postgres-mcp

run_select

Run a read-only SQL query to retrieve data from PostgreSQL. Ensures safety by rejecting writes and multi-statement inputs.

Instructions

Run a single read-only SELECT/WITH/EXPLAIN/SHOW statement and return rows.

This tool rejects non-read and multi-statement SQL and caps returned rows. (In readonly access mode the DB session is also read-only at the engine.)

Tool call template

Replace each placeholder with a value matching the input schema. Only required parameters are included; add optional parameters as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {
      "sql": "<sql>"
    },
    "name": "run_select"
  }
}

Input Schema

Name	Required	Description	Default
`sql`	Yes
`limit`	No

Output Schema

Name	Required	Description	Default
No arguments

Schema Changelog

Changes observed during successful MCP inspections.

First observedv0.2.0Jun 21, 2026

TDQS

A4.2/5.0

Behavior4/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

No annotations provided, so description carries full burden. It discloses that the tool is read-only, rejects non-read and multi-statement SQL, caps returned rows, and notes that the DB session is read-only. This covers core behavioral traits without contradictions.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness5/5

Is the description appropriately sized, front-loaded, and free of redundancy?

Two sentences, no unnecessary words. The main action is front-loaded, and the second sentence adds crucial constraints. Every sentence earns its place.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness4/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

The tool has 2 parameters, no nested objects, and an output schema (not shown but present). The description covers core behavior, restrictions, and one parameter's semantics. Missing details about error handling or edge cases, but for a simple tool it is reasonably complete.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters3/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 0%, so description must compensate. It adds value for the 'sql' parameter by specifying allowed statement types but does not describe the 'limit' parameter explicitly (its role in capping rows is implied). The description partially compensates but leaves the limit parameter under-specified.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description clearly states the tool runs a single read-only SELECT/WITH/EXPLAIN/SHOW statement and returns rows. It distinguishes itself from sibling tools, which are all metadata listing tools, by specifying the exact SQL statement types it executes.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines4/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

The description explicitly lists allowed statement types and mentions restrictions: rejects non-read and multi-statement SQL, caps rows. It provides clear context for when to use this tool (read-only queries) but does not offer explicit exclusions or mention alternative tools beyond what is implied by sibling context.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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