--- title: "Decision: MCP Rust SDK" description: Chose the official modelcontextprotocol/rust-sdk crate rmcp pinned to 2.2.0 for the local history sidecar. --- Evaluated 2026-07-13 for the local CodeVetter history sidecar. ## Decision Use the official `modelcontextprotocol/rust-sdk` crate `rmcp` pinned to `2.2.0`. The production dependency disables default features and enables only `server`, `transport-io`, and `macros`. HTTP/SSE, OAuth, client, child-process, reqwest, and network-server features are absent. The test-only dependency additionally enables `client` for real lifecycle/framing tests. The current stable protocol remains `2025-11-25`. The SDK also contains later revision support, but CodeVetter declares the stable revision and lets the SDK's negotiation layer handle compatible clients. The 2026-07-28 protocol was still a release candidate on the evaluation date and is not the advertised baseline. ## Why this dependency The official SDK owns JSON-RPC framing, initialization/version negotiation, capability routing, cancellation plumbing, MCP error types, schema models, stdio concurrency, and backward-compatible resource-not-found behavior. Reimplementing those protocol mechanics would add more security and interoperability risk than the dependency saves. CodeVetter still owns authorization, canonical queries, redaction, limits, cursor validation, and product semantics. ## License, security, and dependency surface - License: Apache-2.0. - Upstream: - Production feature closure: server + async read/write codec + stdio + schema support and macros. Direct runtime families are Tokio/Tokio-util, Serde, Schemars, Futures, Tracing, Chrono, Thiserror, and the SDK macros. Most were already present in the desktop dependency graph. - Explicitly absent from the selected production features: reqwest, HTTP server, SSE, OAuth, JWT, URL-based elicitation, child-process client, and network transports. - The upstream GitHub security/advisory review and GitHub Advisory Database search on 2026-07-13 found no published advisory specifically for `rmcp 2.2.0`. The local environment did not have `cargo-audit`, so this is a dated review, not a claim that future advisories cannot exist. CI should add an ecosystem advisory scanner when the fleet standard provides one. ## Measured distribution cost The Apple Silicon release sidecar is 7.04 MiB and idles at 12.31 MiB RSS. That binary size includes CodeVetter's shared graph/history/SQLite code and linked desktop crate surface, so it is an upper bound—not an attribution of 6.80 MiB to `rmcp`. Tauri packages the target-suffixed executable beside the app and applies the same app signing/notarization pipeline. A local build is linker ad-hoc signed; the signed release artifact remains the distribution proof. The final release-binary benchmark on 2026-07-14 used a 10,000-event fixture and measured cold initialize at 5.28 ms p50 / 7.90 ms p95. Steady compact queries were 2.16–2.34 ms p50 and 2.43–2.56 ms p95, with 1.4–2.0 KiB responses; resource listing was 2.15 ms. The 25-launch sample contained one 442.81 ms cold outlier. An earlier small fixture measured 5.30 ms / 6.18 ms cold p50/p95 and 1.37–1.40 ms warm p50. Both runs verified zero TCP listeners and no target-repository mutation. The benchmark should be rerun after any SDK, linking, schema, audit, or packaging change. ## Rust versus Go Keep the sidecar in Rust. Its canonical graph/history services and SQLite types already live in Rust, so a Go process would either duplicate product semantics or add another IPC boundary back into Rust. With warm queries around 2.2 ms, 12 MiB RSS, a single 7.0 MiB packaged binary, and no runtime download, there is no measured latency or distribution gap for Go to recover. A Go prototype is justified only if a future isolated workload has an end-to-end benchmark—including process startup, serialization, memory, signing, artifact size, and maintenance—that beats this baseline materially.