feat(host): F9b RoslynScriptedAlarmEvaluator + #107 close engine DI

RoslynScriptedAlarmEvaluator mirrors F8b's pattern for alarm predicates:
caches a compiled ScriptEvaluator<AlarmPredicateContext, bool> per unique
predicate, runs against the dependency dictionary with a 2s timeout, and
turns every failure (compile error, sandbox violation, runtime throw,
ctx.SetVirtualTag attempt — predicates must be pure) into a
ScriptedAlarmEvalResult.Failure. ScriptedAlarmActor preserves prior state
on Failure so a broken predicate can't flip Active/Inactive spuriously.

Program.cs binds both evaluators on driver-role hosts — this fully
satisfies #107 ("bind production VirtualTagEngine + ScriptedAlarmEngine
adapters"). The two Roslyn adapters together replace the F8 + F9 Null
defaults, so VirtualTagActor + ScriptedAlarmActor now run real user
scripts in production.

7 new adapter tests cover: predicate true → Active, predicate false →
Inactive, cache reuse, compile-error denial, write-attempt denial,
empty-predicate denial, post-dispose denial. Host.IntegrationTests now
17/17 green.

Closes #80 + #107. All major v2 follow-ups are now complete; only
cleanup + observability polish remains.

src/Server/ZB.MOM.WW.OtOpcUa.Host/Engines/RoslynScriptedAlarmEvaluator.cs

@@ -0,0 +1,107 @@
+using System.Collections.Concurrent;
+using Microsoft.Extensions.Logging;
+using ZB.MOM.WW.OtOpcUa.Commons.Engines;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+using ZB.MOM.WW.OtOpcUa.Core.ScriptedAlarms;
+using ZB.MOM.WW.OtOpcUa.Core.Scripting;
+using SerilogLogger = Serilog.ILogger;
+using SerilogLog = Serilog.Log;
+
+namespace ZB.MOM.WW.OtOpcUa.Host.Engines;
+
+/// <summary>
+/// F9b — production <see cref="IScriptedAlarmEvaluator"/> binding. Compiles each unique
+/// predicate once via <see cref="ScriptEvaluator{TContext, TResult}"/> against
+/// <see cref="AlarmPredicateContext"/> and caches the resulting evaluator. Predicates are
+/// pure functions returning <c>bool</c>: <see cref="AlarmPredicateContext.SetVirtualTag"/>
+/// throws so a misbehaving script can't smuggle a side effect into alarm evaluation.
+///
+/// Failure modes (compile error, sandbox violation, runtime exception, timeout) all surface
+/// as <see cref="ScriptedAlarmEvalResult.Failure"/>; <see cref="ScriptedAlarmActor"/>
+/// preserves the prior state on failure (does not flip Active/Inactive).
+/// </summary>
+public sealed class RoslynScriptedAlarmEvaluator : IScriptedAlarmEvaluator, IDisposable
+{
+    private static readonly SerilogLogger ScriptLogger = SerilogLog.ForContext<RoslynScriptedAlarmEvaluator>();
+
+    private readonly ConcurrentDictionary<string, ScriptEvaluator<AlarmPredicateContext, bool>> _cache
+        = new(StringComparer.Ordinal);
+    private readonly ILogger<RoslynScriptedAlarmEvaluator> _logger;
+    private readonly TimeSpan _runTimeout;
+    private bool _disposed;
+
+    public RoslynScriptedAlarmEvaluator(ILogger<RoslynScriptedAlarmEvaluator> logger, TimeSpan? runTimeout = null)
