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by io.github.whiteknightonhorse
OverviewDiscussionsAdmin

Nearest UK Postcodes

address.ukpost.nearest
Read-onlyIdempotent

Find nearest UK postcodes to a lat/lon coordinate — returns postcodes sorted by distance with district info. Use for reverse geocoding in the UK (Postcodes.io)

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": {
      "lat": "<lat>",
      "lon": "<lon>"
    },
    "name": "address.ukpost.nearest"
  }
}

Input Schema

NameRequiredDescriptionDefault
latYesLatitude (e.g. 51.5074 for London)
lonYesLongitude (e.g. -0.1278 for London)
limitNoMax results (1-10, default 5)

Output Schema

NameRequiredDescriptionDefault
errorNoPresent only when the call failed. Includes error code, message, request_id, and any provider-specific extras.
resultNoTool response payload. Shape varies per tool — consult the tool description and inputSchema. May be an object, array, string, or number depending on the upstream provider response.

Schema Changelog

Changes observed during successful MCP inspections.

  1. First observed

TDQS

A4/5.0
Behavior3/5

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

Annotations already declare readOnlyHint, destructiveHint false, idempotentHint true, and openWorldHint true. The description adds that results are sorted by distance with district info, but does not disclose additional behavioral traits like rate limits, pagination, or coordinate bounds beyond the implicit UK scope. With strong annotation coverage, the description adds moderate value.

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 two sentences, front-loaded with the primary action, and contains no filler. Every sentence adds value, making it highly concise.

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 complete input schema, output schema present, and rich annotations. The description adds the use case (reverse geocoding) and data source (Postcodes.io). It is complete for a simple reverse geocoding tool, though it could mention behavior for out-of-UK coordinates. Given openWorldHint, this is acceptable.

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?

All three parameters (lat, lon, limit) have full descriptions in the input schema (100% coverage). The description adds no extra parameter-level meaning beyond the schema, so baseline of 3 is appropriate.

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 states the exact function: finding nearest UK postcodes to lat/lon coordinates, sorted by distance with district info. It clearly differentiates from siblings like lookup and validate by specifying 'reverse geocoding' and the data source.

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 says 'Use for reverse geocoding in the UK', which provides clear context. However, it does not explicitly state when not to use this tool or mention alternatives among the three sibling tools, though the purpose is sufficiently distinct.

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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