diff --git a/src/ZB.MOM.WW.ScadaBridge.CentralUI/ServiceCollectionExtensions.cs b/src/ZB.MOM.WW.ScadaBridge.CentralUI/ServiceCollectionExtensions.cs
index 4cbada93..58fb5b69 100644
--- a/src/ZB.MOM.WW.ScadaBridge.CentralUI/ServiceCollectionExtensions.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.CentralUI/ServiceCollectionExtensions.cs
@@ -140,6 +140,7 @@ public static class ServiceCollectionExtensions
         // name collision, no instance binding, no native-alarm-source name reference);
         // the move dialog only SUBMITS the command and renders the returned outcome.
         services.AddScoped<IDataConnectionMoveService, DataConnectionMoveService>();
+        services.AddScoped<IFlowDebugService, FlowDebugService>();
 
         // Schema Library: authoring + read accessors for the reusable named
         // JSON-Schema library (SharedSchema + ISharedSchemaRepository).
diff --git a/src/ZB.MOM.WW.ScadaBridge.CentralUI/Services/FlowDebugService.cs b/src/ZB.MOM.WW.ScadaBridge.CentralUI/Services/FlowDebugService.cs
new file mode 100644
index 00000000..b5de5d72
--- /dev/null
+++ b/src/ZB.MOM.WW.ScadaBridge.CentralUI/Services/FlowDebugService.cs
@@ -0,0 +1,301 @@
+using System.Security.Claims;
+using System.Text.Json;
+using Akka.Actor;
+using Microsoft.AspNetCore.Components.Authorization;
+using Microsoft.Extensions.Logging;
+using ZB.MOM.WW.ScadaBridge.Commons.Entities.Templates;
+using ZB.MOM.WW.ScadaBridge.Commons.Interfaces.Repositories;
+using ZB.MOM.WW.ScadaBridge.Commons.Messages.Management;
+using ZB.MOM.WW.ScadaBridge.Commons.Types.Debugging;
+using ZB.MOM.WW.ScadaBridge.ManagementService;
+using ZB.MOM.WW.ScadaBridge.ScriptAnalysis;
+using ZB.MOM.WW.ScadaBridge.Security;
+
+namespace ZB.MOM.WW.ScadaBridge.CentralUI.Services;
+
+/// <summary>
+/// Default <see cref="IFlowDebugService"/>: prediction runs in-process over the repositories
+/// (method script + instance → template chain → script bodies, DB-only, nothing executes);
+/// the run/replay commands go to the per-node <c>ManagementActor</c> through
+/// <see cref="ManagementActorHolder"/>, exactly as <see cref="DataConnectionMoveService"/>
+/// does, with the current Blazor principal projected to an <see cref="AuthenticatedUser"/>
+/// so the server's Deployer gate runs against the real identity. The Live-run Ask waits up
+/// to <see cref="RunAskTimeout"/> (the server bounds a run by the method timeout + grace and
+/// a value-change by its wait window; both fit inside two minutes).
+/// </summary>
+public sealed class FlowDebugService : IFlowDebugService
+{
+    /// <summary>Ask timeout for the two Live-run commands.</summary>
+    public static readonly TimeSpan RunAskTimeout = TimeSpan.FromMinutes(2);
+
+    /// <summary>Ask timeout for Replay (a central DB read).</summary>
+    public static readonly TimeSpan ReplayAskTimeout = TimeSpan.FromSeconds(30);
+
+    /// <summary>Upper bound on the template parent chain the script resolver walks (cycle guard).</summary>
+    private const int MaxTemplateChain = 16;
+
+    private static readonly JsonSerializerOptions ResultJsonOptions = new()
+    {
+        PropertyNameCaseInsensitive = true,
+        PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
+    };
+
+    private readonly ManagementActorHolder _holder;
+    private readonly AuthenticationStateProvider _auth;
+    private readonly IInboundApiRepository _inboundApi;
+    private readonly ITemplateEngineRepository _templates;
+    private readonly ILogger<FlowDebugService> _logger;
+
+    /// <summary>Initializes the service.</summary>
+    public FlowDebugService(
+        ManagementActorHolder holder,
+        AuthenticationStateProvider auth,
+        IInboundApiRepository inboundApi,
+        ITemplateEngineRepository templates,
+        ILogger<FlowDebugService> logger)
+    {
+        _holder = holder ?? throw new ArgumentNullException(nameof(holder));
+        _auth = auth ?? throw new ArgumentNullException(nameof(auth));
+        _inboundApi = inboundApi ?? throw new ArgumentNullException(nameof(inboundApi));
+        _templates = templates ?? throw new ArgumentNullException(nameof(templates));
+        _logger = logger ?? throw new ArgumentNullException(nameof(logger));
+    }
+
+    /// <inheritdoc/>
+    public async Task<PredictedFlow> PredictMethodAsync(int apiMethodId, CancellationToken cancellationToken = default)
+    {
+        var method = await _inboundApi.GetApiMethodByIdAsync(apiMethodId, cancellationToken);
+        if (method == null)
+            return PredictedFlow.Empty;
+
+        // The predictor is synchronous and takes a synchronous resolver, so the instance →
+        // template → script lookups are pre-fetched lazily through a small memo that blocks
+        // on the repository — acceptable for a handful of constant route targets, and the
+        // predictor never asks for anything else.