+    {
+        _logger = logger;
+        _runTimeout = runTimeout ?? TimeSpan.FromSeconds(2);
+    }
+
+    public ScriptedAlarmEvalResult Evaluate(string alarmId, string predicate, IReadOnlyDictionary<string, object?> dependencies)
+    {
+        if (_disposed) return ScriptedAlarmEvalResult.Failure("evaluator disposed");
+        if (string.IsNullOrWhiteSpace(predicate)) return ScriptedAlarmEvalResult.Failure("empty predicate");
+
+        ScriptEvaluator<AlarmPredicateContext, bool> evaluator;
+        try
+        {
+            evaluator = _cache.GetOrAdd(predicate, ScriptEvaluator<AlarmPredicateContext, bool>.Compile);
+        }
+        catch (CompilationErrorException ex)
+        {
+            _logger.LogWarning(ex, "Alarm {Id}: predicate compile failed", alarmId);
+            return ScriptedAlarmEvalResult.Failure($"compile error: {ex.Message}");
+        }
+        catch (ScriptSandboxViolationException ex)
+        {
+            _logger.LogWarning(ex, "Alarm {Id}: predicate sandbox violation", alarmId);
+            return ScriptedAlarmEvalResult.Failure($"sandbox violation: {ex.Message}");
+        }
+        catch (Exception ex)
+        {
+            _logger.LogWarning(ex, "Alarm {Id}: predicate compile threw", alarmId);
+            return ScriptedAlarmEvalResult.Failure($"compile failure: {ex.Message}");
+        }
+
+        var readCache = BuildReadCache(dependencies);
+        var context = new AlarmPredicateContext(readCache, ScriptLogger);
+
+        try
+        {
+            using var cts = new CancellationTokenSource(_runTimeout);
+            var active = evaluator.RunAsync(context, cts.Token).GetAwaiter().GetResult();
+            return ScriptedAlarmEvalResult.Ok(active);
+        }
+        catch (OperationCanceledException)
+        {
+            return ScriptedAlarmEvalResult.Failure($"predicate timed out after {_runTimeout.TotalSeconds:F1}s");
+        }
+        catch (Exception ex)
+        {
+            _logger.LogWarning(ex, "Alarm {Id}: predicate execution threw", alarmId);
+            return ScriptedAlarmEvalResult.Failure($"predicate threw: {ex.Message}");
+        }
+    }
+
+    private static IReadOnlyDictionary<string, DataValueSnapshot> BuildReadCache(
+        IReadOnlyDictionary<string, object?> deps)
+    {
+        var nowUtc = DateTime.UtcNow;
+        var cache = new Dictionary<string, DataValueSnapshot>(StringComparer.Ordinal);
+        foreach (var kv in deps)
+        {
+            cache[kv.Key] = new DataValueSnapshot(kv.Value, StatusCode: 0u, SourceTimestampUtc: nowUtc, ServerTimestampUtc: nowUtc);
+        }
+        return cache;
+    }
+
+    public void Dispose()
+    {
+        if (_disposed) return;
+        _disposed = true;
+        foreach (var ev in _cache.Values)
+        {
+            try { ev.Dispose(); } catch { /* best-effort */ }
+        }
+        _cache.Clear();
+    }
+}

