diff --git a/crates/ail/tests/snapshots/hello.ll b/crates/ail/tests/snapshots/hello.ll
index 26e1e9d..d2cb677 100644
--- a/crates/ail/tests/snapshots/hello.ll
+++ b/crates/ail/tests/snapshots/hello.ll
@@ -9,6 +9,7 @@ declare i32 @puts(ptr)
 declare ptr @GC_malloc(i64)
 declare i32 @strcmp(ptr, ptr)
 declare zeroext i1 @ail_str_eq(ptr, ptr)
+declare i32 @ail_str_compare(ptr, ptr)
 declare i64 @llvm.fptosi.sat.i64.f64(double)
 
 @ail_hello_main_clos = private unnamed_addr constant { ptr, ptr } { ptr @ail_hello_main_adapter, ptr null }
diff --git a/crates/ail/tests/snapshots/list.ll b/crates/ail/tests/snapshots/list.ll
index de9700f..dd6eb22 100644
--- a/crates/ail/tests/snapshots/list.ll
+++ b/crates/ail/tests/snapshots/list.ll
@@ -9,6 +9,7 @@ declare i32 @puts(ptr)
 declare ptr @GC_malloc(i64)
 declare i32 @strcmp(ptr, ptr)
 declare zeroext i1 @ail_str_eq(ptr, ptr)
+declare i32 @ail_str_compare(ptr, ptr)
 declare i64 @llvm.fptosi.sat.i64.f64(double)
 
 @ail_list_sum_list_clos = private unnamed_addr constant { ptr, ptr } { ptr @ail_list_sum_list_adapter, ptr null }
diff --git a/crates/ail/tests/snapshots/max3.ll b/crates/ail/tests/snapshots/max3.ll
index 8364022..43bac1c 100644
--- a/crates/ail/tests/snapshots/max3.ll
+++ b/crates/ail/tests/snapshots/max3.ll
@@ -9,6 +9,7 @@ declare i32 @puts(ptr)
 declare ptr @GC_malloc(i64)
 declare i32 @strcmp(ptr, ptr)
 declare zeroext i1 @ail_str_eq(ptr, ptr)
+declare i32 @ail_str_compare(ptr, ptr)
 declare i64 @llvm.fptosi.sat.i64.f64(double)
 
 @ail_max3_max_clos = private unnamed_addr constant { ptr, ptr } { ptr @ail_max3_max_adapter, ptr null }
diff --git a/crates/ail/tests/snapshots/sum.ll b/crates/ail/tests/snapshots/sum.ll
index e6920e5..33834fd 100644
--- a/crates/ail/tests/snapshots/sum.ll
+++ b/crates/ail/tests/snapshots/sum.ll
@@ -9,6 +9,7 @@ declare i32 @puts(ptr)
 declare ptr @GC_malloc(i64)
 declare i32 @strcmp(ptr, ptr)
 declare zeroext i1 @ail_str_eq(ptr, ptr)
+declare i32 @ail_str_compare(ptr, ptr)
 declare i64 @llvm.fptosi.sat.i64.f64(double)
 
 @ail_sum_sum_clos = private unnamed_addr constant { ptr, ptr } { ptr @ail_sum_sum_adapter, ptr null }
diff --git a/crates/ail/tests/snapshots/ws_main.ll b/crates/ail/tests/snapshots/ws_main.ll
index 69d4564..406504c 100644
--- a/crates/ail/tests/snapshots/ws_main.ll
+++ b/crates/ail/tests/snapshots/ws_main.ll
@@ -9,6 +9,7 @@ declare i32 @puts(ptr)
 declare ptr @GC_malloc(i64)
 declare i32 @strcmp(ptr, ptr)
 declare zeroext i1 @ail_str_eq(ptr, ptr)
+declare i32 @ail_str_compare(ptr, ptr)
 declare i64 @llvm.fptosi.sat.i64.f64(double)
 
