AI agents use a2a_server_registry to create or update resources in A2A MCP Server — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your A2A MCP Server environment.
The tool both adds and removes server URLs from a registry. Adding is a Write operation; removing could be considered Destructive, but since server URLs can typically be re-added, this is likely reversible. The most severe applicable category considering both operations leans toward Write (reversible modification of configuration data). However, removing a server URL could disrupt agent communication.
From the tool's definition "Add or remove an A2A server URL" — modifies a registry by creating or deleting entries
Documented attack patterns abuse exactly the kind of access a2a_server_registry 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 a2a_server_registry:
{
"version": "1",
"default": "deny",
"tools": {
"a2a_server_registry": {
"limits": [
{
"counter": "a2a_server_registry_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}a2a_server_registry 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.
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Add or remove an A2A server URL. It is categorised as a Write tool in the A2A MCP Server MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the A2A MCP Server MCP server in PolicyLayer and add a rule for a2a_server_registry: 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.
a2a_server_registry is a Write tool with medium risk. Write tools should be rate-limited to prevent accidental bulk modifications.
Yes. Add a rate_limit block to the a2a_server_registry 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 a2a_server_registry. 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.
a2a_server_registry 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.
Free to start. No card required.
3 A2A MCP Server tools catalogued and risk-classified — across an index of 43,000+ MCP servers.