Propagate tool call exceptions through filters (#844)

* Add McpProtocolException specifically for JSON-RPC errors

README.md

@@ -207,7 +207,7 @@ McpServerOptions options = new()
             {
                 if (request.Params.Arguments?.TryGetValue("message", out var message) is not true)
                 {
-                    throw new McpException("Missing required argument 'message'");
+                    throw new McpProtocolException("Missing required argument 'message'", McpErrorCode.InvalidParams);
                 }
 
                 return ValueTask.FromResult(new CallToolResult
@@ -216,7 +216,7 @@ McpServerOptions options = new()
                 });
             }
 
-            throw new McpException($"Unknown tool: '{request.Params?.Name}'");
+            throw new McpProtocolException($"Unknown tool: '{request.Params?.Name}'", McpErrorCode.InvalidRequest);
         }
     }
 };

samples/EverythingServer/Program.cs

@@ -123,7 +123,7 @@ builder.Services
     {
         if (ctx.Params?.Level is null)
         {
-            throw new McpException("Missing required argument 'level'", McpErrorCode.InvalidParams);
+            throw new McpProtocolException("Missing required argument 'level'", McpErrorCode.InvalidParams);
         }
 
         _minimumLoggingLevel = ctx.Params.Level;

src/ModelContextProtocol.AspNetCore/AuthorizationFilterSetup.cs

@@ -78,7 +78,7 @@ internal sealed class AuthorizationFilterSetup(IAuthorizationPolicyProvider? pol
             var authResult = await GetAuthorizationResultAsync(context.User, context.MatchedPrimitive, context.Services, context);
             if (!authResult.Succeeded)
             {
-                throw new McpException("Access forbidden: This tool requires authorization.", McpErrorCode.InvalidRequest);
+                throw new McpProtocolException("Access forbidden: This tool requires authorization.", McpErrorCode.InvalidRequest);
             }
 
             context.Items[AuthorizationFilterInvokedKey] = true;
@@ -170,7 +170,7 @@ internal sealed class AuthorizationFilterSetup(IAuthorizationPolicyProvider? pol
             var authResult = await GetAuthorizationResultAsync(context.User, context.MatchedPrimitive, context.Services, context);
             if (!authResult.Succeeded)
             {
-                throw new McpException("Access forbidden: This resource requires authorization.", McpErrorCode.InvalidRequest);
+                throw new McpProtocolException("Access forbidden: This resource requires authorization.", McpErrorCode.InvalidRequest);
             }
 
             return await next(context, cancellationToken);
@@ -230,7 +230,7 @@ internal sealed class AuthorizationFilterSetup(IAuthorizationPolicyProvider? pol
             var authResult = await GetAuthorizationResultAsync(context.User, context.MatchedPrimitive, context.Services, context);
             if (!authResult.Succeeded)
             {
-                throw new McpException("Access forbidden: This prompt requires authorization.", McpErrorCode.InvalidRequest);
+                throw new McpProtocolException("Access forbidden: This prompt requires authorization.", McpErrorCode.InvalidRequest);
             }
 
             return await next(context, cancellationToken);

src/ModelContextProtocol.Core/McpException.cs

@@ -1,14 +1,21 @@
+using ModelContextProtocol.Protocol;
+
 namespace ModelContextProtocol;
 
 /// <summary>
 /// Represents an exception that is thrown when an Model Context Protocol (MCP) error occurs.
 /// </summary>
 /// <remarks>
-/// This exception is used to represent failures to do with protocol-level concerns, such as invalid JSON-RPC requests,
+/// The <see cref="Exception.Message"/> from a <see cref="McpException"/> may be propagated to the remote
-/// invalid parameters, or internal errors. It is not intended to be used for application-level errors.
+/// endpoint; sensitive information should not be included. If sensitive details need to be included,
-/// <see cref="Exception.Message"/> or <see cref="ErrorCode"/> from a <see cref="McpException"/> may be 
+/// a different exception type should be used.
-/// propagated to the remote endpoint; sensitive information should not be included. If sensitive details need
+///
-/// to be included, a different exception type should be used.
+/// This exception type can be thrown by MCP tools or tool call filters to propagate detailed error messages
+/// from <see cref="Exception.Message"/> when a tool execution fails via a <see cref="CallToolResult"/>.
+/// For non-tool calls, this exception controls the message propogated via a <see cref="JsonRpcError"/>.
+/// 
+/// <see cref="McpProtocolException"/> is a derived type that can be used to also specify the
+/// <see cref="McpErrorCode"/> that should be used for the resulting <see cref="JsonRpcError"/>.
 /// </remarks>
 public class McpException : Exception
 {
@@ -28,46 +35,13 @@ public class McpException : Exception
     }
 
     /// <summary>
-    /// Initializes a new instance of the <see cref="McpException"/> class with a specified error message and a reference to the inner exception that is the cause of this exception.
+    /// Initializes a new instance of the <see cref="McpException"/> class with a specified error message and
+    /// a reference to the inner exception that is the cause of this exception.
     /// </summary>
     /// <param name="message">The message that describes the error.</param>
-    /// <param name="innerException">The exception that is the cause of the current exception, or a null reference if no inner exception is specified.</param>
+    /// <param name="innerException">The exception that is the cause of the current exception, or a null
+    /// reference if no inner exception is specified.</param>
     public McpException(string message, Exception? innerException) : base(message, innerException)
     {
     }
-
-    /// <summary>
-    /// Initializes a new instance of the <see cref="McpException"/> class with a specified error message and JSON-RPC error code.
-    /// </summary>
-    /// <param name="message">The message that describes the error.</param>
-    /// <param name="errorCode">A <see cref="McpErrorCode"/>.</param>
-    public McpException(string message, McpErrorCode errorCode) : this(message, null, errorCode)
-    {
-    }
-
-    /// <summary>
-    /// Initializes a new instance of the <see cref="McpException"/> class with a specified error message, inner exception, and JSON-RPC error code.
-    /// </summary>
-    /// <param name="message">The message that describes the error.</param>
-    /// <param name="innerException">The exception that is the cause of the current exception, or a null reference if no inner exception is specified.</param>
-    /// <param name="errorCode">A <see cref="McpErrorCode"/>.</param>
-    public McpException(string message, Exception? innerException, McpErrorCode errorCode) : base(message, innerException)
-    {
-        ErrorCode = errorCode;
-    }
-
-    /// <summary>
-    /// Gets the error code associated with this exception.
-    /// </summary>
-    /// <remarks>
-    /// This property contains a standard JSON-RPC error code as defined in the MCP specification. Common error codes include:
-    /// <list type="bullet">
-    /// <item><description>-32700: Parse error - Invalid JSON received</description></item>
-    /// <item><description>-32600: Invalid request - The JSON is not a valid Request object</description></item>
-    /// <item><description>-32601: Method not found - The method does not exist or is not available</description></item>
-    /// <item><description>-32602: Invalid params - Invalid method parameters</description></item>
-    /// <item><description>-32603: Internal error - Internal JSON-RPC error</description></item>
-    /// </list>
-    /// </remarks>
-    public McpErrorCode ErrorCode { get; } = McpErrorCode.InternalError;
 }

