diff --git a/crates/ailang-core/src/workspace.rs b/crates/ailang-core/src/workspace.rs
index eac0fe6..27cc41f 100644
--- a/crates/ailang-core/src/workspace.rs
+++ b/crates/ailang-core/src/workspace.rs
@@ -824,6 +824,7 @@ pub(crate) fn validate_canonical_type_names(
                     type_name, mod_name, &local_types, &import_names,
                 )
             })?;
+            check_class_name_fields(def, mod_name)?;
         }
     }
 
@@ -1146,6 +1147,67 @@ where
     }
 }
 
+/// ct.1: reject any `.` in the four class-reference fields.
+/// `ClassDef.name`, `InstanceDef.class`, `SuperclassRef.class`,
+/// `Constraint.class`. Per spec §"Out of scope: Class names": class
+/// names are NOT module-qualified in this milestone, so any
+/// qualified form indicates a half-migrated file.
+fn check_class_name_fields(
+    def: &Def,
+    owning_module: &str,
+) -> Result<(), WorkspaceLoadError> {
+    fn fire(
+        owning_module: &str,
+        name: &str,
+        field: &'static str,
+    ) -> WorkspaceLoadError {
+        WorkspaceLoadError::QualifiedClassName {
+            module: owning_module.to_string(),
+            name: name.to_string(),
+            field,
+        }
+    }
+    match def {
+        Def::Class(cd) => {
+            if cd.name.contains('.') {
+                return Err(fire(owning_module, &cd.name, "ClassDef.name"));
+            }
+            if let Some(sc) = &cd.superclass {
+                if sc.class.contains('.') {
+                    return Err(fire(owning_module, &sc.class, "SuperclassRef.class"));
+                }
+            }
+            for m in &cd.methods {
+                if let Type::Forall { constraints, .. } = &m.ty {
+                    for c in constraints {
+                        if c.class.contains('.') {
+                            return Err(fire(owning_module, &c.class, "Constraint.class"));
+                        }
+                    }
+                }
+            }
+            Ok(())
+        }
+        Def::Instance(id) => {
+            if id.class.contains('.') {
+                return Err(fire(owning_module, &id.class, "InstanceDef.class"));
+            }
+            Ok(())
+        }
+        Def::Fn(fd) => {
+            if let Type::Forall { constraints, .. } = &fd.ty {
+                for c in constraints {
+                    if c.class.contains('.') {
+                        return Err(fire(owning_module, &c.class, "Constraint.class"));
+                    }
+                }
+            }
+            Ok(())
+        }
+        Def::Const(_) | Def::Type(_) => Ok(()),
+    }
+}
+
 /// Iter 22b.1: extract the head-constructor name of a type, for the
 /// "where is this type defined" lookup and for diagnostic-message
 /// rendering.
@@ -2044,4 +2106,129 @@ mod tests {
             other => panic!("expected BareCrossModuleTypeRef, got {other:?}"),
         }
     }
+
+    /// ct.1: a qualified `ClassDef.name` (e.g. `other.Eq`) must fire
+    /// `QualifiedClassName`. Class names stay bare in this milestone.
+    #[test]
+    fn ct1_validator_rejects_qualified_classdef_name() {
+        let m: Module = serde_json::from_value(serde_json::json!({
+            "schema": crate::SCHEMA,
+            "name": "m",
+            "imports": [],
+            "defs": [{
+                "kind": "class",
+                "name": "other.MyEq",
+                "param": "a",
+                "methods": []
+            }],
+        })).unwrap();
+        let mut modules = BTreeMap::new();
+        modules.insert("m".to_string(), m);
+        let err = validate_canonical_type_names(&modules)
+            .expect_err("qualified ClassDef.name must be rejected");
+        match err {
+            WorkspaceLoadError::QualifiedClassName { module, name, field } => {
+                assert_eq!(module, "m");
+                assert_eq!(name, "other.MyEq");
+                assert_eq!(field, "ClassDef.name");
+            }
+            other => panic!("expected QualifiedClassName, got {other:?}"),
+        }
+    }
+
+    /// ct.1: a qualified `InstanceDef.class` must fire
+    /// `QualifiedClassName`.
+    #[test]
+    fn ct1_validator_rejects_qualified_instancedef_class() {
+        let m: Module = serde_json::from_value(serde_json::json!({
+            "schema": crate::SCHEMA,
+            "name": "m",
+            "imports": [],
+            "defs": [{
+                "kind": "instance",
+                "class": "other.MyEq",
+                "type": { "k": "con", "name": "Int" },
+                "methods": []
+            }],
+        })).unwrap();
+        let mut modules = BTreeMap::new();
+        modules.insert("m".to_string(), m);
+        let err = validate_canonical_type_names(&modules)
+            .expect_err("qualified InstanceDef.class must be rejected");
+        match err {
+            WorkspaceLoadError::QualifiedClassName { name, field, .. } => {
+                assert_eq!(name, "other.MyEq");
+                assert_eq!(field, "InstanceDef.class");
+            }
+            other => panic!("expected QualifiedClassName, got {other:?}"),
+        }
+    }
+
+    /// ct.1: a qualified `SuperclassRef.class` must fire
+    /// `QualifiedClassName`.
+    #[test]
+    fn ct1_validator_rejects_qualified_superclassref_class() {
+        let m: Module = serde_json::from_value(serde_json::json!({
+            "schema": crate::SCHEMA,
+            "name": "m",
+            "imports": [],
+            "defs": [{
+                "kind": "class",
+                "name": "MyOrd",
+                "param": "a",
+                "superclass": { "class": "other.MyEq", "type": "a" },
+                "methods": []
+            }],
+        })).unwrap();
+        let mut modules = BTreeMap::new();
+        modules.insert("m".to_string(), m);
+        let err = validate_canonical_type_names(&modules)
+            .expect_err("qualified SuperclassRef.class must be rejected");
+        match err {
+            WorkspaceLoadError::QualifiedClassName { name, field, .. } => {
+                assert_eq!(name, "other.MyEq");
+                assert_eq!(field, "SuperclassRef.class");
+            }
+            other => panic!("expected QualifiedClassName, got {other:?}"),
+        }
+    }
+
+    /// ct.1: a qualified `Constraint.class` (inside a `Type::Forall`)
+    /// must fire `QualifiedClassName`.
+    #[test]
+    fn ct1_validator_rejects_qualified_constraint_class() {
+        let m: Module = serde_json::from_value(serde_json::json!({
+            "schema": crate::SCHEMA,
+            "name": "m",
+            "imports": [],
+            "defs": [{
+                "kind": "fn",
+                "name": "f",
+                "type": {
+                    "k": "forall",
+                    "vars": ["a"],
+                    "constraints": [
+                        { "class": "other.MyEq", "type": { "k": "var", "name": "a" } }
+                    ],
+                    "body": {
+                        "k": "fn", "params": [{ "k": "var", "name": "a" }],
+                        "ret": { "k": "con", "name": "Unit" }, "effects": []
+                    }
+                },
+                "params": ["x"],
+                "body": { "t": "lit", "lit": { "kind": "unit" } }
+            }],
+        })).unwrap();
+        let mut modules = BTreeMap::new();
+        modules.insert("m".to_string(), m);
+        let err = validate_canonical_type_names(&modules)
+            .expect_err("qualified Constraint.class must be rejected");
+        match err {
+            WorkspaceLoadError::QualifiedClassName { name, field, .. } => {
+                assert_eq!(name, "other.MyEq");
+                assert_eq!(field, "Constraint.class");
+            }
+            other => panic!("expected QualifiedClassName, got {other:?}"),
+        }
+    }
 }