diff --git a/src/ZB.MOM.WW.ScadaBridge.ManagementService/FlowDebug/FlowDebugRunner.cs b/src/ZB.MOM.WW.ScadaBridge.ManagementService/FlowDebug/FlowDebugRunner.cs
index 5473b842..a79f0936 100644
--- a/src/ZB.MOM.WW.ScadaBridge.ManagementService/FlowDebug/FlowDebugRunner.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.ManagementService/FlowDebug/FlowDebugRunner.cs
@@ -158,8 +158,7 @@ public sealed class FlowDebugRunner
             }
             catch (OperationCanceledException) when (!ct.IsCancellationRequested)
             {
-                result = new InboundScriptResult(false, null,
-                    $"Flow Debugger run timed out after {timeout.TotalSeconds}s + {RunGrace.TotalSeconds}s grace", TimeoutCode);
+                result = new InboundScriptResult(false, null, TimeoutMessage(timeout), TimeoutCode);
             }
         }
 
@@ -316,13 +315,31 @@ public sealed class FlowDebugRunner
             catch (OperationCanceledException) when (!ct.IsCancellationRequested)
             {
                 timedOut = true;
-                response = new RouteToCallResponse(request.CorrelationId, false, null,
-                    $"Flow Debugger run timed out after {timeout.TotalSeconds}s + {RunGrace.TotalSeconds}s grace", DateTimeOffset.UtcNow);
+                response = new RouteToCallResponse(
+                    request.CorrelationId, false, null, TimeoutMessage(timeout), DateTimeOffset.UtcNow);
             }
         }
 
+        // Everything from here to WriteCentralAsync runs AFTER the site already executed the
+        // script — a real side effect. Nothing in this stretch may throw, or the root row is never
+        // written and the operator sees ManagementActor.MapFault's generic "internal error" for a
+        // call that actually succeeded. So the trace drain skips a faulting event and the return
+        // value falls back to a placeholder, the same way InboundScriptExecutor turns a script's
+        // own faults into a captured error rather than an escaping exception.
         foreach (var e in response.TraceEvents ?? [])
-            collector.Add(e);
+        {
+            try
+            {
+                collector.Add(e);
+            }
+            catch (Exception ex)
+            {
+                _logger.LogWarning(ex,
+                    "Flow Debugger dropped an inline trace event from the run of {Script} on {Instance}",
+                    script.CanonicalName, instance.UniqueName);
+            }
+        }
+
         var events = collector.Complete();
 
         var rootRow = ScadaBridgeAuditEventFactory.Create(
@@ -338,7 +355,7 @@ public sealed class FlowDebugRunner
             durationMs: (int)Math.Min(sw.ElapsedMilliseconds, int.MaxValue),
             errorMessage: response.ErrorMessage,
             requestSummary: cmd.Parameters is { } p ? p.GetRawText() : null,
-            responseSummary: response.ReturnValue is null ? null : JsonSerializer.Serialize(response.ReturnValue, ScriptTraceExtra.JsonOptions),
+            responseSummary: SerializeReturnValue(response.ReturnValue),
             extra: new ScriptTraceExtra { Source = SourceTag, ChildExecutionId = response.ChildExecutionId }.ToJson());
         await WriteCentralAsync(sp, rootRow, ct).ConfigureAwait(false);
 
@@ -477,6 +494,40 @@ public sealed class FlowDebugRunner
         return new FlowDebugRunResult(rootId, true, null, null, events, false, startedAt, DateTimeOffset.UtcNow);
     }
 
