Hello Matt Evans-Lacey,
Could you confirm which type of agent you created: a Copilot Studio agent, an agent in Microsoft 365 Copilot, or the Excel Agent?
That distinction matters. In Copilot Studio, knowledge is primarily used as information the agent can reference, while tools/actions are what allow an agent to perform operations.
If you are using Copilot in Excel instead, it supports directly editing workbook content while keeping the workbook editable. If this is specifically a Copilot Studio agent, I’d also recommend posting the scenario in the Power Platform Copilot Studio forum, which is dedicated to building and troubleshooting Copilot Studio agents.