diff --git a/.ast-grep/rule-tests/__snapshots__/no-string-function-snapshot.yml b/.ast-grep/rule-tests/__snapshots__/no-string-function-snapshot.yml
new file mode 100644
index 0000000..e9713eb
--- /dev/null
+++ b/.ast-grep/rule-tests/__snapshots__/no-string-function-snapshot.yml
@@ -0,0 +1,20 @@
+id: no-string-function
+snapshots:
+  String(123):
+    labels:
+    - source: String(123)
+      style: primary
+      start: 0
+      end: 11
+  String(Date.now()):
+    labels:
+    - source: String(Date.now())
+      style: primary
+      start: 0
+      end: 18
+  String(value):
+    labels:
+    - source: String(value)
+      style: primary
+      start: 0
+      end: 13
diff --git a/.ast-grep/rule-tests/no-string-function-test.yml b/.ast-grep/rule-tests/no-string-function-test.yml
new file mode 100644
index 0000000..814c8dd
--- /dev/null
+++ b/.ast-grep/rule-tests/no-string-function-test.yml
@@ -0,0 +1,13 @@
+id: no-string-function
+valid:
+  # toString() is fine
+  - value.toString()
+  - (123).toString()
+  - date.toLocaleString()
+  # Other functions named String are fine
+  - myString(value)
+invalid:
+  # String() function should be flagged
+  - String(value)
+  - String(123)
+  - String(Date.now())
diff --git a/.ast-grep/rules/no-string-function.yml b/.ast-grep/rules/no-string-function.yml
new file mode 100644
index 0000000..10044df
--- /dev/null
+++ b/.ast-grep/rules/no-string-function.yml
@@ -0,0 +1,7 @@
+id: no-string-function
+language: typescript
+severity: error
+message: "Don't use String() - use .toString() or .toLocaleString() instead."
+note: "String() can have unexpected behavior. Use .toString() for general conversion or .toLocaleString() for locale-aware formatting."
+rule:
+  pattern: String($VAL)