mcp-postgres

pg_database_size

Retrieve the disk space usage of every database in a PostgreSQL server to monitor storage and plan capacity.

Instructions

Show size of all databases.

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": {
      "target": "<target>"
    },
    "name": "pg_database_size"
  }
}

Input Schema

Name	Required	Description	Default
`target`	Yes	Target name from ~/.dbx/targets/

Schema Changelog

Changes observed during successful MCP inspections.

First observedv2026.4.10-3Jul 25, 2026

TDQS

C2.6/5.0

Behavior1/5

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

With no annotations provided, the description carries the full burden of behavioral disclosure. It only states 'Show size of all databases' and fails to mention that a target is required, whether it is read-only, what the output format is, or any side effects.

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

Conciseness3/5

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

The description is extremely concise (one sentence), but it sacrifices necessary detail. It is front-loaded but incomplete, earning a middle score.

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

Completeness2/5

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

Given the lack of output schema and behavioral annotations, the description should explain what the result looks like (e.g., list of database names and sizes). It fails to do so, leaving the agent uncertain about the return format.

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 coverage is 100% with a single parameter 'target' described as 'Target name from ~/.dbx/targets/'. The description adds no additional meaning beyond the schema, as it does not clarify that the target is the server or connection context. Baseline score of 3 is appropriate.

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 'Show size of all databases' clearly indicates the verb (show) and resource (size of all databases), distinguishing it from sibling tools like pg_table_sizes. However, it omits the requirement for a target parameter, which could cause confusion about scope.

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

Usage Guidelines2/5

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

No guidance is provided on when to use this tool vs alternatives such as pg_table_sizes or pg_query. There is no mention of prerequisites (e.g., needing an active connection) or exclusions.

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