Open Index

Look up a postcode

lookup_postcode

Read-onlyIdempotent

The areas a Great Britain postcode sits in (council, constituency, ward, neighbourhood and more) with their codes and pages. A district (E8) or sector (E8 1) gives its councils. Northern Ireland postcodes are not available.

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": {
      "postcode": "<postcode>"
    },
    "name": "lookup_postcode"
  }
}

Input Schema

Name	Required	Description	Default
`postcode`	Yes	A full postcode (E8 1AA), sector (E8 1) or district (E8).

Schema Changelog

Changes observed during successful MCP inspections.

First observedOct 20, 2026

TDQS

A3.8/5.0

Behavior4/5

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

Annotations already declare readOnly, idempotent, non-destructive, non-open-world, so the safety profile is covered. The description adds genuinely non-obvious behavior beyond them: partial inputs (district/sector) return only councils, and NI coverage is absent. Return format/pagination is not mentioned, but the tool is a simple lookup.

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 sentences with no filler, front-loaded with what the tool returns and followed by input-form and coverage caveats. Every clause carries information.

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 no output schema, the description compensates by describing the returned areas and their codes/pages, plus the geographic coverage limit. It is complete for a one-parameter lookup, though it never says what an invalid or unknown postcode yields.

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 parameter's description already enumerates full postcode, sector and district formats. The description's restatement of E8 / E8 1 formats largely duplicates the schema, adding little beyond the output-oriented framing.

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 resource (the areas a GB postcode sits in, with codes and pages) and enumerates the area types returned, so an agent can tell what it produces. It does not name or contrast with any sibling (get_place, search_places, fetch), so the differentiation is left to inference.

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 is implied by the input-form guidance (district, sector, full postcode) and one explicit exclusion ('Northern Ireland postcodes are not available'), which functions as a when-not. There is no guidance on when to choose this over get_place or search_places for geographic lookups.

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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