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

Annotations already cover readOnly/destructive/openWorld/idempotent, yet the description still adds materially: 404 for missing contract, 422 for nonexistent token, both uncharged, plus the billing model ($0.01/call, first 25 free, failed calls free). The note that repeats are not free is consistent with idempotentHint=false.

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?

The core scope and the alternative routing are front-loaded in the first two sentences, and the error/price details are useful rather than filler. It runs slightly long with the pricing and 'changes nothing' clauses, but each sentence carries information.

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 explains the return shape (owner wallet plus collection name and symbol), failure codes, and cost model. An agent has everything needed to invoke this correctly and interpret outcomes.

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 (contract as 0x ERC-721 address, tokenId as a whole number), setting the baseline at 3. The worked example with a real contract address adds mild illustrative value but no semantics beyond the schema.

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 and resource ('Owner of an NFT on Ethereum... the wallet that holds one ERC-721 token id right now (ownerOf)') and explicitly contrasts with siblings eth_nft_balance and eth_nft_metadata. An agent can distinguish this from the two nearest alternatives 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?

Names concrete use cases (verify holder before a trade, mint gate, airdrop) and routes to alternatives with explicit conditions: 'for how many a wallet holds use eth_nft_balance, for the token's metadata eth_nft_metadata.' 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.