build: fix Cargo 1.100's unused dependency warnings

.github/workflows/ci.yml

@@ -177,4 +177,10 @@ jobs:
         # Depth 2 covers every remaining individual feature and pair, while the
         # regular CI all-features tests cover their full union without making
         # this matrix grow exponentially.
-        run: cargo hack check --workspace --feature-powerset --depth 2 --exclude-features default,unstable,wasm_js --no-dev-deps --partition ${{ matrix.partition }}
+        #
+        # There is no `--no-dev-deps`. That flag removes dev-dependencies from
+        # the manifests while cargo-hack runs, so Cargo reports the workspace
+        # dependencies only they inherit as unused. It is also not needed:
+        # with resolver 3, dev-dependencies do not add features to the
+        # library and binary targets this command checks.
+        run: cargo hack check --workspace --feature-powerset --depth 2 --exclude-features default,unstable,wasm_js --partition ${{ matrix.partition }}

md/features.md

@@ -77,7 +77,7 @@ Internal crates request only the core capabilities they use:
 | --- | --- |
 | HTTP client/server | None; transport sides remain opt-in through `client` and `server` |
 | MCP-over-ACP polyfill | `unstable_mcp_over_acp`, with independent draft-v2 and session-fork passthrough features |
-| rmcp integration | `schemars`; one feature per rmcp major version (`rmcp_3`), and an `unstable_mcp_over_acp` passthrough feature. It has no draft-v2 feature: enable `unstable_protocol_v2` on `agent-client-protocol` itself |
+| rmcp integration | `schemars`; one feature per rmcp major version (`rmcp_3`), and an `unstable_mcp_over_acp` passthrough feature. The core crate is only a dependency with an rmcp version feature, so the passthrough does nothing on its own. It has no draft-v2 feature: enable `unstable_protocol_v2` on `agent-client-protocol` itself |
 | Conductor | `process`, `stdio`, and `unstable_mcp_over_acp` for launching components, serving stdio, and classifying MCP traces |
 | YOPO | `process` for launching the agent |
 | Test utilities | `process` and `stdio` for fixture binaries and launch helpers |

src/agent-client-protocol-cookbook/src/lib.rs

@@ -56,6 +56,10 @@
 //! [`Proxy`]: agent_client_protocol::Proxy
 //! [`ConnectTo`]: agent_client_protocol::ConnectTo
 
+// Only rustdoc links and doctests use this crate. Name it here so that Cargo
+// does not report the dependency as unused.
+use agent_client_protocol_rmcp as _;
+
 pub mod ordered_application_dispatch;
 
 pub mod v2_session_coordination {

src/agent-client-protocol-rmcp/Cargo.toml

@@ -18,11 +18,25 @@ rustdoc-args = ["--cfg", "docsrs"]
 # No rmcp version is enabled by default. Each supported rmcp major version has
 # its own feature and module; features are additive.
 default = []
-# rmcp 3.x integration in the `rmcp_3` module.
-rmcp_3 = ["dep:rmcp_3"]
+# rmcp 3.x integration in the `rmcp_3` module. Without an rmcp version feature
+# the crate is empty, so the dependencies the integration uses are optional
+# and enabled here.
+rmcp_3 = [
+    "dep:rmcp_3",
+    "dep:agent-client-protocol",
+    "dep:futures",
+    "dep:futures-concurrency",
+    "dep:schemars",
+    "dep:serde",
+    "dep:serde_json",
+    "dep:tokio",
+    "dep:tokio-util",
+]
 # Passthrough feature for the core crate's `unstable_mcp_over_acp`. The native
-# MCP-over-ACP path follows the core feature however it is enabled.
-unstable_mcp_over_acp = ["agent-client-protocol/unstable_mcp_over_acp"]
+# MCP-over-ACP path follows the core feature however it is enabled. On its own
+# it enables nothing, because the core crate is only a dependency with an
+# rmcp version feature.
+unstable_mcp_over_acp = ["agent-client-protocol?/unstable_mcp_over_acp"]
 
 [[example]]
 name = "with_mcp_server"
@@ -34,16 +48,16 @@ name = "stateless_native_mcp"
 required-features = ["rmcp_3", "unstable_mcp_over_acp"]
 
 [dependencies]
-agent-client-protocol = { workspace = true, features = ["schemars"] }
-futures.workspace = true
-futures-concurrency.workspace = true
+agent-client-protocol = { workspace = true, optional = true, features = ["schemars"] }
+futures = { workspace = true, optional = true }
+futures-concurrency = { workspace = true, optional = true }
 # Keep the version and features in step with the workspace `rmcp` entry.
 rmcp_3 = { package = "rmcp", version = "3.4.0", optional = true, default-features = false, features = ["server", "schemars"] }
-schemars.workspace = true
-serde.workspace = true
-serde_json.workspace = true
-tokio = { workspace = true, features = ["io-util", "macros", "rt"] }
-tokio-util = { workspace = true, features = ["rt"] }
+schemars = { workspace = true, optional = true }
+serde = { workspace = true, optional = true }
+serde_json = { workspace = true, optional = true }
+tokio = { workspace = true, optional = true, features = ["io-util", "macros", "rt"] }
+tokio-util = { workspace = true, optional = true, features = ["rt"] }
 
 [dev-dependencies]
 # The `stateless_native_mcp` example attaches a server to a v2 proxy.

src/agent-client-protocol-rmcp/README.md

@@ -10,7 +10,8 @@ Rust, serve them directly, or attach them to an ACP proxy.
 
 Each supported rmcp major version has its own module behind a matching Cargo
 feature. No rmcp feature is enabled by default; without one, this crate is
-empty. Enable the feature for the rmcp major version your application uses:
+empty and has no dependencies. Enable the feature for the rmcp major version
+your application uses:
 
 | Feature  | Module                                | rmcp |
 | -------- | ------------------------------------- | ---- |