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MCPg - Production-grade PostgreSQL MCP Server

MCPg - Production-grade PostgreSQL MCP Server

by devopam
OverviewSchemaSourceRelated ServersDiscussionsAdmin
Python
Hybrid

Get wait for LSN status

get_wait_for_lsn_status
Read-only

Check whether PostgreSQL 19 WAIT FOR LSN is usable and whether the backend is a standby; get version and recovery status, with poll-loop fallback guidance for older servers.

Instructions

Report whether PG 19's WAIT FOR LSN is usable on this server. Also reports whether the current backend is a standby (the only context where the wait does meaningful work). Never raises. On PG ≤ 18 returns available=false and points at the poll-loop fallback. Returns an object with available (bool), server_version_num (int), server_version, is_in_recovery (bool), and detail.

Example: get_wait_for_lsn_status()

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": "get_wait_for_lsn_status"
  }
}

Input Schema

NameRequiredDescriptionDefault
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
detailYes
availableYes
is_in_recoveryYes
server_versionYes
server_version_numYes

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. First observedv0.6.8

TDQS

A4.1/5.0
Behavior4/5

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

Beyond readOnlyHint/openWorldHint it discloses two meaningful traits: the call never raises (so the agent needn't guard with error handling) and it degrades to available=false on PG ≤ 18 rather than failing. Those are exactly the behaviors annotations cannot express.

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 front-load purpose, then behavior, then the returned field list, plus a call example. Slightly dense in the return-fields enumeration, but nothing is wasted and the most important clause comes first.

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?

With an output schema present the description needn't enumerate return values, yet it does name them anyway, and it covers the version-gating and never-raising behaviors an agent needs. Only the tie-in to wait_for_lsn as the natural next step is left implicit.

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% and the single optional `database` param is fully documented in the schema (configured secondary id vs primary, rejection of plain names). The description adds nothing about the parameter, which is acceptable at this coverage level but earns no bonus.

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 opens with a specific verb-plus-resource statement ('Report whether PG 19's `WAIT FOR LSN` is usable on this server') and adds the secondary purpose (is the backend a standby). It is clearly a capability-probe tool, distinct from the sibling action tool wait_for_lsn and from get_current_wal_lsn.

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?

It contextualizes use well: the wait only does meaningful work on a standby, and on PG ≤ 18 the tool redirects to the poll-loop fallback rather than leaving the agent guessing. It stops short of explicitly saying 'call this before wait_for_lsn', so it is strong but not a full when/when-not/alternatives statement.

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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