Crate flymcprust 

FlyMCPRust: high-performance MCP server SDK for Rust. An ergonomic, safe and fast way to build Model Context Protocol servers, targeting specification revision 2026-07-28 (stateless).

use flymcprust::prelude::*;

/// Adds two numbers.
///
/// # Arguments
/// * `a` - First operand.
/// * `b` - Second operand.
#[tool(read_only, idempotent)]
async fn add(a: f64, b: f64) -> f64 {
    a + b
}

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    Server::builder("calculator", "1.0.0")
        .tool::<add>()
        .build()?
        .serve_stdio()
        .await?;
    Ok(())
}

Features

Feature	Default	Description
macros	yes	The tool, resource and prompt attribute macros.
stdio	no	stdio transport (Server::serve_stdio).
http	no	Streamable HTTP transport (transport::http).
oauth	no	auth::JwtVerifier for OAuth access tokens (implies http).
auto-register	no	ServerBuilder::auto_tools (and auto_resources, auto_prompts) collect every annotated item.
logging	no	logging::init() writes the log, with fix-it hints, to stderr.
test-client	no	In-process testing::TestClient for tests.

See the repository’s docs/ARCHITECTURE.md for design decisions.

Modules

apps

MCP Apps (io.modelcontextprotocol/ui): tools that render an interactive HTML interface in the host.

auth

Authorization: the authenticated caller (Principal), access-token verification (TokenVerifier) and scope checks.

jsonrpc

JSON-RPC 2.0 envelope parsing and response serialization.

logging

Logging setup (feature logging).

message

Builders for prompt messages.

prelude

The most common imports.

security

Helpers for handling untrusted client input safely.

spec

Protocol types of the targeted revision. Protocol types for MCP specification revision 2026-07-28.

testing

In-process test client (feature test-client).

transport

Transports. Each one only moves bytes; all protocol logic lives in Server.

Structs

AppUi

How the host should sandbox a UI resource (_meta.ui).

Completion

Completion values. Results longer than MAX_COMPLETION_VALUES are truncated, with total and hasMore set accordingly.

CompletionRequest

What the client asked to complete.

Ctx

Context of the request being handled. Declare a Ctx parameter in a tool to receive it; it is excluded from the tool’s input schema.

Json

Wraps a value returned as structured content. The tool’s outputSchema is generated from T, and the result carries both structuredContent and the serialized JSON as text, as the specification recommends.

Limits

Resource limits applied to every request.

McpError

A JSON-RPC error returned to the client.

McpRequest

A parsed message plus transport-provided context.

MirroredParam

A tool parameter mirrored into an Mcp-Param-{name} header.

Principal

The caller authenticated by an access token.

PromptArguments

The arguments of a prompts/get request.

PromptDef

A registered prompt: its protocol description plus its entry point.

ResourceDef

A registered resource or resource template.

Server

A built MCP server. Cheap to clone; all clones share one registry.

ServerBuilder

Builds a Server.

State

Shared application state registered with ServerBuilder::state. Declare a State<T> parameter in a tool to receive it; it is excluded from the tool’s input schema. A tool that needs a state type that was never registered makes ServerBuilder::build fail.

TokenError

Why a token was rejected. The reason is logged on the server; clients only learn that the token is invalid.

ToolDef

A registered tool: its protocol description plus its entry point.

ToolError

An error raised by a tool, resource or prompt handler.

TransportContext

Values attached by the transport to a request (for example, authenticated claims or HTTP headers). Tower middleware can populate it before the request reaches the server.

UriTemplate

A parsed URI template.

UriTemplateError

A template that cannot be parsed or is not supported.

UriVars

Variable values extracted from a URI by UriTemplate::matches.

Enums

BuildError

Errors detected while building a server.

Elicitation

The outcome of an elicitation.

ErrorKind

How an error relates to the request, independent of transport. HTTP uses it to pick a status code (400, 404, 200 or 500).

ParamKind

JSON Schema primitive type of a mirrored parameter.

Permission

A browser capability a UI asks the host for. Hosts may refuse.

ToolOutput

What a tool produced.

Visibility

Who may call a tool.

Constants

MIN_REQUEST_STATE_KEY_BYTES

Minimum length of a requestState signing key.

Traits

Extension

A protocol extension supported by the server.

IntoContent

Content that can appear in a message: &str, String (as text) and the specification’s content types.

IntoPromptResult

Conversion of a prompt’s return value into a result.

IntoResourceContents

Conversion of a resource handler’s return value into contents.

IntoToolError

Conversion of tool errors. Implemented for ToolError and for every std::error::Error (treated as internal).

IntoToolResult

Conversion of a tool’s return value into a result.

Prompt

A prompt that can be registered with ServerBuilder::prompt.

Resource

A resource that can be registered with ServerBuilder::resource.

TokenVerifier

Checks an access token and identifies its holder.

Tool

A tool that can be registered with ServerBuilder::tool.

Type Aliases

CompleteFn

A completion provider. Completion runs synchronously and must be fast.

ResourceContents

One item of a resources/read result.

ToolResult

The outcome of a tool invocation.

Attribute Macros

prompt

Turns a free function (sync or async) into an MCP prompt.

resource

Turns a free function (sync or async) into an MCP resource or resource template.

tool

Turns a free function (sync or async) into an MCP tool.