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

Annotations already cover read-only/no-destroy, but the description adds real context they don't: 404 on a missing contract, 422 on a non-ERC-721 contract, both uncharged, and $0.01/call with 25 free calls and repeats being billed. This also explains the idempotentHint=false annotation (each call costs money), which the annotation alone leaves ambiguous.

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 what the tool returns and the sibling it is not, then errors, then pricing — a sensible order. It is dense and the billing/free-tier sentence runs long, but no sentence is pure filler.

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 still says what comes back (count plus collection name and symbol) and covers failure modes and cost, which is everything an agent needs to decide to call it and interpret the result. Nothing material is missing for a two-parameter read tool.

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 both parameter descriptions already say '0x wallet address' and '0x ERC-721 collection contract address'. The description's 'Both inputs are 0x addresses' and its ERC-721 framing restate that rather than adding format, checksum, or validation guidance, so 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 verb+resource+scope: counts NFTs of one ERC-721 collection held by a wallet on Polygon PoS, names the underlying call (balanceOf) and the extra fields returned (name, symbol). It explicitly distinguishes itself from polygon_nft_owner, so an agent can pick between them without opening either 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 two concrete use cases (gate a holder before a token-gated action, check whether a wallet still holds a collection) and the explicit alternative for the other question ('for who holds one token id use polygon_nft_owner'). When-to-use and when-to-use-something-else are both present.

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