diff --git a/build.rs b/build.rs
index f00ab7f..1e200ed 100644
--- a/build.rs
+++ b/build.rs
@@ -8,4 +8,29 @@ fn main() {
         .flag_if_supported("-fno-stack-clash-protection")
         .compile("smarm_canary");
     println!("cargo:rerun-if-changed=canary/canary.c");
+
+    // RFC 010 c6d — build_hash inputs. The compile-time facts a peer must
+    // share for a mesh link: the exact toolchain and the declared (enabled)
+    // feature set. Emitted as a plain string; the hashing (FNV-1a folded
+    // with PROTO_VERSION) happens in src/cluster.rs where the protocol
+    // version actually lives — parsing it out of a source file here would
+    // be a second, fragile copy. Always emitted, even for non-cluster
+    // builds: one env var costs the default build nothing.
+    let rustc = std::env::var("RUSTC").unwrap_or_else(|_| "rustc".to_string());
+    let version = std::process::Command::new(&rustc)
+        .arg("-V")
+        .output()
+        .ok()
+        .map(|o| String::from_utf8_lossy(&o.stdout).trim().to_string())
+        .filter(|v| !v.is_empty())
+        .unwrap_or_else(|| "rustc-unknown".to_string());
+    let mut feats: Vec<String> = std::env::vars()
+        .filter_map(|(k, _)| k.strip_prefix("CARGO_FEATURE_").map(str::to_string))
+        .collect();
+    feats.sort();
+    println!(
+        "cargo:rustc-env=SMARM_BUILD_HASH_INPUTS={version};features={}",
+        feats.join(",")
+    );
+    println!("cargo:rerun-if-env-changed=RUSTC");
 }
diff --git a/src/cluster.rs b/src/cluster.rs
index b3cea4f..aea18d8 100644
--- a/src/cluster.rs
+++ b/src/cluster.rs
@@ -26,6 +26,43 @@ pub use conn::{spawn_established, ConnHandle};
 pub use connect::{dial, spawn_acceptor, AcceptorHandle};
 pub use manager::{Manager, MANAGER};
 
+/// c6d — the derived build hash for [`handshake::LocalNode::build_hash`]:
+/// two builds may mesh only when this matches, and it is a pure function of
+/// the compile-time inputs that define wire compatibility today — the exact
+/// toolchain (`rustc -V`), the declared feature set, and
+/// [`envelope::PROTO_VERSION`]. FNV-1a 64 over the build-script string, then
+/// the proto version folded byte-wise, so a proto bump moves the hash even
+/// on an identical toolchain. The domain is deliberately lean and
+/// tightenable later without a wire change — it is just a `u64`.
+pub const BUILD_HASH: u64 = fold_u32(
+    fnv1a64(env!("SMARM_BUILD_HASH_INPUTS").as_bytes()),
+    envelope::PROTO_VERSION,
+);
+
+/// FNV-1a 64 (const so [`BUILD_HASH`] is a compile-time fact).
+const fn fnv1a64(bytes: &[u8]) -> u64 {
+    let mut h: u64 = 0xcbf2_9ce4_8422_2325;
+    let mut i = 0;
+    while i < bytes.len() {
+        h ^= bytes[i] as u64;
+        h = h.wrapping_mul(0x0000_0100_0000_01b3);
+        i += 1;
+    }
+    h
+}
+
+/// Continue an FNV-1a state over a `u32`'s little-endian bytes.
+const fn fold_u32(mut h: u64, v: u32) -> u64 {
+    let b = v.to_le_bytes();
+    let mut i = 0;
+    while i < b.len() {
+        h ^= b[i] as u64;
+        h = h.wrapping_mul(0x0000_0100_0000_01b3);
+        i += 1;
+    }
+    h
+}
+
 /// A running cluster subtree: an explicitly-started supervisor over the
 /// connection [`Manager`]. Roles will eventually mount this subtree; until the
 /// role mechanism lands it is started by hand (RFC 010 §7). Dropping the handle
@@ -64,3 +101,37 @@ fn manager_child() {
     };
     let _ = monitor(m.pid()).rx.recv();
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// The hash core against the published FNV-1a 64 test vectors — the
+    /// contract is "this is FNV-1a", not "whatever the fn does".
+    #[test]
+    fn fnv1a64_known_vectors() {
+        assert_eq!(fnv1a64(b""), 0xcbf2_9ce4_8422_2325);
+        assert_eq!(fnv1a64(b"a"), 0xaf63_dc4c_8601_ec8c);
+        assert_eq!(fnv1a64(b"foobar"), 0x85944171f73967e8);
+    }
+
+    /// Folding the proto version continues the same FNV state: identical
+    /// inputs with a different version must land on a different hash.
+    #[test]
+    fn proto_version_moves_the_hash() {
+        let base = fnv1a64(b"same-toolchain;features=CLUSTER");
+        assert_ne!(fold_u32(base, 1), fold_u32(base, 2));
+        // And it equals hashing the bytes in one pass — the fold is a
+        // continuation, not a second construction.
+        let mut all = b"same-toolchain;features=CLUSTER".to_vec();
+        all.extend_from_slice(&1u32.to_le_bytes());
+        assert_eq!(fold_u32(base, 1), fnv1a64(&all));
+    }
+
+    /// The derived constant exists, is compile-time, and is not degenerate.
+    #[test]
+    fn build_hash_is_nonzero() {
+        const H: u64 = BUILD_HASH;
+        assert_ne!(H, 0);
+    }
+}