+        var memo = new Dictionary<(string, string), string?>();
+        string? Resolve(string instanceCode, string scriptName)
+        {
+            var key = (instanceCode, scriptName);
+            if (memo.TryGetValue(key, out var cached))
+                return cached;
+            string? body;
+            try
+            {
+                body = ResolveSiteScriptAsync(instanceCode, scriptName, cancellationToken).GetAwaiter().GetResult();
+            }
+            catch (Exception ex)
+            {
+                _logger.LogDebug(ex, "Flow prediction could not resolve {Instance}.{Script}", instanceCode, scriptName);
+                body = null;
+            }
+
+            memo[key] = body;
+            return body;
+        }
+
+        return FlowShapePredictor.PredictInboundMethod(method.Name, method.Script ?? string.Empty, Resolve);
+    }
+
+    /// <inheritdoc/>
+    public async Task<PredictedFlow> PredictValueChangeAsync(int instanceId, string attributeName, CancellationToken cancellationToken = default)
+    {
+        var instance = await _templates.GetInstanceByIdAsync(instanceId, cancellationToken);
+        if (instance == null)
+            return PredictedFlow.Empty;
+
+        var scripts = await CollectEffectiveScriptsAsync(instance.TemplateId, cancellationToken);
+        var triggered = scripts.Values
+            .Where(s => IsValueChangeOn(s, attributeName))
+            .OrderBy(s => s.Name, StringComparer.Ordinal)
+            .ToList();
+
+        // Root: the write itself (Tags lane), then each triggered script predicted as a site script.
+        var nodes = new List<PredictedNode>
+        {
+            new("n1", FlowStepKind.AttributeWrite, FlowLane.Tags, $"{instance.UniqueName}.{attributeName}", false,
+                triggered.Count == 0 ? "no value-change script on this attribute" : null),
+        };
+        var edges = new List<PredictedEdge>();
+        foreach (var script in triggered)
+        {
+            var sub = FlowShapePredictor.PredictSiteScript($"{instance.UniqueName}.{script.Name}", script.Code ?? string.Empty);
+            var offset = nodes.Count;
+            var map = new Dictionary<string, string>(StringComparer.Ordinal);
+            foreach (var n in sub.Nodes)
+            {
+                var id = "n" + (nodes.Count + 1);
+                map[n.Id] = id;
+                nodes.Add(n with { Id = id });
+            }
+
+            if (sub.Nodes.Count > 0)
+                edges.Add(new PredictedEdge("n1", nodes[offset].Id, false));
+            foreach (var e in sub.Edges)
+                edges.Add(e with { From = map[e.From], To = map[e.To] });
+        }
+
+        return new PredictedFlow(nodes, edges);
+    }
+
+    /// <inheritdoc/>
+    public Task<FlowDebugOutcome> RunMethodAsync(int apiMethodId, JsonElement? parameters, CancellationToken cancellationToken = default)
+        => SendAndBuildAsync(new FlowDebugRunMethodCommand(apiMethodId, parameters), RunAskTimeout, cancellationToken);
+
+    /// <inheritdoc/>
+    public Task<FlowDebugOutcome> RunValueChangeAsync(int instanceId, string attributeName, string value, CancellationToken cancellationToken = default)
+        => SendAndBuildAsync(new FlowDebugRunValueChangeCommand(instanceId, attributeName, value), RunAskTimeout, cancellationToken);
+
+    /// <inheritdoc/>
+    public Task<FlowDebugOutcome> ReplayAsync(Guid executionId, CancellationToken cancellationToken = default)
+        => SendAndBuildAsync(new FlowDebugReplayCommand(executionId), ReplayAskTimeout, cancellationToken);
+
+    /// <summary>
+    /// Resolves <c>instanceCode.scriptName</c> to a script body: instance by unique name →
+    /// its template chain (nearest definition wins) → the named script's code.
