C# MCP SDK

Implements bidirectional JSON-RPC 2.0 message serialization using System.Text.Json with custom converters for MCP protocol types. The SDK handles request/response/notification message framing, error serialization with standardized error codes, and automatic message ID generation for request tracking. Built on top of ModelContextProtocol.Core package with pluggable JSON serialization configuration to support custom type converters and null-handling strategies.

Provides a fluent builder API for configuring MCP servers with tool, prompt, and resource capabilities. The ServerOptions builder pattern allows declarative registration of handlers via dependency injection, with automatic parameter resolution from method signatures. Supports both standalone servers and ASP.NET Core integration, with built-in support for request/response filtering, cancellation tokens, and structured error handling. The server manages the full lifecycle including initialization, capability advertisement, and graceful shutdown.

Provides infrastructure for managing tool invocations that take significant time to complete, with built-in progress reporting to clients. Tools can report progress updates during execution, and clients receive notifications of progress changes. The SDK handles progress state management, client notification delivery, and task cancellation. Supports both determinate progress (percentage complete) and indeterminate progress (activity indication).

Enables servers to push resource change notifications to subscribed clients without requiring polling. Clients subscribe to resources with optional filters, and servers send notifications when resource content changes. The SDK manages subscription state, client notification delivery, and cleanup on unsubscription. Supports both full content updates and delta updates for efficient bandwidth usage. Includes automatic resubscription on connection recovery.

Provides seamless integration of MCP servers into ASP.NET Core applications via dedicated middleware and service registration extensions. The integration allows MCP servers to run alongside standard ASP.NET Core endpoints, sharing dependency injection, configuration, and authentication/authorization infrastructure. Supports both HTTP transport and stdio transport for MCP communication. Includes automatic OpenAPI/Swagger documentation generation for MCP capabilities.

Implements comprehensive cancellation support via CancellationToken throughout the SDK, allowing clients to cancel long-running operations. Provides structured error handling with standardized MCP error codes (parse error, invalid request, method not found, etc.) and detailed error messages. Errors include optional error data for additional context. Supports both synchronous and asynchronous error handling with proper exception propagation.

Provides Roslyn-based analyzers that verify MCP server implementations at compile-time, catching common errors before runtime. Source generators emit boilerplate code for tool registration, parameter resolution, and schema generation, eliminating manual code writing. Analyzers check for schema mismatches between tool definitions and implementations, missing required parameters, and invalid capability configurations. Generators produce efficient, reflection-free code for handler invocation.

Implements OAuth 2.0 client-side flows for authenticating with OAuth-protected MCP servers. Handles authorization code flow with automatic redirect URI handling, token exchange, and token refresh. Manages token storage in client session with automatic token refresh before expiration. Supports both interactive (user-initiated) and non-interactive (client credentials) flows. Integrates with platform-specific authentication UI for user consent.

Provides automatic conversion between different content types for resources, allowing servers to expose the same resource in multiple formats (JSON, text, binary, etc.). The SDK includes built-in converters for common types and a pluggable converter interface for custom types. Clients can request specific content types, and the server automatically converts resource content. Supports content negotiation based on client preferences.

Enables developers to define tools as C# methods with automatic JSON Schema generation from parameter types and attributes. The SDK uses Roslyn source generators to create strongly-typed tool wrappers that validate input against the generated schema at invocation time. Tool handlers receive parsed, type-safe parameters and can return structured results or errors. Supports complex parameter types including nested objects, arrays, and enums with full schema documentation.

Allows developers to define reusable prompt templates with named arguments that can be invoked by LLMs. Templates are registered on the server with descriptions and argument schemas, and the SDK handles argument validation and template rendering. Supports both simple string templates and complex multi-part prompts with structured content types. Prompts can be parameterized with user inputs, context data, or results from other tools.

Enables servers to expose resources (documents, files, data sources) with URI-based addressing and optional subscription support for real-time updates. Resources are registered with MIME types and content handlers, and clients can list, read, and subscribe to resource changes. The SDK manages subscription lifecycle including change notifications, unsubscription, and cleanup. Supports both static resources and dynamic content generation based on resource URIs.

Provides a client API for connecting to MCP servers and discovering their capabilities (tools, prompts, resources). The client handles connection establishment over configurable transports (stdio, HTTP), capability negotiation, and session management. Clients can list available tools, prompts, and resources, and invoke them with automatic request/response handling. Supports both synchronous and asynchronous operations with cancellation token support throughout.

Provides a pluggable transport layer supporting multiple communication mechanisms: stdio (for local process communication), HTTP client (for remote servers), and streamable HTTP protocol (for bidirectional communication over HTTP). Each transport implements ITransport with connection lifecycle management, message framing, and error handling. The abstraction allows seamless switching between transports without changing client/server code. Includes automatic reconnection logic and connection pooling for HTTP transports.

Integrates with Microsoft.Extensions.DependencyInjection to automatically resolve handler method parameters from the service container. The SDK uses reflection and source generators to inspect handler signatures and inject registered services, configuration objects, and protocol-specific parameters (like CancellationToken). Supports both constructor injection and method parameter injection, with automatic validation that all dependencies are registered. Enables handlers to depend on any service in the DI container without explicit parameter mapping.

Provides built-in OAuth 2.0 support for protecting MCP server capabilities with authentication and authorization. Servers can declare OAuth requirements in capability metadata, and the SDK handles OAuth flow initiation, token exchange, and credential validation. Supports both authorization code flow and client credentials flow. Integrates with ASP.NET Core authentication middleware for seamless authorization policy enforcement on tools, prompts, and resources.

Implements a middleware-style filtering system for intercepting and modifying requests and responses at the protocol level. Filters can inspect, validate, or transform messages before they reach handlers or are sent to clients. The SDK provides built-in filters for logging, metrics collection, and error handling, with a simple interface for custom filters. Filters execute in a chain with early termination support, allowing filters to reject requests or modify responses.