diff --git a/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Actors/DeploymentManagerActor.cs b/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Actors/DeploymentManagerActor.cs
index d5aa014c..e360dffc 100644
--- a/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Actors/DeploymentManagerActor.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Actors/DeploymentManagerActor.cs
@@ -518,7 +518,11 @@ public class DeploymentManagerActor : ReceiveActor, IWithTimers
         // delete/disable commands, breaking that ordering. A deploy is an
         // infrequent admin command, so briefly holding the singleton for a pure
         // Roslyn compile is acceptable; the central deployer already Asks and
-        // waits for the DeploymentStatusResponse.
+        // waits for the DeploymentStatusResponse. Redeploys and multi-instance
+        // deploys of unchanged scripts hit the process-wide compile cache
+        // (SiteScriptCompileCache), so the synchronous gate recompiles only
+        // genuinely new code and the Instance Actor's PreStart reuses the gate's
+        // compile (N4).
         var errors = _deployCompileValidator.Validate(command.FlattenedConfigurationJson);
         if (errors.Count > 0)
         {
diff --git a/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Scripts/ScriptCompilationService.cs b/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Scripts/ScriptCompilationService.cs
index d08692d9..485323f3 100644
--- a/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Scripts/ScriptCompilationService.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Scripts/ScriptCompilationService.cs
@@ -13,6 +13,13 @@ namespace ZB.MOM.WW.ScadaBridge.SiteRuntime.Scripts;
 /// <see cref="ScriptTrustValidator"/>; this service keeps the real
 /// execution-path compile of the script against <see cref="ScriptGlobals"/> /
 /// <see cref="TriggerExpressionGlobals"/>.
+///
+/// Compile results — success AND failure — are memoised process-wide in
+/// <see cref="SiteScriptCompileCache"/> (keyed on code hash + globals type), so an
+/// unchanged script body compiles once per process lifetime and the deploy gate,
+/// the Instance Actor's PreStart recompile, and SharedScriptLibrary all reuse one
+/// compiled <c>Script&lt;object?&gt;</c>. Error text is name-free by construction, so a
+/// cached failure renders correctly for every caller.
 /// </summary>
 public class ScriptCompilationService
 {
@@ -92,10 +99,33 @@ public class ScriptCompilationService
         => CompileCore(name, expression, typeof(TriggerExpressionGlobals));
 
     /// <summary>
-    /// Shared compilation path: validates the trust model, builds the script
-    /// against the given globals type, and returns the compiled result.
+    /// Shared compilation funnel for every site-side compile (the deploy gate, the
+    /// Instance Actor's PreStart recompile, and SharedScriptLibrary). Results — success
+    /// AND failure — are memoised process-wide in <see cref="SiteScriptCompileCache"/>,
+    /// keyed on code hash + globals type: an unchanged script body compiles once per
+    /// process lifetime and the compiled <c>Script&lt;object?&gt;</c> is shared across all
+    /// call sites (N4). Cached error text is name-free by construction (Roslyn diagnostics /
+    /// the compile-exception message never embed the caller's script name), so a cached
+    /// failure renders correctly for every caller.
     /// </summary>
     private ScriptCompilationResult CompileCore(string name, string code, Type globalsType)
+    {
+        var result = SiteScriptCompileCache.GetOrAdd(code, globalsType,
+            () => CompileUncached(name, code, globalsType));
+        if (!result.IsSuccess)
+            _logger.LogDebug(
+                "Script {Script}: returning cached compile failure ({ErrorCount} error(s))",
+                name, result.Errors.Count); // miss-path logs name the FIRST caller; keep this caller visible
+        return result;
+    }
+
+    /// <summary>
+    /// The uncached compile: validates the trust model, builds the script against the
+    /// given globals type, and returns the compiled result. The verdict is deterministic
+    /// on the code (plus the compile-time-static trust policy and globals surface), so it
+    /// is safe to cache alongside the compiled script.
+    /// </summary>
+    private ScriptCompilationResult CompileUncached(string name, string code, Type globalsType)
     {
         // Validate trust model
         var violations = ValidateTrustModel(code);
diff --git a/tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Scripts/ScriptCompilationServiceTests.cs b/tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Scripts/ScriptCompilationServiceTests.cs
index f02aca5d..d654ff23 100644
--- a/tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Scripts/ScriptCompilationServiceTests.cs
+++ b/tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Scripts/ScriptCompilationServiceTests.cs
@@ -16,6 +16,7 @@ namespace ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests.Scripts;
 /// <c>Stopwatch</c> stays allowed. The real execution-path compile against
 /// <c>ScriptGlobals</c> / <c>TriggerExpressionGlobals</c> is unchanged.
 /// </summary>
+[Collection("SiteScriptCompileCache")]
 public class ScriptCompilationServiceTests
 {
     private readonly ScriptCompilationService _service;
@@ -34,6 +35,28 @@ public class ScriptCompilationServiceTests
         Assert.Empty(result.Errors);
     }
 
+    [Fact]
+    public void Compile_SameCodeTwice_SharesOneRoslynCompile()
+    {
+        SiteScriptCompileCache.Clear();
+        var r1 = _service.Compile("deploy-gate-copy", "return 1 + 1;");
+        var r2 = _service.Compile("prestart-copy", "return 1 + 1;");
+
+        Assert.True(r1.IsSuccess);
+        Assert.Same(r1.CompiledScript, r2.CompiledScript);   // one compile, shared Script<T> (N4)
+        Assert.Equal(1, SiteScriptCompileCache.Hits);
+    }
+
+    [Fact]
+    public void Compile_ScriptAndTriggerExpression_DoNotCrossContaminate()
+    {
+        SiteScriptCompileCache.Clear();
+        var script = _service.Compile("s", "1 > 0");
+        var trigger = _service.CompileTriggerExpression("t", "1 > 0");
+
+        Assert.NotSame(script.CompiledScript, trigger.CompiledScript); // different globals surfaces
+    }
+
     [Fact]
     public void Compile_InvalidSyntax_ReturnsErrors()
     {