+    /// </summary>
+    private async Task<string?> ResolveSiteScriptAsync(string instanceCode, string scriptName, CancellationToken ct)
+    {
+        var instance = await _templates.GetInstanceByUniqueNameAsync(instanceCode, ct);
+        if (instance == null)
+            return null;
+
+        var scripts = await CollectEffectiveScriptsAsync(instance.TemplateId, ct);
+        return scripts.TryGetValue(scriptName, out var script) ? script.Code : null;
+    }
+
+    /// <summary>
+    /// Walks the template parent chain from <paramref name="templateId"/> upward, collecting
+    /// scripts by name with the nearest (most-derived) definition winning — so an override in
+    /// a derived template is what gets predicted, and a base-only script is still found.
+    /// </summary>
+    private async Task<Dictionary<string, TemplateScript>> CollectEffectiveScriptsAsync(int templateId, CancellationToken ct)
+    {
+        var result = new Dictionary<string, TemplateScript>(StringComparer.OrdinalIgnoreCase);
+        int? current = templateId;
+        var hops = 0;
+        var seen = new HashSet<int>();
+        while (current is int id && hops++ < MaxTemplateChain && seen.Add(id))
+        {
+            var scripts = await _templates.GetScriptsByTemplateIdAsync(id, ct);
+            foreach (var s in scripts)
+            {
+                if (!result.ContainsKey(s.Name))
+                    result[s.Name] = s;
+            }
+
+            var template = await _templates.GetTemplateByIdAsync(id, ct);
+            current = template?.ParentTemplateId;
+        }
+
+        return result;
+    }
+
+    /// <summary>True when the script's trigger is a value-change on <paramref name="attributeName"/> (case-insensitive).</summary>
+    private static bool IsValueChangeOn(TemplateScript script, string attributeName)
+    {
+        if (!string.Equals(script.TriggerType, "valuechange", StringComparison.OrdinalIgnoreCase)
+            && !string.Equals(script.TriggerType, "ValueChange", StringComparison.OrdinalIgnoreCase))
+            return false;
+
+        var cfg = script.TriggerConfiguration;
+        if (string.IsNullOrWhiteSpace(cfg))
+            return false;
+
+        try
+        {
+            using var doc = JsonDocument.Parse(cfg);
+            if (doc.RootElement.ValueKind != JsonValueKind.Object)
+                return false;
+            foreach (var prop in doc.RootElement.EnumerateObject())
+            {
+                if (prop.NameEquals("attribute") || prop.NameEquals("Attribute")
+                    || prop.NameEquals("attributeName") || prop.NameEquals("AttributeName"))
+                {
+                    return prop.Value.ValueKind == JsonValueKind.String
+                        && string.Equals(prop.Value.GetString(), attributeName, StringComparison.OrdinalIgnoreCase);
+                }
+            }
+        }
+        catch (JsonException)
+        {
+            // Malformed trigger config — not something prediction should fail over.
+        }
+
+        return false;
+    }
+
+    private async Task<FlowDebugOutcome> SendAndBuildAsync(object command, TimeSpan timeout, CancellationToken ct)
+    {
+        var response = await SendAsync(command, timeout, ct);
+        switch (response)
+        {
+            case ManagementSuccess success:
+            {
+                FlowDebugRunResult? result;
+                try
+                {
+                    result = JsonSerializer.Deserialize<FlowDebugRunResult>(success.JsonData, ResultJsonOptions);
+                }
+                catch (JsonException ex)
+                {
+                    _logger.LogError(ex, "Flow Debugger result could not be parsed for {Command}", command.GetType().Name);
+                    return FlowDebugOutcome.Fail("The server returned an unreadable result.", "BAD_RESULT");
+                }
+
+                if (result == null)
+                    return FlowDebugOutcome.Fail("The server returned no result.", "BAD_RESULT");
+
+                var graph = FlowGraphBuilder.Build(result.Events ?? [], result.Truncated);
+                return new FlowDebugOutcome(graph, result, null, null);
+            }
+            case ManagementUnauthorized unauthorized:
+                return FlowDebugOutcome.Fail(unauthorized.Message, "UNAUTHORIZED");
+            case ManagementError error:
+                return FlowDebugOutcome.Fail(error.Error, error.ErrorCode);
+            default:
+                return FlowDebugOutcome.Fail("Unexpected response from the management service.", "INTERNAL_ERROR");
+        }
+    }
+
+    /// <summary>
+    /// Wraps <paramref name="command"/> in a <see cref="ManagementEnvelope"/> for the current
+    /// principal and Asks the <c>ManagementActor</c>. Transport faults become a synthetic
+    /// <see cref="ManagementError"/> so callers handle one response shape.