src/ModelContextProtocol.Core/McpProtocolException.cs

@@ -0,0 +1,73 @@
+namespace ModelContextProtocol;
+
+/// <summary>
+/// Represents an exception that is thrown when an Model Context Protocol (MCP) error occurs.
+/// </summary>
+/// <remarks>
+/// This exception is used to represent failures to do with protocol-level concerns, such as invalid JSON-RPC requests,
+/// invalid parameters, or internal errors. It is not intended to be used for application-level errors.
+/// <see cref="Exception.Message"/> or <see cref="ErrorCode"/> from a <see cref="McpProtocolException"/> may be 
+/// propagated to the remote endpoint; sensitive information should not be included. If sensitive details need
+/// to be included, a different exception type should be used.
+/// </remarks>
+public sealed class McpProtocolException : McpException
+{
+    /// <summary>
+    /// Initializes a new instance of the <see cref="McpProtocolException"/> class.
+    /// </summary>
+    public McpProtocolException()
+    {
+    }
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="McpProtocolException"/> class with a specified error message.
+    /// </summary>
+    /// <param name="message">The message that describes the error.</param>
+    public McpProtocolException(string message) : base(message)
+    {
+    }
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="McpProtocolException"/> class with a specified error message and a reference to the inner exception that is the cause of this exception.
+    /// </summary>
+    /// <param name="message">The message that describes the error.</param>
+    /// <param name="innerException">The exception that is the cause of the current exception, or a null reference if no inner exception is specified.</param>
+    public McpProtocolException(string message, Exception? innerException) : base(message, innerException)
+    {
+    }
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="McpProtocolException"/> class with a specified error message and JSON-RPC error code.
+    /// </summary>
+    /// <param name="message">The message that describes the error.</param>
+    /// <param name="errorCode">A <see cref="McpErrorCode"/>.</param>
+    public McpProtocolException(string message, McpErrorCode errorCode) : this(message, null, errorCode)
+    {
+    }
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="McpProtocolException"/> class with a specified error message, inner exception, and JSON-RPC error code.
+    /// </summary>
+    /// <param name="message">The message that describes the error.</param>
+    /// <param name="innerException">The exception that is the cause of the current exception, or a null reference if no inner exception is specified.</param>
+    /// <param name="errorCode">A <see cref="McpErrorCode"/>.</param>
+    public McpProtocolException(string message, Exception? innerException, McpErrorCode errorCode) : base(message, innerException)
+    {
+        ErrorCode = errorCode;
+    }
+
+    /// <summary>
+    /// Gets the error code associated with this exception.
+    /// </summary>
+    /// <remarks>
+    /// This property contains a standard JSON-RPC error code as defined in the MCP specification. Common error codes include:
+    /// <list type="bullet">
+    /// <item><description>-32700: Parse error - Invalid JSON received</description></item>
+    /// <item><description>-32600: Invalid request - The JSON is not a valid Request object</description></item>
+    /// <item><description>-32601: Method not found - The method does not exist or is not available</description></item>
+    /// <item><description>-32602: Invalid params - Invalid method parameters</description></item>
+    /// <item><description>-32603: Internal error - Internal JSON-RPC error</description></item>
+    /// </list>
+    /// </remarks>
+    public McpErrorCode ErrorCode { get; } = McpErrorCode.InternalError;
+}

src/ModelContextProtocol.Core/McpSessionHandler.cs

@@ -181,11 +181,17 @@ internal sealed partial class McpSessionHandler : IAsyncDisposable
                         {
                             LogRequestHandlerException(EndpointName, request.Method, ex);
 
-                            JsonRpcErrorDetail detail = ex is McpException mcpe ?
+                            JsonRpcErrorDetail detail = ex is McpProtocolException mcpProtocolException ?
                                 new()
                                 {
-                                    Code = (int)mcpe.ErrorCode,
+                                    Code = (int)mcpProtocolException.ErrorCode,
-                                    Message = mcpe.Message,
+                                    Message = mcpProtocolException.Message,
+                                } : ex is McpException mcpException ?
+                                new()
+                                {
+
+                                    Code = (int)McpErrorCode.InternalError,
+                                    Message = mcpException.Message,
                                 } :
                                 new()
                                 {
@@ -336,7 +342,7 @@ internal sealed partial class McpSessionHandler : IAsyncDisposable
         if (!_requestHandlers.TryGetValue(request.Method, out var handler))
         {
             LogNoHandlerFoundForRequest(EndpointName, request.Method);
-            throw new McpException($"Method '{request.Method}' is not available.", McpErrorCode.MethodNotFound);
+            throw new McpProtocolException($"Method '{request.Method}' is not available.", McpErrorCode.MethodNotFound);
         }
 
         LogRequestHandlerCalled(EndpointName, request.Method);
@@ -446,7 +452,7 @@ internal sealed partial class McpSessionHandler : IAsyncDisposable
             if (response is JsonRpcError error)
             {
                 LogSendingRequestFailed(EndpointName, request.Method, error.Error.Message, error.Error.Code);
-                throw new McpException($"Request failed (remote): {error.Error.Message}", (McpErrorCode)error.Error.Code);
+                throw new McpProtocolException($"Request failed (remote): {error.Error.Message}", (McpErrorCode)error.Error.Code);
             }
 
             if (response is JsonRpcResponse success)
@@ -640,7 +646,7 @@ internal sealed partial class McpSessionHandler : IAsyncDisposable
         }
 
         int? intErrorCode =
-            (int?)((e as McpException)?.ErrorCode) is int errorCode ? errorCode :
+            (int?)((e as McpProtocolException)?.ErrorCode) is int errorCode ? errorCode :
             e is JsonException ? (int)McpErrorCode.ParseError :
             null;
 

