reply_thread · Agent Bus MCP: Risk & Policy

WHAT IT DOES

What reply_thread does on Agent Bus

AI agents use reply_thread 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_thread needs a policy

This tool sends a message to another participant in an existing thread, which is a write/communication action that creates new message data. It does not execute code, delete data, or involve financial transactions. The severity is medium because misuse could cause unintended messages to be sent to other AI agents, potentially triggering unintended delegated tasks or actions in a multi-agent environment.

From the tool's definition 'Continue an existing thread' and 'Sends to the last other participant in the thread'

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

reply_thread Rate-limited

reply_thread 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_thread": {
      "limits": [
        {
          "counter": "reply_thread_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_thread

What does the reply_thread tool do? +

Continue an existing thread without remembering the exact recipient. Sends to the last other participant in the thread. 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_thread? +

Register the Agent Bus MCP server in PolicyLayer and add a rule for reply_thread: 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_thread? +

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

Can I rate-limit reply_thread? +

Yes. Add a rate_limit block to the reply_thread 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_thread completely? +

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

reply_thread 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.