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

Annotations already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false, so the safety profile is covered. The description adds substantial context beyond that: return field composition (kanji, kana readings, romaji, JIS code, GPS coordinates), input flexibility (hyphen handling), coverage size, update cadence ('updated regularly'), licensing (MIT), and auth requirements ('no auth required'). No contradiction with annotations.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

Four sentences, each earning its place: purpose, return contents, input format, then coverage/data source/license/auth. The primary purpose is front-loaded, and there is no waste or repetition of schema details. Well-structured for scanning.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a single-parameter read-only lookup with strong annotations, a 100%-documented schema, and an output schema present, the description is complete: it covers purpose, returns, input format, coverage, freshness, licensing, and auth. The only omission is failure behavior for unknown postcodes, which is minor given the output schema and simple nature of the operation.

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

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

Schema description coverage is 100%, and the postcode parameter's schema description already documents the accepted format, min/max length, and automatic hyphen/space stripping. The tool description's mention of 'with or without hyphen' and the '1500001'/'150-0001' examples reinforce but do not add meaning beyond what the schema already provides. Baseline 3 applies.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

The description states a specific verb ('Look up'), resource ('Japanese postal address'), and input method ('by its 7-digit postcode'). This is immediately distinguishable from its siblings — postcode-japan.postal.search (search) and postcode-japan.reference.prefectures (reference list) — without needing to inspect either schema. The scope is precise and the examples reinforce intent.

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

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

The context is clear: this is the tool to use when you have a 7-digit Japanese postcode, and the coverage statement ('all ~124,000 active Japan Post postcodes') tells the agent it applies to any valid code. It also signals no-auth-needed as a practical precondition. However, it does not explicitly name the alternative (e.g., postcode-japan.postal.search for when only an address is available), so it earns 4 rather than 5.

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