diff --git a/crates/oxc_isolated_declarations/src/diagnostics.rs b/crates/oxc_isolated_declarations/src/diagnostics.rs
index 4577147b7d380..6b6edfdd94652 100644
--- a/crates/oxc_isolated_declarations/src/diagnostics.rs
+++ b/crates/oxc_isolated_declarations/src/diagnostics.rs
@@ -112,7 +112,6 @@ pub fn implicitly_adding_undefined_to_type(span: Span) -> OxcDiagnostic {
     .with_label(span)
 }
 
-#[allow(dead_code)]
 pub fn function_with_assigning_properties(span: Span) -> OxcDiagnostic {
     OxcDiagnostic::error(
         "TS9023: Assigning properties to functions without declaring them is not supported with --isolatedDeclarations. Add an explicit declaration for the properties assigned to this function.",
diff --git a/crates/oxc_isolated_declarations/src/lib.rs b/crates/oxc_isolated_declarations/src/lib.rs
index 285b653b4f96c..b736e8ddbba1a 100644
--- a/crates/oxc_isolated_declarations/src/lib.rs
+++ b/crates/oxc_isolated_declarations/src/lib.rs
@@ -19,11 +19,13 @@ mod types;
 
 use std::{cell::RefCell, collections::VecDeque, mem};
 
+use diagnostics::function_with_assigning_properties;
 use oxc_allocator::Allocator;
 #[allow(clippy::wildcard_imports)]
 use oxc_ast::{ast::*, AstBuilder, Visit};
 use oxc_diagnostics::OxcDiagnostic;
 use oxc_span::{Atom, SourceType, SPAN};
+use rustc_hash::FxHashSet;
 
 use crate::scope::ScopeTree;
 
@@ -105,6 +107,7 @@ impl<'a> IsolatedDeclarations<'a> {
                 }
             }
         }
+        self.report_error_for_expando_function(stmts);
         new_ast_stmts
     }
 
@@ -321,6 +324,7 @@ impl<'a> IsolatedDeclarations<'a> {
                 .push(Statement::from(ModuleDeclaration::ExportNamedDeclaration(empty_export)));
         }
 
+        self.report_error_for_expando_function(stmts);
         new_ast_stmts
     }
 