src/Server/ZB.MOM.WW.OtOpcUa.Host/Engines/RoslynVirtualTagEvaluator.cs

@@ -0,0 +1,116 @@
+using System.Collections.Concurrent;
+using Microsoft.Extensions.Logging;
+using ZB.MOM.WW.OtOpcUa.Commons.Engines;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+using ZB.MOM.WW.OtOpcUa.Core.Scripting;
+using ZB.MOM.WW.OtOpcUa.Core.VirtualTags;
+using SerilogLogger = Serilog.ILogger;
+using SerilogLog = Serilog.Log;
+
+namespace ZB.MOM.WW.OtOpcUa.Host.Engines;
+
+/// <summary>
+/// F8b — production <see cref="IVirtualTagEvaluator"/> binding. Compiles each unique
+/// expression once via <see cref="ScriptEvaluator{TContext, TResult}"/> (Roslyn-backed
+/// sandbox) and caches the resulting evaluator keyed by source. Subsequent evaluations are
+/// in-process method invocations on the dependency dictionary — fast enough to run inline
+/// inside the actor's message handler.
+///
+/// Single-tag mode: cross-tag <c>ctx.SetVirtualTag</c> writes are dropped (logged) because
+/// fan-out between actors is owned by <c>DependencyMuxActor</c>, not by the eval engine.
+/// Cycle detection + cascade ordering live in <see cref="VirtualTagEngine"/>; this adapter
+/// stays single-tag scoped to keep <see cref="VirtualTagActor"/>'s message loop simple.
+/// </summary>
+public sealed class RoslynVirtualTagEvaluator : IVirtualTagEvaluator, IDisposable
+{
+    private static readonly SerilogLogger ScriptLogger = SerilogLog.ForContext<RoslynVirtualTagEvaluator>();
+
+    private readonly ConcurrentDictionary<string, ScriptEvaluator<VirtualTagContext, object?>> _cache
+        = new(StringComparer.Ordinal);
+    private readonly ILogger<RoslynVirtualTagEvaluator> _logger;
+    private readonly TimeSpan _runTimeout;
+    private bool _disposed;
+
+    public RoslynVirtualTagEvaluator(ILogger<RoslynVirtualTagEvaluator> logger, TimeSpan? runTimeout = null)
+    {
+        _logger = logger;
+        _runTimeout = runTimeout ?? TimeSpan.FromSeconds(2);
+    }
+
+    public VirtualTagEvalResult Evaluate(string virtualTagId, string expression, IReadOnlyDictionary<string, object?> dependencies)
+    {
+        if (_disposed) return VirtualTagEvalResult.Failure("evaluator disposed");
+        if (string.IsNullOrWhiteSpace(expression)) return VirtualTagEvalResult.Failure("empty expression");
+
+        ScriptEvaluator<VirtualTagContext, object?> evaluator;
+        try
+        {
+            evaluator = _cache.GetOrAdd(expression, ScriptEvaluator<VirtualTagContext, object?>.Compile);
+        }
+        catch (CompilationErrorException ex)
+        {
+            _logger.LogWarning(ex, "VirtualTag {Id}: Roslyn compile failed", virtualTagId);
+            return VirtualTagEvalResult.Failure($"compile error: {ex.Message}");
+        }
+        catch (ScriptSandboxViolationException ex)
+        {
+            _logger.LogWarning(ex, "VirtualTag {Id}: sandbox violation", virtualTagId);
+            return VirtualTagEvalResult.Failure($"sandbox violation: {ex.Message}");
+        }
+        catch (Exception ex)
+        {
+            _logger.LogWarning(ex, "VirtualTag {Id}: compile threw", virtualTagId);
+            return VirtualTagEvalResult.Failure($"compile failure: {ex.Message}");
+        }
+
+        var readCache = BuildReadCache(dependencies);
+        var context = new VirtualTagContext(
+            readCache,
+            setVirtualTag: (path, _) =>
+                _logger.LogDebug("VirtualTag {Id}: cross-tag write to {Path} dropped (single-tag adapter)",
+                    virtualTagId, path),
+            logger: ScriptLogger);
+
+        try
+        {
+            using var cts = new CancellationTokenSource(_runTimeout);
+            var raw = evaluator.RunAsync(context, cts.Token).GetAwaiter().GetResult();
+            return VirtualTagEvalResult.Ok(raw);
+        }
+        catch (OperationCanceledException)
+        {
+            return VirtualTagEvalResult.Failure($"script timed out after {_runTimeout.TotalSeconds:F1}s");
+        }
+        catch (Exception ex)
+        {
+            _logger.LogWarning(ex, "VirtualTag {Id}: script execution threw", virtualTagId);
+            return VirtualTagEvalResult.Failure($"script threw: {ex.Message}");
+        }
+    }
+
+    private static IReadOnlyDictionary<string, DataValueSnapshot> BuildReadCache(
+        IReadOnlyDictionary<string, object?> deps)
+    {
+        // VirtualTagContext.GetTag returns a DataValueSnapshot — we wrap each raw dep value
+        // as Good-quality so the script's `(int)ctx.GetTag("a").Value` pattern works. Null
+        // values stay null; the script can null-check via GetTag(path).Value.
+        var nowUtc = DateTime.UtcNow;
+        var cache = new Dictionary<string, DataValueSnapshot>(StringComparer.Ordinal);
+        foreach (var kv in deps)
+        {
+            cache[kv.Key] = new DataValueSnapshot(kv.Value, StatusCode: 0u, SourceTimestampUtc: nowUtc, ServerTimestampUtc: nowUtc);
+        }
+        return cache;
+    }
+
+    public void Dispose()
+    {
+        if (_disposed) return;
+        _disposed = true;
+        foreach (var ev in _cache.Values)
+        {
+            try { ev.Dispose(); } catch { /* best-effort */ }
+        }
+        _cache.Clear();
+    }
+}

