diff --git a/src/ScadaLink.AuditLog/Payload/DefaultAuditPayloadFilter.cs b/src/ScadaLink.AuditLog/Payload/DefaultAuditPayloadFilter.cs
new file mode 100644
index 0000000..8682a9f
--- /dev/null
+++ b/src/ScadaLink.AuditLog/Payload/DefaultAuditPayloadFilter.cs
@@ -0,0 +1,124 @@
+using System.Text;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Options;
+using ScadaLink.AuditLog.Configuration;
+using ScadaLink.Commons.Entities.Audit;
+using ScadaLink.Commons.Types.Enums;
+
+namespace ScadaLink.AuditLog.Payload;
+
+/// <summary>
+/// Default <see cref="IAuditPayloadFilter"/>. M5 Bundle A scope: payload
+/// truncation only (RequestSummary / ResponseSummary / ErrorDetail / Extra),
+/// capped at <see cref="AuditLogOptions.DefaultCapBytes"/> on success rows and
+/// <see cref="AuditLogOptions.ErrorCapBytes"/> on error rows. Bundle B layers
+/// header / body / SQL-parameter redaction on top.
+/// </summary>
+/// <remarks>
+/// <para>
+/// Uses <see cref="IOptionsMonitor{TOptions}"/> (not <see cref="IOptions{TOptions}"/>)
+/// so the M5-T8 hot-reload path sees fresh values without re-resolving the
+/// singleton.
+/// </para>
+/// <para>
+/// "Error row" = <see cref="AuditEvent.Status"/> NOT IN (<c>Delivered</c>,
+/// <c>Submitted</c>, <c>Forwarded</c>) — every other status, including the
+/// non-terminal <c>Attempted</c>, the parked/discarded terminals, and the
+/// short-circuit <c>Skipped</c>, receives the larger error cap so a verbose
+/// error body survives.
+/// </para>
+/// <para>
+/// Apply MUST NOT throw — on internal failure the filter over-redacts by
+/// returning the input with <see cref="AuditEvent.PayloadTruncated"/> set and
+/// (Bundle C) increments the <c>AuditRedactionFailure</c> health metric.
+/// </para>
+/// </remarks>
+public sealed class DefaultAuditPayloadFilter : IAuditPayloadFilter
+{
+    private readonly IOptionsMonitor<AuditLogOptions> _options;
+    private readonly ILogger<DefaultAuditPayloadFilter> _logger;
+
+    public DefaultAuditPayloadFilter(
+        IOptionsMonitor<AuditLogOptions> options,
+        ILogger<DefaultAuditPayloadFilter> logger)
+    {
+        _options = options ?? throw new ArgumentNullException(nameof(options));
+        _logger = logger ?? throw new ArgumentNullException(nameof(logger));
+    }
+
+    public AuditEvent Apply(AuditEvent rawEvent)
+    {
+        try
+        {
+            var opts = _options.CurrentValue;
+            var cap = IsErrorStatus(rawEvent.Status) ? opts.ErrorCapBytes : opts.DefaultCapBytes;
+            var truncated = false;
+            var request = TruncateField(rawEvent.RequestSummary, cap, ref truncated);
+            var response = TruncateField(rawEvent.ResponseSummary, cap, ref truncated);
+            var errorDetail = TruncateField(rawEvent.ErrorDetail, cap, ref truncated);
+            var extra = TruncateField(rawEvent.Extra, cap, ref truncated);
+            return rawEvent with
+            {
+                RequestSummary = request,
+                ResponseSummary = response,
+                ErrorDetail = errorDetail,
+                Extra = extra,
+                PayloadTruncated = rawEvent.PayloadTruncated || truncated,
+            };
+        }
+        catch (Exception ex)
+        {
+            // Audit is best-effort: over-redact rather than fail the caller.
+            // Bundle C wires the AuditRedactionFailure health metric here.
+            _logger.LogWarning(
+                ex,
+                "Payload filter failed; returning raw event with PayloadTruncated=true");
+            return rawEvent with { PayloadTruncated = true };
+        }
+    }
+
+    private static string? TruncateField(string? value, int cap, ref bool truncated)
+    {
+        if (value is null)
+        {
+            return null;
+        }
+        var result = TruncateUtf8(value, cap);
+        if (result.Length != value.Length)
+        {
+            truncated = true;
+        }
+        return result;
+    }
+
+    /// <summary>
+    /// UTF-8 byte-safe truncation. Encodes the input to UTF-8, walks back from
+    /// the cap position until the byte is NOT a continuation byte
+    /// (<c>byte &amp; 0xC0 == 0x80</c>), and decodes the resulting prefix —
+    /// guaranteeing the returned string never splits a multi-byte sequence.
