The Model Context Protocol C# SDK uses the [Experimental] attribute to mark APIs that are still in development and may change without notice. For more details on the SDK's versioning policy around experimental APIs, see the Versioning documentation.

Suppressing experimental diagnostics

When you use an experimental API, the compiler produces a diagnostic (e.g., MCPEXP001) to ensure you're aware the API may change. If you want to use the API, suppress the diagnostic in one of these ways:

Project-wide suppression

Add the diagnostic ID to <NoWarn> in your project file:

<PropertyGroup>
  <NoWarn>$(NoWarn);MCPEXP001</NoWarn>
</PropertyGroup>

Per-call suppression

Use #pragma warning disable around specific call sites:

#pragma warning disable MCPEXP001 // The Tasks feature is experimental per the MCP specification and is subject to change.
tool.Execution = new ToolExecution { ... };
#pragma warning restore MCPEXP001

For a full list of experimental diagnostic IDs and their descriptions, see the list of diagnostics.

Serialization behavior

Experimental properties on protocol types are fully serialized and deserialized when using the SDK's built-in serialization via DefaultOptions. This means experimental data is transmitted on the wire even if your application code doesn't directly interact with it, preserving protocol compatibility.

The behavior of experimental properties differs depending on whether you use reflection-based or source-generated serialization:

  • Reflection-based serialization (the default when no JsonSerializerContext is used): Experimental properties are included. No special configuration is needed.
  • Source-generated serialization (using a custom JsonSerializerContext): Experimental properties are not included in your context's serialization contract. This is by design, as it protects your compiled code against binary breaking changes to experimental APIs.

This means that switching between reflection-based and source-generated serialization can silently change which properties are serialized. To avoid this, source-generation users should configure a TypeInfoResolverChain as described below.

Custom JsonSerializerContext

If you define your own JsonSerializerContext that includes MCP protocol types, configure a TypeInfoResolverChain so the SDK's resolver handles MCP types:

using ModelContextProtocol;

JsonSerializerOptions options = new()
{
    TypeInfoResolverChain =
    {
        McpJsonUtilities.DefaultOptions.TypeInfoResolver!,
        MyCustomContext.Default,
    }
};

By placing the SDK's resolver first, MCP types are serialized using the SDK's contract (which includes experimental properties), while your custom context handles your own types. This is recommended even if you aren't currently using experimental APIs, since it ensures your serialization configuration remains correct as new experimental properties are introduced or as you adopt experimental features in the future.

See also

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