+    /// </summary>
+    private async Task<object> SendAsync(object command, TimeSpan timeout, CancellationToken cancellationToken)
+    {
+        var actor = _holder.ActorRef;
+        if (actor is null)
+            return new ManagementError(string.Empty, "Management service is not ready.", "SERVICE_UNAVAILABLE");
+
+        var user = await BuildAuthenticatedUserAsync();
+        var envelope = new ManagementEnvelope(user, command, Guid.NewGuid().ToString("N"));
+
+        try
+        {
+            return await actor.Ask<object>(envelope, timeout, cancellationToken);
+        }
+        catch (OperationCanceledException)
+        {
+            throw;
+        }
+        catch (Exception ex)
+        {
+            _logger.LogError(ex, "ManagementActor Ask failed for {Command}", command.GetType().Name);
+            return new ManagementError(string.Empty, ex.Message, "TRANSPORT_ERROR");
+        }
+    }
+
+    private async Task<AuthenticatedUser> BuildAuthenticatedUserAsync()
+    {
+        var state = await _auth.GetAuthenticationStateAsync();
+        var principal = state.User;
+
+        var username = principal.FindFirst(JwtTokenService.UsernameClaimType)?.Value ?? "unknown";
+        var displayName = principal.FindFirst(JwtTokenService.DisplayNameClaimType)?.Value ?? username;
+        var roles = principal.FindAll(JwtTokenService.RoleClaimType).Select(c => c.Value).ToArray();
+        var permittedSiteIds = principal.FindAll(JwtTokenService.SiteIdClaimType).Select(c => c.Value).ToArray();
+
+        return new AuthenticatedUser(username, displayName, roles, permittedSiteIds);
+    }
+}
diff --git a/src/ZB.MOM.WW.ScadaBridge.CentralUI/Services/IFlowDebugService.cs b/src/ZB.MOM.WW.ScadaBridge.CentralUI/Services/IFlowDebugService.cs
new file mode 100644
index 00000000..7376094a
--- /dev/null
+++ b/src/ZB.MOM.WW.ScadaBridge.CentralUI/Services/IFlowDebugService.cs
@@ -0,0 +1,69 @@
+using System.Text.Json;
+using ZB.MOM.WW.ScadaBridge.Commons.Messages.Management;
+using ZB.MOM.WW.ScadaBridge.Commons.Types.Debugging;
+using ZB.MOM.WW.ScadaBridge.ScriptAnalysis;
+
+namespace ZB.MOM.WW.ScadaBridge.CentralUI.Services;
+
+/// <summary>
+/// Outcome of a Flow Debugger run or replay as the page consumes it: the built
+/// <see cref="FlowGraph"/> (null on failure), the raw server result (null when the command
+/// itself was refused), and a classified error.
+/// </summary>
+/// <param name="Graph">The flow graph built from the returned audit events; null when the command failed outright.</param>
+/// <param name="Raw">The server's <see cref="FlowDebugRunResult"/>; null when no result came back.</param>
+/// <param name="Error">Human-readable failure, null on success.</param>
+/// <param name="ErrorCode">Machine-readable failure code (<c>STANDBY_NODE</c>, <c>NO_TRIGGER_WITHIN_WINDOW</c>, <c>UNAUTHORIZED</c>, <c>SERVICE_UNAVAILABLE</c>, …), null on success.</param>
+public sealed record FlowDebugOutcome(FlowGraph? Graph, FlowDebugRunResult? Raw, string? Error, string? ErrorCode)
+{
+    /// <summary>True when a graph was built (the run may still have finished incomplete — see <see cref="Raw"/>).</summary>
+    public bool Success => Graph != null && Error == null;
+
+    /// <summary>Creates a failed outcome.</summary>
+    public static FlowDebugOutcome Fail(string error, string errorCode) => new(null, null, error, errorCode);
+}
+
+/// <summary>
+/// CentralUI facade over the Flow Debugger (design 2026-09-05 §5.5/§7.1): static shape
+/// prediction from script syntax, and the three management commands (Live run of a method,
+/// Live run of a value change, Replay) dispatched to the per-node <c>ManagementActor</c>
+/// through the in-process <c>ManagementActorHolder</c> — the SAME Ask path the HTTP
+/// <c>/management</c> endpoint uses, so the server stays the single enforcer of the Deployer
+/// gate and the active-node refusal (<c>STANDBY_NODE</c>).
