diff --git a/tests/tools/test_mcp_tool.py b/tests/tools/test_mcp_tool.py
index 5f05b0fc34328..0d7e8358123dd 100644
--- a/tests/tools/test_mcp_tool.py
+++ b/tests/tools/test_mcp_tool.py
@@ -1278,16 +1278,19 @@ def test_windows_location_vars_passed_without_secrets(self):
 
         fake_env = {
             "PATH": r"C:\Windows\System32",
-            "ProgramFiles": r"C:\Program Files",
-            "ProgramData": r"C:\ProgramData",
-            "ProgramW6432": r"C:\Program Files",
+            "PROGRAMFILES": r"C:\Program Files",
+            "PROGRAMDATA": r"C:\ProgramData",
+            "PROGRAMW6432": r"C:\Program Files",
             "LOCALAPPDATA": r"C:\Users\alice\AppData\Local",
             "APPDATA": r"C:\Users\alice\AppData\Roaming",
             "USERPROFILE": r"C:\Users\alice",
             "GITHUB_TOKEN": "ghp_xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx",
             "OPENAI_API_KEY": "sk-proj-abc123",
         }
-        with patch.dict("os.environ", fake_env, clear=True):
+        with (
+            patch("tools.mcp_tool.os.name", "nt"),
+            patch.dict("os.environ", fake_env, clear=True),
+        ):
             result = _build_safe_env(None)
 
         assert result["ProgramFiles"] == r"C:\Program Files"
@@ -1299,6 +1302,20 @@ def test_windows_location_vars_passed_without_secrets(self):
         assert "GITHUB_TOKEN" not in result
         assert "OPENAI_API_KEY" not in result
 
+    def test_posix_preserves_uppercase_windows_location_var_spelling(self):
+        """POSIX environments retain source case for allowed Windows names."""
+        from tools.mcp_tool import _build_safe_env
+
+        fake_env = {"PROGRAMFILES": "/opt/windows-program-files"}
+        with (
+            patch("tools.mcp_tool.os.name", "posix"),
+            patch.dict("os.environ", fake_env, clear=True),
+        ):
+            result = _build_safe_env(None)
+
+        assert result["PROGRAMFILES"] == "/opt/windows-program-files"
+        assert "ProgramFiles" not in result
+
 
 # ---------------------------------------------------------------------------
 # _sanitize_error
diff --git a/tools/mcp_tool.py b/tools/mcp_tool.py
index ffcdff532db1b..676e112044fad 100644
--- a/tools/mcp_tool.py
+++ b/tools/mcp_tool.py
@@ -652,6 +652,24 @@ def _jittered(seconds: float) -> float:
     "WINDIR",
 })
 
+# Canonical (mixed-case) spellings for the Windows location vars whose
+# documented names are not all-uppercase. CPython normalizes every key in
+# ``os.environ`` to UPPERCASE on Windows (``os.name == "nt"``), so iterating it
+# yields e.g. ``"PROGRAMFILES"`` even though the variable is documented and
+# consumed as ``"ProgramFiles"``. Launcher-style tools (and the test contract)
+# expect the canonical spelling, so map the upper-cased key back to it on
+# Windows only. Keys not in this map keep their original spelling
+# (all-uppercase vars like ``APPDATA`` are already canonical).
+_WINDOWS_CANONICAL_ENV_KEYS = {
+    "PROGRAMFILES": "ProgramFiles",
+    "PROGRAMFILES(X86)": "ProgramFiles(x86)",
+    "PROGRAMW6432": "ProgramW6432",
+    "PROGRAMDATA": "ProgramData",
+    "COMMONPROGRAMFILES": "CommonProgramFiles",
+    "COMMONPROGRAMFILES(X86)": "CommonProgramFiles(x86)",
+    "COMMONPROGRAMW6432": "CommonProgramW6432",
+}
+
 # Regex for credential patterns to strip from error messages
 _CREDENTIAL_PATTERN = re.compile(
     r"(?:"
@@ -759,7 +777,14 @@ def _build_safe_env(user_env: Optional[dict]) -> dict:
             or key.startswith("XDG_")
             or (get_secret_source is not None and get_secret_source(key))
         ):
-            env[key] = value
+            # Restore mixed-case Windows location names only where CPython
+            # normalizes os.environ keys. POSIX must retain the source spelling.
+            canonical_key = (
+                _WINDOWS_CANONICAL_ENV_KEYS.get(key.upper(), key)
+                if os.name == "nt"
+                else key
+            )
+            env[canonical_key] = value
     if user_env:
         env.update(user_env)
     return env