rig-mcp 0.1.3

Model Context Protocol bridge for rig-compose tool registries. Wraps the official `rmcp` SDK with rig-compose's transport-agnostic Tool surface.
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# rig-mcp

Model Context Protocol bridge for rig-compose tool registries. Wraps the official `rmcp` SDK with rig-compose's transport-agnostic Tool surface.

[![CI](https://github.com/ForeverAngry/rig-mcp/actions/workflows/ci.yml/badge.svg)](https://github.com/ForeverAngry/rig-mcp/actions/workflows/ci.yml)
[![Crates.io](https://img.shields.io/crates/v/rig-mcp.svg)](https://crates.io/crates/rig-mcp)
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[![License: MIT OR Apache-2.0](https://img.shields.io/badge/license-MIT%20OR%20Apache--2.0-blue.svg)](#license)
[![MSRV](https://img.shields.io/badge/rustc-1.88+-orange.svg)](#status)

## Overview

`rig-mcp` adapts Model Context Protocol endpoints into `rig-compose` tool registries. It uses the official `rmcp` SDK for protocol mechanics and exposes a small Rig-shaped surface: transports list remote tool schemas, call remote tools, and wrap those remote tools as `rig_compose::Tool` values.

Skills that receive a `ToolRegistry` cannot tell whether a tool is local, loopback, or served by a child process over MCP stdio.

## Why It Exists

`rig-compose` keeps tools transport-agnostic. MCP is a transport and protocol concern, so it belongs in a companion crate instead of the kernel. `rig-mcp` fills that gap by turning MCP-served tools into the same `Tool` trait used by local closures and in-process delegates.

It delegates JSON-RPC framing, capability handshakes, and protocol-version negotiation to `rmcp` rather than reimplementing the MCP spec.

## Status

- Crate version: `0.1.1`.
- Rust edition: 2024.
- MSRV: 1.88.
- `rig-compose` dependency: `version = "0.1"`, currently resolved through the sibling path [../rig-compose](../rig-compose) in this workspace.
- `rmcp` dependency: `1.6` with `client`, `server`, `macros`, `transport-io`, and `transport-child-process` features only.
- Current Unreleased work stores the cloneable `rmcp` peer directly in `StdioTransport`, eliminating a transport-level `tokio::sync::Mutex` around concurrent RPCs.

## Feature Flags

`rig-mcp` currently defines no crate features. `just check` runs clippy, tests, and docs with `--all-features` to keep future feature additions covered.

## Key Types

- [src/transport.rs](src/transport.rs): `McpTransport`, the async trait for MCP-like transports. It exposes `endpoint`, `list_tools`, and `call_tool`.
- [src/transport.rs](src/transport.rs): `McpTool`, the adapter that wraps one remote schema and transport as a local `rig_compose::Tool`.
- [src/transport.rs](src/transport.rs): `LoopbackTransport`, an in-process transport over `ToolRegistry` used for tests and embedding.
- [src/stdio.rs](src/stdio.rs): `StdioTransport`, the production child-process stdio MCP client backed by `rmcp`.
- [src/stdio.rs](src/stdio.rs): `serve_stdio`, which exposes a `ToolRegistry` as an MCP tools-only server over the current process's stdin/stdout.

Server-side `tools/list` is answered from `ToolRegistry::schemas`; `tools/call` dispatches to `ToolRegistry::invoke`. Client-side stdio calls are converted back into `ToolSchema` and JSON `Value` results.

## Integration With Rig

`rig-mcp` integrates through `rig-compose`, not directly through `rig-core`. A remote MCP tool becomes a `rig_compose::Tool`, so any `rig-compose` skill or agent can call it through a normal `ToolRegistry`.

This preserves the same call shape across local tools, `LoopbackTransport`, `StdioTransport`, and future transports.

