diff --git a/dotnet/server/integration-tests/ScenarioParityTests.cs b/dotnet/server/integration-tests/ScenarioParityTests.cs
new file mode 100644
index 00000000..c3b22dc7
--- /dev/null
+++ b/dotnet/server/integration-tests/ScenarioParityTests.cs
@@ -0,0 +1,303 @@
+using System.Net.WebSockets;
+using System.Text;
+using System.Text.Json;
+using System.Text.Json.Nodes;
+using Microsoft.AspNetCore.Builder;
+using Microsoft.AspNetCore.Hosting;
+using Microsoft.AspNetCore.TestHost;
+using Microsoft.Extensions.AI;
+using Microsoft.Extensions.DependencyInjection;
+using SmooAI.SmoothOperator.Server.AspNetCore;
+
+namespace SmooAI.SmoothOperator.Server.IntegrationTests;
+
+/// <summary>
+/// Scenario parity runner — the C# port of <c>python/server/tests/test_scenario_parity.py</c>.
+///
+/// Runs every scenario in <c>spec/conformance/scenarios/*.json</c> through the C# server and asserts
+/// the normalized protocol output matches. This is the shared corpus that holds the five native servers
+/// (Rust · C# · Python · TypeScript · Go) to parity: when all five run this corpus green, the servers
+/// are at protocol parity. The turn is deterministic because the engine runs on the same scripted mock
+/// chat client the scenario declares (<c>mockLlmScript</c>) — no gateway, no flakiness.
+/// </summary>
+public class ScenarioParityTests
+{
+    private static readonly string ScenariosDir = ResolveScenariosDir();
+
+    public static IEnumerable<object[]> Scenarios()
+    {
+        foreach (var path in Directory.EnumerateFiles(ScenariosDir, "*.json").OrderBy(p => p, StringComparer.Ordinal))
+        {
+            yield return new object[] { Path.GetFileName(path), path };
+        }
+    }
+
+    [Theory]
+    [MemberData(nameof(Scenarios))]
+    public async Task ScenarioParity(string name, string path)
+    {
+        _ = name; // surfaced as the test id via MemberData
+        var scenario = JsonNode.Parse(await File.ReadAllTextAsync(path))!.AsObject();
+        var chat = BuildMock(scenario["mockLlmScript"]?.AsArray());
+
+        await using var app = BuildApp(chat);
+        await app.StartAsync();
+        using var socket = await ConnectAsync(app.GetTestServer());
+
+        var vars = new Dictionary<string, JsonNode?>();
+        foreach (var step in scenario["steps"]!.AsArray())
+        {
+            var stepObj = step!.AsObject();
+            var send = Subst(stepObj["send"]!, vars);
+            await SendAsync(socket, send.ToJsonString());
+            await MatchExpectedAsync(socket, stepObj["expect"]!.AsArray(), vars);
+        }
+
+        await socket.CloseAsync(WebSocketCloseStatus.NormalClosure, "done", CancellationToken.None);
+        await app.StopAsync();
+    }
+
+    /// <summary>Seed the scripted mock chat client from the scenario's <c>mockLlmScript</c>.</summary>
+    private static MockChatClient BuildMock(JsonArray? script)
+    {
+        var mock = new MockChatClient();
+        if (script is null)
+        {
+            return mock;
+        }
+
+        foreach (var entry in script)
+        {
+            var obj = entry!.AsObject();
+            var kind = obj["kind"]!.GetValue<string>();
+            switch (kind)
+            {
+                case "text":
+                    mock.PushText(obj["text"]!.GetValue<string>());
+                    break;
+                case "toolCall":
+                    mock.PushToolCall(
+                        obj["id"]?.GetValue<string>() ?? "call-1",
+                        obj["name"]!.GetValue<string>(),
+                        ParseToolArguments(obj["arguments"]));
+                    break;
+                default:
+                    throw new InvalidOperationException($"unknown mockLlmScript kind: '{kind}'");
+            }
+        }
+
+        return mock;
+    }
+
+    /// <summary>
+    /// A scenario tool call's <c>arguments</c> is a JSON-object string (per the spec) — parse it into the
+    /// argument dictionary the mock expects. Tolerate an inline object too.
