Crate tower_mcp_types 

Standalone MCP protocol and error types – zero runtime dependencies.

tower-mcp-types ships Model Context Protocol types and JSON-RPC 2.0 primitives as a self-contained crate. It has no dependency on Tower, Tokio, axum, or any async runtime – only serde, serde_json, thiserror, and base64.

This makes it suitable for:

• WASM targets (browser clients, edge runtimes, editor extensions)
• Code generation tools that need to inspect or emit MCP messages
• Thin clients and proxies that only need to parse/serialize MCP wire format without running a full server
• Testing libraries that want MCP types without pulling in a server stack

The inspection module provides strict, transport-free classification of JSON-RPC requests, notifications, success responses, error responses, and batches for intermediaries that observe both sides of a connection. Its inspection::McpInspector layers exact-revision MCP method, direction, batching, and typed parameter checks on top without selecting a revision implicitly or changing an application’s runtime allowlist.

Stateless protocol support (2026-07-28)

This crate also represents the released 2026-07-28 protocol, which adds stateless operation, per-request _meta, and the server/discover RPC. Key items:

• protocol::PROTOCOL_VERSION_2026_07_28 – the canonical "2026-07-28" wire-version constant. Runtime enablement is a separate concern in the parent tower-mcp crate.
• protocol::KNOWN_PROTOCOL_VERSIONS – every version whose wire format is represented here, independent of runtime compile-time features.
• protocol::DiscoverResult / protocol::DiscoverParams – types for the server/discover RPC (SEP-2575), which returns the server’s supported protocol versions and capabilities without requiring an initialize handshake.
• error::UnsupportedProtocolVersionData – the structured error data attached to a -32022 (UnsupportedProtocolVersion) response. Contains the supported versions list and the requested version string.
• error::McpErrorCode::HeaderMismatch (-32020) and error::McpErrorCode::UnsupportedProtocolVersion (-32022) – spec-assigned error codes (SEP-2243 and SEP-2575, renumbered by the upstream error-code allocation in spec PR modelcontextprotocol#2907). See the error module for the full error code table.

The only standalone Rust MCP types crate

Most Rust MCP libraries bundle their protocol types with their runtime. tower-mcp-types ships independently so that tools with narrower requirements – WASM, codegen, linters, CLI utilities – can take a dependency without pulling in an async server stack. The Zed editor team raised exactly this concern for the official rmcp crate, which has not yet split types from runtime.

Feature flags

Feature	What it adds
(default: none)	Core protocol types + serde serialization
testing	Wire-format assertion helpers for JSON-RPC test cases

Usage

Add to Cargo.toml:

[dependencies]
tower-mcp-types = "0.25"

Parse an incoming JSON-RPC request:

use tower_mcp_types::protocol::JsonRpcRequest;

let json = r#"{"jsonrpc":"2.0","id":1,"method":"tools/list","params":null}"#;
let req: JsonRpcRequest = serde_json::from_str(json).unwrap();
assert_eq!(req.method, "tools/list");

Construct a JSON-RPC error response:

use tower_mcp_types::error::{ErrorCode, JsonRpcError};

let err = JsonRpcError::new(ErrorCode::MethodNotFound, "tools/call not found");
assert_eq!(err.code, -32601);
let serialized = serde_json::to_string(&err).unwrap();
assert!(serialized.contains("-32601"));

Compatibility with tower-mcp

tower-mcp re-exports everything from this crate, so migrating from tower-mcp-types to the full server library requires only a dependency change – no import paths need to change:

tower-mcp = { version = "0.25", features = ["http"] }

WASM support

tower-mcp-types compiles for wasm32-unknown-unknown out of the box. This is verified by a CI job on every commit.

The parent tower-mcp crate is not WASM-compatible today; it pulls in Tokio networking features that do not build for wasm32. See tower-mcp issue #777 for progress on client-side WASM support in the full crate.

Re-exports

pub use error::BoxError;

pub use error::Error;

pub use error::ErrorCode;

pub use error::JsonRpcError;

pub use error::McpErrorCode;

pub use error::MissingRequiredClientCapabilityData;

pub use error::Result;

pub use error::ResultExt;

pub use error::ToolError;

pub use protocol::CallToolResult;

pub use protocol::Content;

pub use protocol::GetPromptResult;

pub use protocol::ReadResourceResult;

Modules

error

Error types for MCP

inspection

Semantic inspection of JSON-RPC wire values.

protocol

MCP protocol types based on JSON-RPC 2.0

tasks

Exact wire types for the final MCP Tasks extension (SEP-2663).