+/// </summary>
+public interface IFlowDebugService
+{
+    /// <summary>Predicts the flow shape of an inbound API method from its script and the site scripts its constant routes resolve to.</summary>
+    /// <param name="apiMethodId">The method id.</param>
+    /// <param name="cancellationToken">Cancellation token.</param>
+    /// <returns>The predicted flow; <see cref="PredictedFlow.Empty"/> when the method does not exist.</returns>
+    Task<PredictedFlow> PredictMethodAsync(int apiMethodId, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Predicts the flow a value change on <paramref name="attributeName"/> would trigger: the
+    /// instance's template scripts whose value-change trigger names that attribute, each predicted
+    /// as a site script.
+    /// </summary>
+    /// <param name="instanceId">The instance id.</param>
+    /// <param name="attributeName">Canonical attribute name.</param>
+    /// <param name="cancellationToken">Cancellation token.</param>
+    Task<PredictedFlow> PredictValueChangeAsync(int instanceId, string attributeName, CancellationToken cancellationToken = default);
+
+    /// <summary>Runs an inbound API method with tracing on the active central node and returns the flow.</summary>
+    /// <param name="apiMethodId">The method id.</param>
+    /// <param name="parameters">Flat JSON object of parameters, or null.</param>
+    /// <param name="cancellationToken">Cancellation token.</param>
+    Task<FlowDebugOutcome> RunMethodAsync(int apiMethodId, JsonElement? parameters, CancellationToken cancellationToken = default);
+
+    /// <summary>Writes an attribute through the traced path and returns the flow the change triggered.</summary>
+    /// <param name="instanceId">The instance id.</param>
+    /// <param name="attributeName">Canonical attribute name.</param>
+    /// <param name="value">Value in canonical string form.</param>
+    /// <param name="cancellationToken">Cancellation token.</param>
+    Task<FlowDebugOutcome> RunValueChangeAsync(int instanceId, string attributeName, string value, CancellationToken cancellationToken = default);
+
+    /// <summary>Rebuilds a past flow from the central audit store.</summary>
+    /// <param name="executionId">Any execution id in the chain.</param>
+    /// <param name="cancellationToken">Cancellation token.</param>
+    Task<FlowDebugOutcome> ReplayAsync(Guid executionId, CancellationToken cancellationToken = default);
+}
diff --git a/tests/ZB.MOM.WW.ScadaBridge.CentralUI.Tests/Services/FlowDebugServiceTests.cs b/tests/ZB.MOM.WW.ScadaBridge.CentralUI.Tests/Services/FlowDebugServiceTests.cs
new file mode 100644
index 00000000..5cb04144
--- /dev/null
+++ b/tests/ZB.MOM.WW.ScadaBridge.CentralUI.Tests/Services/FlowDebugServiceTests.cs
@@ -0,0 +1,276 @@
+using System.Security.Claims;
+using System.Text.Json;
+using Akka.Actor;
+using Microsoft.AspNetCore.Components.Authorization;
+using Microsoft.Extensions.Logging.Abstractions;
+using NSubstitute;
+using ZB.MOM.WW.Audit;
+using ZB.MOM.WW.ScadaBridge.CentralUI.Services;
+using ZB.MOM.WW.ScadaBridge.Commons.Entities.InboundApi;
+using ZB.MOM.WW.ScadaBridge.Commons.Entities.Instances;
+using ZB.MOM.WW.ScadaBridge.Commons.Entities.Templates;
+using ZB.MOM.WW.ScadaBridge.Commons.Interfaces.Repositories;
+using ZB.MOM.WW.ScadaBridge.Commons.Messages.Management;
+using ZB.MOM.WW.ScadaBridge.Commons.Types.Audit;
+using ZB.MOM.WW.ScadaBridge.Commons.Types.Debugging;
+using ZB.MOM.WW.ScadaBridge.Commons.Types.Enums;
+using ZB.MOM.WW.ScadaBridge.ManagementService;
+using ZB.MOM.WW.ScadaBridge.Security;
+
+namespace ZB.MOM.WW.ScadaBridge.CentralUI.Tests.Services;
+
+/// <summary>
+/// Tests for <see cref="FlowDebugService"/> (Flow Debugger design 2026-09-05 §5.5/§7.1):
+/// the management-command bridge (holder null, error mapping, success → graph) and the
+/// prediction resolver wiring over substituted repositories.
