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

Annotations already declare read-only, non-destructive, open-world. The description adds genuinely new behavior: 404 for a missing contract, 422 for a nonexistent token, both uncharged; $0.01 per call via x402 or a card key with the first 25 free and failed calls free; and repeats are re-charged, which explains the non-idempotent hint. It stops short of describing return shape ordering or chain-reorg caveats, so a 4 rather than 5.

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 the definition, then alternatives, then error/pricing facts — a logical order with no filler. It is dense and slightly overpacked (example and pricing detail in the same breath as semantics), keeping it just under a 5.

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 names the returned data (owner wallet, collection name and symbol), covers failure modes, and states the cost model. An agent has everything needed to call it correctly and interpret the result.

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 the baseline is 3; the description still adds value with a worked example (a real Uniswap V3 position-manager contract plus tokenId=1) that shows the expected address format and that tokenId is a bare integer. It doesn't explain edge cases such as burned tokens or non-ERC-721 contracts, so it stays at 4.

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: returns the owner (ownerOf) of one ERC-721 token id on Arbitrum One, plus the collection name and symbol. It explicitly contrasts with arb_nft_balance and arb_nft_metadata, so an agent can pick it 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?

Gives concrete occasions to use it (verifying a holder before a trade, mint gate, or airdrop) and names the two sibling alternatives with the condition that selects each. Nothing is left to inference.

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