AI agents use replace_function_body to create or update resources in Ast Editor — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Ast Editor environment.
The tool modifies function implementations reversibly through AST-based editing. This is Write (not Execute) because it edits code text rather than running it; it's reversible (not Destructive). Severity is high because incorrect modifications could corrupt critical code logic, though impact depends on which functions are modified.
From the tool's definition Tool name 'replace_function_body' indicates modification of code. Server description states it 'provides AI coding agents with the ability to edit files surgically via Abstract Syntax Trees' — the core purpose is reversible file editing.
Documented attack patterns abuse exactly the kind of access replace_function_body gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and Ast Editor, and nothing reaches the server without passing your rules. This is the rule we recommend for replace_function_body:
{
"version": "1",
"default": "deny",
"tools": {
"replace_function_body": {
"limits": [
{
"counter": "replace_function_body_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}replace_function_body 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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replace_function_body. It is categorised as a Write tool in the Ast Editor MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the Ast Editor MCP server in PolicyLayer and add a rule for replace_function_body: 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 Ast Editor. Nothing to install.
replace_function_body 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 replace_function_body 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 replace_function_body. 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.
replace_function_body is provided by the Ast Editor MCP server (kambleakash0/agent-skills). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from Ast Editor, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.
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59 Ast Editor tools catalogued and risk-classified — across an index of 43,000+ MCP servers.