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

Beyond the annotations it discloses failure semantics (404/422, both uncharged), billing behavior ($0.01/call via x402 USDC or card key, first 25 free, failed calls free), and the fact that repeats cost money again — which explains the otherwise surprising idempotentHint=false. It also confirms read-only, consistent with readOnlyHint=true.

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 with purpose front-loaded, followed by alternatives, error codes and pricing. Every clause carries information, though the pricing/billing run-on is heavy enough that it slightly crowds the core call semantics.

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 two-address read tool with no output schema, the description covers inputs, chain, return contents (balance plus collection name and symbol), error behavior, cost and safety. Nothing an agent needs in order to invoke it correctly is missing.

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 both parameters are already documented in the schema. The description only restates that both inputs are 0x addresses and that the contract must be an ERC-721 collection, adding little beyond the structured field descriptions. 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, standard and chain: count of NFTs from one ERC-721 collection held by a wallet on Optimism via balanceOf, plus the name/symbol returned. It explicitly differentiates from the sibling op_nft_owner ('for who holds one token id use op_nft_owner'), so the agent can choose 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 concrete when-to-use cases (pre-token-gated-action holder check, verifying a wallet still holds a collection) and names the alternative tool with the condition that selects it. It also supplies when-not conditions in the form of error outcomes (404 for missing contract, 422 for non-ERC-721).

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