pathological CPU-bound expression is not
+                // fully interruptible — acceptable because trigger expressions are
+                // authored by trusted Design-role users and compile-checked pre-deploy.
+                using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(2));
+                var state = await expression.RunAsync(new TriggerExpressionGlobals(snapshot), cancellationToken: cts.Token);
+                return state.ReturnValue is bool b && b;
+            }
+            catch (Exception ex)
+            {
+                // OperationCanceledException (timeout) falls through here too and is
+                // treated as false. LogExpressionError touches only thread-safe
+                // members (_healthCollector Interlocked, _logger, DI-resolved
+                // singleton logger) so it is safe off the actor thread.
+                LogExpressionError(ex);
+                return false;
+            }
+        }, CancellationToken.None, TaskCreationOptions.DenyChildAttach,
+            ScriptExecutionScheduler.Shared(_options)).Unwrap().PipeTo(self,
+            success: r => new ExpressionEvalResult(r));
+    }
 
-        if (exprConfig.Mode == TriggerMode.WhileTrue)
+    /// <summary>
+    /// Applies an off-dispatcher trigger-expression result on the actor thread:
+    /// runs the OnTrue / WhileTrue edge logic against <see cref="_lastExpressionResult"/>,
+    /// clears the in-flight flag, and drains a coalesced pending evaluation (against
+    /// the now-current snapshot) if one was requested while this one ran.
+    /// </summary>
+    private void HandleExpressionEvalResult(ExpressionEvalResult msg)
+    {
+        _evalInFlight = false;
+        if (_triggerConfig is ExpressionTriggerConfig exprConfig)
         {
-            HandleWhileTrueTransition(result, _lastExpressionResult);
+            if (exprConfig.Mode == TriggerMode.WhileTrue)
+                HandleWhileTrueTransition(msg.Result, _lastExpressionResult);
+            else if (msg.Result && !_lastExpressionResult)
+                TrySpawnExecution(null);
+            _lastExpressionResult = msg.Result;
         }
-        else if (result && !_lastExpressionResult)
-        {
-            TrySpawnExecution(null);
-        }
-
-        _lastExpressionResult = result;
+        if (_evalPending) { _evalPending = false; StartExpressionEvaluation(); }
     }
 
     /// <summary>
@@ -576,6 +603,13 @@ public class ScriptActor : ReceiveActor, IWithTimers
     }
 
     internal record ScriptExecutionCompleted(string ScriptName, bool Success, string? Error);
+
+    /// <summary>
+    /// Piped back to self from an off-dispatcher trigger-expression evaluation (P1);
+    /// carries the boolean truth value so the OnTrue/WhileTrue edge logic runs on
+    /// the actor thread.
+    /// </summary>
+    private sealed record ExpressionEvalResult(bool Result);
 }
 
 // ── Trigger config types ──
diff --git a/tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Actors/ScriptActorTests.cs b/tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Actors/ScriptActorTests.cs
index b82bc95c..7be99989 100644
--- a/tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Actors/ScriptActorTests.cs
+++ b/tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Actors/ScriptActorTests.cs
@@ -303,6 +303,43 @@ public class ScriptActorTests : TestKit, IDisposable
     private Script<object?> CompileTriggerExpression(string expression) =>
         _compilationService.CompileTriggerExpression("trigger-expr", expression).CompiledScript!;
 
+    /// <summary>
+    /// Compiles a trigger expression WITHOUT the trust validator so a test can use
+    /// <c>System.Threading.Thread</c> (forbidden in real scripts) to observe which
+    /// thread the evaluation runs on. Mirrors the real compile against
+    /// <see cref="TriggerExpressionGlobals"/>.
+    /// </summary>
+    private static Script<object?> CompileRawTriggerExpression(string expression)
+    {
+        var opts = ScriptOptions.Default
+            .WithReferences(typeof(object).Assembly, typeof(Enumerable).Assembly)
+            .WithImports("System", "System.Collections.Generic", "System.Linq", "System.Threading.Tasks");
+        var s = CSharpScript.Create<object?>(expression, opts, typeof(TriggerExpressionGlobals));
+        s.Compile();
+        return s;
+    }
+
+    [Fact]
+    public void ExpressionTrigger_EvaluatesOnScriptSchedulerThread_AndStillFires()
+    {
+        // The expression is TRUE only when evaluated on a script-execution thread.
+        // Before P1 it ran synchronously on the actor's dispatcher thread (name is
+        // NOT "script-execution-*") → false → no fire. After P1 it runs on the
+        // script scheduler → true → fire.
+        var expr = CompileRawTriggerExpression(
+            "System.Threading.Thread.CurrentThread.Name != null && " +
+            "System.Threading.Thread.CurrentThread.Name.StartsWith(\"script-execution-\")");
+        var (actor, instance) = CreateTriggeredActor(
+            "ExprThread",
+            "Expression",
+            "{\"expression\":\"true\",\"mode\":\"OnTrue\"}",
+            minTimeBetweenRuns: null,
+            expr);
+
+        actor.Tell(Change("Any", "1"));
+        instance.ExpectMsg<SetStaticAttributeCommand>(TimeSpan.FromSeconds(10)); // fired ⇒ evaluated off-dispatcher
+    }
+
     [Fact]
     public void ScriptActor_ConditionalWhileTrue_FiresOnEdgeThenReFiresWhileConditionHolds()
     {