+/// </summary>
+public sealed class FlowDebugServiceTests : IDisposable
+{
+    private static readonly JsonSerializerOptions ServerJson = new()
+    {
+        PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
+        DefaultIgnoreCondition = System.Text.Json.Serialization.JsonIgnoreCondition.WhenWritingNull,
+    };
+
+    private readonly ActorSystem _system = ActorSystem.Create("flow-debug-service-tests");
+    private readonly IInboundApiRepository _inboundApi = Substitute.For<IInboundApiRepository>();
+    private readonly ITemplateEngineRepository _templates = Substitute.For<ITemplateEngineRepository>();
+    private readonly List<ManagementEnvelope> _received = new();
+
+    public void Dispose() => _system.Terminate().Wait(TimeSpan.FromSeconds(5));
+
+    private FlowDebugService CreateService(object? reply, string role = "Deployer")
+    {
+        var holder = new ManagementActorHolder();
+        if (reply != null)
+            holder.ActorRef = _system.ActorOf(Props.Create(() => new ScriptedManagementActor(reply, _received)));
+
+        var claims = new[]
+        {
+            new Claim(JwtTokenService.UsernameClaimType, "tester"),
+            new Claim(JwtTokenService.RoleClaimType, role),
+        };
+        var principal = new ClaimsPrincipal(new ClaimsIdentity(claims, "TestAuth"));
+        return new FlowDebugService(holder, new StubAuth(principal), _inboundApi, _templates, NullLogger<FlowDebugService>.Instance);
+    }
+
+    private static string SuccessPayload(int events, bool completed = true, bool truncated = false)
+    {
+        var root = Guid.NewGuid();
+        var child = Guid.NewGuid();
+        var t0 = DateTime.UtcNow;
+        var list = new List<AuditEvent>
+        {
+            ScadaBridgeAuditEventFactory.Create(AuditChannel.ApiInbound, AuditKind.InboundRequest, AuditStatus.Delivered,
+                occurredAtUtc: t0, target: "POST /api/M", executionId: root, durationMs: 100,
+                requestSummary: """{"MachineCode":"Z28061"}""", responseSummary: """{"Result":true}"""),
+        };
+        if (events > 1)
+            list.Add(ScadaBridgeAuditEventFactory.Create(AuditChannel.ScriptTrace, AuditKind.RouteCall, AuditStatus.Delivered,
+                occurredAtUtc: t0.AddMilliseconds(1), target: "I.S", executionId: root, durationMs: 90,
+                extra: new ScriptTraceExtra { ChildExecutionId = child }.ToJson()));
+        if (events > 2)
+            list.Add(ScadaBridgeAuditEventFactory.Create(AuditChannel.ScriptTrace, AuditKind.ScriptRun, AuditStatus.Delivered,
+                occurredAtUtc: t0.AddMilliseconds(2), target: "S", executionId: child, parentExecutionId: root,
+                sourceSiteId: "site-a", sourceInstanceId: "I", durationMs: 80));
+
+        var result = new FlowDebugRunResult(root, completed, null, null, list, truncated, t0, t0.AddSeconds(1));
+        return JsonSerializer.Serialize(result, ServerJson);
+    }
+
+    [Fact]
+    public async Task RunMethod_HolderNull_ReportsServiceUnavailable()
+    {
+        var svc = CreateService(reply: null);
+
+        var outcome = await svc.RunMethodAsync(4, null);
+
+        Assert.False(outcome.Success);
+        Assert.Null(outcome.Graph);
+        Assert.Equal("SERVICE_UNAVAILABLE", outcome.ErrorCode);
+        Assert.Empty(_received);
+    }
+
+    [Fact]
+    public async Task RunMethod_ManagementError_MapsCodeAndMessageVerbatim()
+    {
+        var svc = CreateService(new ManagementError("c", "This node is standby.", "STANDBY_NODE"));
+
+        var outcome = await svc.RunMethodAsync(4, null);
+
+        Assert.False(outcome.Success);
+        Assert.Equal("STANDBY_NODE", outcome.ErrorCode);
+        Assert.Equal("This node is standby.", outcome.Error);
+    }
+
+    [Fact]
+    public async Task RunMethod_Unauthorized_MapsToUnauthorizedCode()
+    {
+        var svc = CreateService(new ManagementUnauthorized("c", "Deployer role required."), role: "Viewer");
+
+        var outcome = await svc.RunMethodAsync(4, null);
+
+        Assert.Equal("UNAUTHORIZED", outcome.ErrorCode);
+        Assert.Equal("Deployer role required.", outcome.Error);
+    }
+
+    [Fact]
+    public async Task RunMethod_Success_BuildsGraphWithTheReturnedSteps_AndSendsTheTypedCommandForThePrincipal()
+    {
+        var svc = CreateService(new ManagementSuccess("c", SuccessPayload(events: 3)));
+        using var doc = JsonDocument.Parse("""{"MachineCode":"Z28061"}""");
+
+        var outcome = await svc.RunMethodAsync(4, doc.RootElement.Clone());
+
+        Assert.True(outcome.Success);
+        Assert.NotNull(outcome.Graph);
+        Assert.NotNull(outcome.Raw);
+        Assert.True(outcome.Raw!.Completed);
+        // 3 returned events + the synthesized Response node.
