ab_assign · Yaver MCP: Risk & Policy

WHAT IT DOES

What ab_assign does on Yaver

AI agents use ab_assign to create or update resources in Yaver — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Yaver environment.

PARAMETERS

Parameter	Type	Required	Description
`key`	string	Yes
`userId`	string	Yes

Parameters from the server's own tool schema.

WHY IT NEEDS A POLICY

Why ab_assign needs a policy

An AI agent can call ab_assign faster than any human can review — one bad instruction and it creates or modifies resources in Yaver by the hundred, each call as confident as the last.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

ab_assign Rate-limited

ab_assign 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.

View as policy code

policy.json

{
  "version": "1",
  "default": "deny",
  "tools": {
    "ab_assign": {
      "limits": [
        {
          "counter": "ab_assign_rate",
          "window": "minute",
          "max": 30,
          "scope": "grant"
        }
      ]
    }
  }
}

LIMIT THIS TOOL →

Instant setup, no code required.

PolicyLayer is an MCP gateway: every tool call is checked against your rule before it reaches Yaver, so you can run agents against it safely.

FAQ

Questions about ab_assign

What does the ab_assign tool do? +

Deterministically assign a userId to a variant. Returns the variant name; empty if the experiment is not running. Logs an exposure event. It is categorised as a Write tool in the Yaver MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

What parameters does ab_assign accept? +

ab_assign accepts 2 parameters: key, userId. Required: key, userId. The full parameter table on this page comes from the server's own tool schema.

How do I enforce a policy on ab_assign? +

Register the Yaver MCP server in PolicyLayer and add a rule for ab_assign: 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 Yaver. Nothing to install.

What risk level is ab_assign? +

ab_assign is a Write tool with medium risk. Write tools should be rate-limited to prevent accidental bulk modifications.

Can I rate-limit ab_assign? +

Yes. Add a rate_limit block to the ab_assign 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.

How do I block ab_assign completely? +

Set action: deny in the PolicyLayer policy for ab_assign. 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 ab_assign? +

ab_assign is provided by the Yaver MCP server (yaver-cli). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.