AI agents use extract_function to create or update resources in LSP MCP Server — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your LSP MCP Server environment.
This tool creates a new function from selected code and modifies existing code to call it. This is a reversible code transformation (Write), not irreversible destruction. It modifies source files but the changes can be undone. Severity is medium because misuse could unintentionally restructure code across files.
From the tool's definition "Extract selected code into a new function"
Documented attack patterns abuse exactly the kind of access extract_function gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and LSP MCP Server, and nothing reaches the server without passing your rules. This is the rule we recommend for extract_function:
{
"version": "1",
"default": "deny",
"tools": {
"extract_function": {
"limits": [
{
"counter": "extract_function_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}extract_function 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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Extract selected code into a new function. It is categorised as a Write tool in the LSP MCP Server MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the LSP MCP Server MCP server in PolicyLayer and add a rule for extract_function: 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 LSP MCP Server. Nothing to install.
extract_function 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 extract_function 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 extract_function. 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.
extract_function is provided by the LSP MCP Server MCP server (sminnee/lsp-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from LSP 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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5 LSP MCP Server tools catalogued and risk-classified — across an index of 43,000+ MCP servers.