diff --git a/crates/ty_python_semantic/resources/mdtest/generics/legacy/classes.md b/crates/ty_python_semantic/resources/mdtest/generics/legacy/classes.md
index bc6ccdb7c1..30d6a89ec0 100644
--- a/crates/ty_python_semantic/resources/mdtest/generics/legacy/classes.md
+++ b/crates/ty_python_semantic/resources/mdtest/generics/legacy/classes.md
@@ -860,9 +860,6 @@ reveal_type(Sub)  # revealed: <class 'Sub'>
 U = TypeVar("U")
 
 class Base2(Generic[T, U]): ...
-
-# TODO: no error
-# error: [unsupported-base] "Unsupported class base with type `<class 'Base2[Sub2, U@Sub2]'> | <class 'Base2[Sub2[Unknown], U@Sub2]'>`"
 class Sub2(Base2["Sub2", U]): ...
 ```
 
@@ -888,8 +885,6 @@ from typing_extensions import Generic, TypeVar
 
 T = TypeVar("T")
 
-# TODO: no error "Unsupported class base with type `<class 'list[Derived[T@Derived]]'> | <class 'list[@Todo]'>`"
-# error: [unsupported-base]
 class Derived(list[Derived[T]], Generic[T]): ...
 ```
 
diff --git a/crates/ty_python_semantic/src/types.rs b/crates/ty_python_semantic/src/types.rs
index 54beaf3037..e2050ec45f 100644
--- a/crates/ty_python_semantic/src/types.rs
+++ b/crates/ty_python_semantic/src/types.rs
@@ -912,8 +912,19 @@ impl<'db> Type<'db> {
         previous: Self,
         cycle: &salsa::Cycle,
     ) -> Self {
-        UnionType::from_elements_cycle_recovery(db, [self, previous])
-            .recursive_type_normalized(db, cycle)
+        // Avoid unioning two generic aliases of the same class together; this union will never
+        // simplify and is likely to cause downstream problems. This introduces the theoretical
+        // possibility of cycle oscillation involving such types (because we are not strictly
+        // widening the type on each iteration), but so far we have not seen an example of that.
+        match (previous, self) {
+            (Type::GenericAlias(prev_alias), Type::GenericAlias(curr_alias))
+                if prev_alias.origin(db) == curr_alias.origin(db) =>
+            {
+                self
+            }
+            _ => UnionType::from_elements_cycle_recovery(db, [self, previous]),
+        }
+        .recursive_type_normalized(db, cycle)
     }
 
     fn is_none(&self, db: &'db dyn Db) -> bool {