[benbrandt]

CI on main fails with the current stable toolchain. Cargo 1.100 adds two manifest lints that warn by default, and CI denies warnings (CARGO_BUILD_WARNINGS=deny, set by setup-rust-toolchain). The last green run on main used 1.99.1. #499 is the first run on 1.100, and its Build job and all eight feature powerset jobs fail for this reason.

No warning is relaxed or allowed here. Each one is fixed where it comes from.

Fixes

• cargo::unused_workspace_dependencies in the feature powerset jobs. cargo hack --no-dev-deps removes dev-dependencies from the manifests while it runs. Nine [workspace.dependencies] entries are only inherited by dev-dependencies, so Cargo reported them as unused in every combination. The flag is gone. It is not needed with resolver 3: a library-only check of agent-client-protocol with --no-default-features resolves no features, even though its dev-dependency agent-client-protocol-test depends back on it with process and stdio.
• cargo::unused_dependencies in agent-client-protocol-rmcp. With the flag gone, the powerset reports that the crate declares dependencies it does not use without rmcp_3, where the crate is empty. Its eight dependencies are now optional and rmcp_3 enables them. No new feature names appear, because the feature refers to them with dep:.
• cargo::unused_dependencies in the cookbook. Only rustdoc links and doctests use agent-client-protocol-rmcp there, and rustdoc resolves links against regular dependencies only, so it cannot move to [dev-dependencies]. The cookbook now names the crate with use agent_client_protocol_rmcp as _;.

Behavior change

agent-client-protocol-rmcp with only unstable_mcp_over_acp, and no rmcp_3, no longer enables the core crate's feature. The passthrough is now agent-client-protocol?/unstable_mcp_over_acp, and the core crate is not a dependency in that configuration. Nothing in the workspace uses that combination. The README and md/features.md say so.

Verification

Run locally on Rust 1.100 with CARGO_BUILD_WARNINGS=deny, as in CI:

• The powerset command with --each-feature instead of --feature-powerset --depth 2 (33 combinations), and the full powerset of agent-client-protocol-rmcp. I did not run the full depth-2 powerset locally.
• cargo check --workspace --all-targets --all-features --locked and the wasm checks from the MSRV job
• cargo fmt -- --check, cargo clippy, cargo clippy --all-targets --all-features, and the wasm clippy commands
• the docs.rs build on nightly and the WASI docs build
• just test

An earlier run of just test on this branch failed once in acp_agent::tests::protocol_eof_still_reports_nonzero_child_exit. That test is flaky on main (3 failures in 60 local runs of the acp_agent tests) and unrelated to this change.