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

Goes well beyond the readOnly/destructive annotations: it discloses error semantics (404 for a missing contract, 422 for a non-ERC-721 contract), that failed calls are uncharged, the pricing model ($0.01/call via x402 USDC or card key, first 25 free), and that repeats are not free — which explains the otherwise puzzling idempotentHint=false for a read. Read-only nature is restated ('reads or computes only and changes nothing').

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-loads purpose, then the sibling routing, then parameter/error constraints, then pricing — a sensible order. It is dense and the pricing/refund clause is long, but each clause carries information an agent needs (error codes, cost, non-free repeats).

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 2-param read tool with no output schema, the description still says what comes back (count plus collection name and symbol), how failures surface, and what it costs. Nothing needed to call 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 baseline is 3; the description adds meaning by stating both inputs are 0x addresses and by constraining the contract to an ERC-721 collection (non-ERC-721 yields 422). It does not add format examples or checksum guidance, so it stops short of the top mark.

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: count of NFTs held in one ERC-721 collection on zkSync Era, naming the underlying call (balanceOf) and the returned metadata (name, symbol). It also explicitly distinguishes itself from the sibling zksync_nft_owner ('for who holds one token id use ...'), so an agent can route 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 (checking a holder before a token-gated action, verifying a wallet still holds a collection) and names the exact alternative plus the condition that selects it (per-token-id ownership). The when-not case is implicit in that routing sentence but is unambiguous.

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