Excel (xlsx) skill

Spreadsheets are where a language model's weaknesses and a script's strengths diverge most sharply. Asking a model to reason its way through a large workbook is slow, expensive in context, and unreliable on exactly the arithmetic it looks most confident about. Handing the workbook to code that reads cells, evaluates ranges and writes results is deterministic and cheap. This skill encodes that division of labour.

The governance angle is less about the code and more about what the code is pointed at. Spreadsheets in a business are where the sensitive data that never made it into a governed system lives — salary models, customer lists, forecasts, board material, pricing. An agent with a working Excel skill and filesystem access is an efficient reader of all of it, and every cell it reads becomes context that goes wherever that session goes.

Formulas add a second dimension. A workbook can contain external references and links that pull data from other locations when evaluated, so 'read this spreadsheet' is not necessarily a self-contained operation. A workbook received from outside the organisation should be treated as untrusted input, not as a data file.

The context cost is worth planning for too: a large workbook rendered into text can consume an enormous share of the window, which is precisely why the script-based approach — extract what is needed, summarise the rest — is the right shape.