diff --git a/crates/ailang-codegen/src/lib.rs b/crates/ailang-codegen/src/lib.rs
index 936b463..11dae95 100644
--- a/crates/ailang-codegen/src/lib.rs
+++ b/crates/ailang-codegen/src/lib.rs
@@ -1851,15 +1851,18 @@ impl<'a> Emitter<'a> {
             return Ok((dst, "i1".into()));
         }
         if name == "float_to_str" {
-            // Deferred to iter 5+: needs runtime-allocated Str
-            // (currently the codegen Str path uses only static
-            // `@.str_*` globals; no malloc-backed dynamic-Str
-            // infrastructure). The typecheck path in iter 3
-            // installs `float_to_str : (Float) -> Str`; until the
-            // runtime gets a Str allocator, calling it crashes
-            // codegen. This is intentional: don't ship an
-            // unimplemented call to the LLM-author surface.
-            unimplemented!("Floats milestone iter 5+: float_to_str needs dynamic Str allocation");
+            // Codegen lowering is deferred to a follow-up iteration:
+            // it needs runtime-allocated Str, and the codegen Str
+            // path currently uses only static `@.str_*` globals (no
+            // malloc-backed dynamic-Str infrastructure). The
+            // typecheck path installs `float_to_str : (Float) -> Str`
+            // so the symbol resolves, but calling it has no codegen
+            // path yet — surface a structured error rather than
+            // panicking the compiler.
+            return Err(CodegenError::Internal(
+                "`float_to_str` codegen lowering is not yet implemented \
+                 (requires dynamic Str allocation in the runtime)".into(),
+            ));
         }
 
         // Cross-module call: exactly one dot in the name → resolve via import map.