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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive, open-world), so the bar is lower, yet the description adds real behavioral context: row limits via offset/limit, the ~80-TLD coverage boundary, and a free CSV with the full slice linked in the answer. It does not describe the exact return fields, though it hints at domains plus global rank.

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?

A single dense paragraph, front-loaded with the core purpose before the caveats. Every sentence carries information (scope, exclusion rule, availability, pagination, cost), though the packing of caveats makes it slightly heavy rather than crisply segmented.

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?

With no output schema, the description appropriately covers what comes back (domains with global rank, plus a linked full-CSV slice), the pagination ceiling, the TLD coverage limit, and that either selector param works. Combined with annotations handling safety, an agent has everything needed to call it correctly.

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 coverage is 100%, so the baseline is 3, but the description adds meaning beyond the schema: tld vs. country are alternate selectors ('Pass tld or country'), and country's ccTLD-only interpretation is explicit. Pagination semantics (up to 1,000 rows via offset/limit) are also stated, going beyond the raw min/max bounds.

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 (most popular domains of one TLD or country, a slice of the NetAPI Top 1M) with a concrete verb-like framing and examples (Germany/.de, .io). It is clearly distinguishable from siblings such as top_1m (global), tld_stats (TLD metrics), and domain_rank (single domain), which an agent can tell apart without opening a schema.

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?

Gives concrete usage conditions: pass either tld or country, and clarifies that a 'country' means its ccTLD so .com sites of that country are excluded. It also flags availability (~80 TLDs with ≥1,000 domains) and cost ('Free'). It stops short of explicitly naming which sibling to use for a global or per-domain question.

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