+    /// </summary>
+    private static string TruncateUtf8(string value, int capBytes)
+    {
+        if (string.IsNullOrEmpty(value))
+        {
+            return value;
+        }
+        var bytes = Encoding.UTF8.GetBytes(value);
+        if (bytes.Length <= capBytes)
+        {
+            return value;
+        }
+        var boundary = capBytes;
+        while (boundary > 0 && (bytes[boundary] & 0xC0) == 0x80)
+        {
+            boundary--;
+        }
+        return Encoding.UTF8.GetString(bytes, 0, boundary);
+    }
+
+    private static bool IsErrorStatus(AuditStatus status) => status switch
+    {
+        AuditStatus.Delivered or AuditStatus.Submitted or AuditStatus.Forwarded => false,
+        _ => true,
+    };
+}
diff --git a/src/ScadaLink.AuditLog/ServiceCollectionExtensions.cs b/src/ScadaLink.AuditLog/ServiceCollectionExtensions.cs
index 346ea0f..d42f299 100644
--- a/src/ScadaLink.AuditLog/ServiceCollectionExtensions.cs
+++ b/src/ScadaLink.AuditLog/ServiceCollectionExtensions.cs
@@ -5,6 +5,7 @@ using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Options;
 using ScadaLink.AuditLog.Central;
 using ScadaLink.AuditLog.Configuration;
+using ScadaLink.AuditLog.Payload;
 using ScadaLink.AuditLog.Site;
 using ScadaLink.AuditLog.Site.Telemetry;
 using ScadaLink.Commons.Interfaces.Services;
@@ -59,6 +60,15 @@ public static class ServiceCollectionExtensions
             .ValidateOnStart();
         services.AddSingleton<IValidateOptions<AuditLogOptions>, AuditLogOptionsValidator>();
 
+        // M5 Bundle A: payload filter — truncates oversized RequestSummary /
+        // ResponseSummary / ErrorDetail / Extra fields between event
+        // construction and persistence. Bundle B layers header / body /
+        // SQL-parameter redaction onto the same singleton; Bundle C wires it
+        // into the FallbackAuditWriter / CentralAuditWriter / IngestActor
+        // paths. Singleton — the filter is stateless and the IOptionsMonitor
+        // dependency picks up M5-T8 hot reloads on its own.
+        services.AddSingleton<IAuditPayloadFilter, DefaultAuditPayloadFilter>();
+
         // M2 Bundle E: site writer + telemetry options bindings.
         // BindConfiguration is not used because the configuration root supplied
         // by the caller may not be the application root — we go through the
diff --git a/tests/ScadaLink.AuditLog.Tests/Payload/TruncationTests.cs b/tests/ScadaLink.AuditLog.Tests/Payload/TruncationTests.cs
new file mode 100644
index 0000000..d747336
--- /dev/null
+++ b/tests/ScadaLink.AuditLog.Tests/Payload/TruncationTests.cs
@@ -0,0 +1,226 @@
+using System.Linq;
+using System.Text;
+using Microsoft.Extensions.Logging.Abstractions;
+using Microsoft.Extensions.Options;
+using ScadaLink.AuditLog.Configuration;
+using ScadaLink.AuditLog.Payload;
+using ScadaLink.Commons.Entities.Audit;
+using ScadaLink.Commons.Types.Enums;
+
+namespace ScadaLink.AuditLog.Tests.Payload;
+
+/// <summary>
+/// Bundle A (M5-T2) tests for <see cref="DefaultAuditPayloadFilter"/> truncation.
+/// The filter caps RequestSummary / ResponseSummary / ErrorDetail / Extra at
+/// <see cref="AuditLogOptions.DefaultCapBytes"/> (8 KiB) on success rows and
+/// <see cref="AuditLogOptions.ErrorCapBytes"/> (64 KiB) on error rows. "Error
+/// row" = <see cref="AuditEvent.Status"/> NOT IN (<c>Delivered</c>,
+/// <c>Submitted</c>, <c>Forwarded</c>). Truncation must respect UTF-8 character
+/// boundaries (never split a multi-byte sequence mid-character) and must set
+/// <see cref="AuditEvent.PayloadTruncated"/> true when any field is shortened.
