Installation

Add the library with cargo add. The crate is published as protocol-droid on crates.io, and the macro crate protocol-droid-macros is a transitive dependency that cargo resolves automatically.

cargo add protocol-droid

The crate compiles on stable Rust 1.98.1 — see rust-toolchain.toml for the exact pin. A cargo invocation that picks a different toolchain will be auto-redirected by the file, so a clone of the repo is reproducible end-to-end.

Feature flags

The default build is the domain, the IR, every codec, and the stdio transport. Everything else is additive and does not change behaviour on the default path:

Feature	What it adds
`stdio`	The stdio transport, server and client. On by default
`http`	Streamable HTTP and the `2026-07-28` stateless lifecycle, client and server
`legacy-sse`	The deprecated `2024-11-05` HTTP+SSE transport, client and server. Implies `http`
`oauth`	The OAuth 2.1 resource server and client (PKCE, discovery, registration, refresh, browser sign-in). Implies `http`
`random-ids`	CSPRNG session ids instead of a counter. An HTTP server needs ids from somewhere
`a2a`	The A2A v1.0 server, client, and cross-protocol bridge. Implies `http`
`acp-experimental`	Zed’s Agent Client Protocol v1.25.0. Pre-1.0, no semver guarantee

The CLI is a separate crate (protocol-droid-cli); its bridge feature, on by default, pulls in http, stdio, legacy-sse, random-ids and oauth.

All schema revisions are always compiled. They are data, not optional features — a --no-default-features build still speaks every revision, it just does not bring transports or OAuth along with it.

Toolchain

cargo build --workspace --all-features
cargo test  --workspace --all-features
cargo l                                                   # the full clippy profile
cargo deny --all-features check

cargo l is an alias for the strict lint profile. It is the one that gates commits; a bare cargo build is also warning-free because .cargo/config.toml sets build.warnings = "deny". Full tooling guidance lives in the Development guide.

Vendored schemas

Every wire type is generated from the official schema.json / schema.ts, and those files are pinned by commit and locked by SHA-256. A build refuses to compile if the tree has drifted from the manifest, and no build ever touches the network.

To re-fetch a schema after a new MCP release:

cargo xtask refresh-spec 2026-07-28      # re-fetch a tagged revision
cargo xtask refresh-spec draft --newest   # re-resolve main, repin, and re-fetch

Full details — layout, what each field is for, how to vendor a new revision — are in crates/protocol-droid-schema/spec/README.md.