 @ail_ws_lib_add_clos = private unnamed_addr constant { ptr, ptr } { ptr @ail_ws_lib_add_adapter, ptr null }
diff --git a/crates/ailang-codegen/src/lib.rs b/crates/ailang-codegen/src/lib.rs
index 8c07f99..35f5b2c 100644
--- a/crates/ailang-codegen/src/lib.rs
+++ b/crates/ailang-codegen/src/lib.rs
@@ -495,6 +495,15 @@ fn lower_workspace_inner(ws: &Workspace, alloc: AllocStrategy) -> Result<String>
     // by `ail build` (see `crates/ail/src/main.rs::build_to`). The
     // `zeroext i1` return matches clang's lowering of C `_Bool`.
     out.push_str("declare zeroext i1 @ail_str_eq(ptr, ptr)\n");
+    // Iter 23.3: `runtime/str.c::ail_str_compare` backs the prelude's
+    // `compare__Str` mono symbol (see `try_emit_primitive_instance_body`
+    // `compare__Str` arm below). Declared unconditionally on the same
+    // rationale as `@ail_str_eq` — the .o is supplied by the
+    // unconditionally-linked `runtime/str.c`, and clang -O2 dead-
+    // strips the symbol when no caller exists. Returns i32 normalised
+    // to {-1, 0, +1} so the branch ladder in the intercept can compare
+    // against constant 0 directly.
+    out.push_str("declare i32 @ail_str_compare(ptr, ptr)\n");
     // Floats iter 4.4: saturating fp-to-int intrinsic for
     // float_to_int_truncate. NaN → 0, +Inf → i64::MAX, -Inf →
     // i64::MIN, finite-out-of-range saturates, finite-in-range
@@ -3653,4 +3662,38 @@ mod tests {
             "eq__Str body must call @ail_str_eq; ir was:\n{ir}"
         );
     }
+
+    /// Iter 23.3: `runtime/str.c::ail_str_compare` backs the prelude's
+    /// `compare__Str` mono symbol (see `try_emit_primitive_instance_body`).
+    /// The declaration is unconditional in the IR header alongside
+    /// `@ail_str_eq`; this test pins the header line so a regression
+    /// that drops it would fail at link time only on programs that
+    /// actually call `compare` on a Str.
+    #[test]
+    fn ir_header_declares_ail_str_compare() {
+        let m = Module {
+            schema: SCHEMA.into(),
+            name: "t".into(),
+            imports: vec![],
+            defs: vec![Def::Fn(FnDef {
+                name: "main".into(),
+                ty: Type::Fn {
+                    params: vec![],
+                    ret: Box::new(Type::unit()),
+                    effects: vec![],
+                    param_modes: vec![],
+                    ret_mode: ParamMode::Implicit,
+                },
+                params: vec![],
+                body: Term::Lit { lit: Literal::Unit },
+                suppress: vec![],
+                doc: None,
+            })],
+        };
+        let ir = emit_ir(&m).unwrap();
+        assert!(
+            ir.contains("declare i32 @ail_str_compare(ptr, ptr)"),
+            "header missing @ail_str_compare declaration; ir was:\n{ir}"
+        );
+    }
 }
diff --git a/runtime/str.c b/runtime/str.c
index d238d4c..113899b 100644
--- a/runtime/str.c
+++ b/runtime/str.c
@@ -24,3 +24,17 @@
 bool ail_str_eq(const char *a, const char *b) {
     return strcmp(a, b) == 0;
 }
+
+/* Three-way comparison of NUL-terminated strings. Normalises libc
+ * strcmp's signed-int output to exactly -1 / 0 / +1 so the codegen
+ * caller can branch on `icmp slt i32 result, 0` and
+ * `icmp eq i32 result, 0` against constants directly without
+ * worrying about strcmp implementations that return values outside
+ * {-1, 0, +1}. Heap-Str ABI milestone will replace the body with
+ * length-prefixed comparison without changing the signature. */
+int ail_str_compare(const char *a, const char *b) {
+    int r = strcmp(a, b);
+    if (r < 0) return -1;
+    if (r > 0) return 1;
+    return 0;
+}