Fix Excel attach: bind via GetActiveObject, not xlwings hwnd-walk

The server timed out on every Excel-touching call once the ModelChoice
add-in was loaded into Excel. Root cause: xlwings binds to Excel by walking
window handles (AccessibleObjectFromWindow -> .Application), and that path
fails with OLE 0x800A01A8 — and can hang the call outright — when the
Excel-DNA add-in is present. The ROT-based GetActiveObject('Excel.Application')
dispatch is reliable.

_book now injects that dispatch into app.impl._xl before any worksheet
access (per call, so an Excel restart can't strand a handle). Verified live
end to end against a loaded add-in: write_tree, render, list_trees, roll_up,
run_evpi, run_evii, and verify_rollback all work (root EV 1460 from the
add-in's MC_V_ cells matched the Python rollback exactly).

No-ops off Windows / if win32com is unavailable, so the fake-bridge tests
are unaffected. 50 tests pass; ruff + mypy clean. Version 0.0.13.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: vosesoft

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "modelchoice-mcp"
-version = "0.0.12"
+version = "0.0.13"
 description = "An open Model Context Protocol server for Vose Software's ModelChoice decision-tree add-in for Excel."
 readme = "README.md"
 requires-python = ">=3.11"

src/modelchoice_mcp/__init__.py

@@ -1,4 +1,4 @@
 """ModelChoice MCP — an open Model Context Protocol server for Vose
 Software's ModelChoice decision-tree add-in for Excel."""
 
-__version__ = "0.0.12"
+__version__ = "0.0.13"

src/modelchoice_mcp/bridge.py

@@ -51,13 +51,33 @@ def _book(self, workbook: str | None) -> Any:
             raise ExcelNotRunningError(
                 "No running Excel instance found. Open the workbook in Excel first."
             )
+        self._harden_attach(app)
         if workbook is None:
             return app.books.active
         for b in app.books:
             if b.name == workbook:
                 return b
         raise ModelChoiceNotFoundError(f"Workbook {workbook!r} is not open.")
 
+    @staticmethod
+    def _harden_attach(app: Any) -> None:
+        """Make the COM attach robust against a loaded ModelChoice add-in.
+
+        xlwings binds to Excel by walking window handles
+        (``AccessibleObjectFromWindow`` → ``.Application``). Once ModelChoice's
+        Excel-DNA add-in is loaded, that path fails with OLE ``0x800A01A8``
+        (and can hang the call entirely). The ROT-based
+        ``GetActiveObject('Excel.Application')`` dispatch is reliable, so we
+        inject it as the app's COM object before any worksheet access. Done
+        per call so an Excel restart can't leave a stale handle. No-ops off
+        Windows or if win32com isn't importable."""
+        try:
+            import win32com.client  # type: ignore[import-untyped]
+
+            app.impl._xl = win32com.client.GetActiveObject("Excel.Application")
+        except Exception:
+            pass  # fall back to xlwings' default attach
+
     def read_store_raw(self, workbook: str | None = None) -> str:
         """Return the reassembled ``_MC_Store`` A1 payload, or '' if the
         sheet is absent."""

tests/test_server.py

@@ -7,7 +7,7 @@
 
 
 def test_version_set() -> None:
-    assert __version__ == "0.0.12"
+    assert __version__ == "0.0.13"
 
 
 def test_server_name() -> None: