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MCPg - Production-grade PostgreSQL MCP Server

MCPg - Production-grade PostgreSQL MCP Server

by devopam
OverviewSchemaSourceRelated ServersDiscussionsAdmin
Python
Hybrid

List databases

list_databases
Read-only

Show all configured PostgreSQL databases, including the primary and any read-only secondaries, with their IDs, read-only status, and reachability.

Instructions

List every database this MCPg server is configured to serve: the primary (the default target of every tool) plus any read-only secondaries from MCPG_SECONDARY_DATABASE_URLS. Read-capable tools accept an optional database argument naming one of these ids; omitting it targets the primary. Secondaries are read-only (PostgreSQL-enforced) — writes / DDL / shell / migrate always run against the primary. Returns an object with primary_id, database_ids (primary first), and databases — a list of objects with id, is_primary, read_only, reachable (a live SELECT 1 probe), and detail (an error string when unreachable).

Tool call template

This tool declares no required parameters. Add optional parameters from the input schema as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {},
    "name": "list_databases"
  }
}

Input Schema

NameRequiredDescriptionDefault

No arguments

Output Schema

NameRequiredDescriptionDefault
databasesYes
primary_idYes
database_idsYes

Schema Changelog

Changes observed during successful MCP inspections.

  1. First observedv0.6.8

TDQS

A4.5/5.0
Behavior4/5

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

Annotations declare readOnlyHint=true, and the description reinforces that this is a read-only operation. It adds transparency about secondaries being read-only (PostgreSQL-enforced) and describes the return object structure. No contradictions with annotations.

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?

The description is concise, with three sentences covering purpose, usage context, and return format. No redundant information; every sentence adds value. Well-structured and easy to parse.

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

Completeness5/5

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

Given the tool has no parameters and an output schema (though not shown in input, context says true), the description fully explains the tool's behavior, return structure, and how it relates to other tools. No gaps.

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

Parameters4/5

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

There are 0 parameters (schema coverage 100%), so the baseline is elevated. The description adds meaning by explaining the role of primary and secondary databases and describes the return object (primary_id, database_ids, databases list with fields). This provides context beyond the schema.

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 'List every database this MCPg server is configured to serve', specifying the verb 'list' and resource 'databases'. It distinguishes from sibling tools by detailing the primary and secondary database concept, which is unique to this tool.

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 explains when to use this tool: to see available databases and understand which database is targeted by default. It provides context that read-capable tools accept an optional `database` argument and that writes target the primary. No explicit exclusions or alternatives are given, but the context is clear.

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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