reply · Agent Bus MCP: Risk & Policy

WHAT IT DOES

What reply does on Agent Bus

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

WHY IT NEEDS A POLICY

Why reply needs a policy

The tool writes a response message to the shared local message bus, fulfilling a pending ask. This is a reversible write operation (sending a message), not destructive, financial, or code execution. Severity is medium because an AI agent could send misleading or incorrect responses to other agents, potentially corrupting collaborative workflows.

From the tool's definition 'Answer a pending ask' — the tool sends a reply message to an existing pending ask, creating/modifying communication state on the message bus.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

reply Rate-limited

reply 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": {
    "reply": {
      "limits": [
        {
          "counter": "reply_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 Agent Bus, so you can run agents against it safely.

FAQ

Questions about reply

What does the reply tool do? +

Answer a pending ask. The original ask. It is categorised as a Write tool in the Agent Bus MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on reply? +

Register the Agent Bus MCP server in PolicyLayer and add a rule for reply: 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 Agent Bus. Nothing to install.

What risk level is reply? +

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

Can I rate-limit reply? +

Yes. Add a rate_limit block to the reply 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 reply completely? +

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

reply is provided by the Agent Bus MCP server (mustaphasteph/agent-bus). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.