fix(auditlog): capture request/response payloads on outbound API audit rows

The outbound ApiCall emitter hard-coded RequestSummary/ResponseSummary to null,
so audited API calls carried no inputs/outputs — contrary to the Audit Log
payload-capture spec. Thread the call arguments into the sync ApiCall emitter
and the cached immediate-completion path (CachedSubmit / ApiCallCached /
CachedResolve), and stamp the response body from ExternalCallResult.ResponseJson.
The writer's payload filter still applies the size cap + redaction downstream.

The S&F retry-loop cached rows are unchanged — request data is not threaded
through the store-and-forward buffer (same boundary as SourceScript).

src/ScadaLink.CentralUI/Components/Audit/AuditFilterBar.razor

@@ -6,20 +6,23 @@
 
 <div class="card mb-3" data-test="audit-filter-bar">
     <div class="card-body py-2">
-        @* All filters sit in one wrapped row. The four multi-value dimensions
+        @* All filters sit in one wrapped row. Kind / Status / Site use compact
-           (Channel / Kind / Status / Site) use compact MultiSelectDropdown
+           MultiSelectDropdown controls; Channel is a single-select because the
-           controls so the bar stays a row or two tall instead of four stacked
+           Kind options narrow to the chosen channel — so the bar stays a row or
-           blocks of chip buttons. *@
+           two tall instead of four stacked blocks of chip buttons. *@
         <div class="row g-2 align-items-end">
+            @* Single-select: one channel at a time, so the Kind options below
+               narrow cleanly to that channel. "All channels" clears it. *@
             <div class="col-auto" data-test="filter-channel">
-                <label class="form-label small mb-1">Channel</label>
+                <label class="form-label small mb-1" for="audit-channel">Channel</label>
-                <div>
+                <select id="audit-channel" data-test="filter-channel-select"
-                    <MultiSelectDropdown TValue="AuditChannel"
+                        class="form-select form-select-sm" @bind="SelectedChannel">
-                                         Items="_channels"
+                    <option value="">All channels</option>
-                                         Selected="_model.Channels"
+                    @foreach (var channel in _channels)
-                                         SelectionChanged="OnChannelsChanged"
+                    {
-                                         DataTest="filter-channel-ms" />
+                        <option value="@channel">@channel</option>
-                </div>
+                    }
+                </select>
             </div>
 
             @* Kind options are narrowed by the Channel selection (VisibleKinds). *@

src/ScadaLink.CentralUI/Components/Audit/AuditFilterBar.razor.cs

@@ -8,13 +8,14 @@ namespace ScadaLink.CentralUI.Components.Audit;
 /// <summary>
 /// Filter bar for the central Audit Log page (#23 M7-T2). Owns the
 /// <see cref="AuditQueryModel"/> binding state and renders the filter controls
-/// — Channel / Kind / Status / Site as compact
+/// — Channel as a single-select (one channel at a time, so the Kind options
+/// narrow to it cleanly); Kind / Status / Site as compact
 /// <see cref="ScadaLink.CentralUI.Components.Shared.MultiSelectDropdown{TValue}"/>
-/// controls, plus the time range, free-text searches and the Errors-only
+/// controls; plus the time range, free-text searches and the Errors-only
 /// toggle — and publishes an <see cref="AuditLogQueryFilter"/> via
-/// <see cref="OnFilterChanged"/> when the user clicks Apply. The four
+/// <see cref="OnFilterChanged"/> when the user clicks Apply. The selected
-/// multi-value dimensions map straight through to the filter's list fields;
+/// dimensions map through to the filter's list fields; see
-/// see <see cref="AuditQueryModel"/> for the Errors-only and time-range rules.
+/// <see cref="AuditQueryModel"/> for the Errors-only and time-range rules.
 /// </summary>
 public partial class AuditFilterBar
 {
@@ -82,8 +83,29 @@ public partial class AuditFilterBar
     }
 
