diff --git a/Cargo.toml b/Cargo.toml
index 8b101b6..362b77d 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -33,6 +33,11 @@ budget-accounting = []
 # and unflagged; only the optional gen_server transport sits behind this, so a
 # release build pays nothing for an observer it never starts.
 observer = []
+# RFC 010 c1: clustering. Off by default — the default build stays libc-only,
+# byte-for-byte (gate checked per phase). serde is the payload contract,
+# postcard the payload codec; both minimal (no default features). Everything
+# cluster-shaped lives behind this flag.
+cluster = ["dep:serde", "dep:postcard"]
 # Run-queue selection: exactly one, compile-time (see src/run_queue.rs).
 # Non-default variants need --no-default-features (features are additive).
 rq-mutex = []
@@ -44,6 +49,9 @@ cc = "1"
 
 [dependencies]
 libc = "0.2"
+# RFC 010 §2 — only compiled under `--features cluster`.
+serde = { version = "1", default-features = false, optional = true }
+postcard = { version = "1", default-features = false, optional = true }
 
 [target.'cfg(loom)'.dependencies]
 loom = "0.7"
diff --git a/src/cluster.rs b/src/cluster.rs
new file mode 100644
index 0000000..9326694
--- /dev/null
+++ b/src/cluster.rs
@@ -0,0 +1,6 @@
+//! RFC 010 — clustering (smarm⇄smarm, explicit remote boundary).
+//!
+//! c1: feature flag + optional deps only. The owned envelope (c2), transport
+//! trait (c3), and everything above them land in later chunks. This module is
+//! intentionally empty so the `cluster` feature's default-build invariance is
+//! reviewable in isolation.
diff --git a/src/lib.rs b/src/lib.rs
index 7dfb3f4..3a6ee80 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -14,6 +14,8 @@
 pub mod actor;
 pub mod causal;
 pub mod channel;
+#[cfg(feature = "cluster")]
+pub mod cluster;
 pub mod context;
 pub mod gen_server;
 pub mod gen_statem;