AI agents call readonly_query to retrieve information from Prometheus MCP Server without modifying anything — typically the context-gathering step in research, monitoring, and reporting workflows, before the agent takes action elsewhere.
The 'readonly_' prefix strongly signals this tool retrieves or queries data without side effects. Despite the empty description reducing confidence slightly, the naming convention is a reliable indicator. AWS Managed Prometheus queries are typically metric lookups and time-series reads, which are non-destructive operations. No write, execute, or destructive operations are implied.
From the tool's definition Tool name 'readonly_query' explicitly indicates read-only semantics; it queries Prometheus (AWS Managed Prometheus) metrics without modification capabilities
Documented attack patterns abuse exactly the kind of access readonly_query gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and Prometheus MCP Server, and nothing reaches the server without passing your rules. This is the rule we recommend for readonly_query:
{
"version": "1",
"default": "deny",
"tools": {
"readonly_query": {}
}
}readonly_query is read-only, so it stays allowed — but everything else on the server is denied unless you say otherwise.
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readonly_query. It is categorised as a Read tool in the Prometheus MCP Server MCP Server, which means it retrieves data without modifying state.
Register the Prometheus MCP Server MCP server in PolicyLayer and add a rule for readonly_query: 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 Prometheus MCP Server. Nothing to install.
readonly_query 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 readonly_query 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 readonly_query. 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.
readonly_query is provided by the Prometheus MCP Server MCP server (awslabs.prometheus-mcp-server). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from Prometheus 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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805 Prometheus MCP Server tools catalogued and risk-classified — across an index of 43,000+ MCP servers.