fix(filesystem): move a file across filesystems by copying it on EXDEV

.changeset/5188-filesystem-move-exdev.md

@@ -0,0 +1,5 @@
+---
+"@modelcontextprotocol/server-filesystem": patch
+---
+
+`move_file` now moves a regular file between allowed directories on different filesystems (mounts, disks or shares), where it used to fail with `EXDEV`: it copies the file to a newly created destination, keeping its permissions and modification time, then removes the source, and removes a partial copy if anything fails. A directory still cannot be moved across filesystems, and now fails with an error that says so.

src/filesystem/README.md

@@ -161,6 +161,12 @@ The server's directory access control follows this flow:
     - `source` (string)
     - `destination` (string)
   - Fails if destination exists
+  - Across filesystems (the source and destination on different mounts or
+    disks), a regular file is moved by copying it to a newly created
+    destination, with its permissions and modification time, and then removing
+    the source. If the copy fails, the partial destination is removed and the
+    source is left in place. A directory cannot be moved across filesystems
+    and fails with an error; move its files one at a time instead
 
 - **search_files**
   - Recursively search for files/directories that match or do not match patterns

src/filesystem/__tests__/lib.test.ts

@@ -703,6 +703,40 @@ describe("Lib Functions", () => {
 
         expect(mockPinned.renameIntoDirectory).not.toHaveBeenCalled();
       });
+
+      it("copies and removes instead when the rename crosses a device (EXDEV)", async () => {
+        mockFs.lstat.mockRejectedValueOnce(
+          Object.assign(new Error("not found"), { code: "ENOENT" }),
+        );
+        mockPinned.renameIntoDirectory.mockImplementationOnce(() => {
+          throw Object.assign(new Error("EXDEV: cross-device link"), {
+            code: "EXDEV",
+          });
+        });
+
+        await moveFile("/test/source.txt", "/mnt/other/dest.txt");
+
+        expect(mockPinned.moveFileAcrossDevices).toHaveBeenCalledWith(
+          "/test/source.txt",
+          "/mnt/other/dest.txt",
+        );
+      });
+
+      it("reports a rename failure other than EXDEV as is, without copying", async () => {
+        mockFs.lstat.mockRejectedValueOnce(
+          Object.assign(new Error("not found"), { code: "ENOENT" }),
+        );
+        mockPinned.renameIntoDirectory.mockImplementationOnce(() => {
+          throw Object.assign(new Error("EACCES: permission denied"), {
+            code: "EACCES",
+          });
+        });
+
+        await expect(
+          moveFile("/test/source.txt", "/test/dest.txt"),
+        ).rejects.toThrow("EACCES: permission denied");
+        expect(mockPinned.moveFileAcrossDevices).not.toHaveBeenCalled();
+      });
     });
   });