AI agents use format_number 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.
Based on the server's stated purpose of performing Excel file operations and the 'format_' prefix matching sibling formatting tools, this tool modifies cell number formatting, which is a reversible write operation. The empty description prevents full certainty, but the context of format operations on an Excel server strongly indicates a Write classification rather than Read or Execute.
From the tool's definition Tool name 'format_number' with empty description; sibling tools include write operations like 'format_fill', 'format_font', 'create_sheet', and 'create_workbook' on an Excel MCP server that 'perform Excel file operations (create, read, write, format)'.
Documented attack patterns abuse exactly the kind of access format_number 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 format_number:
{
"version": "1",
"default": "deny",
"tools": {
"format_number": {
"limits": [
{
"counter": "format_number_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}format_number 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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format_number. 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 format_number: 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.
format_number 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 format_number 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 format_number. 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.
format_number 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.