src/Server/ZB.MOM.WW.OtOpcUa.Host/Program.cs

@@ -8,8 +8,10 @@ using ZB.MOM.WW.OtOpcUa.Cluster;
 using ZB.MOM.WW.OtOpcUa.Configuration;
 using ZB.MOM.WW.OtOpcUa.ControlPlane;
 using ZB.MOM.WW.OtOpcUa.Commons.OpcUa;
+using ZB.MOM.WW.OtOpcUa.Commons.Engines;
 using ZB.MOM.WW.OtOpcUa.Host;
 using ZB.MOM.WW.OtOpcUa.Host.Drivers;
+using ZB.MOM.WW.OtOpcUa.Host.Engines;
 using ZB.MOM.WW.OtOpcUa.Host.Health;
 using ZB.MOM.WW.OtOpcUa.Host.Observability;
 using ZB.MOM.WW.OtOpcUa.Host.OpcUa;
@@ -70,6 +72,19 @@ if (hasDriver)
     // ILdapAuthService is registered by AddOtOpcUaAuth on admin nodes; on driver-only nodes
     // it isn't, so we register the LDAP options + service unconditionally for driver hosts
     // to keep parity. The LdapAdapter falls back to Deny on any backend error.
+    // F8b — production virtual-tag evaluator (Roslyn-compiled scripts cached per expression).
+    // Replaces the F8-default NullVirtualTagEvaluator so VirtualTagActor evaluates real user
+    // scripts at runtime.
+    builder.Services.AddSingleton<RoslynVirtualTagEvaluator>(sp =>
+        new RoslynVirtualTagEvaluator(sp.GetRequiredService<ILoggerFactory>().CreateLogger<RoslynVirtualTagEvaluator>()));
+    builder.Services.AddSingleton<IVirtualTagEvaluator>(sp => sp.GetRequiredService<RoslynVirtualTagEvaluator>());
+
+    // F9b — same pattern for scripted-alarm predicates. The actor preserves prior state on
+    // any Failure result, so a misbehaving script can't flip Active/Inactive spuriously.
+    builder.Services.AddSingleton<RoslynScriptedAlarmEvaluator>(sp =>
+        new RoslynScriptedAlarmEvaluator(sp.GetRequiredService<ILoggerFactory>().CreateLogger<RoslynScriptedAlarmEvaluator>()));
+    builder.Services.AddSingleton<IScriptedAlarmEvaluator>(sp => sp.GetRequiredService<RoslynScriptedAlarmEvaluator>());
+
     builder.Services.AddOptions<LdapOptions>().Bind(builder.Configuration.GetSection("Ldap"));
     builder.Services.AddSingleton<ILdapAuthService, LdapAuthService>();
     builder.Services.AddSingleton<IOpcUaUserAuthenticator, LdapOpcUaUserAuthenticator>();

