sync_bridge

THE RISK

Medium Risk

Why sync_bridge needs a policy

This tool fetches tools from an external MCP server and registers them as skills within AvatarBook. It creates new skill registrations (Write), but the security concern is high because it ingests arbitrary external tools and registers them as executable agent capabilities, expanding the attack surface significantly.

From the tool's definition Sync tools from an external MCP server and register them as AvatarBook skills

Documented attack patterns abuse exactly the kind of access sync_bridge gives an agent:

POLICY

How to control sync_bridge

PolicyLayer is an MCP gateway — it sits between your AI agents and AvatarBook MCP Server, and nothing reaches the server without passing your rules. This is the rule we recommend for sync_bridge:

policy.json

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

sync_bridge 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.

Create a free account and register AvatarBook MCP Server — nothing to install.
Add this policy — paste it, or build it visually.
Point your MCP client (Claude, Cursor, anything) at your gateway URL.

LIMIT THIS TOOL →

Free to start. No card required.

EXPLORE

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Go deeper

The MCP Attack Database → Documented attack patterns against MCP deployments — and the policies that stop them.
Argument validation → Checking tool call arguments against schema and policy before they run.
Scoped token → Per-person credentials that grant a defined subset of servers and tools.
Rate Limiting MCP Tool Calls: A Practical Guide →
MCP Security: Why Prompt Guardrails Aren't Enough →

FAQ

Questions about sync_bridge

What does the sync_bridge tool do? +

Sync tools from an external MCP server and register them as AvatarBook skills. It is categorised as a Write tool in the AvatarBook MCP Server MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on sync_bridge? +

Register the AvatarBook MCP Server MCP server in PolicyLayer and add a rule for sync_bridge: 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 AvatarBook MCP Server. Nothing to install.

What risk level is sync_bridge? +

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

Can I rate-limit sync_bridge? +

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

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

sync_bridge is provided by the AvatarBook MCP Server MCP server (noritaka88ta/avatarbook). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.

Enforce policy on every AvatarBook MCP Server tool call.

Start from AvatarBook MCP Server, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.

CHECK YOUR STACK →