diff --git a/crates/oxc_transformer/src/lib.rs b/crates/oxc_transformer/src/lib.rs
index 54cbf7e3bf6bb..3abe163c2a6c1 100644
--- a/crates/oxc_transformer/src/lib.rs
+++ b/crates/oxc_transformer/src/lib.rs
@@ -141,12 +141,12 @@ impl<'a> Traverse<'a> for Transformer<'a> {
     }
 
     fn enter_expression(&mut self, expr: &mut Expression<'a>, _ctx: &TraverseCtx<'a>) {
-        self.x0_typescript.transform_expression(expr);
         self.x1_react.transform_expression(expr);
         self.x3_es2015.transform_expression(expr);
     }
 
     fn exit_expression(&mut self, expr: &mut Expression<'a>, _ctx: &TraverseCtx<'a>) {
+        self.x0_typescript.transform_expression_on_exit(expr);
         self.x3_es2015.transform_expression_on_exit(expr);
     }
 
@@ -245,4 +245,8 @@ impl<'a> Traverse<'a> for Transformer<'a> {
     ) {
         self.x0_typescript.transform_module_declaration(decl);
     }
+
+    fn enter_ts_type_assertion(&mut self, node: &mut TSTypeAssertion<'a>, _ctx: &TraverseCtx<'a>) {
+        self.x0_typescript.transform_ts_type_assertion(node);
+    }
 }
diff --git a/crates/oxc_transformer/src/typescript/annotations.rs b/crates/oxc_transformer/src/typescript/annotations.rs
index c82db5b210bea..6e802aa883666 100644
--- a/crates/oxc_transformer/src/typescript/annotations.rs
+++ b/crates/oxc_transformer/src/typescript/annotations.rs
@@ -11,7 +11,10 @@ use oxc_span::{Atom, SPAN};
 use oxc_syntax::operator::AssignmentOperator;
 use rustc_hash::FxHashSet;
 
-use super::collector::TypeScriptReferenceCollector;
+use super::{
+    collector::TypeScriptReferenceCollector,
+    diagnostics::{type_assertion_reserved, type_parameters_reserved},
+};
 
 pub struct TypeScriptAnnotations<'a> {
     #[allow(dead_code)]
@@ -197,6 +200,20 @@ impl<'a> TypeScriptAnnotations<'a> {
     }
 
     pub fn transform_arrow_expression(&mut self, expr: &mut ArrowFunctionExpression<'a>) {
+        if self.is_mts_or_ctx_file() {
+            // <T>() => {} is invalid, but <T, >() => {} is valid
+            if let Some(type_parameters) = &expr.type_parameters {
+                if type_parameters.params.len() == 1
+                    && !type_parameters
+                        .span
+                        .source_text(self.ctx.source_text)
+                        .trim_end()
+                        .ends_with(',')
+                {
+                    self.ctx.error(type_parameters_reserved(expr.span));
+                }
+            }
+        }
         expr.type_parameters = None;
         expr.return_type = None;
     }
@@ -240,7 +257,7 @@ impl<'a> TypeScriptAnnotations<'a> {
         });
     }
 
-    pub fn transform_expression(&mut self, expr: &mut Expression<'a>) {
+    pub fn transform_expression_on_exit(&mut self, expr: &mut Expression<'a>) {
         *expr = self.ctx.ast.copy(expr.get_inner_expression());
     }
 
@@ -396,4 +413,15 @@ impl<'a> TypeScriptAnnotations<'a> {
     pub fn transform_jsx_fragment(&mut self, _elem: &mut JSXFragment<'a>) {
         self.has_jsx_fragment = true;
     }
+
+    pub fn transform_ts_type_assertion(&mut self, node: &mut TSTypeAssertion<'a>) {
+        if self.is_mts_or_ctx_file() {
+            self.ctx.error(type_assertion_reserved(node.span));
+        }
+    }
+
+    pub fn is_mts_or_ctx_file(&self) -> bool {
+        let extension = self.ctx.source_path.extension();
+        extension.is_some_and(|ext| matches!(ext.to_str(), Some("mts" | "cts")))
+    }
 }
diff --git a/crates/oxc_transformer/src/typescript/diagnostics.rs b/crates/oxc_transformer/src/typescript/diagnostics.rs
index 381bf515f9536..a09d7ceef5a08 100644
--- a/crates/oxc_transformer/src/typescript/diagnostics.rs
+++ b/crates/oxc_transformer/src/typescript/diagnostics.rs
@@ -15,3 +15,11 @@ pub fn ambient_module_nested(span0: Span) -> OxcDiagnostic {
     OxcDiagnostic::warning("Ambient modules cannot be nested in other modules or namespaces.")
         .with_label(span0)
 }
+
+pub fn type_assertion_reserved(span: Span) -> OxcDiagnostic {
+    OxcDiagnostic::warning("This syntax is reserved in files with the .mts or .cts extension. Use an `as` expression instead.").with_label(span)
+}
+
+pub fn type_parameters_reserved(span: Span) -> OxcDiagnostic {
+    OxcDiagnostic::warning("This syntax is reserved in files with the .mts or .cts extension. Add a trailing comma, as in `<T,>() => ...`.").with_label(span)
+}
diff --git a/crates/oxc_transformer/src/typescript/mod.rs b/crates/oxc_transformer/src/typescript/mod.rs
index 776f35ac51904..b587e88eb871c 100644
--- a/crates/oxc_transformer/src/typescript/mod.rs
+++ b/crates/oxc_transformer/src/typescript/mod.rs
@@ -100,8 +100,8 @@ impl<'a> TypeScript<'a> {
         self.reference_collector.visit_transform_export_named_declaration(decl);
     }
 