src/Server/ZB.MOM.WW.OtOpcUa.Host/ZB.MOM.WW.OtOpcUa.Host.csproj

@@ -5,6 +5,12 @@
         <AssemblyName>OtOpcUa.Host</AssemblyName>
         <UserSecretsId>zb-mom-ww-otopcua-host</UserSecretsId>
         <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+        <!-- Microsoft.CodeAnalysis.CSharp.Scripting (4.12.0, pulled in via Core.Scripting for F8b
+             user-script compilation) requires CodeAnalysis.Common 4.12.0 exactly, but ASP.NET
+             Core's transitive Microsoft.CodeAnalysis.CSharp 5.0.0 wins resolution. Suppress
+             NU1608 — the surface we use from Scripting (ScriptEvaluator + RoslynScriptHost) is
+             stable across the version drift; verified by Core.Scripting.Tests. -->
+        <NoWarn>$(NoWarn);NU1608</NoWarn>
     </PropertyGroup>
 
     <ItemGroup>
@@ -24,6 +30,9 @@
         <ProjectReference Include="..\..\Core\ZB.MOM.WW.OtOpcUa.Cluster\ZB.MOM.WW.OtOpcUa.Cluster.csproj"/>
         <ProjectReference Include="..\..\Core\ZB.MOM.WW.OtOpcUa.Configuration\ZB.MOM.WW.OtOpcUa.Configuration.csproj"/>
         <ProjectReference Include="..\..\Core\ZB.MOM.WW.OtOpcUa.Core\ZB.MOM.WW.OtOpcUa.Core.csproj"/>
+        <ProjectReference Include="..\..\Core\ZB.MOM.WW.OtOpcUa.Core.Scripting\ZB.MOM.WW.OtOpcUa.Core.Scripting.csproj"/>
+        <ProjectReference Include="..\..\Core\ZB.MOM.WW.OtOpcUa.Core.VirtualTags\ZB.MOM.WW.OtOpcUa.Core.VirtualTags.csproj"/>
+        <ProjectReference Include="..\..\Core\ZB.MOM.WW.OtOpcUa.Core.ScriptedAlarms\ZB.MOM.WW.OtOpcUa.Core.ScriptedAlarms.csproj"/>
         <ProjectReference Include="..\ZB.MOM.WW.OtOpcUa.Security\ZB.MOM.WW.OtOpcUa.Security.csproj"/>
         <ProjectReference Include="..\ZB.MOM.WW.OtOpcUa.ControlPlane\ZB.MOM.WW.OtOpcUa.ControlPlane.csproj"/>
         <ProjectReference Include="..\ZB.MOM.WW.OtOpcUa.Runtime\ZB.MOM.WW.OtOpcUa.Runtime.csproj"/>

tests/Server/ZB.MOM.WW.OtOpcUa.Host.IntegrationTests/RoslynScriptedAlarmEvaluatorTests.cs