@@ -393,6 +397,85 @@ impl<'a> IsolatedDeclarations<'a> {
         })
     }
 
+    pub fn report_error_for_expando_function(&self, stmts: &oxc_allocator::Vec<'a, Statement<'a>>) {
+        let mut can_expando_function_names = FxHashSet::default();
+        for stmt in stmts {
+            match stmt {
+                Statement::ExportNamedDeclaration(decl) => match decl.declaration.as_ref() {
+                    Some(Declaration::FunctionDeclaration(func)) => {
+                        if func.body.is_some() {
+                            if let Some(id) = func.id.as_ref() {
+                                can_expando_function_names.insert(id.name.clone());
+                            }
+                        }
+                    }
+                    Some(Declaration::VariableDeclaration(decl)) => {
+                        for declarator in &decl.declarations {
+                            if declarator.id.type_annotation.is_none()
+                                && declarator.init.as_ref().is_some_and(Expression::is_function)
+                            {
+                                if let Some(name) = declarator.id.get_identifier() {
+                                    can_expando_function_names.insert(name.clone());
+                                }
+                            }
+                        }
+                    }
+                    _ => (),
+                },
+                Statement::ExportDefaultDeclaration(decl) => {
+                    if let ExportDefaultDeclarationKind::FunctionDeclaration(func) =
+                        &decl.declaration
+                    {
+                        if func.body.is_some() {
+                            if let Some(id) = func.id.as_ref() {
+                                can_expando_function_names.insert(id.name.clone());
+                            }
+                        }
+                    }
+                }
+                Statement::FunctionDeclaration(func) => {
+                    if func.body.is_some() {
+                        if let Some(id) = func.id.as_ref() {
+                            if self.scope.has_reference(&id.name) {
+                                can_expando_function_names.insert(id.name.clone());
+                            }
+                        }
+                    }
+                }
+                Statement::VariableDeclaration(decl) => {
+                    for declarator in &decl.declarations {
+                        if declarator.id.type_annotation.is_none()
+                            && declarator.init.as_ref().is_some_and(Expression::is_function)
+                        {
+                            if let Some(name) = declarator.id.get_identifier() {
+                                if self.scope.has_reference(&name) {
+                                    can_expando_function_names.insert(name.clone());
+                                }
+                            }
+                        }
+                    }
+                }
+                Statement::ExpressionStatement(stmt) => {
+                    if let Expression::AssignmentExpression(assignment) = &stmt.expression {
+                        if let AssignmentTarget::StaticMemberExpression(static_member_expr) =
+                            &assignment.left
+                        {
+                            if let Expression::Identifier(ident) = &static_member_expr.object {
+                                if can_expando_function_names.contains(&ident.name) {
+                                    self.error(function_with_assigning_properties(
+                                        static_member_expr.span,
+                                    ));
+                                }
+                            }
+                        }
+                    }
+                }
+
+                _ => {}
+            }
+        }
+    }
+
     pub fn is_declare(&self) -> bool {
         // If we are in a module block, we don't need to add declare
         !self.scope.is_ts_module_block_flag()
diff --git a/crates/oxc_isolated_declarations/tests/fixtures/expando-function.ts b/crates/oxc_isolated_declarations/tests/fixtures/expando-function.ts
new file mode 100644
index 0000000000000..8633029cc6b0f
--- /dev/null
+++ b/crates/oxc_isolated_declarations/tests/fixtures/expando-function.ts
@@ -0,0 +1,15 @@
+export function foo(): void {}
+foo.apply = () => {}
+
+export const bar = (): void => {}
+bar.call = ()=> {}
+
+
+export namespace NS {
+  export const goo = (): void => {}
+  goo.length = 10
+}
+
+// unexported
+const zoo = (): void => {}
+zoo.toString = ()=> {}
\ No newline at end of file
diff --git a/crates/oxc_isolated_declarations/tests/snapshots/expando-function.snap b/crates/oxc_isolated_declarations/tests/snapshots/expando-function.snap
new file mode 100644
index 0000000000000..9f41e74cc5f6c
--- /dev/null
+++ b/crates/oxc_isolated_declarations/tests/snapshots/expando-function.snap
@@ -0,0 +1,44 @@
+---
+source: crates/oxc_isolated_declarations/tests/mod.rs
+input_file: crates/oxc_isolated_declarations/tests/fixtures/expando-function.ts
+---
+==================== .D.TS ====================
+
+export declare function foo(): void;
+export declare const bar: () => void;
+export declare module NS {
+	export const goo: () => void;
+}
+
+
+==================== Errors ====================
+
+  x TS9023: Assigning properties to functions without declaring them is not
+  | supported with --isolatedDeclarations. Add an explicit declaration for the
+  | properties assigned to this function.
+    ,-[10:3]
+  9 |   export const goo = (): void => {}
+ 10 |   goo.length = 10
+    :   ^^^^^^^^^^
+ 11 | }
+    `----
+
+  x TS9023: Assigning properties to functions without declaring them is not
+  | supported with --isolatedDeclarations. Add an explicit declaration for the
+  | properties assigned to this function.
+   ,-[2:1]
+ 1 | export function foo(): void {}
+ 2 | foo.apply = () => {}
+   : ^^^^^^^^^
+ 3 | 
+   `----
+
+  x TS9023: Assigning properties to functions without declaring them is not
+  | supported with --isolatedDeclarations. Add an explicit declaration for the
+  | properties assigned to this function.
+   ,-[5:1]
+ 4 | export const bar = (): void => {}
+ 5 | bar.call = ()=> {}
+   : ^^^^^^^^
+ 6 | 
+   `----