AI agents call list_agents to retrieve information from A2A MCP Server without modifying anything — typically the context-gathering step in research, monitoring, and reporting workflows, before the agent takes action elsewhere.
This tool queries and returns information about available agents without modifying, executing, or deleting any data. It is a pure read operation that retrieves metadata about agents in the A2A protocol registry.
From the tool's definition Tool name is 'list_agents' and description states it 'List available agents with their agent cards' — a retrieval operation with no side effects.
Documented attack patterns abuse exactly the kind of access list_agents gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and A2A MCP Server, and nothing reaches the server without passing your rules. This is the rule we recommend for list_agents:
{
"version": "1",
"default": "deny",
"tools": {
"list_agents": {}
}
}list_agents is read-only, so it stays allowed — but everything else on the server is denied unless you say otherwise.
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List available agents with their agent cards. It is categorised as a Read tool in the A2A MCP Server MCP Server, which means it retrieves data without modifying state.
Register the A2A MCP Server MCP server in PolicyLayer and add a rule for list_agents: 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 A2A MCP Server. Nothing to install.
list_agents is a Read tool with low risk. Read-only tools are generally safe to allow by default.
Yes. Add a rate_limit block to the list_agents 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 list_agents. 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.
list_agents is provided by the A2A MCP Server MCP server (regismesquita/mcp_a2a). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from A2A MCP Server, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.
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3 A2A MCP Server tools catalogued and risk-classified — across an index of 43,000+ MCP servers.