     /// <summary>
-    /// Runs after a Channel selection changes. Drops any Kind selections that fell
+    /// Single-select Channel binding for the filter bar. The Audit Log filters one
-    /// outside the new visible set — without this, removing a channel could leave
+    /// channel at a time so the Kind options narrow cleanly to it; the model still
+    /// stores the selection as a set (0 or 1 entry) so <see cref="AuditQueryModel.ToFilter"/>
+    /// and <see cref="AuditQueryModel.VisibleKinds"/> are unchanged. <c>null</c> = all channels.
+    /// </summary>
+    private AuditChannel? SelectedChannel
+    {
+        get => _model.Channels.Count > 0 ? _model.Channels.First() : null;
+        set
+        {
+            _model.Channels.Clear();
+            if (value is { } channel)
+            {
+                _model.Channels.Add(channel);
+            }
+
+            OnChannelsChanged();
+        }
+    }
+
+    /// <summary>
+    /// Runs after the Channel selection changes. Drops any Kind selections that fell
+    /// outside the new visible set — without this, changing the channel could leave
     /// stale Kind selections that no longer match any visible option.
     /// </summary>
     private void OnChannelsChanged()

src/ScadaLink.SiteRuntime/Scripts/ScriptRuntimeContext.cs

@@ -420,7 +420,7 @@ public class ScriptRuntimeContext
             {
                 var elapsedMs = (int)((Stopwatch.GetTimestamp() - startTicks)
                     * 1000d / Stopwatch.Frequency);
-                EmitCallAudit(systemName, methodName, occurredAtUtc, elapsedMs, result, thrown);
+                EmitCallAudit(systemName, methodName, occurredAtUtc, elapsedMs, result, thrown, parameters);
             }
         }
 
@@ -458,7 +458,7 @@ public class ScriptRuntimeContext
             // Submitted row even if the immediate-delivery attempt happens to
             // resolve before this method returns.
             await EmitCachedSubmitTelemetryAsync(
-                systemName, methodName, target, trackedId, occurredAtUtc, cancellationToken)
+                systemName, methodName, target, trackedId, occurredAtUtc, parameters, cancellationToken)
                 .ConfigureAwait(false);
 
             // Hand off to the existing cached-call path. The TrackedOperationId
@@ -503,7 +503,7 @@ public class ScriptRuntimeContext
             if (result is { WasBuffered: false })
             {
                 await EmitImmediateTerminalTelemetryAsync(
-                    systemName, methodName, target, trackedId, result, cancellationToken)
+                    systemName, methodName, target, trackedId, result, parameters, cancellationToken)
                     .ConfigureAwait(false);
             }
 
