apibase

UK Postcode Lookup

address.ukpost.lookup

Read-onlyIdempotent

Look up a UK postcode — returns district, region, country, ward, parish, parliamentary constituency, and lat/lon coordinates. Backed by ONS/Ordnance Survey data. Example: "SW1A 1AA" → Westminster, London (Postcodes.io)

Input Schema

TableJSON Schema

Name	Required	Description	Default
`postcode`	Yes	UK postcode to look up (e.g. "SW1A 1AA" for Westminster, "EC2R 8AH" for City of London, "M1 1AA" for Manchester). Returns district, region, country, lat/lon, parliamentary constituency

Output Schema

TableJSON Schema

Name	Required	Description	Default
`error`	No	Present only when the call failed. Includes error code, message, request_id, and any provider-specific extras.
`result`	No	Tool 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.

First observedSep 22, 2026

TDQS

A3.7/5.0

Behavior3/5

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

Annotations already indicate readOnlyHint, idempotentHint, and destructiveHint false. The description adds data source context (ONS/Ordnance Survey) and an example, but no additional behavioral traits beyond what annotations convey.

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?

Two concise sentences and an example, front-loaded with the core purpose. Every sentence is informative with no filler.

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?

Output schema exists, so return values are documented elsewhere. The description lists the key data fields returned. For a simple lookup tool, the description is complete.

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?

Input schema has 100% description coverage for the postcode parameter. The description adds an example but no extra meaning beyond the schema's description.

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 the verb 'look up' and resource 'UK postcode', lists the returned data (district, region, country, etc.), and includes an example. It differentiates from siblings like address.ukpost.nearest and address.ukpost.validate by specifying detailed reverse-lookup output.

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 given on when to use this tool versus siblings (e.g., nearest or validate). The description does not mention typical use cases or alternatives.

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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