+        Assert.Equal(4, outcome.Graph!.Steps.Count);
+        Assert.Contains(outcome.Graph.Steps, s => s.Kind == FlowStepKind.ScriptRun && s.Lane == FlowLane.Site);
+
+        var envelope = Assert.Single(_received);
+        var cmd = Assert.IsType<FlowDebugRunMethodCommand>(envelope.Command);
+        Assert.Equal(4, cmd.ApiMethodId);
+        Assert.Equal("Z28061", cmd.Parameters!.Value.GetProperty("MachineCode").GetString());
+        Assert.Equal("tester", envelope.User.Username);
+        Assert.Contains("Deployer", envelope.User.Roles);
+    }
+
+    [Fact]
+    public async Task RunValueChange_AndReplay_SendTheirCommands()
+    {
+        var svc = CreateService(new ManagementSuccess("c", SuccessPayload(events: 1)));
+        var exec = Guid.NewGuid();
+
+        var vc = await svc.RunValueChangeAsync(7, "Speed", "42");
+        var rp = await svc.ReplayAsync(exec);
+
+        Assert.True(vc.Success);
+        Assert.True(rp.Success);
+        Assert.Equal(2, _received.Count);
+        var vcCmd = Assert.IsType<FlowDebugRunValueChangeCommand>(_received[0].Command);
+        Assert.Equal((7, "Speed", "42"), (vcCmd.InstanceId, vcCmd.AttributeName, vcCmd.Value));
+        var rpCmd = Assert.IsType<FlowDebugReplayCommand>(_received[1].Command);
+        Assert.Equal(exec, rpCmd.ExecutionId);
+    }
+
+    [Fact]
+    public async Task Run_TruncatedResult_FlagsTheGraph()
+    {
+        var svc = CreateService(new ManagementSuccess("c", SuccessPayload(events: 2, truncated: true)));
+
+        var outcome = await svc.RunMethodAsync(4, null);
+
+        Assert.True(outcome.Success);
+        Assert.True(outcome.Graph!.Truncated);
+    }
+
+    [Fact]
+    public async Task PredictMethod_ResolvesConstantRouteTargets_ThroughInstanceTemplateChain()
+    {
+        _inboundApi.GetApiMethodByIdAsync(4, Arg.Any<CancellationToken>())
+            .Returns(new ApiMethod("Delmia", """return await Route.To("Sim01").Call("Process", new { });""") { Id = 4 });
+        _templates.GetInstanceByUniqueNameAsync("Sim01", Arg.Any<CancellationToken>())
+            .Returns(new Instance("Sim01") { Id = 9, TemplateId = 20, SiteId = 1 });
+        // Derived template 20 has no scripts of its own; its base 10 defines Process.