@@ -521,6 +521,7 @@ public class ScriptRuntimeContext
             string target,
             TrackedOperationId trackedId,
             DateTime occurredAtUtc,
+            IReadOnlyDictionary<string, object?>? parameters,
             CancellationToken cancellationToken)
         {
             if (_cachedForwarder == null)
@@ -544,6 +545,8 @@ public class ScriptRuntimeContext
                         SourceScript = _sourceScript,
                         Target = target,
                         Status = AuditStatus.Submitted,
+                        // Submit precedes the call — request args only, no response yet.
+                        RequestSummary = SerializeRequest(parameters),
                         ForwardState = AuditForwardState.Pending,
                     },
                     Operational: new SiteCallOperational(
@@ -599,6 +602,7 @@ public class ScriptRuntimeContext
             string target,
             TrackedOperationId trackedId,
             ExternalCallResult result,
+            IReadOnlyDictionary<string, object?>? parameters,
             CancellationToken cancellationToken)
         {
             if (_cachedForwarder == null)
@@ -653,6 +657,8 @@ public class ScriptRuntimeContext
                         Status = AuditStatus.Attempted,
                         HttpStatus = httpStatus,
                         ErrorMessage = result.Success ? null : result.ErrorMessage,
+                        RequestSummary = SerializeRequest(parameters),
+                        ResponseSummary = result.ResponseJson,
                         ForwardState = AuditForwardState.Pending,
                     },
                     Operational: new SiteCallOperational(
@@ -712,6 +718,8 @@ public class ScriptRuntimeContext
                         Status = auditTerminalStatus,
                         HttpStatus = httpStatus,
                         ErrorMessage = result.Success ? null : result.ErrorMessage,
+                        RequestSummary = SerializeRequest(parameters),
+                        ResponseSummary = result.ResponseJson,
                         ForwardState = AuditForwardState.Pending,
                     },
                     Operational: new SiteCallOperational(
@@ -762,7 +770,8 @@ public class ScriptRuntimeContext
             DateTime occurredAtUtc,
             int durationMs,
             ExternalCallResult? result,
-            Exception? thrown)
+            Exception? thrown,
+            IReadOnlyDictionary<string, object?>? parameters)
         {
             if (_auditWriter == null)
             {
@@ -772,7 +781,8 @@ public class ScriptRuntimeContext
             AuditEvent evt;
             try
             {
-                evt = BuildCallAuditEvent(systemName, methodName, occurredAtUtc, durationMs, result, thrown);
+                evt = BuildCallAuditEvent(
+                    systemName, methodName, occurredAtUtc, durationMs, result, thrown, parameters);
             }
             catch (Exception buildEx)
             {
@@ -828,7 +838,8 @@ public class ScriptRuntimeContext
             DateTime occurredAtUtc,
             int durationMs,
             ExternalCallResult? result,
-            Exception? thrown)
+            Exception? thrown,
+            IReadOnlyDictionary<string, object?>? parameters)
         {
             // Status: Delivered on a Success result; Failed otherwise (the
             // ExternalSystemClient already maps HTTP non-2xx + transient
@@ -885,13 +896,41 @@ public class ScriptRuntimeContext
                 DurationMs = durationMs,
                 ErrorMessage = errorMessage,
                 ErrorDetail = errorDetail,
-                RequestSummary = null,
+                // Payload capture: the request arguments and the response body.
-                ResponseSummary = null,
+                // The audit writer's payload filter applies the configured size
+                // cap and header/secret redaction downstream — the emitter just
+                // hands over the raw values.
+                RequestSummary = SerializeRequest(parameters),
+                ResponseSummary = result?.ResponseJson,
                 PayloadTruncated = false,
                 Extra = null,
                 ForwardState = AuditForwardState.Pending,
             };
         }
+
+        /// <summary>
+        /// Serialises the outbound-call argument dictionary into the JSON
+        /// <c>RequestSummary</c> stamped on <c>ApiOutbound</c> audit rows.
+        /// Returns <c>null</c> for a null/empty argument set. Serialization
+        /// failure is swallowed (returns <c>null</c>) — a payload that cannot be
+        /// summarised must never abort the best-effort audit emission.
+        /// </summary>
+        private static string? SerializeRequest(IReadOnlyDictionary<string, object?>? parameters)
+        {
+            if (parameters is null || parameters.Count == 0)
+            {
+                return null;
+            }
+
+            try
+            {
+                return JsonSerializer.Serialize(parameters);
+            }
+            catch (Exception)
+            {
+                return null;
+            }
+        }
     }
 
     /// <summary>

tests/ScadaLink.CentralUI.PlaywrightTests/Audit/AuditLogPageTests.cs

@@ -91,9 +91,8 @@ public class AuditLogPageTests
             await page.WaitForLoadStateAsync(LoadState.NetworkIdle);
 
             // Pre-Apply, both rows are absent because the grid stays empty until
-            // the user filters. Open the Channel dropdown, tick ApiOutbound, Apply.
+            // the user filters. Pick the ApiOutbound channel, then Apply.
-            await page.Locator("[data-test='filter-channel-ms-toggle']").ClickAsync();
+            await page.Locator("[data-test='filter-channel-select']").SelectOptionAsync("ApiOutbound");
-            await page.Locator("[data-test='filter-channel-ms-opt-ApiOutbound']").ClickAsync();
             await page.Locator("[data-test='filter-apply']").ClickAsync();
             await page.WaitForLoadStateAsync(LoadState.NetworkIdle);
 

