Crate fastmcp_server

Expand description

MCP server implementation for FastMCP.

This crate provides the server-side implementation:

Server builder pattern
Tool, resource, and prompt registration
Request routing and dispatching
Session management

MCP 2026-07-28 support is under implementation and remains unverified. The public protocol constant is still 2024-11-05; server source presence is not aggregate conformance or release evidence.

§Example

use fastmcp_rust::prelude::*;

#[tool]
async fn greet(ctx: &McpContext, name: String) -> McpResult<String> {
    ctx.checkpoint()?;
    Ok(format!("Hello, {name}!"))
}

fn main() {
    Server::new("my-server", "1.0.0")
        .tool(Greet)
        .build()
        .run_stdio();
}

§Role in the System

fastmcp-server is the execution engine for MCP servers. It ties together:

Protocol types (fastmcp-protocol) for requests and responses
Transports (fastmcp-transport) for stdio/SSE/WebSocket/HTTP/memory I/O
Core context + cancellation (fastmcp-core) for budgets and checkpoints
Console output (fastmcp-console) for human-friendly stderr rendering

The façade package fastmcp-rust re-exports this API, so most users interact with Server via fastmcp_rust::prelude::*.

§Extension panic-containment boundary

Unwinding extension callback panics (including handler, auth, middleware, and lifecycle callbacks) are caught at their framework boundaries. Request-path failures are mapped to a fixed payload-free peer error. FastMCP also installs a redacting panic hook for local diagnostics, but Rust’s hook is process-global and replaceable: an embedding application that installs another hook afterward controls subsequent panic diagnostics. Such replacement does not change the fixed peer error, but FastMCP cannot promise payload-free local diagnostics after its hook has been replaced.

Re-exports§

pub use extensions::ExtensionHandler;
pub use extensions::ExtensionHandlerInvocationError;
pub use extensions::ExtensionHandlerKey;
pub use extensions::ExtensionHandlerLookupError;
pub use extensions::ExtensionHandlerRegistrationError;
pub use extensions::ExtensionHandlerRegistry;
pub use bidirectional::PendingRequests;
pub use bidirectional::RequestSender;
pub use bidirectional::TransportElicitationSender;
pub use bidirectional::TransportRootsProvider;
pub use bidirectional::TransportSamplingSender;

Modules§

bidirectional: Bidirectional request handling for server-to-client communication.
caching: Response caching middleware for MCP servers.
extensions: Typed server handlers for negotiated MCP extensions.
http_admission: HTTP-02 A: modern server HTTP admission and validation pipeline.
legacy_2024: Exact MCP 2024-11-05 server lifecycle adapter.
oauth: OAuth 2.0/2.1 authorization-server implementation code for MCP.
oidc: OpenID Connect (OIDC) Provider for MCP.
providers: Built-in resource providers for common use cases.
rate_limiting: Rate limiting middleware for protecting FastMCP servers from abuse.
transform: Tool transformations for dynamic schema modification.

Structs§

AllowAllAuthProvider: Default allow-all provider (returns anonymous auth context).
AuthRequest: Authentication request view used by providers.
BidirectionalSenders: Configuration for bidirectional senders to attach to context.
BoundHttpServer: A bound, caller-owned HTTP server lifecycle.
ConsoleConfig: Shared configuration for FastMCP console output and host integrations.
FinalElicitation: One final-era elicitation request that a handler returns as MRTR input.
FinalRoots: One final-era roots request that a handler returns as MRTR input.
FinalSampling: One final-era sampling request that a handler returns as MRTR input.
HttpNonquiescentShutdown: Caller-owned settlement authority for a bounded, nonquiescent HTTP stop.
HttpServerConfig: Configuration for the live HTTP listener used by Server::run_http.
InboundRequestContext: Sanitized, immutable ingress facts for one server dispatch.
LifespanHooks: Lifecycle hooks for server startup and shutdown.
LoggingConfig: Logging configuration for the server.
MountResult: Result of a mount operation.
ProgressNotificationSender: A notification sender that sends progress notifications via a callback.
Router: Routes MCP requests to the appropriate handlers.
Server: An MCP server instance.
ServerBuilder: Builder for configuring an MCP server.
ServerHttpEndpoint: A live server composition over a bounded HTTP endpoint.
ServerHttpRequestCancellation: Cancellation view shared by one public HTTP response body and its MCP request authority.
ServerHttpSession: A server-owned session opened through ServerHttpEndpoint.
ServerHttpSseResponse: A server-bound modern SSE response body.
ServerStats: Thread-safe server statistics collector.
Session: An MCP session between client and server.
StaticTokenVerifier: Static token verifier backed by fixed-width token digests.
StatsSnapshot: Point-in-time snapshot of server statistics.
SubscriptionListenHandle: Caller-owned in-process subscriptions/listen stream.
TagFilters: Tag filtering parameters for list operations.
TokenAuthProvider: Token-based authentication provider.

Enums§

BannerStyle: Style variants for the startup banner
DuplicateBehavior: Behavior when registering a component with a name that already exists.
FinalMethodOutcome: One application-authored result of a final method that may require input.
FinalResourceReadCacheHintProvenance: Identifies who selected a complete final resource-read cache policy.
FinalToolOutcome: One application-authored outcome of a final tools/call handler.
FinalToolSchemaAuthority: Legacy diagnostic label for an exact-final tool’s schema ownership.
HttpServerShutdown: The complete outcome of BoundHttpServer::serve.
HttpShutdownSettlement: The state of a caller-owned nonquiescent HTTP shutdown handle.
InboundRequestTransport: Allowlisted transport provenance attached to a sanitized inbound request.
MiddlewareDecision: Result of middleware request interception.
ServerExtensionConfigurationError: Failure while installing the server-owned extension registry.
ServerExtensionNegotiationError: Failure while deriving a current-exchange extension capability set.
ServerHttpEndpointError: Failure while constructing or operating a public server HTTP endpoint.
ServerHttpEndpointResponse: A response emitted by ServerHttpSession::handle.
ServerLaunchPolicyError: The selected launch policy is malformed, not valid Unicode, or unavailable in the compiled server feature set.
ToolErrorKind: Closed framework error classes that an output-schema tool must represent.
TrafficVerbosity: Verbosity levels for traffic logging

Traits§

AuthProvider: Authentication provider interface.
CompletionHandler: Handler for completion/complete in both supported protocol eras.
FinalElicitationContextExt: Final-only elicitation construction for McpContext.
FinalRootsContextExt: Final-only roots construction for McpContext.
FinalSamplingContextExt: Final-only sampling construction for McpContext.
Middleware: Middleware hook trait for request/response interception.
PromptHandler: Handler for a prompt.
ResourceHandler: Handler for a resource.
TokenVerifier: Token verifier interface used by token-based auth providers.
ToolHandler: Handler for a tool.

Functions§

create_context_with_progress: Helper to create an McpContext with optional progress reporting and session state.
create_context_with_progress_and_senders: Helper to create an McpContext with optional progress reporting, session state, and bidirectional senders.
promote_legacy_prompt_messages: Promotes legacy prompt messages for a final handler default.
promote_legacy_resource_contents: Promotes legacy resource contents for a final handler default.
promote_legacy_tool_content: Promotes an exact legacy tool payload into the final complete-result algebra.

Type Aliases§

BoxFuture: A boxed future for async handler results.
NotificationSender: Type alias for a notification sender callback.
ShutdownHook: Type alias for shutdown hook function.
StartupHook: Type alias for startup hook function.