GOV.UK

Look Up UK Postcode

govuk_lookup_postcode

Read-onlyIdempotent

Look up a UK postcode to retrieve its local authority, region, constituency, and other administrative geography.

Useful for determining which council area, parliamentary constituency, or NHS region a postcode falls within. Commonly used to direct users to the correct local service on GOV.UK (e.g. council tax, planning, waste).

Uses the postcodes.io public API (no key required).

Input Schema

TableJSON Schema

Name	Required	Description	Default
`postcode`	Yes	UK postcode, e.g. 'SW1A 2AA' or 'NG1 1AA'. Spaces optional.

Output Schema

TableJSON Schema

Name	Required	Description
`codes`	No	GSS codes for all administrative geographies covering this postcode.
`region`	No	ONS region, e.g. 'East Midlands'.
`country`	No	Country, e.g. 'England', 'Scotland', 'Wales', 'Northern Ireland'.
`latitude`	No	Latitude in decimal degrees (WGS84).
`postcode`	No	Canonicalised postcode as returned by postcodes.io.
`longitude`	No	Longitude in decimal degrees (WGS84).
`admin_county`	No	Administrative county, where applicable (null in unitary areas).
`local_authority`	No	Local authority / council covering the postcode.
`nhs_integrated_care_board`	No	NHS Integrated Care Board, where available.
`parliamentary_constituency`	No	Parliamentary constituency (pre-2025 boundary).
`parliamentary_constituency_2025`	No	Parliamentary constituency under the 2025 boundaries.

TDQS

A4.2/5.0

Behavior4/5

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

Annotations already declare read-only, idempotent, and non-destructive. The description adds valuable context that it uses the postcodes.io public API with no key required, but does not mention rate limits or invalid input handling. This is sufficient for a simple read-only 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?

Three short, focused paragraphs with no redundancy. The first sentence gives the core operation, the second adds usage context, and the third provides a technical detail. Every sentence earns its place.

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?

Given the tool's simplicity (one parameter, full schema coverage, output schema present), the description adequately covers what, when, and external API details. It could mention edge cases like invalid postcodes, but for a direct lookup this is sufficient.

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?

The schema provides 100% coverage of the single parameter, including format, length, and examples. The description adds little beyond what the schema already states, so the baseline of 3 applies.

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 action 'Look up' and the resource 'UK postcode' with specific outputs like local authority, region, and constituency. This clearly distinguishes it from sibling tools focused on content, organisations, or search.

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 explicitly states when to use the tool ('Useful for determining which council area...') and gives practical examples like directing users to local services. However, it does not mention alternative tools or when-not-to-use, though no sibling tool offers postcode lookup.

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

All GOV.UK tools Inspect

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TDQS

A4.6/5.0

Disambiguation5/5

Each tool targets a distinct operation: page metadata, sections, content search, organisation profiles, organisation listing, postcode lookup, and full-text search. No two tools overlap in purpose, so an agent can clearly differentiate them.

Naming Consistency5/5

All tools follow the 'govuk_verb_noun' pattern (e.g., govuk_get_content, govuk_list_organisations) using imperative verbs. The naming is uniform and predictable, with only minor variation in verb choice (get, grep, lookup) but no mixing of conventions.

Tool Count5/5

Seven tools is ideal for the scope of reading GOV.UK data: page content (3 tools), organisations (2), postcode lookup (1), and search (1). Each tool serves a necessary function without being excessive or insufficient.

Completeness5/5

The tool set covers the primary read operations for the domain: retrieving page structure and content, searching across content, browsing and fetching organisations, and looking up postcodes. While there is no single tool to fetch all sections at once, the combination of get_content and get_section covers it adequately. No obvious gaps for a read-only API.

Resources

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