diff --git a/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Actors/DeploymentManagerActor.cs b/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Actors/DeploymentManagerActor.cs
index 9b9cf6ff..b2c8d54c 100644
--- a/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Actors/DeploymentManagerActor.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Actors/DeploymentManagerActor.cs
@@ -35,6 +35,10 @@ public class DeploymentManagerActor : ReceiveActor, IWithTimers
 {
     private readonly SiteStorageService _storage;
     private readonly ScriptCompilationService _compilationService;
+    // Pure, off-thread deploy-time compile gate. A site-side compile failure
+    // must reject the deployment (spec: no partial state applied), so every
+    // DeployInstanceCommand is validated before any actor/persistence work.
+    private readonly DeployCompileValidator _deployCompileValidator;
     private readonly SharedScriptLibrary _sharedScriptLibrary;
     private readonly SiteStreamManager? _streamManager;
     private readonly SiteRuntimeOptions _options;
@@ -125,6 +129,7 @@ public class DeploymentManagerActor : ReceiveActor, IWithTimers
     {
         _storage = storage;
         _compilationService = compilationService;
+        _deployCompileValidator = new DeployCompileValidator(compilationService);
         _sharedScriptLibrary = sharedScriptLibrary;
         _streamManager = streamManager;
         _options = options;
@@ -387,6 +392,41 @@ public class DeploymentManagerActor : ReceiveActor, IWithTimers
     /// apply still replies to the right actor.
     /// </param>
     private void HandleDeploy(DeployInstanceCommand command, IActorRef replyTo)
+    {
+        // Site-side compile gate (S3): a compile failure must reject the
+        // deployment with NO partial state applied (no Instance Actor, no
+        // persisted config) — the design spec's contract. Validation runs
+        // synchronously on the actor thread: it is a pure prefix step of the
+        // deploy handler, so the existing redeploy-supersede / delete-during-
+        // redeploy ordering (which depends on strict mailbox FIFO) is preserved
+        // exactly. It is NOT run off-thread — piping the verdict back to self
+        // reorders concurrent deploys relative to each other and to
+        // 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.
+        var errors = _deployCompileValidator.Validate(command.FlattenedConfigurationJson);
+        if (errors.Count > 0)
+        {
+            var instanceName = command.InstanceUniqueName;
+            var joined = string.Join(" | ", errors);
+            LogDeploymentEvent("Error", instanceName,
+                $"Instance {instanceName} deploy rejected — site compile validation failed (deploymentId={command.DeploymentId})",
+                joined);
+            replyTo.Tell(new DeploymentStatusResponse(
+                command.DeploymentId, instanceName, DeploymentStatus.Failed,
+                "Script compilation failed on site: " + joined, DateTimeOffset.UtcNow));
+            return;
+        }
+
+        ProceedWithDeploy(command, replyTo);
+    }
+
+    /// <summary>
+    /// Core deploy application (redeploy-supersede + fresh apply), reached only
+    /// after the synchronous compile gate in <see cref="HandleDeploy"/> passes.
+    /// </summary>
+    private void ProceedWithDeploy(DeployInstanceCommand command, IActorRef replyTo)
     {
         var instanceName = command.InstanceUniqueName;
         _logger.LogInformation(
diff --git a/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Actors/InstanceActor.cs b/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Actors/InstanceActor.cs
index f0fa95cb..0e79f3e2 100644
--- a/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Actors/InstanceActor.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Actors/InstanceActor.cs
@@ -1336,7 +1336,9 @@ public class InstanceActor : ReceiveActor
     /// Creates child Script Actors and Alarm Actors from the flattened configuration.
     /// Script Actors spawned per script definition.
     /// Alarm Actors spawned per alarm definition, as peers to Script Actors.
-    /// Compilation errors reject entire instance deployment (logged but actor still starts).
+    /// Compile failures are rejected at deploy time by the Deployment Manager's
+    /// validation gate; a failure reaching this point (startup load of a legacy
+    /// config) is logged and the offending script/alarm skipped.
     ///
     /// Each child is seeded from a private point-in-time snapshot
     /// of <c>_attributes</c>, NOT the live dictionary. The snapshot is taken here on
diff --git a/tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Actors/DeploymentManagerActorTests.cs b/tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Actors/DeploymentManagerActorTests.cs
index 1c6925a4..1f82cf81 100644
--- a/tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Actors/DeploymentManagerActorTests.cs
+++ b/tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Actors/DeploymentManagerActorTests.cs
@@ -232,6 +232,40 @@ public class DeploymentManagerActorTests : TestKit, IDisposable
         Assert.Equal("NewPump", response.InstanceUniqueName);
     }
 
+    // ── S3: site-side compile failure rejects the deployment (no partial state) ──
+
+    private static string MakeConfigWithBrokenScriptJson(string instanceName) =>
+        JsonSerializer.Serialize(new FlattenedConfiguration
+        {
+            InstanceUniqueName = instanceName,
+            Attributes = [new ResolvedAttribute { CanonicalName = "TestAttr", Value = "42", DataType = "Int32" }],
+            Scripts = [new ResolvedScript { CanonicalName = "S1", Code = "this is not C# ~~~" }]
+        });
+
+    [Fact]
+    public async Task Deploy_WithNonCompilingScript_ReportsFailed_AndCreatesNoInstanceActor()
+    {
+        var actor = CreateDeploymentManager();
+        await Task.Delay(500); // wait for empty startup
+
+        actor.Tell(new DeployInstanceCommand(
+            "dep-bad", "bad-instance", "r1",
+            MakeConfigWithBrokenScriptJson("bad-instance"), "admin", DateTimeOffset.UtcNow));
+
+        var resp = ExpectMsg<DeploymentStatusResponse>(TimeSpan.FromSeconds(10));
+        Assert.Equal(DeploymentStatus.Failed, resp.Status);
+        Assert.NotNull(resp.ErrorMessage);
+        Assert.Contains("compil", resp.ErrorMessage!, StringComparison.OrdinalIgnoreCase);
+
+        // No partial state applied: instance actor must not exist …
+        Sys.ActorSelection(actor.Path / "bad-instance").Tell(new Identify(1));
+        Assert.Null(ExpectMsg<ActorIdentity>().Subject);
+
+        // … and no config must have been persisted.
+        var configs = await _storage.GetAllDeployedConfigsAsync();
+        Assert.DoesNotContain(configs, c => c.InstanceUniqueName == "bad-instance");
+    }
+
     // ── M1.6: site event log `deployment` category ─────────────────────────
 
     [Fact]