Modify an existing interception rule.
AI agents use proxy_update_rule to create or update resources in Proxy — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Proxy environment.
This tool modifies existing interception rules in a MITM proxy. While it's a reversible write operation, the blast radius is high because misconfigured interception rules could silently alter, block, or redirect network traffic for connected devices (Chrome, CLI tools, Docker containers, Android devices), potentially enabling data exfiltration, credential theft, or traffic manipulation at scale.
From the tool's definition Modify an existing interception rule
Documented attack patterns abuse exactly the kind of access proxy_update_rule gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and Proxy, and nothing reaches the server without passing your rules. This is the rule we recommend for proxy_update_rule:
{
"version": "1",
"default": "deny",
"tools": {
"proxy_update_rule": {
"limits": [
{
"counter": "proxy_update_rule_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}proxy_update_rule 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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Modify an existing interception rule. It is categorised as a Write tool in the Proxy MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the Proxy MCP server in PolicyLayer and add a rule for proxy_update_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 Proxy. Nothing to install.
proxy_update_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 proxy_update_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 proxy_update_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.
proxy_update_rule is provided by the Proxy MCP server (yfe404/proxy-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from Proxy, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.
Free to start. No card required.
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