Attach a file (PDF, image) to an existing Xero bill. NOTE: Xero has a 10 attachment limit per bill - create multiple bills if you have more than 9 expenses.
AI agents use xero_attach_file to create or update resources in Xero Expenses MCP — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Xero Expenses MCP environment.
This tool creates or modifies data reversibly by attaching files to bills in Xero accounting software. It is a Write operation rather than Read (which would only retrieve data), Execute (which would run arbitrary code), Destructive (attachments can be removed), or Financial (it does not move money or create financial obligations itself).
From the tool's definition Tool name 'xero_attach_file' and description 'Attach a file...to an existing Xero bill' indicates a modification operation that adds data to an existing record.
Attacks that exploit this kind of access
Attach a file (PDF, image) to an existing Xero bill. NOTE: Xero has a 10 attachment limit per bill - create multiple bills if you have more than 9 expenses. It is categorised as a Write tool in the Xero Expenses MCP MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the Xero Expenses MCP server in PolicyLayer and add a rule for xero_attach_file: 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 Xero Expenses MCP. Nothing to install.
xero_attach_file 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 xero_attach_file 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 xero_attach_file. 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.
xero_attach_file is provided by the Xero Expenses MCP server (muness/xero-expenses-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Every MCP server has a record like this.
Type a name, get the same breakdown: verified identity, auth posture, risk grade, capabilities, recommended policy.
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