src/ModelContextProtocol.Core/Server/McpServer.Methods.cs

@@ -291,7 +291,7 @@ public abstract partial class McpServer : McpSession, IMcpServer
     /// <param name="type">The type of the schema being built.</param>
     /// <param name="serializerOptions">The serializer options to use.</param>
     /// <returns>The built request schema.</returns>
-    /// <exception cref="McpException"></exception>
+    /// <exception cref="McpProtocolException"></exception>
     private static ElicitRequestParams.RequestSchema BuildRequestSchema(Type type, JsonSerializerOptions serializerOptions)
     {
         var schema = new ElicitRequestParams.RequestSchema();
@@ -301,7 +301,7 @@ public abstract partial class McpServer : McpSession, IMcpServer
 
         if (typeInfo.Kind != JsonTypeInfoKind.Object)
         {
-            throw new McpException($"Type '{type.FullName}' is not supported for elicitation requests.");
+            throw new McpProtocolException($"Type '{type.FullName}' is not supported for elicitation requests.");
         }
 
         foreach (JsonPropertyInfo pi in typeInfo.Properties)
@@ -319,33 +319,33 @@ public abstract partial class McpServer : McpSession, IMcpServer
     /// <param name="type">The type to create the schema for.</param>
     /// <param name="serializerOptions">The serializer options to use.</param>
     /// <returns>The created primitive schema definition.</returns>
-    /// <exception cref="McpException">Thrown when the type is not supported.</exception>
+    /// <exception cref="McpProtocolException">Thrown when the type is not supported.</exception>
     private static ElicitRequestParams.PrimitiveSchemaDefinition CreatePrimitiveSchema(Type type, JsonSerializerOptions serializerOptions)
     {
         if (type.IsGenericType && type.GetGenericTypeDefinition() == typeof(Nullable<>))
         {
-            throw new McpException($"Type '{type.FullName}' is not a supported property type for elicitation requests. Nullable types are not supported.");
+            throw new McpProtocolException($"Type '{type.FullName}' is not a supported property type for elicitation requests. Nullable types are not supported.");
         }
 
         var typeInfo = serializerOptions.GetTypeInfo(type);
 
         if (typeInfo.Kind != JsonTypeInfoKind.None)
         {
-            throw new McpException($"Type '{type.FullName}' is not a supported property type for elicitation requests.");
+            throw new McpProtocolException($"Type '{type.FullName}' is not a supported property type for elicitation requests.");
         }
 
         var jsonElement = AIJsonUtilities.CreateJsonSchema(type, serializerOptions: serializerOptions);
 
         if (!TryValidateElicitationPrimitiveSchema(jsonElement, type, out var error))
         {
-            throw new McpException(error);
+            throw new McpProtocolException(error);
         }
 
         var primitiveSchemaDefinition =
             jsonElement.Deserialize(McpJsonUtilities.JsonContext.Default.PrimitiveSchemaDefinition);
 
         if (primitiveSchemaDefinition is null)
-            throw new McpException($"Type '{type.FullName}' is not a supported property type for elicitation requests.");
+            throw new McpProtocolException($"Type '{type.FullName}' is not a supported property type for elicitation requests.");
 
         return primitiveSchemaDefinition;
     }

src/ModelContextProtocol.Core/Server/McpServerImpl.cs

@@ -288,7 +288,7 @@ internal sealed partial class McpServerImpl : McpServer
 
         listResourcesHandler ??= (static async (_, __) => new ListResourcesResult());
         listResourceTemplatesHandler ??= (static async (_, __) => new ListResourceTemplatesResult());
-        readResourceHandler ??= (static async (request, _) => throw new McpException($"Unknown resource URI: '{request.Params?.Uri}'", McpErrorCode.InvalidParams));
+        readResourceHandler ??= (static async (request, _) => throw new McpProtocolException($"Unknown resource URI: '{request.Params?.Uri}'", McpErrorCode.InvalidParams));
         subscribeHandler ??= (static async (_, __) => new EmptyResult());
         unsubscribeHandler ??= (static async (_, __) => new EmptyResult());
         var listChanged = resourcesCapability?.ListChanged;
@@ -452,7 +452,7 @@ internal sealed partial class McpServerImpl : McpServer
         ServerCapabilities.Prompts = new();
 
         listPromptsHandler ??= (static async (_, __) => new ListPromptsResult());
-        getPromptHandler ??= (static async (request, _) => throw new McpException($"Unknown prompt: '{request.Params?.Name}'", McpErrorCode.InvalidParams));
+        getPromptHandler ??= (static async (request, _) => throw new McpProtocolException($"Unknown prompt: '{request.Params?.Name}'", McpErrorCode.InvalidParams));
         var listChanged = promptsCapability?.ListChanged;
 
         // Handle tools provided via DI by augmenting the handlers to incorporate them.
@@ -540,7 +540,7 @@ internal sealed partial class McpServerImpl : McpServer
         ServerCapabilities.Tools = new();
 
         listToolsHandler ??= (static async (_, __) => new ListToolsResult());
-        callToolHandler ??= (static async (request, _) => throw new McpException($"Unknown tool: '{request.Params?.Name}'", McpErrorCode.InvalidParams));
+        callToolHandler ??= (static async (request, _) => throw new McpProtocolException($"Unknown tool: '{request.Params?.Name}'", McpErrorCode.InvalidParams));
         var listChanged = toolsCapability?.ListChanged;
 
         // Handle tools provided via DI by augmenting the handlers to incorporate them.
@@ -580,7 +580,7 @@ internal sealed partial class McpServerImpl : McpServer
 
         listToolsHandler = BuildFilterPipeline(listToolsHandler, options.Filters.ListToolsFilters);
         callToolHandler = BuildFilterPipeline(callToolHandler, options.Filters.CallToolFilters, handler =>
-            (request, cancellationToken) =>
+            async (request, cancellationToken) =>
             {
                 // Initial handler that sets MatchedPrimitive
                 if (request.Params?.Name is { } toolName && tools is not null &&
@@ -589,37 +589,23 @@ internal sealed partial class McpServerImpl : McpServer
                     request.MatchedPrimitive = tool;
                 }
 
