Context

The current Python→Rust sidecar boundary uses HTTP/JSON over a Unix Domain Socket — it works, but it is undocumented, untyped, and Python-to-Rust only. ADR-044 (docs/docs/architecture/adr/044-module-communication-protocol.md) proposes replacing it with a typed protobuf contract over a Unix Domain Socket that any language can implement.

Goal

Explore whether gRPC-over-UDS is a viable replacement for the HTTP/JSON IPC boundary, what the performance trade-offs look like in practice, and how much of ADR-044's intended value can be demonstrated with the current Rust sidecar architecture.

Scope

• Implement a McpRuntime gRPC service (Invoke, InvokeStream, CloseSession, HealthCheck) with a Rust tonic server and Python grpc.aio client
• Gate everything behind a grpc-uds Cargo feature flag — zero overhead when disabled
• Keep HTTP/JSON IPC as the default; gRPC-UDS opt-in via MCP_RUST_GRPC_UDS
• Benchmark the new boundary against the existing HTTP/JSON IPC and Python baseline

Non-Goals

• Production-ready implementation
• Catalog change subscriptions or session broadcast streaming (mentioned in ADR-044, out of scope for this POC)
• Multi-language validation (Go/Java module plugging into the same contract)
• Merging as-is without team review

Outcome

• Document findings in the PRs
• Decide whether to invest further or keep HTTP/JSON IPC as the long-term default and revisit gRPC-UDS when a second language module joins the platform