fix: validate FileSystemProvider root; use fresh dict on reload

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/fastmcp/server/providers/filesystem.py

@@ -89,6 +89,14 @@ class FileSystemProvider(LocalProvider):
     ) -> None:
         super().__init__(on_duplicate="replace")
         self._root = Path(root).resolve()
+        if not self._root.exists():
+            raise FileNotFoundError(
+                f"FileSystemProvider root does not exist: {self._root}"
+            )
+        if not self._root.is_dir():
+            raise NotADirectoryError(
+                f"FileSystemProvider root is not a directory: {self._root}"
+            )
         self._reload = reload
         self._loaded = False
         # Track files we've warned about: path -> mtime when warned
@@ -102,10 +110,6 @@ class FileSystemProvider(LocalProvider):
 
     def _load_components(self) -> None:
         """Discover and register all components from the filesystem."""
-        # Clear existing components if reloading
-        if self._loaded:
-            self._components.clear()
-
         result = discover_and_import(self._root)
 
         # Log warnings for failed files (only once per file version)
@@ -126,6 +130,9 @@ class FileSystemProvider(LocalProvider):
         for fp in successful_files:
             self._warned_files.pop(fp, None)
 
+        # Fresh dict (not .clear()) so in-flight iterators over the old dict aren't disrupted.
+        self._components = {}
+
         for file_path, component in result.components:
             try:
                 self._register_component(component)

tests/fs/test_provider.py

@@ -3,6 +3,8 @@
 import time
 from pathlib import Path
 
+import pytest
+
 from fastmcp import FastMCP
 from fastmcp.client import Client
 from fastmcp.server.providers import FileSystemProvider
@@ -16,6 +18,19 @@ class TestFileSystemProvider:
         provider = FileSystemProvider(tmp_path)
         assert repr(provider).startswith("FileSystemProvider")
 
+    def test_provider_raises_on_missing_root(self, tmp_path: Path):
+        """Provider should raise FileNotFoundError for non-existent root."""
+        missing = tmp_path / "does_not_exist"
+        with pytest.raises(FileNotFoundError, match="does not exist"):
+            FileSystemProvider(missing)
+
+    def test_provider_raises_on_file_root(self, tmp_path: Path):
+        """Provider should raise NotADirectoryError when root is a file."""
+        file_path = tmp_path / "not_a_dir.py"
+        file_path.write_text("x = 1")
+        with pytest.raises(NotADirectoryError, match="not a directory"):
+            FileSystemProvider(file_path)
+
     def test_provider_discovers_tools(self, tmp_path: Path):
         """Provider should discover @tool decorated functions."""
         tools_dir = tmp_path / "tools"