diff --git a/pyrefly/lib/alt/attr.rs b/pyrefly/lib/alt/attr.rs
index f56dd13c35..461fb47397 100644
--- a/pyrefly/lib/alt/attr.rs
+++ b/pyrefly/lib/alt/attr.rs
@@ -2017,6 +2017,24 @@ impl<'a, Ans: LookupAnswer> AnswersSolver<'a, Ans> {
             {
                 acc.not_found(NotFoundOn::ClassInstance(cls.class_object().clone(), base))
             }
+            // A bound method is a `types.MethodType`, which does not override
+            // `__setattr__`/`__delattr__`, so it does not accept arbitrary attribute
+            // assignment (this fails at runtime). Without this arm the base would fall
+            // through to the general lookup and resolve the dunder via
+            // `MethodType.__getattr__`, incorrectly permitting the assignment.
+            AttributeBase1::BoundMethod(_)
+                if (*dunder_name == dunder::SETATTR || *dunder_name == dunder::DELATTR)
+                    && self.field_is_inherited_from(
+                        self.stdlib.method_type().class_object(),
+                        dunder_name,
+                        (ModuleName::builtins().as_str(), "object"),
+                    ) =>
+            {
+                acc.not_found(NotFoundOn::ClassInstance(
+                    self.stdlib.method_type().class_object().clone(),
+                    base,
+                ))
+            }
             AttributeBase1::ShapedArrayInstance(tensor)
                 if (*dunder_name == dunder::SETATTR
                     || *dunder_name == dunder::DELATTR
diff --git a/pyrefly/lib/test/attributes.rs b/pyrefly/lib/test/attributes.rs
index 437dc6640e..3ffef0ce8a 100644
--- a/pyrefly/lib/test/attributes.rs
+++ b/pyrefly/lib/test/attributes.rs
@@ -3055,3 +3055,16 @@ class C:
     x = untyped(1)  # E: implicitly inferred to be `Any`
 "#,
 );
+
+testcase!(
+    test_bound_method_no_arbitrary_attr_set,
+    r#"
+class Foo:
+    def real_method(self) -> None: ...
+f: Foo = Foo()
+f.real_method.test = None  # E: has no attribute `test`
+del f.real_method.test  # E: has no attribute `test`
+# Known method attributes are still accessible.
+name: str = f.real_method.__name__
+"#,
+);