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

Annotations already declare readOnly/idempotent/non-destructive and open-world, so safety is covered; the description adds the behavioral facts annotations cannot carry: exact price (0.01 USDC via x402 or 5 credits of a Factline key) and the free tier of 5 paid-tool calls per day with no key needed. It also enumerates the returned record fields, so the agent knows the payload shape despite there being no output schema.

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?

Front-loaded with purpose and return fields, then the practical constraints; every sentence (including cost and the free-call allowance) earns its place. It is a dense single block with no visual separation between purpose, input and pricing, which costs a little scannability.

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 paid, read-only lookup with no output schema and no required params, it covers everything an agent needs to decide and call: identity of the resource, alternative identifiers, coverage limits, price, free-tier escape hatch, and the returned record fields. Only edge cases such as ambiguous name matches or not-found behavior are unstated.

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 baseline 3, but the description adds real meaning beyond the schema: it states that name, ticker or CIK are alternative lookup keys (the schema marks nothing required and gives no anyOf/logic). The api_key mechanic is left mostly to the schema, which documents it well.

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?

Specific verb and resource: 'verify a US listed company or SEC registrant: finds it by name, ticker or CIK and returns its SEC record.' The US/EDGAR scope plus the explicit exclusion 'Private US companies are not in EDGAR' distinguishes it from the generic company_lookup and from eu_company_check/china_supplier_check without opening any 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?

Says clearly when to use it (verifying a US listed company/SEC registrant) and when not (private US companies, which are not in EDGAR), plus the payment prerequisites. It stops short of naming a specific sibling as the fallback for non-US or private companies, so there is no explicit alternative routing.

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