-                return handler(request, cancellationToken);
+                try
-            }, handler =>
-            async (request, cancellationToken) =>
-            {
-                // Final handler that provides exception handling only for tool execution
-                // Only wrap tool execution in try-catch, not tool resolution
-                if (request.MatchedPrimitive is McpServerTool)
                 {
-                    try
+                    return await handler(request, cancellationToken);
-                    {
-                        return await handler(request, cancellationToken).ConfigureAwait(false);
-                    }
-                    catch (Exception e) when (e is not OperationCanceledException)
-                    {
-                        ToolCallError(request.Params?.Name ?? string.Empty, e);
-
-                        string errorMessage = e is McpException ?
-                            $"An error occurred invoking '{request.Params?.Name}': {e.Message}" :
-                            $"An error occurred invoking '{request.Params?.Name}'.";
-
-                        return new()
-                        {
-                            IsError = true,
-                            Content = [new TextContentBlock { Text = errorMessage }],
-                        };
-                    }
                 }
-                else
+                catch (Exception e) when (e is not OperationCanceledException and not McpProtocolException)
                 {
-                    // For unmatched tools, let exceptions bubble up as protocol errors
+                    ToolCallError(request.Params?.Name ?? string.Empty, e);
-                    return await handler(request, cancellationToken).ConfigureAwait(false);
+
+                    string errorMessage = e is McpException ?
+                        $"An error occurred invoking '{request.Params?.Name}': {e.Message}" :
+                        $"An error occurred invoking '{request.Params?.Name}'.";
+
+                    return new()
+                    {
+                        IsError = true,
+                        Content = [new TextContentBlock { Text = errorMessage }],
+                    };
                 }
             });
 
@@ -735,16 +721,10 @@ internal sealed partial class McpServerImpl : McpServer
     private static McpRequestHandler<TParams, TResult> BuildFilterPipeline<TParams, TResult>(
         McpRequestHandler<TParams, TResult> baseHandler,
         List<McpRequestFilter<TParams, TResult>> filters,
-        McpRequestFilter<TParams, TResult>? initialHandler = null,
+        McpRequestFilter<TParams, TResult>? initialHandler = null)
-        McpRequestFilter<TParams, TResult>? finalHandler = null)
     {
         var current = baseHandler;
 
-        if (finalHandler is not null)
-        {
-            current = finalHandler(current);
-        }
-
         for (int i = filters.Count - 1; i >= 0; i--)
         {
             current = filters[i](current);

tests/ModelContextProtocol.AspNetCore.Tests/AuthorizeAttributeTests.cs

@@ -36,7 +36,7 @@ public class AuthorizeAttributeTests(ITestOutputHelper testOutputHelper) : Kestr
 
         var client = await ConnectAsync();
 
-        var exception = await Assert.ThrowsAsync<McpException>(async () =>
+        var exception = await Assert.ThrowsAsync<McpProtocolException>(async () =>
             await client.CallToolAsync(
                 "authorized_tool",
                 new Dictionary<string, object?> { ["message"] = "test" },
@@ -101,7 +101,7 @@ public class AuthorizeAttributeTests(ITestOutputHelper testOutputHelper) : Kestr
 
         var client = await ConnectAsync();
 
-        var exception = await Assert.ThrowsAsync<McpException>(async () =>
+        var exception = await Assert.ThrowsAsync<McpProtocolException>(async () =>
             await client.CallToolAsync(
                 "admin_tool",
                 new Dictionary<string, object?> { ["message"] = "test" },
@@ -188,7 +188,7 @@ public class AuthorizeAttributeTests(ITestOutputHelper testOutputHelper) : Kestr
 
         var client = await ConnectAsync();
 
-        var exception = await Assert.ThrowsAsync<McpException>(async () =>
+        var exception = await Assert.ThrowsAsync<McpProtocolException>(async () =>
             await client.GetPromptAsync(
                 "authorized_prompt",
                 new Dictionary<string, object?> { ["message"] = "test" },
@@ -235,7 +235,7 @@ public class AuthorizeAttributeTests(ITestOutputHelper testOutputHelper) : Kestr
 
         var client = await ConnectAsync();
 
-        var exception = await Assert.ThrowsAsync<McpException>(async () =>
+        var exception = await Assert.ThrowsAsync<McpProtocolException>(async () =>
             await client.ReadResourceAsync(
                 "resource://authorized",
                 cancellationToken: TestContext.Current.CancellationToken));
@@ -277,7 +277,7 @@ public class AuthorizeAttributeTests(ITestOutputHelper testOutputHelper) : Kestr
         await using var app = await StartServerWithoutAuthFilters(builder => builder.WithTools<AuthorizationTestTools>());
         var client = await ConnectAsync();
 
-        var exception = await Assert.ThrowsAsync<McpException>(async () =>
+        var exception = await Assert.ThrowsAsync<McpProtocolException>(async () =>
             await client.ListToolsAsync(cancellationToken: TestContext.Current.CancellationToken));
 
         Assert.Equal("Request failed (remote): An error occurred.", exception.Message);
@@ -289,21 +289,23 @@ public class AuthorizeAttributeTests(ITestOutputHelper testOutputHelper) : Kestr
     }
 
     [Fact]
-    public async Task CallTool_WithoutAuthFilters_ThrowsInvalidOperationException()
+    public async Task CallTool_WithoutAuthFilters_ReturnsError()
     {
         _mockLoggerProvider.LogMessages.Clear();
         await using var app = await StartServerWithoutAuthFilters(builder => builder.WithTools<AuthorizationTestTools>());
         var client = await ConnectAsync();
 
-        var exception = await Assert.ThrowsAsync<McpException>(async () =>
+        var toolResult = await client.CallToolAsync(
-            await client.CallToolAsync(
                 "authorized_tool",
                 new Dictionary<string, object?> { ["message"] = "test" },
-                cancellationToken: TestContext.Current.CancellationToken));
+                cancellationToken: TestContext.Current.CancellationToken);
 
-        Assert.Equal("Request failed (remote): An error occurred.", exception.Message);
+        Assert.True(toolResult.IsError);
+
+        var errorContent = Assert.IsType<TextContentBlock>(Assert.Single(toolResult.Content));
+        Assert.Equal("An error occurred invoking 'authorized_tool'.", errorContent.Text);
         Assert.Contains(_mockLoggerProvider.LogMessages, log =>
-            log.LogLevel == LogLevel.Warning &&
+            log.LogLevel == LogLevel.Error &&
             log.Exception is InvalidOperationException &&
             log.Exception.Message.Contains("Authorization filter was not invoked for tools/call operation") &&
             log.Exception.Message.Contains("Ensure that AddAuthorizationFilters() is called"));
@@ -316,7 +318,7 @@ public class AuthorizeAttributeTests(ITestOutputHelper testOutputHelper) : Kestr
         await using var app = await StartServerWithoutAuthFilters(builder => builder.WithPrompts<AuthorizationTestPrompts>());
         var client = await ConnectAsync();
 
-        var exception = await Assert.ThrowsAsync<McpException>(async () =>
+        var exception = await Assert.ThrowsAsync<McpProtocolException>(async () =>
             await client.ListPromptsAsync(cancellationToken: TestContext.Current.CancellationToken));
 