## Usage

The loopback path is covered by tests in [src/transport.rs](src/transport.rs). This example mirrors `mcp_tool_indistinguishable_from_local`.

```rust,no_run
use std::sync::Arc;

use rig_compose::{LocalTool, ToolRegistry, ToolSchema};
use rig_mcp::{LoopbackTransport, McpTool, McpTransport};
use serde_json::json;

# async fn run() -> Result<(), rig_compose::KernelError> {
let server = ToolRegistry::new();
server.register(Arc::new(LocalTool::new(
    ToolSchema {
        name: "math.add".into(),
        description: "add two integers".into(),
        args_schema: json!({"type": "object"}),
        result_schema: json!({"type": "integer"}),
    },
    |args| async move {
        let a = args.get("a").and_then(serde_json::Value::as_i64).unwrap_or(0);
        let b = args.get("b").and_then(serde_json::Value::as_i64).unwrap_or(0);
        Ok(json!(a + b))
    },
)));

let transport: Arc<dyn McpTransport> =
    Arc::new(LoopbackTransport::new("loopback://test", server));

let client = ToolRegistry::new();
for tool in McpTool::from_transport(transport).await? {
    client.register(tool);
}

let output = client.invoke("math.add", json!({"a": 10, "b": 32})).await?;
assert_eq!(output, json!(42));
# Ok(()) }
```

Production stdio behavior is implemented in [src/stdio.rs](src/stdio.rs). `serve_stdio` exposes a local `ToolRegistry`; `StdioTransport::spawn` starts a child process and speaks MCP over its stdio.

## Validation

Canonical validation is `just check`.

That recipe runs formatter checks, `cargo clippy --all-targets --all-features -- -D warnings`, `cargo test --all-targets --all-features`, and rustdoc with all features and `-D warnings -D rustdoc::broken_intra_doc_links`.

## Gotchas

- Error handling surfaces `rig_compose::KernelError`; the crate does not introduce a separate public error enum.
- The `rmcp` feature surface is intentionally tight. Do not enable extra transports or HTTP/TLS by default without a concrete need.
- `StdioTransport` caches the cloneable `rmcp::Peer` and keeps the running service alive with an `Arc<RunningService<...>>`; dropping the transport drops the service and closes the child stdio.
- `StdioTransport::call_tool` accepts object or null arguments. Other JSON shapes return `KernelError::InvalidArgument`.

## Ecosystem

These companion crates are maintained as separate repositories. Together they form a small stack around the upstream Rig project: `rig-compose` provides the kernel surface, `rig-resources` contributes reusable skills and tools, `rig-mcp` moves tools across MCP, and `rig-memvid` connects Rig agents to persistent `.mv2` memory.

```mermaid
flowchart TD
    rig["rig / rig-core"]
    compose["rig-compose 0.1.x"]
    resources["rig-resources 0.1.x"]
    mcp["rig-mcp 0.1.x"]
    memvid["rig-memvid 0.1.x"]

    compose -. "Rig-shaped kernel; no direct rig-core dep" .-> rig
    resources -- "rig-compose = 0.2; features: security, graph, full" --> compose
    mcp -- "rig-compose = 0.2; rmcp stdio bridge" --> compose
    memvid -- "rig-core = 0.36.0; features: lex, vec, api_embed, temporal, encryption" --> rig
```

Pinned Rig-facing dependencies from the current manifests:

| Crate | Direct Rig-facing dependency | Notes |
| --- | --- | --- |
| `rig-compose` | none | Defines a Rig-shaped kernel surface without depending on `rig-core`. |
| `rig-resources` | `rig-compose = 0.2` | Provides reusable skills, resource tools, and security helpers. |
| `rig-mcp` | `rig-compose = 0.2` | Bridges `rig-compose` tools over MCP stdio and loopback transports. |
| `rig-memvid` | `rig-core = 0.36.0` | Implements Rig vector-store and prompt-hook flows over Memvid. |

The concrete multi-crate workflow tested today is the MCP loopback path: a `rig_compose::ToolRegistry` is exposed through `rig_mcp::LoopbackTransport`, remote schemas are wrapped as `rig_mcp::McpTool`, and the wrapped tools are registered back into another `ToolRegistry`. That proves a local `rig-compose` tool and an MCP-adapted tool are indistinguishable to callers. The backing test is `mcp_tool_indistinguishable_from_local` in [rig-mcp/src/transport.rs](https://github.com/ForeverAngry/rig-mcp/blob/main/src/transport.rs).

## License

Licensed under either Apache-2.0 or MIT, at your option.