[benbrandt]

Summary

agent-client-protocol-rmcp now has one module per supported rmcp major version, each behind a matching Cargo feature, so future rmcp majors can land as minor releases instead of forcing an ACP major.

• rmcp is now an optional dependency renamed to rmcp_3 (package = "rmcp", 3.4.0, server + schemars, same as the workspace entry). Feature rmcp_3 = ["dep:rmcp_3"]. No rmcp feature is on by default.
• The whole integration (McpServerExt, McpServerBuilder, re-exports of McpTool, EnabledTools, tool_fn!, tool_fn_mut!) moves to agent_client_protocol_rmcp::rmcp_3. The crate root only holds docs, and nothing there depends on which rmcp features are on. Enabling a future rmcp_4 would only add an rmcp_4 module.
• With no rmcp feature, the crate compiles as an empty crate, and its root docs say which feature to enable. There is no compile_error!.
• The unstable_protocol_v2 and unstable_mcp_over_acp passthrough features are unchanged and apply to every rmcp module.
• docs.rs: all-features = true plus --cfg docsrs, and a doc(cfg) badge on the module, the same setup as the core crate.

Naming: rmcp_3 for the feature, module and dependency

The crate's existing features are snake_case (unstable_mcp_over_acp, unstable_protocol_v2), so a hyphenated rmcp-3 would be the odd one out. Using one identifier for the feature, the module and the renamed dependency means cfg(feature = "rmcp_3"), rmcp_3::McpServerExt and dep:rmcp_3 all read the same and grep together. Inside the crate, each module file imports use ::rmcp_3 as rmcp;, so the existing rmcp:: paths are unchanged.

Tests and examples use a dev-dependency also named rmcp_3 (with macros), because Cargo rejects the same package under two names. Each one does extern crate rmcp_3 as rmcp; so rmcp's macros keep resolving.

Dependents

• Conductor and cookbook dev-deps enable rmcp_3 and import from rmcp_3::. Their own direct rmcp dependencies are unchanged. Testy (agent-client-protocol-test) uses rmcp directly and not the integration crate, so it needs no change.
• The ignore doctests in the core crate that mention the integration now use the new path.

Docs

• The rmcp README has a feature/module table, a rewritten "Versioning" section (one module per rmcp major, no default, a new rmcp major is a minor release, dropping one is breaking and waits for an ACP major), and an adjusted "Why a Separate Crate?".
• md/migration-v4.md has a new section on enabling rmcp_3, with the before/after import table.
• design.md, features.md, transport-architecture.md and mcp-over-acp.md mention the feature, and the example commands include rmcp_3.
• The .release-plz.toml comment now says rmcp majors arrive as features in minor releases.

CI

• The default WASI check of the rmcp crate now passes --features rmcp_3; otherwise it would only check an empty crate.
• The docs.rs build check also covers agent-client-protocol-rmcp.
• just test also runs cargo test -p agent-client-protocol-rmcp --features rmcp_3, which covers the integration without the unstable passthrough features.

Test plan

• cargo fmt -- --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings, plain cargo clippy -- -D warnings, and -p agent-client-protocol-rmcp --no-default-features (lib and all targets)
• cargo hack check --workspace --feature-powerset --depth 2 --exclude-features default,unstable,wasm_js --no-dev-deps
• cargo test --workspace --all-features --locked --no-fail-fast (only agent-client-protocol-derive --lib fails, a local dyld/runner issue that also happens on main)
• cargo test -p agent-client-protocol-rmcp --features rmcp_3 --locked, and with no features
• RUSTDOCFLAGS="-D warnings" cargo doc -p agent-client-protocol-rmcp, with all features and with none; nightly --cfg docsrs docs build
• cargo check -p agent-client-protocol-rmcp --lib --target wasm32-wasip2 --features rmcp_3
• mdbook build