         Assert.Equal("Request failed (remote): An error occurred.", exception.Message);
@@ -334,7 +336,7 @@ public class AuthorizeAttributeTests(ITestOutputHelper testOutputHelper) : Kestr
         await using var app = await StartServerWithoutAuthFilters(builder => builder.WithPrompts<AuthorizationTestPrompts>());
         var client = await ConnectAsync();
 
-        var exception = await Assert.ThrowsAsync<McpException>(async () =>
+        var exception = await Assert.ThrowsAsync<McpProtocolException>(async () =>
             await client.GetPromptAsync(
                 "authorized_prompt",
                 new Dictionary<string, object?> { ["message"] = "test" },
@@ -355,7 +357,7 @@ public class AuthorizeAttributeTests(ITestOutputHelper testOutputHelper) : Kestr
         await using var app = await StartServerWithoutAuthFilters(builder => builder.WithResources<AuthorizationTestResources>());
         var client = await ConnectAsync();
 
-        var exception = await Assert.ThrowsAsync<McpException>(async () =>
+        var exception = await Assert.ThrowsAsync<McpProtocolException>(async () =>
             await client.ListResourcesAsync(cancellationToken: TestContext.Current.CancellationToken));
 
         Assert.Equal("Request failed (remote): An error occurred.", exception.Message);
@@ -373,7 +375,7 @@ public class AuthorizeAttributeTests(ITestOutputHelper testOutputHelper) : Kestr
         await using var app = await StartServerWithoutAuthFilters(builder => builder.WithResources<AuthorizationTestResources>());
         var client = await ConnectAsync();
 
-        var exception = await Assert.ThrowsAsync<McpException>(async () =>
+        var exception = await Assert.ThrowsAsync<McpProtocolException>(async () =>
             await client.ReadResourceAsync(
                 "resource://authorized",
                 cancellationToken: TestContext.Current.CancellationToken));
@@ -393,7 +395,7 @@ public class AuthorizeAttributeTests(ITestOutputHelper testOutputHelper) : Kestr
         await using var app = await StartServerWithoutAuthFilters(builder => builder.WithResources<AuthorizationTestResources>());
         var client = await ConnectAsync();
 
-        var exception = await Assert.ThrowsAsync<McpException>(async () =>
+        var exception = await Assert.ThrowsAsync<McpProtocolException>(async () =>
             await client.ListResourceTemplatesAsync(cancellationToken: TestContext.Current.CancellationToken));
 
         Assert.Equal("Request failed (remote): An error occurred.", exception.Message);

tests/ModelContextProtocol.AspNetCore.Tests/HttpServerIntegrationTests.cs

@@ -240,7 +240,7 @@ public abstract class HttpServerIntegrationTests : LoggedTest, IClassFixture<Sse
 
         // act
         await using var client = await GetClientAsync();
-        await Assert.ThrowsAsync<McpException>(async () => await client.GetPromptAsync("non_existent_prompt", null, cancellationToken: TestContext.Current.CancellationToken));
+        await Assert.ThrowsAsync<McpProtocolException>(async () => await client.GetPromptAsync("non_existent_prompt", null, cancellationToken: TestContext.Current.CancellationToken));
     }
 
     [Fact]

tests/ModelContextProtocol.AspNetCore.Tests/MapMcpStreamableHttpTests.cs

@@ -2,6 +2,7 @@
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Primitives;
 using ModelContextProtocol.Client;
+using System.Collections.Concurrent;
 
 namespace ModelContextProtocol.AspNetCore.Tests;
 
@@ -148,7 +149,7 @@ public class MapMcpStreamableHttpTests(ITestOutputHelper outputHelper) : MapMcpT
     [Fact]
     public async Task StreamableHttpClient_SendsMcpProtocolVersionHeader_AfterInitialization()
     {
-        var protocolVersionHeaderValues = new List<string?>();
+        var protocolVersionHeaderValues = new ConcurrentQueue<string?>();
 
         Builder.Services.AddMcpServer().WithHttpTransport(ConfigureStateless).WithTools<EchoHttpContextUserTools>();
 
@@ -160,7 +161,7 @@ public class MapMcpStreamableHttpTests(ITestOutputHelper outputHelper) : MapMcpT
             {
                 if (!StringValues.IsNullOrEmpty(context.Request.Headers["mcp-protocol-version"]))
                 {
-                    protocolVersionHeaderValues.Add(context.Request.Headers["mcp-protocol-version"]);
+                    protocolVersionHeaderValues.Enqueue(context.Request.Headers["mcp-protocol-version"]);
                 }
 
                 await next(context);

tests/ModelContextProtocol.TestServer/Program.cs

@@ -170,7 +170,7 @@ internal static class Program
             {
                 if (request.Params?.Arguments is null || !request.Params.Arguments.TryGetValue("message", out var message))
                 {
-                    throw new McpException("Missing required argument 'message'", McpErrorCode.InvalidParams);
+                    throw new McpProtocolException("Missing required argument 'message'", McpErrorCode.InvalidParams);
                 }
                 return new CallToolResult
                 {
@@ -190,7 +190,7 @@ internal static class Program
                     !request.Params.Arguments.TryGetValue("prompt", out var prompt) ||
                     !request.Params.Arguments.TryGetValue("maxTokens", out var maxTokens))
                 {
-                    throw new McpException("Missing required arguments 'prompt' and 'maxTokens'", McpErrorCode.InvalidParams);
+                    throw new McpProtocolException("Missing required arguments 'prompt' and 'maxTokens'", McpErrorCode.InvalidParams);
                 }
                 var sampleResult = await request.Server.SampleAsync(CreateRequestSamplingParams(prompt.ToString(), "sampleLLM", Convert.ToInt32(maxTokens.GetRawText())),
                     cancellationToken);
@@ -209,7 +209,7 @@ internal static class Program
             }
             else
             {
-                throw new McpException($"Unknown tool: {request.Params?.Name}", McpErrorCode.InvalidParams);
+                throw new McpProtocolException($"Unknown tool: {request.Params?.Name}", McpErrorCode.InvalidParams);
             }
         };
     }
@@ -287,7 +287,7 @@ internal static class Program
             }
             else
             {
-                throw new McpException($"Unknown prompt: {request.Params?.Name}", McpErrorCode.InvalidParams);
+                throw new McpProtocolException($"Unknown prompt: {request.Params?.Name}", McpErrorCode.InvalidParams);
             }
 
             return new GetPromptResult
@@ -305,7 +305,7 @@ internal static class Program
         {
             if (request.Params?.Level is null)
             {
-                throw new McpException("Missing required argument 'level'", McpErrorCode.InvalidParams);
+                throw new McpProtocolException("Missing required argument 'level'", McpErrorCode.InvalidParams);
             }
 