+    /// <summary>
+    /// The timed-out message shared by both Live entry points, so the two paths never drift.
+    /// </summary>
+    private static string TimeoutMessage(TimeSpan timeout) =>
+        $"Flow Debugger run timed out after {timeout.TotalSeconds}s + {RunGrace.TotalSeconds}s grace";
+
+    /// <summary>
+    /// Serializes a routed call's return value for the root row's <c>ResponseSummary</c>, NEVER
+    /// throwing: <see cref="ScriptTraceExtra.JsonOptions"/> rejects named floating-point literals,
+    /// so a script returning <c>double.NaN</c> / an infinity — or any other value the serializer
+    /// cannot represent — would otherwise take down the audit write of a call that already ran.
+    /// The placeholder keeps the row (and the operator's Live result) intact.
+    ///
+    /// <para>
+    /// The catch is deliberately broad (as <see cref="WriteCentralAsync"/>'s already is): the value
+    /// crossing this seam came from a user-authored script, and the serializer's failure mode varies
+    /// by value — <c>ArgumentException</c> for a non-finite number, <see cref="JsonException"/> for a
+    /// cycle, <see cref="NotSupportedException"/> for an unmappable type. A best-effort audit summary
+    /// must not be the thing that fails a call the site already ran.
+    /// </para>
+    /// </summary>
+    private static string? SerializeReturnValue(object? returnValue)
+    {
+        if (returnValue is null) return null;
+        try
+        {
+            return JsonSerializer.Serialize(returnValue, ScriptTraceExtra.JsonOptions);
+        }
+        catch (Exception ex)
+        {
+            return $"<unserializable return value: {ex.GetType().Name}>";
+        }
+    }
+
     private static void EnsureActiveNode(IServiceProvider sp)
     {
         var gate = sp.GetService<IActiveNodeGate>();
diff --git a/tests/ZB.MOM.WW.ScadaBridge.ManagementService.Tests/FlowDebugRunnerTests.cs b/tests/ZB.MOM.WW.ScadaBridge.ManagementService.Tests/FlowDebugRunnerTests.cs
index 1afa25de..acc3b345 100644
--- a/tests/ZB.MOM.WW.ScadaBridge.ManagementService.Tests/FlowDebugRunnerTests.cs
+++ b/tests/ZB.MOM.WW.ScadaBridge.ManagementService.Tests/FlowDebugRunnerTests.cs
@@ -489,10 +489,20 @@ public class FlowDebugRunnerTests
         RouteToSetAttributesRequest? write = null;
         _router.RouteToSetAttributesAsync("site-a", Arg.Do<RouteToSetAttributesRequest>(r => write = r), Arg.Any<CancellationToken>())
             .Returns(ci => new RouteToSetAttributesResponse(((RouteToSetAttributesRequest)ci[1]).CorrelationId, true, null, DateTimeOffset.UtcNow));
+        // Mirrors RunValueChange_TracedWrite_ThenPollsUntilSpawnedRunSettles: nothing on the first
+        // poll, then the SAME row (fixed event id) from the second on — a fresh id per poll would
+        // re-arm the settle window forever and the loop would only ever end on the deadline.
+        var calls = 0;
         puller.PullAsync("site-a", Arg.Any<IReadOnlyCollection<Guid>>(), Arg.Any<DateTime>(), Arg.Any<int>(), Arg.Any<CancellationToken>())
-            .Returns(ci => (IReadOnlyList<AuditEvent>)[ScadaBridgeAuditEventFactory.Create(AuditChannel.ScriptTrace, AuditKind.ScriptRun, AuditStatus.Delivered,
-                eventId: Guid.Parse("bbbbbbbb-0000-0000-0000-000000000001"),
-                executionId: Guid.NewGuid(), parentExecutionId: ((IReadOnlyCollection<Guid>)ci[1]).First(), target: "OnLevel")]);
+            .Returns(ci =>
+            {
+                calls++;
+                return calls >= 2
+                    ? (IReadOnlyList<AuditEvent>)[ScadaBridgeAuditEventFactory.Create(AuditChannel.ScriptTrace, AuditKind.ScriptRun, AuditStatus.Delivered,
+                        eventId: Guid.Parse("bbbbbbbb-0000-0000-0000-000000000001"),
+                        executionId: Guid.NewGuid(), parentExecutionId: ((IReadOnlyCollection<Guid>)ci[1]).First(), target: "OnLevel")]
+                    : Array.Empty<AuditEvent>();
+            });
 
