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

Annotations already declare readOnlyHint=true and openWorldHint=true, so the safety and scope profile is covered. The description usefully adds scale (22,700+ entries) and cost framing, but says nothing about the shard/page pagination model that governs how the index must actually be traversed — the itemized behavior lives in the schema, not the description.

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?

One sentence, front-loaded with the resource and scope, with the cost differentiator trailing. Every clause earns its place; nothing is padded.

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?

An output schema exists, so return values need no explanation, and annotations cover safety. However, for a sharded paginated enumerator the description omits how to traverse the index (shard discovery, iteration strategy), leaving the agent to infer the crawl behavior from the schema alone.

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%, with page, shard, and page_size all documented including defaults and ranges. The description adds no syntax or format meaning beyond the schema, so the 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?

States a specific resource (Subway global store-URL index) with a concrete scale (22,700+ entries) and verb-like intent ('enumerate Subway locations'). This is clearly distinguishable from siblings like subway_store and subway_nearby, which return detail or proximity rather than a full URL index, though the description never names those siblings directly.

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 phrase 'the cheapest way to enumerate Subway locations' gives a selection criterion: use this when you want bulk enumeration rather than targeted lookup. It does not explicitly state when NOT to use it or name the alternative (e.g., subway_nearby for a specific area), so the guidance is clear but incomplete.

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