AI agents use write_formula to create or update resources in Excel MCP Server — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Excel MCP Server environment.
This tool creates or modifies cell contents by inserting formulas, which is reversible through undo or overwriting. While formulas can trigger calculations with side effects, the tool itself performs a write operation rather than execute (which would be running external code/scripts).
From the tool's definition Tool name is 'write_formula' and description states 'Write a formula to a cell.' The verb 'write' and the explicit mention of writing to a cell confirm this is a data modification operation.
Documented attack patterns abuse exactly the kind of access write_formula gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and Excel MCP Server, and nothing reaches the server without passing your rules. This is the rule we recommend for write_formula:
{
"version": "1",
"default": "deny",
"tools": {
"write_formula": {
"limits": [
{
"counter": "write_formula_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}write_formula stays usable, but capped — an agent stuck in a loop can't make hundreds of changes a minute. Everything else on the server is denied unless you say otherwise.
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Write a formula to a cell. It is categorised as a Write tool in the Excel MCP Server MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the Excel MCP Server MCP server in PolicyLayer and add a rule for write_formula: allow, deny, rate-limit, or require approval. Point your MCP client at the PolicyLayer proxy URL and the rule is enforced on every call, before it reaches Excel MCP Server. Nothing to install.
write_formula is a Write tool with medium risk. Write tools should be rate-limited to prevent accidental bulk modifications.
Yes. Add a rate_limit block to the write_formula rule in your PolicyLayer policy. For example, setting max: 10 and window: 60 limits the tool to 10 calls per minute. Rate limits are tracked per agent session and reset automatically.
Set action: deny in the PolicyLayer policy for write_formula. The AI agent will receive a policy violation error and cannot call the tool. You can also include a reason field to explain why the tool is blocked.
write_formula is provided by the Excel MCP Server MCP server (mort-lab/excel-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from Excel MCP Server, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.
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17 Excel MCP Server tools catalogued and risk-classified — across an index of 43,000+ MCP servers.