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Postal Lookup MCP

by io.github.Lulu-The-Narwhal
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Server Details

Free, keyless postal/ZIP code lookup: place name(s), state/region, and coordinates.

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Status
Healthy
Last Tested
Transport
Streamable HTTP · MCP 2025-11-25
URL

TDQS

A4.6/5.0

Scored across 1 tool

Disambiguation5/5

With only one tool, there is no possibility of confusion between tools. The tool's purpose is clearly defined and distinct from any hypothetical alternatives.

Naming Consistency5/5

The single tool name 'lookup_postal_code' follows a clear verb_noun pattern, and since there is only one tool, there are no inconsistencies or mixed conventions.

Tool Count2/5

The server offers only one tool, which feels too few for the broad scope of 'postal lookup'. A more complete set would at least include a reverse lookup (place to postal code) or validation, so the single tool is thin.

Completeness3/5

The tool covers the core function of resolving postal codes to places, but the description explicitly excludes reverse lookup and full address lookup, which are natural expectations for a postal lookup server. These gaps are notable but not catastrophic since the primary forward lookup is well implemented.

Available Tools

1 tool
lookup_postal_codeA
Read-only
Inspect

Resolve a postal/ZIP code to its place name(s), state/region, and coordinates. country_code is a 2-letter ISO code (US, GB, DE, ...); postal_code format varies by country (e.g. "90210" for the US, "SW1A 1AA" style outward codes for the UK). Use for "what city is ZIP 90210 in", "where is postal code X in country Y", or any question that needs a place name/region/lat-lon from a postal code -- not for the reverse (place name to postal code) or for full street address lookup. Some postal codes span multiple places, in which case all of them are returned. Returns an error dict (never raises) if the code isn't recognized for that country.

ParametersJSON Schema
NameRequiredDescriptionDefault
postal_codeYes
country_codeYes

Output Schema

ParametersJSON Schema
NameRequiredDescription

No output parameters

TDQS

A5/5.0
Behavior5/5

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

The readOnlyHint annotation is present, and the description adds valuable behavioral context beyond it: description of multi-place postal codes, retourning all matches, and the error dict behavior (never raises). These are non-obvious behaviors that help the agent anticipate outcomes.

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 well-structured and front-loaded with purpose, followed by usage exclusions and behavioral edge cases. Every sentence adds distinct value, with no redundancy or 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?

Given the tool's moderate complexity (2 params, multiple return possibilities), the description covers purpose, parameter formats, usage exclusions, edge cases, and error behavior. It provides a complete picture even without an output schema.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters5/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 0%, but the description fully compensates by explaining both parameters: country_code as a 2-letter ISO code with examples, and postal_code with country-specific format examples. This provides essential semantic detail missing from the schema.

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 uses a specific verb ('Resolve') and clearly identifies the resource (postal/ZIP code to place names, state/region, coordinates). It distinguishes itself from reverse lookups and full street address lookup, making the tool's purpose unambiguous.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines5/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

The description provides explicit when-to-use examples ('what city is ZIP 90210 in') and states what it is not for ('not for the reverse... or for full street address lookup'). This clearly guides an agent on when to select this tool over alternatives.

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

Tool Schema Changelog

Recent tool additions, removals, and schema changes observed during successful MCP inspections.

  1. 1 tool update
    • First observedlookup_postal_code

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