             _minimumLoggingLevel = request.Params.Level;
@@ -387,7 +387,7 @@ internal static class Program
                 }
                 catch (Exception e)
                 {
-                    throw new McpException($"Invalid cursor: '{request.Params.Cursor}'", e, McpErrorCode.InvalidParams);
+                    throw new McpProtocolException($"Invalid cursor: '{request.Params.Cursor}'", e, McpErrorCode.InvalidParams);
                 }
             }
 
@@ -409,7 +409,7 @@ internal static class Program
         {
             if (request.Params?.Uri is null)
             {
-                throw new McpException("Missing required argument 'uri'", McpErrorCode.InvalidParams);
+                throw new McpProtocolException("Missing required argument 'uri'", McpErrorCode.InvalidParams);
             }
 
             if (request.Params.Uri.StartsWith("test://dynamic/resource/"))
@@ -417,7 +417,7 @@ internal static class Program
                 var id = request.Params.Uri.Split('/').LastOrDefault();
                 if (string.IsNullOrEmpty(id))
                 {
-                    throw new McpException($"Invalid resource URI: '{request.Params.Uri}'", McpErrorCode.InvalidParams);
+                    throw new McpProtocolException($"Invalid resource URI: '{request.Params.Uri}'", McpErrorCode.InvalidParams);
                 }
 
                 return new ReadResourceResult
@@ -434,7 +434,7 @@ internal static class Program
             }
 
             ResourceContents contents = resourceContents.FirstOrDefault(r => r.Uri == request.Params.Uri)
-                ?? throw new McpException($"Resource not found: '{request.Params.Uri}'", McpErrorCode.InvalidParams);
+                ?? throw new McpProtocolException($"Resource not found: '{request.Params.Uri}'", McpErrorCode.InvalidParams);
 
             return new ReadResourceResult
             {
@@ -446,12 +446,12 @@ internal static class Program
         {
             if (request?.Params?.Uri is null)
             {
-                throw new McpException("Missing required argument 'uri'", McpErrorCode.InvalidParams);
+                throw new McpProtocolException("Missing required argument 'uri'", McpErrorCode.InvalidParams);
             }
             if (!request.Params.Uri.StartsWith("test://static/resource/")
                 && !request.Params.Uri.StartsWith("test://dynamic/resource/"))
             {
-                throw new McpException($"Invalid resource URI: '{request.Params.Uri}'", McpErrorCode.InvalidParams);
+                throw new McpProtocolException($"Invalid resource URI: '{request.Params.Uri}'", McpErrorCode.InvalidParams);
             }
 
             _subscribedResources.TryAdd(request.Params.Uri, true);
@@ -463,12 +463,12 @@ internal static class Program
         {
             if (request?.Params?.Uri is null)
             {
-                throw new McpException("Missing required argument 'uri'", McpErrorCode.InvalidParams);
+                throw new McpProtocolException("Missing required argument 'uri'", McpErrorCode.InvalidParams);
             }
             if (!request.Params.Uri.StartsWith("test://static/resource/")
                 && !request.Params.Uri.StartsWith("test://dynamic/resource/"))
             {
-                throw new McpException($"Invalid resource URI: '{request.Params.Uri}'", McpErrorCode.InvalidParams);
+                throw new McpProtocolException($"Invalid resource URI: '{request.Params.Uri}'", McpErrorCode.InvalidParams);
             }
 
             _subscribedResources.TryRemove(request.Params.Uri, out _);
@@ -509,7 +509,7 @@ internal static class Program
                     return new CompleteResult { Completion = new() { Values = values, HasMore = false, Total = values.Length } };
 
                 default:
-                    throw new McpException($"Unknown reference type: '{request.Params?.Ref.Type}'", McpErrorCode.InvalidParams);
+                    throw new McpProtocolException($"Unknown reference type: '{request.Params?.Ref.Type}'", McpErrorCode.InvalidParams);
             }
         };
     }

tests/ModelContextProtocol.TestSseServer/Program.cs

@@ -158,13 +158,13 @@ public class Program
             {
                 if (request.Params is null)
                 {
-                    throw new McpException("Missing required parameter 'name'", McpErrorCode.InvalidParams);
+                    throw new McpProtocolException("Missing required parameter 'name'", McpErrorCode.InvalidParams);
                 }
                 if (request.Params.Name == "echo")
                 {
                     if (request.Params.Arguments is null || !request.Params.Arguments.TryGetValue("message", out var message))
                     {
-                        throw new McpException("Missing required argument 'message'", McpErrorCode.InvalidParams);
+                        throw new McpProtocolException("Missing required argument 'message'", McpErrorCode.InvalidParams);
                     }
                     return new CallToolResult
                     {
@@ -184,7 +184,7 @@ public class Program
                         !request.Params.Arguments.TryGetValue("prompt", out var prompt) ||
                         !request.Params.Arguments.TryGetValue("maxTokens", out var maxTokens))
                     {
-                        throw new McpException("Missing required arguments 'prompt' and 'maxTokens'", McpErrorCode.InvalidParams);
+                        throw new McpProtocolException("Missing required arguments 'prompt' and 'maxTokens'", McpErrorCode.InvalidParams);
                     }
                     var sampleResult = await request.Server.SampleAsync(CreateRequestSamplingParams(prompt.ToString(), "sampleLLM", Convert.ToInt32(maxTokens.ToString())),
                         cancellationToken);
@@ -196,7 +196,7 @@ public class Program
                 }
                 else
                 {
-                    throw new McpException($"Unknown tool: '{request.Params.Name}'", McpErrorCode.InvalidParams);
+                    throw new McpProtocolException($"Unknown tool: '{request.Params.Name}'", McpErrorCode.InvalidParams);
                 }
             },
             ListResourceTemplatesHandler = async (request, cancellationToken) =>
@@ -226,7 +226,7 @@ public class Program
                     }
                     catch (Exception e)
                     {
-                        throw new McpException($"Invalid cursor: '{requestParams.Cursor}'", e, McpErrorCode.InvalidParams);
+                        throw new McpProtocolException($"Invalid cursor: '{requestParams.Cursor}'", e, McpErrorCode.InvalidParams);
                     }
                 }
 