+        _templates.GetScriptsByTemplateIdAsync(20, Arg.Any<CancellationToken>())
+            .Returns(Task.FromResult<IReadOnlyList<TemplateScript>>([]));
+        _templates.GetTemplateByIdAsync(20, Arg.Any<CancellationToken>())
+            .Returns(new Template("Derived") { Id = 20, ParentTemplateId = 10 });
+        _templates.GetScriptsByTemplateIdAsync(10, Arg.Any<CancellationToken>())
+            .Returns(Task.FromResult<IReadOnlyList<TemplateScript>>(
+            [
+                new TemplateScript("Process", """await Attributes.SetAsync("Cmd", 1); await Attributes.WaitAsync("Ack", true, System.TimeSpan.FromSeconds(1));""") { Id = 1, TemplateId = 10 },
+            ]));
+        _templates.GetTemplateByIdAsync(10, Arg.Any<CancellationToken>())
+            .Returns(new Template("Base") { Id = 10, ParentTemplateId = null });
+
+        var svc = CreateService(reply: null);
+        var flow = await svc.PredictMethodAsync(4);
+
+        Assert.Equal(
+        [
+            FlowStepKind.InboundRequest, FlowStepKind.RouteCall, FlowStepKind.ScriptRun,
+            FlowStepKind.AttributeWrite, FlowStepKind.AttributeWait, FlowStepKind.Response,
+        ], flow.Nodes.Select(n => n.Kind).ToList());
+        Assert.Equal("Sim01.Process", flow.Nodes[2].Title);
+        Assert.Equal("Sim01.Cmd", flow.Nodes[3].Title);
+    }
+
+    [Fact]
+    public async Task PredictMethod_UnknownMethod_IsEmpty()
+    {
+        _inboundApi.GetApiMethodByIdAsync(99, Arg.Any<CancellationToken>()).Returns((ApiMethod?)null);
+        var svc = CreateService(reply: null);
+
+        var flow = await svc.PredictMethodAsync(99);
+
+        Assert.Empty(flow.Nodes);
+    }
+
+    [Fact]
+    public async Task PredictValueChange_RootsAtTheWrite_AndAppendsEachTriggeredScript()
+    {
+        _templates.GetInstanceByIdAsync(9, Arg.Any<CancellationToken>())
+            .Returns(new Instance("Sim01") { Id = 9, TemplateId = 10, SiteId = 1 });
+        _templates.GetScriptsByTemplateIdAsync(10, Arg.Any<CancellationToken>())
+            .Returns(Task.FromResult<IReadOnlyList<TemplateScript>>(
+            [
+                new TemplateScript("OnFlag", """Attributes["Ack"] = true;""") { Id = 1, TemplateId = 10, TriggerType = "valuechange", TriggerConfiguration = """{"attributeName":"Flag"}""" },
+                new TemplateScript("OnOther", "return 1;") { Id = 2, TemplateId = 10, TriggerType = "valuechange", TriggerConfiguration = """{"attributeName":"Other"}""" },
+                new TemplateScript("Manual", "return 1;") { Id = 3, TemplateId = 10, TriggerType = "call" },
+            ]));
+        _templates.GetTemplateByIdAsync(10, Arg.Any<CancellationToken>())
+            .Returns(new Template("Base") { Id = 10 });
+
+        var svc = CreateService(reply: null);
+        var flow = await svc.PredictValueChangeAsync(9, "Flag");
+
+        Assert.Equal(
+        [
+            (FlowStepKind.AttributeWrite, "Sim01.Flag"),
+            (FlowStepKind.ScriptRun, "Sim01.OnFlag"),
+            (FlowStepKind.AttributeWrite, "Ack"),
+        ], flow.Nodes.Select(n => (n.Kind, n.Title)).ToList());
+        Assert.Equal(2, flow.Edges.Count);
+        Assert.Equal(("n1", "n2"), (flow.Edges[0].From, flow.Edges[0].To));
+        Assert.Equal(("n2", "n3"), (flow.Edges[1].From, flow.Edges[1].To));
+    }
+
+    [Fact]
+    public async Task PredictValueChange_NoTriggeredScript_NotesIt()
+    {
+        _templates.GetInstanceByIdAsync(9, Arg.Any<CancellationToken>())
+            .Returns(new Instance("Sim01") { Id = 9, TemplateId = 10, SiteId = 1 });
+        _templates.GetScriptsByTemplateIdAsync(10, Arg.Any<CancellationToken>())
+            .Returns(Task.FromResult<IReadOnlyList<TemplateScript>>([]));
+        _templates.GetTemplateByIdAsync(10, Arg.Any<CancellationToken>()).Returns(new Template("Base") { Id = 10 });
+
+        var svc = CreateService(reply: null);
+        var flow = await svc.PredictValueChangeAsync(9, "Flag");
+
+        var root = Assert.Single(flow.Nodes);
+        Assert.Contains("no value-change script", root.Note);
+    }
+
+    private sealed class StubAuth : AuthenticationStateProvider
+    {
+        private readonly ClaimsPrincipal _user;
+        public StubAuth(ClaimsPrincipal user) => _user = user;
+        public override Task<AuthenticationState> GetAuthenticationStateAsync() => Task.FromResult(new AuthenticationState(_user));
+    }
+
+    private sealed class ScriptedManagementActor : ReceiveActor
+    {
+        public ScriptedManagementActor(object reply, List<ManagementEnvelope> received)
+        {
+            Receive<ManagementEnvelope>(env =>
+            {
+                lock (received) received.Add(env);
+                Sender.Tell(reply);
+            });
+        }
+    }
+}