Create a standing instruction that auto-triggers actions when incoming packages match conditions. Rules run on every new package and execute the specified action if all conditions match. Use requires_approval to add a human review step before execution.
Risk signalsHigh parameter count (10 properties) · Bulk/mass operation — affects multiple targets
Part of the Mailbox server.
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AI agents use create_rule to create or modify resources in Mailbox. Write operations carry medium risk because an autonomous agent could trigger bulk unintended modifications. Rate limits prevent a single agent session from making hundreds of changes in rapid succession. Argument validation ensures the agent passes expected values.
Without a policy, an AI agent could call create_rule repeatedly, creating or modifying resources faster than any human could review. PolicyLayer's rate limiting ensures write operations happen at a controlled pace, and argument validation catches malformed or unexpected inputs before they reach Mailbox.
Write tools can modify data. A rate limit prevents runaway bulk operations from AI agents.
{
"version": "1",
"default": "deny",
"tools": {
"create_rule": {
"limits": [
{
"counter": "create_rule_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}See the full Mailbox policy for all 29 tools.
These attack patterns abuse exactly the kind of access create_rule gives an agent. Each links to the full case and the policy that stops it:
Other write tools across the catalogue. The same approach applies to each: rate-limit and validate the arguments.
Create a standing instruction that auto-triggers actions when incoming packages match conditions. Rules run on every new package and execute the specified action if all conditions match. Use requires_approval to add a human review step before execution.. It is categorised as a Write tool in the Mailbox MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the Mailbox MCP server in PolicyLayer and add a rule for create_rule: 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 Mailbox. Nothing to install.
create_rule 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 create_rule 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 create_rule. 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.
create_rule is provided by the Mailbox MCP server (https://mailbox.bot/api/mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Deterministic rules across all 29 Mailbox tools. Per-identity grants. Full audit log. Live in minutes. Nothing to install.
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4,600+ MCP servers and 31,000+ tools scanned and risk-classified.