MCP

Reusable Rust client support for the Model Context Protocol. The workspace provides a tools-only client, stdio and Streamable HTTP transports, OAuth, a strict named local registry, and a standalone b10x-mcp operator CLI.

Both transports are exercised here rather than only in a consumer: crates/b10x-mcp-stdio/tests/transport.rs and crates/b10x-mcp-http/tests/transport.rs drive a real pipe and a real loopback socket against the controlled server b10x-mcp-testkit compiles from standard-library source, and assert the negotiated version, the frozen snapshot and one tool call on each.

The client prefers MCP 2026-07-28 and falls back to 2025-11-25. Consumers retain authority: Harness supplies envelopes and approvals; Connectors supplies catalog, grants, egress, and hosted credential custody. MCP server annotations never grant either consumer anything.

cargo xtask gate
cargo run -p b10x-mcp-cli -- connections list

The default registry is $XDG_CONFIG_HOME/b10x/mcp.toml (falling back to the XDG location below HOME). OAuth material lives separately under $XDG_STATE_HOME/b10x/mcp, with owner-only permissions. A minimal registry looks like this:

[connections.local_files]
transport = "stdio"
program = "/absolute/path/to/mcp-server"
args = ["--stdio"]
cwd = "/absolute/working/directory"
inherit-env = []

[connections.remote]
transport = "http"
url = "https://mcp.example.com/mcp"

[connections.remote.auth]
kind = "oauth"
resource-url = "https://mcp.example.com/mcp"
redirect-uri = "http://127.0.0.1:38123/callback"
client-name = "b10x MCP client"
scopes = []
application-type = "native"

Use b10x-mcp auth login remote, then b10x-mcp tools snapshot remote. A bearer token may instead come from an explicitly named environment variable or JSON file/pointer; inline credentials and ambient credential discovery are not part of the schema.

Consumer boundary

The library returns lossless descriptors and results. It does not convert MCP annotations into permissions, risk, idempotency, grants, or approvals. An embedding consumer must review and attach those facts itself. See the boundary design.

Hosts with their own egress and secret boundary can use connect_http_with_client; the supplied client performs every HTTP exchange while the foundation retains protocol negotiation, discovery, snapshotting, bounds, and calls.

License

Apache-2.0.

Documentation

MCP documentation · Start · Ecosystem · Impact · Releases