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There is no confirmed universal “two-line fix” for a .NET MCP tool class crash: the right change depends on the exception, the SDK version, and whether the failure happens at startup or when a tool is called. The supported pattern is to mark the class and methods with the MCP attributes, then register the class explicitly or discover it from an assembly. Those steps are a useful first check, not a diagnosis of every runtime failure.
How the .NET MCP SDK finds attribute-based tools
For attribute-based tools, put [McpServerToolType] on the containing class and [McpServerTool] on each method you want to expose. The SDK guide’s example is:
[McpServerToolType]
public class MyTools
{
[McpServerTool, Description("Echoes the input message back")]
public static string Echo(string message) => $"Echo: {message}";
}The attributes describe candidate tools; the server also needs a registration path so it can find them. The official tool guide shows explicit type registration:
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builder.Services.AddMcpServer()
.WithHttpTransport(o => o.Stateless = true)
.WithTools<MyTools>();For a server whose tools are in the scanned assembly, the getting-started guide demonstrates .WithToolsFromAssembly(). It discovers classes marked with [McpServerToolType] and registers methods marked with [McpServerTool]. The same guide shows it alongside AddMcpServer() and WithStdioServerTransport() for a stdio server. See the official tools guide and getting-started guide.
Choose explicit registration or assembly discovery
| Approach | Scope | When it fits |
|---|---|---|
.WithTools<MyTools>() |
Registers the named tool type. | A small server or a project where you want the registered tool types to be explicit. |
.WithToolsFromAssembly() |
Discovers attributed tool classes in the assembly being scanned. | A server that keeps its tools together in an assembly, including a shared tool assembly. |
The documentation establishes both choices, but does not say one is universally safer or faster. Check that your chosen registration path actually includes the class you expect; an attributed class that is never registered or discovered will not be available as a server tool.
Dependency injection does not identify the cause by itself
The SDK allows tool methods to accept services registered with dependency injection. If an instance tool fails during construction, inspect its constructor and the registrations for the services it requests. If the tool is discovered but fails only when invoked, inspect the method’s arguments and its service dependencies. The presence of DI alone does not establish why a class crashed, and the documented static example does not mean every valid tool must be static. The tools guide also describes type and factory registration approaches; follow the approach appropriate to the class’s construction needs.
Rank #2
Find the failure stage before changing code
Capture the complete exception and stack trace, then establish when it occurs. Startup or discovery, tool invocation, and transport connection are separate parts of the server path; a change to tool attributes will not necessarily fix a transport mismatch.
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- Check the installed MCP package and version, and confirm the relevant attributes and namespaces are present.
- Confirm the class is included through
.WithTools<YourToolType>()or the assembly passed to.WithToolsFromAssembly(). - If the class needs an instance, check its constructor and the dependencies available through the registration approach you use.
Crash when a client calls a tool
- Check whether the incoming JSON arguments can bind to the method’s parameters.
- Inspect the method body and any injected services it uses for the actual exception.
- Distinguish a normal exception from an MCP protocol exception: the SDK documentation describes different error behavior for these cases. That behavior does not identify the cause of a particular crash.
Failure to connect or exchange messages
Check transport configuration independently of tool discovery. The official getting-started guide shows separate setup paths for stdio and HTTP; make sure the server’s transport and the client’s connection configuration match the path you intend to use.
Rank #3
Check the SDK version before applying version-specific advice
Microsoft announced MCP C# SDK v2.0 on July 28, 2026, describing it as implementing the 2026-07-28 revision of the MCP specification. The announcement says stable v1 code continues to compile and run after upgrading. It lists the v2 package target frameworks as net8.0, net9.0, net10.0, and netstandard2.0. It identifies ModelContextProtocol as the normal package for hosted or stdio use, ModelContextProtocol.AspNetCore for HTTP servers, and ModelContextProtocol.Core for client or low-level APIs. Check your installed package and the release notes before applying advice tied to a particular version. See Microsoft’s v2.0 announcement.
Why the exact two-line fix cannot be named yet
The official guidance documents the attribute and registration patterns, but it does not connect this title’s crash to a specific SDK defect or reproducible two-line patch. Without the affected SDK version, the code, and the full exception, naming a particular attribute, static modifier, constructor, or registration call as the cause would be guesswork. Treat the patterns above as checks; the exception and failure stage determine the fix.
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