diff --git a/core/src/schema/ir/frozen/cas/blob.rs b/core/src/schema/ir/frozen/cas/blob.rs
index ca40e0e..fdcdc56 100644
--- a/core/src/schema/ir/frozen/cas/blob.rs
+++ b/core/src/schema/ir/frozen/cas/blob.rs
@@ -7,6 +7,27 @@ use crate::package::build::cas::storage::Hash;
 use crate::schema::ir::frozen::unit::FrozenUnit;
 use eyre::{eyre, Result};
 
+/// A canonical, cross-language-stable digest of one schema's frozen units:
+/// BLAKE3 over their `bincode` encoding — the same serialization the CAS uses
+/// for blobs (see [`frozen_unit_to_blob`]) — folded to 64 bits for the
+/// connection handshake's `ir_hash`.
+///
+/// Deterministic for a given frozen IR: independent of the generator, the host,
+/// and whether the units were read from the working tree or a committed
+/// version. Two ends of a connection generated from the same schema — in any
+/// target language — get the same value, so a generator embeds this verbatim
+/// rather than hashing its own (language-specific) rendering.
+pub fn schema_ir_hash(units: &[FrozenUnit]) -> u64 {
+    let bytes = bincode::serialize(units)
+        .expect("FrozenUnit implements Serialize; in-memory encoding cannot fail");
+    let digest = blake3::hash(&bytes);
+    u64::from_le_bytes(
+        digest.as_bytes()[..8]
+            .try_into()
+            .expect("a BLAKE3 digest is 32 bytes"),
+    )
+}
+
 /// Convert a FrozenUnit to a Blob
 pub fn frozen_unit_to_blob(unit: &FrozenUnit) -> Result<Blob> {
     // Serialize FrozenUnit using bincode
diff --git a/core/src/schema/ir/frozen/mod.rs b/core/src/schema/ir/frozen/mod.rs
index 8455f03..cec0120 100644
--- a/core/src/schema/ir/frozen/mod.rs
+++ b/core/src/schema/ir/frozen/mod.rs
@@ -2,3 +2,6 @@
 pub mod unit;
 // Removed: basic_storage - replaced by CAS
 pub mod cas;  // CAS module (public for tests and build)
+
+/// Canonical frozen-schema digest for the connection handshake's `ir_hash`.
+pub use cas::blob::schema_ir_hash;
diff --git a/core/tests/cas/mod.rs b/core/tests/cas/mod.rs
index a82a554..dc15817 100644
--- a/core/tests/cas/mod.rs
+++ b/core/tests/cas/mod.rs
@@ -7,3 +7,4 @@ pub mod commit_tests;
 pub mod config_in_commit_tests;
 pub mod refs_tests;
 pub mod span_hash_sensitivity_tests;
+pub mod schema_ir_hash_tests;
diff --git a/core/tests/cas/schema_ir_hash_tests.rs b/core/tests/cas/schema_ir_hash_tests.rs
new file mode 100644
index 0000000..673da47
--- /dev/null
+++ b/core/tests/cas/schema_ir_hash_tests.rs
@@ -0,0 +1,45 @@
+// `schema_ir_hash` — the canonical frozen-schema digest a generator embeds as
+// the connection handshake's `IR_HASH`. It must be deterministic for a given
+// frozen IR and move when the schema's meaning (or, like the CAS blob hash,
+// its formatting) changes.
+
+use comline_core::schema::ir::compiler::interpreter::incremental::IncrementalInterpreter;
+use comline_core::schema::ir::compiler::Compile;
+use comline_core::schema::ir::frozen::schema_ir_hash;
+
+#[test]
+fn identical_source_hashes_identically() {
+    let source = "struct User {\n    id: u64\n    name: str\n}\n";
+    let a = IncrementalInterpreter::from_source(source);
+    let b = IncrementalInterpreter::from_source(source);
+
+    assert_eq!(schema_ir_hash(&a), schema_ir_hash(&b));
+}
+
+#[test]
+fn a_semantic_change_moves_the_hash() {
+    let before = IncrementalInterpreter::from_source("struct User {\n    id: u64\n}\n");
+    let after = IncrementalInterpreter::from_source("struct User {\n    id: u64\n    name: str\n}\n");
+
+    assert_ne!(schema_ir_hash(&before), schema_ir_hash(&after));
+}
+
+#[test]
+fn unit_order_is_significant() {
+    let ab = IncrementalInterpreter::from_source(
+        "struct A {\n    x: u64\n}\nstruct B {\n    y: u64\n}\n",
+    );
+    let ba = IncrementalInterpreter::from_source(
+        "struct B {\n    y: u64\n}\nstruct A {\n    x: u64\n}\n",
+    );
+
+    // Different declaration order ⇒ different frozen IR ⇒ different identity.
+    assert_ne!(schema_ir_hash(&ab), schema_ir_hash(&ba));
+}
+
+#[test]
+fn an_empty_schema_still_hashes() {
+    let empty: Vec<comline_core::schema::ir::frozen::unit::FrozenUnit> = Vec::new();
+    // Just needs to not panic and be stable.
+    assert_eq!(schema_ir_hash(&empty), schema_ir_hash(&empty));
+}