tests/ScadaLink.CentralUI.Tests/Components/Audit/AuditFilterBarTests.cs

@@ -13,10 +13,11 @@ namespace ScadaLink.CentralUI.Tests.Components.Audit;
 /// <summary>
 /// bUnit tests for <see cref="AuditFilterBar"/> (#23 M7-T2 / Bundle B).
 ///
-/// The bar carries the 10 spec filter elements plus the Errors-only toggle. The
+/// The bar carries the 10 spec filter elements plus the Errors-only toggle.
-/// Channel / Kind / Status / Site dimensions are rendered as
+/// Channel is a single-select <c>&lt;select data-test="filter-channel-select"&gt;</c>;
+/// Kind / Status / Site are
 /// <see cref="ScadaLink.CentralUI.Components.Shared.MultiSelectDropdown{TValue}"/>
-/// controls; each option is a checkbox tagged
+/// controls whose options are checkboxes tagged
 /// <c>data-test="filter-&lt;dim&gt;-ms-opt-&lt;value&gt;"</c>. Tests pin:
 /// (1) the full filter set renders; (2) Apply raises <c>OnFilterChanged</c> with
 /// the selected values; (3) the Channel→Kind narrowing map drives Kind option
@@ -75,8 +76,8 @@ public class AuditFilterBarTests : BunitContext
         var cut = Render<AuditFilterBar>(p => p
             .Add(c => c.OnFilterChanged, EventCallback.Factory.Create<AuditLogQueryFilter>(this, f => captured = f)));
 
-        // Drive UI: tick a Channel option, type in the Target search box, click Apply.
+        // Drive UI: pick a Channel, type in the Target search box, click Apply.
-        cut.Find("[data-test=\"filter-channel-ms-opt-ApiOutbound\"]").Change(true);
+        cut.Find("[data-test=\"filter-channel-select\"]").Change("ApiOutbound");
         cut.Find("[data-test=\"filter-target\"] input").Change("Plant-A-OPC");
         cut.Find("[data-test=\"filter-apply\"]").Click();
 
@@ -86,23 +87,25 @@ public class AuditFilterBarTests : BunitContext
     }
 
     [Fact]
-    public void Apply_WithMultipleChannelChips_PassesAllSelectedChannels()
+    public void ChangingChannel_ReplacesTheSelection_SingleSelect()
     {
-        // Task 9: ToFilter no longer collapses the chip multi-select — every
+        // Channel is single-select: picking a second channel replaces the first
-        // selected channel chip reaches the filter's Channels list.
+        // rather than adding to it (the page filters one channel at a time).
         AuditLogQueryFilter? captured = null;
         var cut = Render<AuditFilterBar>(p => p
             .Add(c => c.OnFilterChanged, EventCallback.Factory.Create<AuditLogQueryFilter>(this, f => captured = f)));
 
-        cut.Find("[data-test=\"filter-channel-ms-opt-ApiOutbound\"]").Change(true);
+        cut.Find("[data-test=\"filter-channel-select\"]").Change("ApiOutbound");
-        cut.Find("[data-test=\"filter-channel-ms-opt-Notification\"]").Change(true);
+        cut.Find("[data-test=\"filter-channel-select\"]").Change("Notification");
         cut.Find("[data-test=\"filter-apply\"]").Click();
 
         Assert.NotNull(captured);
-        Assert.NotNull(captured!.Channels);
+        Assert.Equal(new[] { AuditChannel.Notification }, captured!.Channels);
-        Assert.Equal(2, captured.Channels!.Count);
+
-        Assert.Contains(AuditChannel.ApiOutbound, captured.Channels);
+        // Selecting "All channels" clears the channel filter entirely.
-        Assert.Contains(AuditChannel.Notification, captured.Channels);
+        cut.Find("[data-test=\"filter-channel-select\"]").Change(string.Empty);
+        cut.Find("[data-test=\"filter-apply\"]").Click();
+        Assert.Null(captured!.Channels);
     }
 
     [Fact]
@@ -116,8 +119,8 @@ public class AuditFilterBarTests : BunitContext
             Assert.Contains($"data-test=\"filter-kind-ms-opt-{kind}\"", cut.Markup);
         }
 
-        // Select only ApiOutbound; Kind options outside the channel-kind map drop out.
+        // Select ApiOutbound; Kind options outside the channel-kind map drop out.
-        cut.Find("[data-test=\"filter-channel-ms-opt-ApiOutbound\"]").Change(true);
+        cut.Find("[data-test=\"filter-channel-select\"]").Change("ApiOutbound");
 
         var apiKinds = AuditQueryModel.KindsByChannel[AuditChannel.ApiOutbound];
         foreach (var kind in apiKinds)

tests/ScadaLink.SiteRuntime.Tests/Scripts/ExternalSystemCachedCallEmissionTests.cs