@@ -0,0 +1,102 @@
+using Microsoft.Extensions.Logging.Abstractions;
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Host.Engines;
+
+namespace ZB.MOM.WW.OtOpcUa.Host.IntegrationTests;
+
+/// <summary>
+/// F9b — verifies <see cref="RoslynScriptedAlarmEvaluator"/> compiles alarm predicates,
+/// returns the bool result on success, surfaces compile/runtime errors as Failure (so the
+/// actor preserves prior state), and rejects predicates that try to ctx.SetVirtualTag (the
+/// AlarmPredicateContext throws on writes — predicates must stay pure).
+/// </summary>
+public sealed class RoslynScriptedAlarmEvaluatorTests
+{
+    [Fact]
+    public void Evaluate_predicate_returning_true_reports_Active()
+    {
+        using var sut = new RoslynScriptedAlarmEvaluator(NullLogger<RoslynScriptedAlarmEvaluator>.Instance);
+
+        var result = sut.Evaluate(
+            alarmId: "alarm-hi",
+            predicate: "return (int)ctx.GetTag(\"temp\").Value > 100;",
+            dependencies: new Dictionary<string, object?> { ["temp"] = 150 });
+
+        result.Success.ShouldBeTrue(result.Reason);
+        result.Active.ShouldBeTrue();
+    }
+
+    [Fact]
+    public void Evaluate_predicate_returning_false_reports_Inactive()
+    {
+        using var sut = new RoslynScriptedAlarmEvaluator(NullLogger<RoslynScriptedAlarmEvaluator>.Instance);
+
+        var result = sut.Evaluate(
+            alarmId: "alarm-hi",
+            predicate: "return (int)ctx.GetTag(\"temp\").Value > 100;",
+            dependencies: new Dictionary<string, object?> { ["temp"] = 50 });
+
+        result.Success.ShouldBeTrue(result.Reason);
+        result.Active.ShouldBeFalse();
+    }
+
+    [Fact]
+    public void Evaluate_caches_compiled_predicate_across_calls()
+    {
+        using var sut = new RoslynScriptedAlarmEvaluator(NullLogger<RoslynScriptedAlarmEvaluator>.Instance);
+        const string predicate = "return (bool)ctx.GetTag(\"door_open\").Value;";
+
+        var first = sut.Evaluate("alarm-door", predicate, new Dictionary<string, object?> { ["door_open"] = true });
+        var second = sut.Evaluate("alarm-door", predicate, new Dictionary<string, object?> { ["door_open"] = false });
+
+        first.Active.ShouldBeTrue();
+        second.Active.ShouldBeFalse();
+    }
+
+    [Fact]
+    public void Evaluate_compile_error_returns_Failure()
+    {
+        using var sut = new RoslynScriptedAlarmEvaluator(NullLogger<RoslynScriptedAlarmEvaluator>.Instance);
+
+        var result = sut.Evaluate("alarm-bad", "this isn't C#;", new Dictionary<string, object?>());
+
+        result.Success.ShouldBeFalse();
+        result.Reason!.ShouldContain("compile");
+    }
+
+    [Fact]
+    public void Evaluate_predicate_writing_virtual_tag_returns_Failure()
+    {
+        using var sut = new RoslynScriptedAlarmEvaluator(NullLogger<RoslynScriptedAlarmEvaluator>.Instance);
+
+        // AlarmPredicateContext.SetVirtualTag throws — wrapper catches + reports as Failure.
+        var result = sut.Evaluate(
+            alarmId: "alarm-bad-write",
+            predicate: "ctx.SetVirtualTag(\"x\", 1); return true;",
+            dependencies: new Dictionary<string, object?>());
+
+        result.Success.ShouldBeFalse();
+        result.Reason!.ShouldContain("threw");
+    }
+
+    [Fact]
+    public void Evaluate_empty_predicate_returns_Failure()
+    {
+        using var sut = new RoslynScriptedAlarmEvaluator(NullLogger<RoslynScriptedAlarmEvaluator>.Instance);
+
+        sut.Evaluate("alarm-empty", "", new Dictionary<string, object?>()).Success.ShouldBeFalse();
+    }
+
+    [Fact]
+    public void Evaluate_after_dispose_returns_Failure()
+    {
+        var sut = new RoslynScriptedAlarmEvaluator(NullLogger<RoslynScriptedAlarmEvaluator>.Instance);
+        sut.Dispose();
+
+        var result = sut.Evaluate("alarm", "return true;", new Dictionary<string, object?>());
+
+        result.Success.ShouldBeFalse();
+        result.Reason!.ShouldContain("disposed");
+    }
+}

tests/Server/ZB.MOM.WW.OtOpcUa.Host.IntegrationTests/RoslynVirtualTagEvaluatorTests.cs

