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MCPg - Production-grade PostgreSQL MCP Server

MCPg - Production-grade PostgreSQL MCP Server

by devopam
OverviewSchemaSourceRelated ServersDiscussionsAdmin
Python
Hybrid

Describe table

describe_table
Read-only

Retrieve a PostgreSQL table's columns in ordinal order, including data type, nullability, default, and vector dimension, with an optional fresh read to reflect schema changes.

Instructions

Describe the columns of a table, in ordinal order. Set fresh=true to bypass the cache and re-read live (e.g. after a schema change). Returns a list of objects with name, data_type, nullable, default, and vector_dimension (set only for pgvector vector(N) columns).

Example: describe_table(schema='public', table='users')

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": {
      "schema": "<schema>",
      "table": "<table>"
    },
    "name": "describe_table"
  }
}

Input Schema

NameRequiredDescriptionDefault
freshNo
tableYes
schemaYes
databaseNoOptional: a database id from list_databases, to target a configured secondary (read-only) database; omit for the primary. Database names that are not configured ids are rejected.

Output Schema

NameRequiredDescriptionDefault
resultYes

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed1 schema field changedv0.8.4
    • changedInput schema / properties / database / description
      Previous value: -"Optional: target a configured secondary (read-only) database by name; omit for the primary. Call list_databases to see the configured ids."New value: +"Optional: a database id from list_databases, to target a configured secondary (read-only) database; omit for the primary. Database names that are not configured ids are rejected."
  2. Changed1 schema field changed
    • addedInput schema / properties / fresh
      Added value: +{
+  "default": false,
+  "title": "Fresh",
+  "type": "boolean"
+}
  3. First observedv0.6.8

TDQS

A3.8/5.0
Behavior4/5

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

Annotations already declare readOnlyHint=true and openWorldHint=false, so the safety profile is covered. The description adds genuinely non-obvious behavior beyond that: results are cached by default and fresh=true forces a live re-read after DDL. It does not discuss what happens for a nonexistent table or schema, which is a minor remaining gap.

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

Conciseness4/5

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

Three short sentences plus a concrete example, with the core action and output shape front-loaded and the fresh-flag caveat immediately after. The example is mildly redundant since both named parameters are required and self-explanatory, but it earns its place as a call-shape illustration.

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?

For a read-only introspection tool with an output schema present, the description covers the action, ordering, caching behavior, and result shape, which is close to sufficient. The only notable omission is error behavior for missing schemas or tables, which an agent may need when probing unknown objects.

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?

Schema description coverage is only 25%, with just `database` documented inline, so the description must compensate and largely does: it explains the cache-bypass meaning of `fresh` and demonstrates schema/table usage in the example. The optional `database` targeting is handled by the schema itself, leaving no parameter entirely unexplained across the two sources.

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

Purpose4/5

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

The description states a specific verb and resource — describing the columns of a table, returned in ordinal order — which is far more precise than the title alone. It doesn't explicitly distinguish itself from nearby siblings like list_tables, list_generated_columns, or describe_ao_table, so an agent must infer the boundary, but the column-level intent is unambiguous.

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

Usage Guidelines3/5

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

Usage guidance exists but is scoped to one parameter: set fresh=true after a schema change to bypass the cache. There is no statement of when to choose describe_table over list_tables or another metadata sibling, and no exclusions or prerequisites are given.

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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