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

Beyond readOnlyHint/openWorldHint, the description discloses the upstream source (official Firebase API, public, no key required), the failure mode (deleted or unknown ids return 404), and a concrete cost model ($0.002 USDC per successful call via x402 on Base/Solana/Arc, failed calls not charged). Cost and billing behavior is exactly the kind of information annotations cannot convey.

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?

Two tight sentences, front-loaded with the operation and source, followed by the failure mode and price. The only waste is the enumerated field list ('type, title, author, time, score…'), which an output schema already carries.

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 an output schema present and annotations covering safety, the description fills the remaining gaps an agent needs: auth (no key), error behavior (404), and payment requirements. Rate limits and the relationship to search_hacker_news are the only unaddressed items.

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% and the schema already documents the single 'id' parameter with an example (8863). The description only reinforces that ids are numeric and that unknown ids 404; it adds no format nuance such as string-vs-integer coercion. 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 operation on a specific resource: fetch one Hacker News item by numeric id from the official Firebase HN API, and it enumerates the fields returned. That is far clearer than the tool name suggests ('search_hackernews_items' actually fetches by id rather than searching). It does not, however, distinguish itself from the closely named sibling search_hacker_news, which an agent could easily pick instead.

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?

Usage is only implied: the phrase 'by numeric id' signals this is a lookup tool used when you already have an id. There is no explicit 'use this when…/use X instead' guidance and no mention of the sibling search_hacker_news that performs actual searching.

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