@@ -94,6 +94,42 @@ public class ExternalSystemCachedCallEmissionTests
         Assert.Null(packet.Operational.TerminalAtUtc);
     }
 
+    [Fact]
+    public async Task CachedCall_ImmediateCompletion_CapturesRequestArgs_AndResponseBody()
+    {
+        var client = new Mock<IExternalSystemClient>();
+        client
+            .Setup(c => c.CachedCallAsync(
+                "ERP", "GetOrder",
+                It.IsAny<IReadOnlyDictionary<string, object?>?>(),
+                InstanceName,
+                It.IsAny<CancellationToken>(),
+                It.IsAny<TrackedOperationId?>()))
+            .ReturnsAsync(new ExternalCallResult(true, "{\"ok\":true}", null, WasBuffered: false));
+        var forwarder = new CapturingForwarder();
+
+        var helper = CreateHelper(client.Object, forwarder);
+        var args = new Dictionary<string, object?> { ["orderId"] = 42 };
+        await helper.CachedCall("ERP", "GetOrder", args);
+
+        // Immediate completion (WasBuffered=false) emits Submit, Attempted, Resolve.
+        Assert.Equal(3, forwarder.Telemetry.Count);
+        var submit = forwarder.Telemetry.Single(t => t.Audit.Kind == AuditKind.CachedSubmit);
+        var attempted = forwarder.Telemetry.Single(t => t.Audit.Kind == AuditKind.ApiCallCached);
+        var resolve = forwarder.Telemetry.Single(t => t.Audit.Kind == AuditKind.CachedResolve);
+
+        // Every row carries the request args; the two post-call rows also carry
+        // the response body (Submit precedes the call, so it has no response).
+        Assert.Equal("{\"orderId\":42}", submit.Audit.RequestSummary);
+        Assert.Null(submit.Audit.ResponseSummary);
+
+        Assert.Equal("{\"orderId\":42}", attempted.Audit.RequestSummary);
+        Assert.Equal("{\"ok\":true}", attempted.Audit.ResponseSummary);
+
+        Assert.Equal("{\"orderId\":42}", resolve.Audit.RequestSummary);
+        Assert.Equal("{\"ok\":true}", resolve.Audit.ResponseSummary);
+    }
+
     [Fact]
     public async Task CachedCall_ReturnsTrackedOperationId()
     {

tests/ScadaLink.SiteRuntime.Tests/Scripts/ExternalSystemCallAuditEmissionTests.cs

@@ -81,6 +81,29 @@ public class ExternalSystemCallAuditEmissionTests
         Assert.Equal(DateTimeKind.Utc, evt.OccurredAtUtc.Kind);
         Assert.NotEqual(Guid.Empty, evt.EventId);
         Assert.False(evt.PayloadTruncated);
+        // No call arguments → null request summary; the response body is captured.
+        Assert.Null(evt.RequestSummary);
+        Assert.Equal("{}", evt.ResponseSummary);
+    }
+
+    [Fact]
+    public async Task Call_CapturesRequestArgs_AndResponseBody_OnTheAuditRow()
+    {
+        var client = new Mock<IExternalSystemClient>();
+        client
+            .Setup(c => c.CallAsync("Weather", "GetCurrent", It.IsAny<IReadOnlyDictionary<string, object?>?>(), It.IsAny<CancellationToken>()))
+            .ReturnsAsync(new ExternalCallResult(true, "{\"tempC\":11.4}", null));
+        var writer = new CapturingAuditWriter();
+
+        var helper = CreateHelper(client.Object, writer);
+        var args = new Dictionary<string, object?> { ["city"] = "Dublin" };
+        await helper.Call("Weather", "GetCurrent", args);
+
+        var evt = Assert.Single(writer.Events);
+        // RequestSummary is the serialized argument dictionary; ResponseSummary
+        // is the verbatim response body. (Cap + redaction are the writer's job.)
+        Assert.Equal("{\"city\":\"Dublin\"}", evt.RequestSummary);
+        Assert.Equal("{\"tempC\":11.4}", evt.ResponseSummary);
     }
 
     [Fact]