+    /// </summary>
+    private static IDictionary<string, object?> ParseToolArguments(JsonNode? arguments)
+    {
+        if (arguments is null)
+        {
+            return new Dictionary<string, object?>();
+        }
+
+        var obj = arguments is JsonValue value && value.TryGetValue<string>(out var json)
+            ? JsonNode.Parse(json)!.AsObject()
+            : arguments.AsObject();
+
+        var result = new Dictionary<string, object?>();
+        foreach (var (key, node) in obj)
+        {
+            result[key] = node is null ? null : JsonSerializer.Deserialize<object?>(node.ToJsonString());
+        }
+
+        return result;
+    }
+
+    /// <summary>Match the outbound event stream against an ordered list of matchers (ports the Python runner).</summary>
+    private static async Task MatchExpectedAsync(WebSocket socket, JsonArray matchers, Dictionary<string, JsonNode?> vars)
+    {
+        JsonObject? pending = null; // one-event lookahead when a `repeat` matcher overruns
+        foreach (var matcherNode in matchers)
+        {
+            var matcher = matcherNode!.AsObject();
+            var type = matcher["type"]!.GetValue<string>();
+            var repeat = matcher["repeat"]?.GetValue<bool>() ?? false;
+            var accumulateField = matcher["accumulate"]?.GetValue<string>();
+            var accumulated = new StringBuilder();
+
+            while (true)
+            {
+                var ev = pending ?? await NextEventAsync(socket);
+                pending = null;
+
+                if (repeat && ev["type"]!.GetValue<string>() != type)
+                {
+                    // the repeated run ended; this event belongs to the next matcher
+                    pending = ev;
+                    break;
+                }
+
+                Assert.Equal(type, ev["type"]!.GetValue<string>());
+
+                if (matcher["status"] is JsonNode status)
+                {
+                    Assert.Equal(status.GetValue<int>(), ev["status"]!.GetValue<int>());
+                }
+
+                if (matcher["statusGte"] is JsonNode statusGte)
+                {
+                    Assert.True(ev["status"]!.GetValue<int>() >= statusGte.GetValue<int>(),
+                        $"{type}: status {ev["status"]!.GetValue<int>()} < {statusGte.GetValue<int>()}");
+                }
+
+                if (matcher["assert"] is JsonObject asserts)
+                {
+                    foreach (var (dotPath, expected) in asserts)
+                    {
+                        var actual = Dot(ev, dotPath);
+                        Assert.True(JsonEquals(actual, expected),
+                            $"{type}: {dotPath} = {actual?.ToJsonString() ?? "null"} != {expected?.ToJsonString() ?? "null"}");
+                    }
+                }
+
+                if (matcher["capture"] is JsonObject captures)
+                {
+                    foreach (var (var, dotPathNode) in captures)
+                    {
+                        vars[var] = Dot(ev, dotPathNode!.GetValue<string>());
+                    }
+                }
+
+                if (accumulateField is not null)
+                {
+                    accumulated.Append(Dot(ev, accumulateField)!.GetValue<string>());
+                }
+
+                if (!repeat)
+                {
+                    break;
+                }
+            }
+
+            if (matcher["assertAccumulated"] is JsonNode assertAccumulated)
+            {
+                Assert.Equal(assertAccumulated.GetValue<string>(), accumulated.ToString());
+            }
+        }
+    }
+
+    /// <summary>Next protocol event, skipping non-semantic keepalive/pong frames (matches the Python runner).</summary>
+    private static async Task<JsonObject> NextEventAsync(WebSocket socket)
+    {
+        while (true)
+        {
+            var ev = await ReceiveAsync(socket);
+            var type = ev["type"]?.GetValue<string>();
+            if (type is not ("keepalive" or "pong"))
+            {
+                return ev;
+            }
+        }
+    }
+
+    /// <summary>Resolve a dotted path (<c>data.data.response.responseParts</c>) into a nested node.</summary>
+    private static JsonNode? Dot(JsonObject root, string path)
+    {
+        JsonNode? cur = root;
+        foreach (var part in path.Split('.'))
