diff --git a/crates/my-lang/src/stdlib.rs b/crates/my-lang/src/stdlib.rs
index 07ffd17..8b8d3bb 100644
--- a/crates/my-lang/src/stdlib.rs
+++ b/crates/my-lang/src/stdlib.rs
@@ -1471,6 +1471,37 @@ fn register_fs_functions(define: &mut impl FnMut(String, Value)) {
             },
         }),
     );
+
+    // fs_list_dir(path) -> Array<String> - entry names (not full paths) in
+    // `path`, sorted for deterministic tooling output. Errors if `path` is not
+    // a readable directory. See hyperpolymath/my-lang#55.
+    define(
+        "fs_list_dir".to_string(),
+        Value::NativeFunction(NativeFunction {
+            name: "fs_list_dir".to_string(),
+            arity: 1,
+            func: |args| match &args[0] {
+                Value::String(path) => {
+                    let rd = std::fs::read_dir(path).map_err(|e| {
+                        RuntimeError::Custom(format!("fs_list_dir({}) failed: {}", path, e))
+                    })?;
+                    let mut names: Vec<String> = Vec::new();
+                    for entry in rd {
+                        let entry = entry.map_err(|e| {
+                            RuntimeError::Custom(format!("fs_list_dir({}) failed: {}", path, e))
+                        })?;
+                        names.push(entry.file_name().to_string_lossy().into_owned());
+                    }
+                    names.sort();
+                    Ok(Value::Array(names.into_iter().map(Value::String).collect()))
+                }
+                _ => Err(RuntimeError::TypeError {
+                    expected: "string".to_string(),
+                    got: format!("{:?}", args[0]),
+                }),
+            },
+        }),
+    );
 }
 
 // ============================================================================
@@ -1880,6 +1911,7 @@ pub fn stdlib_functions() -> Vec<&'static str> {
         "fs_read_file",
         "fs_create_dir_all",
         "fs_exists",
+        "fs_list_dir",
         // Env extras
         "env_args",
         // Map / dict
diff --git a/examples/fs_list_dir.my b/examples/fs_list_dir.my
new file mode 100644
index 0000000..ae9a23a
--- /dev/null
+++ b/examples/fs_list_dir.my
@@ -0,0 +1,12 @@
+// Solo fs_list_dir() builtin (hyperpolymath/my-lang#55).
+//
+//   my run examples/fs_list_dir.my
+//
+// Lists this repo's examples/ directory, sorted. Output includes the other
+// example files (date.my, json.my, map.my, fs_list_dir.my, ...).
+
+fn main() -> Int {
+    let entries = fs_list_dir("examples");
+    println(str_join(entries, "\n"));
+    return 0;
+}
diff --git a/tests/integration_test.rs b/tests/integration_test.rs
index 11f3012..79a3c39 100644
--- a/tests/integration_test.rs
+++ b/tests/integration_test.rs
@@ -293,6 +293,27 @@ fn test_eval_date_today() {
     }
 }
 
+// Solo fs_list_dir() stdlib builtin (hyperpolymath/my-lang#55): enumerate a
+// directory, sorted entry names. Self-contained: builds a dir with fs_* builtins
+// then lists it (exercises fs_create_dir_all/fs_write_file/fs_list_dir/str_join).
+#[test]
+fn test_eval_fs_list_dir() {
+    let source = r#"
+        fn main() -> String {
+            let dir = "target/.it_fs_list_dir";
+            fs_create_dir_all(dir);
+            fs_write_file("target/.it_fs_list_dir/b.txt", "x");
+            fs_write_file("target/.it_fs_list_dir/a.txt", "y");
+            let names = fs_list_dir(dir);
+            return str_join(names, ",");
+        }
+    "#;
+    match eval(source) {
+        Ok(Value::String(s)) => assert_eq!(s, "a.txt,b.txt"),
+        other => panic!("expected sorted dir listing, got {:?}", other),
+    }
+}
+
 // AI Runtime tests (require API keys, so just test initialization)
 #[test]
 fn test_ai_runtime_creation() {