diff --git a/crates/ailang-core/src/lib.rs b/crates/ailang-core/src/lib.rs
index b366c6e..9471f6b 100644
--- a/crates/ailang-core/src/lib.rs
+++ b/crates/ailang-core/src/lib.rs
@@ -59,6 +59,7 @@ pub mod canonical;
 pub mod desugar;
 pub mod hash;
 pub mod pretty;
+pub mod primitives;
 pub mod workspace;
 
 pub use ast::{
diff --git a/crates/ailang-core/src/primitives.rs b/crates/ailang-core/src/primitives.rs
new file mode 100644
index 0000000..24418f6
--- /dev/null
+++ b/crates/ailang-core/src/primitives.rs
@@ -0,0 +1,35 @@
+//! The primitive-name set: `Int`, `Bool`, `Str`, `Unit`. Single
+//! source of truth for "is this name a built-in zero-arity type
+//! constructor?" Every typecheck and codegen site that gates
+//! behaviour on the primitive set goes through this module.
+//!
+//! Adding a primitive: append to BOTH functions below; the four
+//! consumers compile-fail-loudly if the predicate is forgotten
+//! (the failure is a missed branch, not a panic), but
+//! `primitive_surface_name` will silently return `None` for the
+//! new name unless its `match` is extended too. Keep the two
+//! functions in lockstep.
+
+/// Returns `true` iff `name` is one of the built-in zero-arity
+/// primitive type constructors. Cross-crate predicate consumed by
+/// `ailang-check` (`linearity::is_heap_type`, `lib::check_type_well_formed`,
+/// `lib::qualify_local_types`) and `ailang-codegen`
+/// (`subst::qualify_local_types_codegen`).
+pub fn is_primitive_name(name: &str) -> bool {
+    matches!(name, "Int" | "Bool" | "Str" | "Unit")
+}
+
+/// Returns the static-lifetime surface name iff `name` is a
+/// primitive. Used by `mono::primitive_surface_name` to embed the
+/// human-readable form in monomorphised symbol names; the static
+/// lifetime is what makes the symbol-builder's `&'static str`
+/// signature work.
+pub fn primitive_surface_name(name: &str) -> Option<&'static str> {
+    match name {
+        "Int" => Some("Int"),
+        "Bool" => Some("Bool"),
+        "Str" => Some("Str"),
+        "Unit" => Some("Unit"),
+        _ => None,
+    }
+}