Call BASIC subroutines on the server
Risk signalsExecutes server-side code
Part of the U2 MCP Server server.
Free to start. No card required.
AI agents invoke call_subroutine to trigger processes or run actions in U2 MCP Server. Execute operations can have side effects beyond the immediate call -- triggering builds, sending notifications, or starting workflows. Rate limits and argument validation are essential to prevent runaway execution.
call_subroutine can trigger processes with real-world consequences. An uncontrolled agent might start dozens of builds, send mass notifications, or kick off expensive compute jobs. PolicyLayer enforces rate limits and validates arguments to keep execution within safe bounds.
Execute tools trigger processes. Rate-limit and validate arguments to prevent unintended side effects.
{
"version": "1",
"default": "deny",
"tools": {
"call_subroutine": {
"limits": [
{
"counter": "call_subroutine_rate",
"window": "minute",
"max": 10,
"scope": "grant"
}
]
}
}
}See the full U2 MCP Server policy for all 22 tools.
These attack patterns abuse exactly the kind of access call_subroutine gives an agent. Each links to the full case and the policy that stops it:
Other execute tools across the catalogue. The same approach applies to each: rate-limit and validate the arguments.
Call BASIC subroutines on the server. It is categorised as a Execute tool in the U2 MCP Server MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.
Register the U2 MCP Server MCP server in PolicyLayer and add a rule for call_subroutine: 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 U2 MCP Server. Nothing to install.
call_subroutine is a Execute tool with high risk. Execute tools should be rate-limited and have argument validation enabled.
Yes. Add a rate_limit block to the call_subroutine 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 call_subroutine. 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.
call_subroutine is provided by the U2 MCP Server MCP server (pypi:u2-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Deterministic rules across all 22 U2 MCP Server tools. Per-identity grants. Full audit log. Live in minutes. Nothing to install.
Free to start. No card required.
4,600+ MCP servers and 31,000+ tools scanned and risk-classified.