+/// </summary>
+public class TruncationTests
+{
+    private static IOptionsMonitor<AuditLogOptions> Monitor(AuditLogOptions? opts = null)
+    {
+        var snapshot = opts ?? new AuditLogOptions();
+        return new StaticMonitor(snapshot);
+    }
+
+    private static DefaultAuditPayloadFilter Filter(AuditLogOptions? opts = null) =>
+        new(Monitor(opts), NullLogger<DefaultAuditPayloadFilter>.Instance);
+
+    private static AuditEvent NewEvent(
+        AuditStatus status = AuditStatus.Delivered,
+        string? request = null,
+        string? response = null,
+        string? errorDetail = null,
+        string? extra = null,
+        bool payloadTruncated = false) => new()
+    {
+        EventId = Guid.NewGuid(),
+        OccurredAtUtc = DateTime.UtcNow,
+        Channel = AuditChannel.ApiOutbound,
+        Kind = AuditKind.ApiCall,
+        Status = status,
+        RequestSummary = request,
+        ResponseSummary = response,
+        ErrorDetail = errorDetail,
+        Extra = extra,
+        PayloadTruncated = payloadTruncated,
+    };
+
+    [Fact]
+    public void SuccessRow_10KB_RequestSummary_TruncatedTo8KB_PayloadTruncatedTrue()
+    {
+        var input = new string('a', 10 * 1024);
+        var evt = NewEvent(AuditStatus.Delivered, request: input);
+
+        var result = Filter().Apply(evt);
+
+        Assert.NotNull(result.RequestSummary);
+        Assert.Equal(8192, Encoding.UTF8.GetByteCount(result.RequestSummary!));
+        Assert.True(result.PayloadTruncated);
+    }
+
+    [Fact]
+    public void ErrorRow_10KB_RequestSummary_NotTruncated_UnderErrorCap()
+    {
+        var input = new string('b', 10 * 1024);
+        var evt = NewEvent(AuditStatus.Failed, request: input);
+
+        var result = Filter().Apply(evt);
+
+        Assert.Equal(input, result.RequestSummary);
+        Assert.False(result.PayloadTruncated);
+    }
+
+    [Fact]
+    public void ErrorRow_70KB_RequestSummary_TruncatedTo64KB_PayloadTruncatedTrue()
+    {
+        var input = new string('c', 70 * 1024);
+        var evt = NewEvent(AuditStatus.Failed, request: input);
+
+        var result = Filter().Apply(evt);
+
+        Assert.NotNull(result.RequestSummary);
+        Assert.Equal(65536, Encoding.UTF8.GetByteCount(result.RequestSummary!));
+        Assert.True(result.PayloadTruncated);
+    }
+
+    [Fact]
+    public void Multibyte_UTF8_TruncatedAtCharacterBoundary_NotMidByte()
+    {
+        // U+1F600 (grinning face) encodes to 4 UTF-8 bytes; 2000 of them = 8000 bytes,
+        // safely under the 8192 default cap so the boundary scan kicks in mid-character
+        // when we push past it. Pad with a few extra emoji so the *input* is > 8192 bytes
+        // and forces truncation.
+        var emoji = "😀"; // surrogate pair => one code point => 4 UTF-8 bytes
+        var sb = new StringBuilder();
+        for (int i = 0; i < 2100; i++)
+        {
+            sb.Append(emoji);
+        }
+        var input = sb.ToString();
+        Assert.True(Encoding.UTF8.GetByteCount(input) > 8192);
+
+        var evt = NewEvent(AuditStatus.Delivered, request: input);
+
+        var result = Filter().Apply(evt);
+
+        Assert.NotNull(result.RequestSummary);
+        var resultBytes = Encoding.UTF8.GetByteCount(result.RequestSummary!);
+        Assert.True(resultBytes <= 8192, $"expected <= 8192 bytes, got {resultBytes}");
+        // 4-byte emoji boundary: the kept byte length must be a multiple of 4.