@@ -248,7 +248,7 @@ public class Program
             {
                 if (request.Params?.Uri is null)
                 {
-                    throw new McpException("Missing required argument 'uri'", McpErrorCode.InvalidParams);
+                    throw new McpProtocolException("Missing required argument 'uri'", McpErrorCode.InvalidParams);
                 }
 
                 if (request.Params.Uri.StartsWith("test://dynamic/resource/"))
@@ -256,7 +256,7 @@ public class Program
                     var id = request.Params.Uri.Split('/').LastOrDefault();
                     if (string.IsNullOrEmpty(id))
                     {
-                        throw new McpException($"Invalid resource URI: '{request.Params.Uri}'", McpErrorCode.InvalidParams);
+                        throw new McpProtocolException($"Invalid resource URI: '{request.Params.Uri}'", McpErrorCode.InvalidParams);
                     }
 
                     return new ReadResourceResult
@@ -273,7 +273,7 @@ public class Program
                 }
 
                 ResourceContents? contents = resourceContents.FirstOrDefault(r => r.Uri == request.Params.Uri) ??
-                    throw new McpException($"Resource not found: '{request.Params.Uri}'", McpErrorCode.InvalidParams);
+                    throw new McpProtocolException($"Resource not found: '{request.Params.Uri}'", McpErrorCode.InvalidParams);
 
                 return new ReadResourceResult
                 {
@@ -317,7 +317,7 @@ public class Program
             {
                 if (request.Params is null)
                 {
-                    throw new McpException("Missing required parameter 'name'", McpErrorCode.InvalidParams);
+                    throw new McpProtocolException("Missing required parameter 'name'", McpErrorCode.InvalidParams);
                 }
                 List<PromptMessage> messages = new();
                 if (request.Params.Name == "simple_prompt")
@@ -354,7 +354,7 @@ public class Program
                 }
                 else
                 {
-                    throw new McpException($"Unknown prompt: {request.Params.Name}", McpErrorCode.InvalidParams);
+                    throw new McpProtocolException($"Unknown prompt: {request.Params.Name}", McpErrorCode.InvalidParams);
                 }
 
                 return new GetPromptResult

tests/ModelContextProtocol.Tests/ClientIntegrationTests.cs

@@ -193,7 +193,7 @@ public partial class ClientIntegrationTests : LoggedTest, IClassFixture<ClientIn
 
         // act
         await using var client = await _fixture.CreateClientAsync(clientId);
-        await Assert.ThrowsAsync<McpException>(async () => 
+        await Assert.ThrowsAsync<McpProtocolException>(async () => 
             await client.GetPromptAsync("non_existent_prompt", null, cancellationToken: TestContext.Current.CancellationToken));
     }
 

tests/ModelContextProtocol.Tests/Configuration/McpServerBuilderExtensionsFilterTests.cs

@@ -58,7 +58,18 @@ public class McpServerBuilderExtensionsFilterTests : ClientServerTestBase
                 var logger = GetLogger(request.Services, "CallToolFilter");
                 var primitiveId = request.MatchedPrimitive?.Id ?? "unknown";
                 logger.LogInformation($"CallToolFilter executed for tool: {primitiveId}");
-                return await next(request, cancellationToken);
+                try
+                {
+                    return await next(request, cancellationToken);
+                }
+                catch (Exception ex)
+                {
+                    return new CallToolResult
+                    {
+                        Content = [new TextContentBlock { Type = "text", Text = $"Error from filter: {ex.Message}" }],
+                        IsError = true
+                    };
+                }
             })
             .AddListPromptsFilter((next) => async (request, cancellationToken) =>
             {
@@ -162,6 +173,20 @@ public class McpServerBuilderExtensionsFilterTests : ClientServerTestBase
         Assert.Equal("CallToolFilter", logMessage.Category);
     }
 
+    [Fact]
+    public async Task AddCallToolFilter_Catches_Exception_From_Tool()
+    {
+        await using McpClient client = await CreateMcpClientForServer();
+
+        var result = await client.CallToolAsync("throwing_tool_method", cancellationToken: TestContext.Current.CancellationToken);
+
+        Assert.True(result.IsError);
+        Assert.NotNull(result.Content);
+        var textContent = Assert.Single(result.Content);
+        var textBlock = Assert.IsType<TextContentBlock>(textContent);
+        Assert.Equal("Error from filter: This tool always throws an exception", textBlock.Text);
+    }
+
     [Fact]
     public async Task AddListPromptsFilter_Logs_When_ListPrompts_Called()
     {
@@ -286,6 +311,12 @@ public class McpServerBuilderExtensionsFilterTests : ClientServerTestBase
         {
             return "test result";
         }
+
+        [McpServerTool]
+        public static string ThrowingToolMethod()
+        {
+            throw new InvalidOperationException("This tool always throws an exception");
+        }
     }
 
     [McpServerPromptType]

tests/ModelContextProtocol.Tests/Configuration/McpServerBuilderExtensionsPromptsTests.cs

@@ -62,7 +62,7 @@ public partial class McpServerBuilderExtensionsPromptsTests : ClientServerTestBa
                                 };
 
                             default:
-                                throw new McpException($"Unexpected cursor: '{cursor}'", McpErrorCode.InvalidParams);
+                                throw new McpProtocolException($"Unexpected cursor: '{cursor}'", McpErrorCode.InvalidParams);
                         }
                     })
         .WithGetPromptHandler(async (request, cancellationToken) =>
@@ -78,7 +78,7 @@ public partial class McpServerBuilderExtensionsPromptsTests : ClientServerTestBa
                     };
 
                 default:
-                    throw new McpException($"Unknown prompt '{request.Params?.Name}'", McpErrorCode.InvalidParams);
+                    throw new McpProtocolException($"Unknown prompt '{request.Params?.Name}'", McpErrorCode.InvalidParams);
             }
         })
         .WithPrompts<SimplePrompts>();
@@ -190,7 +190,7 @@ public partial class McpServerBuilderExtensionsPromptsTests : ClientServerTestBa
     {
         await using McpClient client = await CreateMcpClientForServer();
 
-        await Assert.ThrowsAsync<McpException>(async () => await client.GetPromptAsync(
+        await Assert.ThrowsAsync<McpProtocolException>(async () => await client.GetPromptAsync(
             nameof(SimplePrompts.ThrowsException),
             cancellationToken: TestContext.Current.CancellationToken));
     }
@@ -200,7 +200,7 @@ public partial class McpServerBuilderExtensionsPromptsTests : ClientServerTestBa
     {
         await using McpClient client = await CreateMcpClientForServer();
 
-        var e = await Assert.ThrowsAsync<McpException>(async () => await client.GetPromptAsync(
+        var e = await Assert.ThrowsAsync<McpProtocolException>(async () => await client.GetPromptAsync(
             "NotRegisteredPrompt",
             cancellationToken: TestContext.Current.CancellationToken));
 
@@ -212,7 +212,7 @@ public partial class McpServerBuilderExtensionsPromptsTests : ClientServerTestBa
     {
         await using McpClient client = await CreateMcpClientForServer();
 
