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

Annotations only say readOnly/openWorld; the description adds the real behavioral detail an agent needs: the 5-capitalised-name cap, the per-entity found/not-found result with Wikidata ID, the separate year-sentence listing, and the x402 payment model ($0.003 USDC on Base/Solana/Arc, failed calls not charged). The charge/no-charge asymmetry is exactly the kind of cost context 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 sentences, front-loaded with the core behavior and the limitation before cost. The final payment sentence is dense (three chains, two price conditions) but every clause carries operational value; no filler to cut.

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-input read-only check with an existing output schema, nothing an agent needs is missing: input shape, extraction limit, result semantics, known limitation, and billing behavior are all present. Return-value detail is partly redundant with the output schema but harmless.

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?

Only one parameter at 100% schema coverage, so the schema already documents 'text' as the model answer with a 10–5000 char range. The description adds meaning beyond it by clarifying that only capitalised names (max 5) are extracted from that text, which tells the agent how to shape its input.

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 verb (checks), a specific resource (named entities in an AI answer) and the exact backing source (Wikidata), plus the output shape (found/not-found with Wikidata ID and description). An agent can tell this apart from citation_verifier or fact_brief_synthesizer without opening the 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?

Explicitly delimits scope: 'It checks that entities exist, not that claims about them are true, so a found entity can still have a wrong fact.' That tells the agent when this tool is and is not sufficient, plus the 5-name ceiling. It stops short of naming an alternative sibling for the fact-truth case, so not a 5.

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