fix: validate tool download destination before network

tests/test_tool_manager.py

@@ -333,13 +333,58 @@ def test_download_tool_returns_error_when_destination_is_a_file(tm, tmp_path, mo
 
     download_root = tmp_path / "downloads"
     download_root.mkdir()
-    (download_root / "universal_mail_cleaner").write_text("blocked", encoding="utf-8")
+    sentinel = download_root / "universal_mail_cleaner"
+    sentinel.write_text("blocked", encoding="utf-8")
     monkeypatch.setattr(tool_manager, "_DOWNLOAD_DIR", download_root)
 
+    network_calls = []
+
+    def fail_fast_urlopen(*args, **kwargs):
+        network_calls.append((args, kwargs))
+        raise AssertionError("unexpected network request for an invalid destination")
+
+    monkeypatch.setattr(tool_manager.urllib.request, "urlopen", fail_fast_urlopen)
+    err = tm.download_tool("universal_mail_cleaner")
+
+    assert err is not None
+    assert err.startswith("Download error: cannot prepare destination:")
+    assert network_calls == []
+    assert sentinel.read_text(encoding="utf-8") == "blocked"
+
+
+def test_download_tool_returns_error_when_destination_stat_fails(tm, tmp_path, monkeypatch):
+    import tool_manager
+
+    download_root = tmp_path / "downloads"
+    download_root.mkdir()
+    dest_dir = download_root / "universal_mail_cleaner"
+    dest_dir.mkdir()
+    sentinel = dest_dir / "keep.txt"
+    sentinel.write_text("keep", encoding="utf-8")
+    monkeypatch.setattr(tool_manager, "_DOWNLOAD_DIR", download_root)
+
+    original_stat = Path.stat
+
+    def fail_destination_stat(path, *args, **kwargs):
+        if path == dest_dir:
+            raise PermissionError("synthetic destination stat failure")
+        return original_stat(path, *args, **kwargs)
+
+    monkeypatch.setattr(Path, "stat", fail_destination_stat)
+    network_calls = []
+
+    def fail_fast_urlopen(*args, **kwargs):
+        network_calls.append((args, kwargs))
+        raise AssertionError("unexpected network request after a destination stat failure")
+
+    monkeypatch.setattr(tool_manager.urllib.request, "urlopen", fail_fast_urlopen)
     err = tm.download_tool("universal_mail_cleaner")
 
     assert err is not None
     assert err.startswith("Download error: cannot prepare destination:")
+    assert "synthetic destination stat failure" in err
+    assert network_calls == []
+    assert sentinel.read_text(encoding="utf-8") == "keep"
 
 
 def test_download_tool_extracts_flat_archive(tm, tmp_path, monkeypatch):

tool_manager.py

@@ -4,6 +4,7 @@
 import os
 import re
 import shutil
+import stat
 import subprocess
 import sys
 import urllib.request
@@ -302,6 +303,22 @@ def download_tool(self, tool_id: str,
             return "No GitHub repo configured for this tool"
         repo = meta["github_repo"]
 
+        # Validate the local destination before making any network request.
+        dest_dir = _DOWNLOAD_DIR / tool_id
+        for candidate in (dest_dir, *dest_dir.parents):
+            try:
+                destination_stat = candidate.stat()
+            except FileNotFoundError:
+                continue
+            except OSError as exc:
+                return f"Download error: cannot prepare destination: {exc}"
+            if not stat.S_ISDIR(destination_stat.st_mode):
+                return (
+                    "Download error: cannot prepare destination: "
+                    f"not a directory: {candidate}"
+                )
+            break
+
         # Fetch release metadata
         api_url = f"https://api.github.com/repos/{repo}/releases/latest"
         try:
@@ -320,7 +337,6 @@ def download_tool(self, tool_id: str,
             return "No zipball_url in release info"
 
         # Prepare download directory
-        dest_dir = _DOWNLOAD_DIR / tool_id
         try:
             dest_dir.mkdir(parents=True, exist_ok=True)
         except OSError as exc: