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

Annotations already declare readOnly/destructive, but the description adds error semantics (404 for missing contract, 422 for non-ERC-20), billing behavior ($0.01/call, first 25 free, failed calls free), and explains why repeats aren't free — which aligns with idempotentHint=false. Only gap is no detail on response shape/pagination, but with an output-less simple read that's minor.

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 purpose, then usage, then semantics, then billing — all information earns its place. It is dense and long for a single description, but nothing is redundant; the pricing sentence is the only slightly tacked-on element.

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 3-param read with no output schema, the description covers purpose, alternatives, accepted token formats, error codes, and cost behavior. Nothing an agent needs 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 the baseline is 3, but the description adds value by clarifying that 'token' accepts a ticker or a 0x contract and what happens on an invalid one, and by naming Permit2 as the typical spender. It doesn't add much for 'owner', but overall it goes slightly 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 ('ERC-20 token allowance on zkSync Era') and enumerates the returned values (raw and token-unit allowance, unlimited flag). It explicitly separates itself from the sibling zksync_token_balance, so an 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 two concrete use cases ('check an approval before a swap', 'audit risky open approvals') and names the alternative for the adjacent need ('for the owner's balance use zksync_token_balance'). The when/when-not split is explicit.

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