+        Assert.Equal(0, resultBytes % 4);
+        // And round-tripping the result must not introduce a U+FFFD replacement char.
+        Assert.DoesNotContain('�', result.RequestSummary);
+        Assert.True(result.PayloadTruncated);
+    }
+
+    [Fact]
+    public void NullSummary_PassesThrough_AsNull()
+    {
+        var evt = NewEvent(AuditStatus.Delivered, request: null, response: null, errorDetail: null, extra: null);
+
+        var result = Filter().Apply(evt);
+
+        Assert.Null(result.RequestSummary);
+        Assert.Null(result.ResponseSummary);
+        Assert.Null(result.ErrorDetail);
+        Assert.Null(result.Extra);
+        Assert.False(result.PayloadTruncated);
+    }
+
+    [Fact]
+    public void RawEventAlreadyTruncated_PayloadTruncatedRemainsTrue()
+    {
+        // Small payload that requires no truncation, but the caller already
+        // flagged PayloadTruncated upstream — the filter must not clear it.
+        var evt = NewEvent(AuditStatus.Delivered, request: "small", payloadTruncated: true);
+
+        var result = Filter().Apply(evt);
+
+        Assert.Equal("small", result.RequestSummary);
+        Assert.True(result.PayloadTruncated);
+    }
+
+    [Fact]
+    public void StatusAttempted_TreatedAsError_UsesErrorCap()
+    {
+        // 10 KB is under the 64 KB error cap; if Attempted were a success status
+        // the value would be truncated to 8 KB. We assert it is NOT truncated.
+        var input = new string('d', 10 * 1024);
+        var evt = NewEvent(AuditStatus.Attempted, request: input);
+
+        var result = Filter().Apply(evt);
+
+        Assert.Equal(input, result.RequestSummary);
+        Assert.False(result.PayloadTruncated);
+    }
+
+    [Fact]
+    public void StatusParked_TreatedAsError_UsesErrorCap()
+    {
+        var input = new string('e', 10 * 1024);
+        var evt = NewEvent(AuditStatus.Parked, request: input);
+
+        var result = Filter().Apply(evt);
+
+        Assert.Equal(input, result.RequestSummary);
+        Assert.False(result.PayloadTruncated);
+    }
+
+    [Fact]
+    public void StatusSkipped_TreatedAsError_UsesErrorCap()
+    {
+        var input = new string('f', 10 * 1024);
+        var evt = NewEvent(AuditStatus.Skipped, request: input);
+
+        var result = Filter().Apply(evt);
+
+        Assert.Equal(input, result.RequestSummary);
+        Assert.False(result.PayloadTruncated);
+    }
+
+    [Fact]
+    public void ErrorDetail_AndExtra_Truncated_Independently()
+    {
+        // Each field is capped on its own — a 10 KB RequestSummary and a 10 KB
+        // ErrorDetail on the same Delivered row should both be cut to 8 KB and
+        // the row flagged truncated.
+        var input = new string('g', 10 * 1024);
+        var evt = NewEvent(
+            AuditStatus.Delivered,
+            request: input,
+            response: input,
+            errorDetail: input,
+            extra: input);
+
+        var result = Filter().Apply(evt);
+
+        Assert.Equal(8192, Encoding.UTF8.GetByteCount(result.RequestSummary!));
+        Assert.Equal(8192, Encoding.UTF8.GetByteCount(result.ResponseSummary!));
+        Assert.Equal(8192, Encoding.UTF8.GetByteCount(result.ErrorDetail!));
+        Assert.Equal(8192, Encoding.UTF8.GetByteCount(result.Extra!));
+        Assert.True(result.PayloadTruncated);
+    }
+
+    /// <summary>
+    /// IOptionsMonitor test double — returns the same snapshot on every read,
+    /// no change-token plumbing required for these tests (Bundle D wires the
+    /// real hot-reload path).
+    /// </summary>
+    private sealed class StaticMonitor : IOptionsMonitor<AuditLogOptions>
+    {
+        private readonly AuditLogOptions _value;
+
+        public StaticMonitor(AuditLogOptions value) => _value = value;
+
+        public AuditLogOptions CurrentValue => _value;
+
+        public AuditLogOptions Get(string? name) => _value;
+
+        public IDisposable? OnChange(Action<AuditLogOptions, string?> listener) => null;
+    }
+}