-    pub fn transform_expression(&mut self, expr: &mut Expression<'a>) {
-        self.annotations.transform_expression(expr);
+    pub fn transform_expression_on_exit(&mut self, expr: &mut Expression<'a>) {
+        self.annotations.transform_expression_on_exit(expr);
     }
 
     pub fn transform_formal_parameter(&mut self, param: &mut FormalParameter<'a>) {
@@ -211,4 +211,8 @@ impl<'a> TypeScript<'a> {
     pub fn transform_jsx_fragment(&mut self, elem: &mut JSXFragment<'a>) {
         self.annotations.transform_jsx_fragment(elem);
     }
+
+    pub fn transform_ts_type_assertion(&mut self, node: &mut TSTypeAssertion<'a>) {
+        self.annotations.transform_ts_type_assertion(node);
+    }
 }
diff --git a/tasks/transform_conformance/babel.snap.md b/tasks/transform_conformance/babel.snap.md
index fead6a775b13c..8ded029893b64 100644
--- a/tasks/transform_conformance/babel.snap.md
+++ b/tasks/transform_conformance/babel.snap.md
@@ -1,6 +1,6 @@
 commit: 4bd1b2c2
 
-Passed: 309/362
+Passed: 313/362
 
 # All Passed:
 * babel-preset-react
@@ -16,12 +16,8 @@ Passed: 309/362
 * spec/newableArrowFunction-default/input.js
 * spec/newableArrowFunction-vs-spec-false/input.js
 
-# babel-preset-typescript (7/16)
+# babel-preset-typescript (11/16)
 * node-extensions/import-in-cts/input.cts
-* node-extensions/type-assertion-in-cts/input.cts
-* node-extensions/type-assertion-in-mts/input.mts
-* node-extensions/type-param-arrow-in-cts/input.mts
-* node-extensions/type-param-arrow-in-mts/input.mts
 * node-extensions/with-in-mts/input.mts
 * opts/allowDeclareFields/input.ts
 * opts/optimizeConstEnums/input.ts
diff --git a/tasks/transform_conformance/src/test_case.rs b/tasks/transform_conformance/src/test_case.rs
index fd12136e8d4ff..d39821aef98c4 100644
--- a/tasks/transform_conformance/src/test_case.rs
+++ b/tasks/transform_conformance/src/test_case.rs
@@ -467,6 +467,8 @@ fn get_babel_error(error: &str) -> String {
         "Duplicate __self prop found." => "Duplicate __self prop found. You are most likely using the deprecated transform-react-jsx-self Babel plugin. Both __source and __self are automatically set when using the automatic runtime. Please remove transform-react-jsx-source and transform-react-jsx-self from your Babel config.",
         "Duplicate __source prop found." => "Duplicate __source prop found. You are most likely using the deprecated transform-react-jsx-source Babel plugin. Both __source and __self are automatically set when using the automatic runtime. Please remove transform-react-jsx-source and transform-react-jsx-self from your Babel config.",
         "Expected `>` but found `/`" => "Unexpected token, expected \",\"",
+        "This syntax is reserved in files with the .mts or .cts extension. Use an `as` expression instead." => "This syntax is reserved in files with the .mts or .cts extension. Use an `as` expression instead. (1:0)",
+        "This syntax is reserved in files with the .mts or .cts extension. Add a trailing comma, as in `<T,>() => ...`." => "This syntax is reserved in files with the .mts or .cts extension. Add a trailing comma, as in `<T,>() => ...`. (1:0)",
         _ => error
     }.to_string()
 }