Stabilize directory resource listings

docs/servers/resources.md

@@ -126,6 +126,7 @@ The same rule applies to anything else JSON-serialisable: a list, a Pydantic mod
     `mcp.server.mcpserver.resources` has ready-made `Resource` classes (`TextResource`,
     `BinaryResource`, `FileResource`, `HttpResource`, `DirectoryResource`) that you register
     with `mcp.add_resource(...)`.
+    `DirectoryResource` returns sorted relative file paths with `/` separators on every platform.
 
 A client can also **subscribe** to a resource and be notified when it changes; that's the client's half of the story and it lives in **[The Client](../client/index.md)**.
 

src/mcp/server/mcpserver/resources/types.py

@@ -206,13 +206,13 @@ class DirectoryResource(Resource):
 
     @pydantic.field_validator("path")
     @classmethod
-    def validate_absolute_path(cls, path: Path) -> Path:  # pragma: no cover
+    def validate_absolute_path(cls, path: Path) -> Path:
         """Ensure path is absolute."""
         if not path.is_absolute():
             raise ValueError("Path must be absolute")
         return path
 
-    def list_files(self) -> list[Path]:  # pragma: no cover
+    def list_files(self) -> list[Path]:
         """List files in the directory."""
         if not self.path.exists():
             raise FileNotFoundError(f"Directory not found: {self.path}")
@@ -223,8 +223,9 @@ def list_files(self) -> list[Path]:  # pragma: no cover
             return list(self.path.glob(self.pattern)) if not self.recursive else list(self.path.rglob(self.pattern))
         return list(self.path.glob("*")) if not self.recursive else list(self.path.rglob("*"))
 
-    async def read(self) -> str:  # Always returns JSON string  # pragma: no cover
+    async def read(self) -> str:
         """Read the directory listing."""
-        files = await anyio.to_thread.run_sync(self.list_files)
-        file_list = [str(f.relative_to(self.path)) for f in files if f.is_file()]
+        file_list = await anyio.to_thread.run_sync(
+            lambda: sorted(f.relative_to(self.path).as_posix() for f in self.list_files() if f.is_file())
+        )
         return json.dumps({"files": file_list}, indent=2)

tests/server/mcpserver/resources/test_directory_resources.py

@@ -0,0 +1,68 @@
+import json
+from pathlib import Path
+
+import pytest
+from pydantic import ValidationError
+
+from mcp.server.mcpserver.resources import DirectoryResource
+
+
+@pytest.fixture
+def directory_with_files(tmp_path: Path) -> Path:
+    (tmp_path / "z.txt").write_text("z", encoding="utf-8")
+    (tmp_path / "a.txt").write_text("a", encoding="utf-8")
+    (tmp_path / "nested").mkdir()
+    (tmp_path / "nested" / "b.txt").write_text("b", encoding="utf-8")
+    return tmp_path
+
+
+@pytest.fixture
+def regular_file(tmp_path: Path) -> Path:
+    path = tmp_path / "file.txt"
+    path.write_text("content", encoding="utf-8")
+    return path
+
+
+@pytest.mark.anyio
+@pytest.mark.parametrize(
+    ("recursive", "pattern", "expected"),
+    [
+        (False, None, ["a.txt", "z.txt"]),
+        (True, None, ["a.txt", "nested/b.txt", "z.txt"]),
+        (False, "*.txt", ["a.txt", "z.txt"]),
+        (True, "*.txt", ["a.txt", "nested/b.txt", "z.txt"]),
+    ],
+)
+async def test_directory_resource_returns_sorted_posix_file_paths(
+    directory_with_files: Path, recursive: bool, pattern: str | None, expected: list[str]
+) -> None:
+    """SDK-defined listings contain sorted relative file paths for each glob mode."""
+    resource = DirectoryResource(
+        uri="test://directory", name="directory", path=directory_with_files, recursive=recursive, pattern=pattern
+    )
+
+    assert json.loads(await resource.read()) == {"files": expected}
+
+
+@pytest.mark.anyio
+async def test_directory_resource_rejects_missing_directory(tmp_path: Path) -> None:
+    """SDK-defined directory reads report a missing path."""
+    resource = DirectoryResource(uri="test://directory", name="directory", path=tmp_path / "missing")
+
+    with pytest.raises(FileNotFoundError):
+        await resource.read()
+
+
+@pytest.mark.anyio
+async def test_directory_resource_rejects_regular_file(regular_file: Path) -> None:
+    """SDK-defined directory reads reject a regular file path."""
+    resource = DirectoryResource(uri="test://directory", name="directory", path=regular_file)
+
+    with pytest.raises(NotADirectoryError):
+        await resource.read()
+
+
+def test_directory_resource_requires_absolute_path() -> None:
+    """SDK-defined resource validation rejects relative directory paths."""
+    with pytest.raises(ValidationError):
+        DirectoryResource(uri="test://directory", name="directory", path=Path("relative"))