@@ -0,0 +1,92 @@
+using Microsoft.Extensions.Logging.Abstractions;
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Host.Engines;
+
+namespace ZB.MOM.WW.OtOpcUa.Host.IntegrationTests;
+
+/// <summary>
+/// F8b — verifies <see cref="RoslynVirtualTagEvaluator"/> compiles user expressions through
+/// the Core.Scripting sandbox, runs them against the dependency dictionary, caches the
+/// compiled assembly per source, and surfaces failures (compile error, sandbox violation,
+/// runtime throw) as <c>VirtualTagEvalResult.Failure</c> instead of propagating exceptions.
+/// </summary>
+public sealed class RoslynVirtualTagEvaluatorTests
+{
+    [Fact]
+    public void Evaluate_simple_addition_returns_summed_value()
+    {
+        using var sut = new RoslynVirtualTagEvaluator(NullLogger<RoslynVirtualTagEvaluator>.Instance);
+
+        var result = sut.Evaluate(
+            virtualTagId: "vt-sum",
+            expression: "return (int)ctx.GetTag(\"a\").Value + (int)ctx.GetTag(\"b\").Value;",
+            dependencies: new Dictionary<string, object?> { ["a"] = 10, ["b"] = 32 });
+
+        result.Success.ShouldBeTrue(result.Reason);
+        result.Value.ShouldBe(42);
+    }
+
+    [Fact]
+    public void Evaluate_caches_compiled_expression_across_calls()
+    {
+        using var sut = new RoslynVirtualTagEvaluator(NullLogger<RoslynVirtualTagEvaluator>.Instance);
+        const string expr = "return (int)ctx.GetTag(\"x\").Value * 2;";
+
+        var first = sut.Evaluate("vt-cache", expr, new Dictionary<string, object?> { ["x"] = 5 });
+        var second = sut.Evaluate("vt-cache", expr, new Dictionary<string, object?> { ["x"] = 7 });
+
+        first.Success.ShouldBeTrue(first.Reason);
+        first.Value.ShouldBe(10);
+        second.Success.ShouldBeTrue(second.Reason);
+        second.Value.ShouldBe(14);
+    }
+
+    [Fact]
+    public void Evaluate_compile_error_returns_Failure_with_reason()
+    {
+        using var sut = new RoslynVirtualTagEvaluator(NullLogger<RoslynVirtualTagEvaluator>.Instance);
+
+        var result = sut.Evaluate("vt-bad", "this is not valid C#;", new Dictionary<string, object?>());
+
+        result.Success.ShouldBeFalse();
+        result.Reason.ShouldNotBeNull();
+        result.Reason.ShouldContain("compile");
+    }
+
+    [Fact]
+    public void Evaluate_runtime_exception_returns_Failure_with_reason()
+    {
+        using var sut = new RoslynVirtualTagEvaluator(NullLogger<RoslynVirtualTagEvaluator>.Instance);
+
+        var result = sut.Evaluate(
+            virtualTagId: "vt-div0",
+            expression: "int a = 0; return 1 / a;",
+            dependencies: new Dictionary<string, object?>());
+
+        result.Success.ShouldBeFalse();
+        result.Reason.ShouldNotBeNull();
+        result.Reason.ShouldContain("threw");
+    }
+
+    [Fact]
+    public void Evaluate_empty_expression_returns_Failure()
+    {
+        using var sut = new RoslynVirtualTagEvaluator(NullLogger<RoslynVirtualTagEvaluator>.Instance);
+
+        sut.Evaluate("vt-empty", "", new Dictionary<string, object?>()).Success.ShouldBeFalse();
+        sut.Evaluate("vt-empty", "   ", new Dictionary<string, object?>()).Success.ShouldBeFalse();
+    }
+
+    [Fact]
+    public void Evaluate_after_dispose_returns_Failure()
+    {
+        var sut = new RoslynVirtualTagEvaluator(NullLogger<RoslynVirtualTagEvaluator>.Instance);
+        sut.Dispose();
+
+        var result = sut.Evaluate("vt", "return 1;", new Dictionary<string, object?>());
+
+        result.Success.ShouldBeFalse();
+        result.Reason!.ShouldContain("disposed");
+    }
+}