explain_query · Server MCP: Risk & Policy

WHAT IT DOES

What explain_query does on Server

AI agents invoke explain_query to trigger actions in Server. What it does depends on the arguments the agent supplies, and its effects often reach beyond the immediate call — builds kicked off, notifications sent, workflows started.

WHY IT NEEDS A POLICY

Why explain_query needs a policy

EXPLAIN ANALYZE physically executes the SQL statement on the database engine (unlike plain EXPLAIN which only plans). This means any DML or DDL in the query (INSERT, UPDATE, DELETE, DROP) would run for real, making this an Execute-class tool with high blast radius. An AI agent could inadvertently or maliciously run destructive SQL under the guise of 'explaining' it.

From the tool's definition 'Run EXPLAIN (ANALYZE, BUFFERS) on a SQL query' — EXPLAIN ANALYZE actually executes the query against the database to collect real timing and buffer statistics

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

explain_query Rate-limited

explain_query stays usable, but rate-capped — a runaway agent can't fire it dozens of times a minute. Everything else on the server is denied unless you say otherwise.

View as policy code

policy.json

{
  "version": "1",
  "default": "deny",
  "tools": {
    "explain_query": {
      "limits": [
        {
          "counter": "explain_query_rate",
          "window": "minute",
          "max": 10,
          "scope": "grant"
        }
      ]
    }
  }
}

RATE-LIMIT THIS TOOL →

Instant setup, no code required.

PolicyLayer is an MCP gateway: every tool call is checked against your rule before it reaches Server, so you can run agents against it safely.

FAQ

Questions about explain_query

What does the explain_query tool do? +

Run EXPLAIN (ANALYZE, BUFFERS) on a SQL query to show the execution plan with actual timing,. It is categorised as a Execute tool in the Server MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.

How do I enforce a policy on explain_query? +

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

What risk level is explain_query? +

explain_query is a Execute tool with high risk. Execute tools should be rate-limited and have argument validation enabled.

Can I rate-limit explain_query? +

Yes. Add a rate_limit block to the explain_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.

How do I block explain_query completely? +

Set action: deny in the PolicyLayer policy for explain_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.

What MCP server provides explain_query? +

explain_query is provided by the Server MCP server (@nest-mcp/server). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.