// MCP TOOL REFERENCE

call_subroutine

Call BASIC subroutines on the server

Executes server-side code

Part of the U2 MCP Server MCP server. Enforce policies on this tool with Intercept, the open-source MCP proxy.

u2-mcp Execute Risk 5/5

// WHEN AI AGENTS USE THIS TOOL

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.

// WHY ENFORCE A POLICY ON CALL_SUBROUTINE

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. Intercept enforces rate limits and validates arguments to keep execution within safe bounds.

// RECOMMENDED POLICY

Execute tools trigger processes. Rate-limit and validate arguments to prevent unintended side effects.

u2-mcp-server.yaml

tools:
  call_subroutine:
    rules:
      - action: allow
        rate_limit:
          max: 10
          window: 60
        validate:
          required_args: true

See the full U2 MCP Server policy for all 22 tools.

// DETAILS

Tool Name call_subroutine

Category Execute

MCP Server U2 MCP Server MCP Server

Risk Level High

// MORE U2 MCP SERVER TOOLS

Destructive delete_knowledge Destructive delete_record Execute execute_query Execute execute_tcl Write connect Write disconnect Write save_knowledge Write write_record

View all 22 tools →

// WHAT CAN GO WRONG

Agents calling execute-class tools like call_subroutine have been implicated in these attack patterns. Read the full case and prevention policy for each:

Browse the full MCP Attack Database →

// OTHER EXECUTE TOOLS ACROSS MCP SERVERS

Other tools in the Execute risk category across the catalogue. The same policy patterns (rate-limit, validate) apply to each.

Agent Passport System — Cryptographic Identity for AI Agents commerce_preflight Agent Passport System — Cryptographic Identity for AI Agents execute_with_context Agent Passport System — Cryptographic Identity for AI Agents gateway_execute_approval Agent Passport System — Cryptographic Identity for AI Agents governance_360 Agent Passport System — Cryptographic Identity for AI Agents parse_governance_block_html AgentClear — API Marketplace for AI Agents call_service

// IN CONTEXT OF HIGH RISK

call_subroutine is one of the high-risk operations in U2 MCP Server. For the full severity-focused view — only the high-risk tools with their recommended policies — see the breakdown for this server, or browse all high-risk tools across every MCP server.

// RELATED READING

// FAQ

What does the call_subroutine tool do? +

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.

How do I enforce a policy on call_subroutine? +

Add a rule in your Intercept YAML policy under the tools section for call_subroutine. You can allow, deny, rate-limit, or validate arguments. Then run Intercept as a proxy in front of the U2 MCP Server MCP server.

What risk level is call_subroutine? +

call_subroutine is a Execute tool with high risk. Execute tools should be rate-limited and have argument validation enabled.

Can I rate-limit call_subroutine? +

Yes. Add a rate_limit block to the call_subroutine rule in your Intercept 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.

How do I block call_subroutine completely? +

Set action: deny in the Intercept 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.

What MCP server provides call_subroutine? +

call_subroutine is provided by the U2 MCP Server MCP server (u2-mcp). Intercept sits as a proxy in front of this server to enforce policies before tool calls reach the server.

call_subroutine

Enforce policies on U2 MCP Server