-        var e = await Assert.ThrowsAsync<McpException>(async () => await client.GetPromptAsync(
+        var e = await Assert.ThrowsAsync<McpProtocolException>(async () => await client.GetPromptAsync(
             "returns_chat_messages",
             cancellationToken: TestContext.Current.CancellationToken));
 

tests/ModelContextProtocol.Tests/Configuration/McpServerBuilderExtensionsResourcesTests.cs

@@ -62,7 +62,7 @@ public partial class McpServerBuilderExtensionsResourcesTests : ClientServerTest
                                 };
 
                             default:
-                                throw new McpException($"Unexpected cursor: '{cursor}'", McpErrorCode.InvalidParams);
+                                throw new McpProtocolException($"Unexpected cursor: '{cursor}'", McpErrorCode.InvalidParams);
                         }
                     })
                 .WithListResourceTemplatesHandler(async (request, cancellationToken) =>
@@ -91,7 +91,7 @@ public partial class McpServerBuilderExtensionsResourcesTests : ClientServerTest
                                     }],
                                 };
                             default:
-                                throw new McpException($"Unexpected cursor: '{cursor}'", McpErrorCode.InvalidParams);
+                                throw new McpProtocolException($"Unexpected cursor: '{cursor}'", McpErrorCode.InvalidParams);
                         }
                     })
         .WithReadResourceHandler(async (request, cancellationToken) =>
@@ -109,7 +109,7 @@ public partial class McpServerBuilderExtensionsResourcesTests : ClientServerTest
                     };
             }
 
-            throw new McpException($"Resource not found: {request.Params?.Uri}");
+            throw new McpProtocolException($"Resource not found: {request.Params?.Uri}");
         })
         .WithResources<SimpleResources>();
     }
@@ -235,7 +235,7 @@ public partial class McpServerBuilderExtensionsResourcesTests : ClientServerTest
     {
         await using McpClient client = await CreateMcpClientForServer();
 
-        await Assert.ThrowsAsync<McpException>(async () => await client.ReadResourceAsync(
+        await Assert.ThrowsAsync<McpProtocolException>(async () => await client.ReadResourceAsync(
             $"resource://mcp/{nameof(SimpleResources.ThrowsException)}",
             cancellationToken: TestContext.Current.CancellationToken));
     }
@@ -245,7 +245,7 @@ public partial class McpServerBuilderExtensionsResourcesTests : ClientServerTest
     {
         await using McpClient client = await CreateMcpClientForServer();
 
-        var e = await Assert.ThrowsAsync<McpException>(async () => await client.ReadResourceAsync(
+        var e = await Assert.ThrowsAsync<McpProtocolException>(async () => await client.ReadResourceAsync(
             "test:///NotRegisteredResource",
             cancellationToken: TestContext.Current.CancellationToken));
 

tests/ModelContextProtocol.Tests/Configuration/McpServerBuilderExtensionsToolsTests.cs

@@ -90,7 +90,7 @@ public partial class McpServerBuilderExtensionsToolsTests : ClientServerTestBase
                         };
 
                     default:
-                        throw new McpException($"Unexpected cursor: '{cursor}'", McpErrorCode.InvalidParams);
+                        throw new McpProtocolException($"Unexpected cursor: '{cursor}'", McpErrorCode.InvalidParams);
                 }
             })
             .WithCallToolHandler(async (request, cancellationToken) =>
@@ -106,7 +106,7 @@ public partial class McpServerBuilderExtensionsToolsTests : ClientServerTestBase
                         };
 
                     default:
-                        throw new McpException($"Unknown tool: '{request.Params?.Name}'", McpErrorCode.InvalidParams);
+                        throw new McpProtocolException($"Unknown tool: '{request.Params?.Name}'", McpErrorCode.InvalidParams);
                 }
             })
             .WithTools<EchoTool>(serializerOptions: BuilderToolsJsonContext.Default.Options);
@@ -388,7 +388,7 @@ public partial class McpServerBuilderExtensionsToolsTests : ClientServerTestBase
     {
         await using McpClient client = await CreateMcpClientForServer();
 
-        var e = await Assert.ThrowsAsync<McpException>(async () => await client.CallToolAsync(
+        var e = await Assert.ThrowsAsync<McpProtocolException>(async () => await client.CallToolAsync(
             "NotRegisteredTool",
             cancellationToken: TestContext.Current.CancellationToken));
 

tests/ModelContextProtocol.Tests/DiagnosticTests.cs

@@ -87,7 +87,7 @@ public class DiagnosticTests
             await RunConnected(async (client, server) =>
             {
                 await client.CallToolAsync("Throw", cancellationToken: TestContext.Current.CancellationToken);
-                await Assert.ThrowsAsync<McpException>(async () => await client.CallToolAsync("does-not-exist", cancellationToken: TestContext.Current.CancellationToken));
+                await Assert.ThrowsAsync<McpProtocolException>(async () => await client.CallToolAsync("does-not-exist", cancellationToken: TestContext.Current.CancellationToken));
             }, new List<string>());
         }
 

tests/ModelContextProtocol.Tests/Protocol/ElicitationTypedTests.cs

@@ -248,7 +248,7 @@ public partial class ElicitationTypedTests : ClientServerTestBase
             },
         });
 
-        var ex = await Assert.ThrowsAsync<McpException>(async() =>
+        var ex = await Assert.ThrowsAsync<McpProtocolException>(async() =>
             await client.CallToolAsync("TestElicitationUnsupportedType", cancellationToken: TestContext.Current.CancellationToken));
 
         Assert.Contains(typeof(UnsupportedForm.Nested).FullName!, ex.Message);
@@ -270,7 +270,7 @@ public partial class ElicitationTypedTests : ClientServerTestBase
             }
         });
 
-        var ex = await Assert.ThrowsAsync<McpException>(async () =>
+        var ex = await Assert.ThrowsAsync<McpProtocolException>(async () =>
             await client.CallToolAsync("TestElicitationNullablePropertyForm", cancellationToken: TestContext.Current.CancellationToken));
     }
 
@@ -290,7 +290,7 @@ public partial class ElicitationTypedTests : ClientServerTestBase
             }
         });
 
-        var ex = await Assert.ThrowsAsync<McpException>(async () =>
+        var ex = await Assert.ThrowsAsync<McpProtocolException>(async () =>
             await client.CallToolAsync("TestElicitationNonObjectGenericType", cancellationToken: TestContext.Current.CancellationToken));
 
         Assert.Contains(typeof(string).FullName!, ex.Message);