+        {
+            cur = cur!.AsObject()[part];
+        }
+
+        return cur;
+    }
+
+    /// <summary>Replace <c>{{name}}</c> placeholders in string fields from captured vars (recursively).</summary>
+    private static JsonNode Subst(JsonNode value, Dictionary<string, JsonNode?> vars)
+    {
+        switch (value)
+        {
+            case JsonObject obj:
+            {
+                var result = new JsonObject();
+                foreach (var (key, child) in obj)
+                {
+                    result[key] = child is null ? null : Subst(child, vars);
+                }
+
+                return result;
+            }
+            case JsonValue v when v.TryGetValue<string>(out var s) && s.StartsWith("{{", StringComparison.Ordinal) && s.EndsWith("}}", StringComparison.Ordinal):
+            {
+                var resolved = vars[s[2..^2]];
+                return resolved is null ? JsonValue.Create((string?)null)! : resolved.DeepClone();
+            }
+            default:
+                return value.DeepClone();
+        }
+    }
+
+    /// <summary>Structural JSON equality — compares scalars and arrays/objects by their canonical text.</summary>
+    private static bool JsonEquals(JsonNode? a, JsonNode? b)
+    {
+        if (a is null || b is null)
+        {
+            return a is null && b is null;
+        }
+
+        return JsonNode.DeepEquals(a, b);
+    }
+
+    private static WebApplication BuildApp(IChatClient chat)
+    {
+        var builder = WebApplication.CreateBuilder();
+        builder.WebHost.UseTestServer();
+        builder.Services.AddSingleton(chat);
+        builder.Services.AddSmoothOperatorServer();
+
+        var app = builder.Build();
+        app.MapSmoothOperatorWebSocket("/ws");
+        return app;
+    }
+
+    private static async Task<WebSocket> ConnectAsync(TestServer server)
+    {
+        var client = server.CreateWebSocketClient();
+        return await client.ConnectAsync(new Uri(server.BaseAddress, "ws"), CancellationToken.None);
+    }
+
+    private static Task SendAsync(WebSocket socket, string json) =>
+        socket.SendAsync(Encoding.UTF8.GetBytes(json), WebSocketMessageType.Text, endOfMessage: true, CancellationToken.None);
+
+    private static async Task<JsonObject> ReceiveAsync(WebSocket socket)
+    {
+        var buffer = new byte[16 * 1024];
+        using var stream = new MemoryStream();
+        WebSocketReceiveResult result;
+        do
+        {
+            result = await socket.ReceiveAsync(buffer, CancellationToken.None);
+            stream.Write(buffer, 0, result.Count);
+        }
+        while (!result.EndOfMessage);
+        return JsonNode.Parse(Encoding.UTF8.GetString(stream.ToArray()))!.AsObject();
+    }
+
+    /// <summary>Walk up from the test assembly to the repo root and resolve the shared scenarios dir.</summary>
+    private static string ResolveScenariosDir()
+    {
+        var dir = AppContext.BaseDirectory;
+        while (dir is not null)
+        {
+            var candidate = Path.Combine(dir, "spec", "conformance", "scenarios");
+            if (Directory.Exists(candidate))
+            {
+                return candidate;
+            }
+
+            dir = Path.GetDirectoryName(dir.TrimEnd(Path.DirectorySeparatorChar));
+        }
+
+        throw new DirectoryNotFoundException("could not locate spec/conformance/scenarios from " + AppContext.BaseDirectory);
+    }
+}
diff --git a/dotnet/server/src/FrameDispatcher.cs b/dotnet/server/src/FrameDispatcher.cs
index f76e3c8d..b86cfab9 100644
--- a/dotnet/server/src/FrameDispatcher.cs
+++ b/dotnet/server/src/FrameDispatcher.cs
@@ -119,7 +119,7 @@ private async Task HandleGetSessionAsync(JsonObject frame, string? requestId, Ac
         var session = await _store.GetSessionAsync(frame["sessionId"]?.GetValue<string>() ?? string.Empty, cancellationToken).ConfigureAwait(false);
         if (session is null)
         {
-            sink(ProtocolEvents.Error(requestId, "NOT_FOUND", "Session not found"));
+            sink(ProtocolEvents.Error(requestId, "SESSION_NOT_FOUND", "Session not found"));
             return;
         }
 
@@ -139,7 +139,7 @@ private async Task HandleSendMessageAsync(JsonObject frame, string? requestId, A
         var session = await _store.GetSessionAsync(frame["sessionId"]?.GetValue<string>() ?? string.Empty, cancellationToken).ConfigureAwait(false);
         if (session is null)
         {
-            sink(ProtocolEvents.Error(requestId, "NOT_FOUND", "Session not found"));
+            sink(ProtocolEvents.Error(requestId, "SESSION_NOT_FOUND", "Session not found"));
             return;
         }
 
diff --git a/dotnet/server/src/ProtocolEvents.cs b/dotnet/server/src/ProtocolEvents.cs
index 8f0c83ef..05533666 100644
--- a/dotnet/server/src/ProtocolEvents.cs
+++ b/dotnet/server/src/ProtocolEvents.cs
@@ -91,10 +91,16 @@ public static JsonObject EventualResponse(string requestId, int status, string m
 
     public static JsonObject Error(string? requestId, string code, string message)
     {
+        // The {code, message} descriptor is duplicated at the envelope top level (`error`) and nested
+        // under `data.error`, per spec/events/error.schema.json — the top-level copy is "kept for clients
+        // that pattern-match on the envelope-level `error` field". Mirrors the Python reference server.
+        var data = new JsonObject { ["error"] = new JsonObject { ["code"] = code, ["message"] = message } };
+        if (requestId is not null) data["requestId"] = requestId;
         var ev = new JsonObject
         {
             ["type"] = "error",
-            ["data"] = new JsonObject { ["error"] = new JsonObject { ["code"] = code, ["message"] = message } },
+            ["error"] = new JsonObject { ["code"] = code, ["message"] = message },
+            ["data"] = data,
             ["timestamp"] = NowMs(),
         };
         if (requestId is not null) ev["requestId"] = requestId;