         var result = await Runner().RunScriptAsync(_services.BuildServiceProvider(),
             new FlowDebugRunScriptCommand(5, "onlevel", null, "7", WaitSeconds: 10), Deployer, CancellationToken.None);
@@ -500,7 +510,12 @@ public class FlowDebugRunnerTests
         Assert.True(result.Completed, result.Error);
         Assert.Equal("7", write!.AttributeValues["Level"]);
         Assert.True(write.Trace);
+        Assert.Equal(result.RootExecutionId, write.ParentExecutionId);
+        Assert.Equal(2, result.Events.Count);
         Assert.Equal(AuditKind.AttributeWrite, result.Events[0].AsRow().Kind);
+        Assert.Equal(AuditKind.ScriptRun, result.Events[1].AsRow().Kind);
+        // Settled: at least one quiet poll (2 s window) after the last new row, then stopped.
+        Assert.True(calls >= 3, $"expected settle polls, got {calls}");
         await _router.DidNotReceiveWithAnyArgs().RouteToCallAsync(default!, default!, default);
     }
 
@@ -599,6 +614,58 @@ public class FlowDebugRunnerTests
         await pipeline.DidNotReceiveWithAnyArgs().FlattenAndValidateAsync(default, default, default, default);
     }
 
+    [Fact]
+    public async Task RunScript_CallKind_UnserializableReturnValue_StillWritesTheRootRow()
+    {
+        // The site already RAN the script by the time the root row is built, so a return value the
+        // serializer refuses (ScriptTraceExtra.JsonOptions rejects named floating-point literals)
+        // must not lose the audit row and surface as MapFault's generic "internal error".
+        WireScript(CallScript("Manual"));
+        _router.RouteToCallAsync("site-a", Arg.Any<RouteToCallRequest>(), Arg.Any<CancellationToken>())
+            .Returns(ci => new RouteToCallResponse(((RouteToCallRequest)ci[1]).CorrelationId, true, double.NaN, null, DateTimeOffset.UtcNow));
+
+        var result = await Runner().RunScriptAsync(_services.BuildServiceProvider(),
+            new FlowDebugRunScriptCommand(5, "Manual", null, null), Deployer, CancellationToken.None);
+
+        Assert.True(result.Completed, result.Error);
+        Assert.Null(result.ErrorCode);
+        var root = Assert.Single(result.Events).AsRow();
+        Assert.Equal(AuditKind.RouteCall, root.Kind);
+        Assert.Contains("unserializable", root.ResponseSummary);
+        var written = Assert.Single(_central);
+        Assert.Equal(AuditKind.RouteCall, written.AsRow().Kind);
+    }
+
+    /// <summary>A routed call that never answers until its token is cancelled — the timeout path.</summary>
+    private static async Task<RouteToCallResponse> NeverAnswersAsync(CancellationToken ct)
+    {
+        await Task.Delay(Timeout.Infinite, ct);
+        throw new InvalidOperationException("unreachable");
+    }
+
+    [Fact]
+    public async Task RunScript_CallKind_Timeout_ReportsTimeoutCode()
+    {
+        // NOTE: this test really waits ExecutionTimeoutSeconds (1 s) + RunGrace (10 s) — the runner
+        // arms a real CancelAfter, which the injectable delay/clock does not drive. ~11 s is the
+        // price of the only coverage of this path.
+        WireScript(CallScript("Manual", timeout: 1));
+        _router.RouteToCallAsync("site-a", Arg.Any<RouteToCallRequest>(), Arg.Any<CancellationToken>())
+            .Returns(ci => NeverAnswersAsync((CancellationToken)ci[2]));
+
+        var result = await Runner().RunScriptAsync(_services.BuildServiceProvider(),
+            new FlowDebugRunScriptCommand(5, "Manual", null, null), Deployer, CancellationToken.None);
+
+        Assert.False(result.Completed);
+        Assert.Equal(FlowDebugRunner.TimeoutCode, result.ErrorCode);
+        Assert.Contains("timed out", result.Error);
+        var root = Assert.Single(result.Events).AsRow();
+        Assert.Equal(AuditKind.RouteCall, root.Kind);
+        Assert.Equal(AuditStatus.Failed, root.Status);
+        var written = Assert.Single(_central);
+        Assert.Equal(AuditStatus.Failed, written.AsRow().Status);
+    }
+
     [Fact]
     public async Task RunScript_CallKind_RootRow_IsRedacted_WhenARedactorIsRegistered()
     {