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

Beyond the readOnly and destructive annotations, the description discloses that the balance is read at the latest block, returned in wei and POL, malformed input costs nothing, each successful call is charged including repeats, and the first 25 calls are free. The billing note is consistent with idempotentHint=false, since repeated reads incur repeated charges.

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 key scope, alternatives, and output format are front-loaded, and most sentences earn their place. The pricing details are useful but slightly repetitive: 'each call is charged, so repeats are not free' restates the earlier per-call price and free-call terms.

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 one-parameter read-only balance tool with no output schema, the description covers scope, alternatives, return units, error behavior, and cost model. Nothing essential for correct invocation 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 single address parameter is already documented. The description adds useful meaning with the 'any 0x wallet address' phrasing, a concrete example address, and the malformed-address rejection behavior.

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?

The description states a specific verb and resource: returns the native POL balance (eth_getBalance) of a 0x wallet on Polygon PoS at the latest block, in wei and POL. It explicitly distinguishes itself from polygon_token_balance for ERC-20 tokens and wallet_portfolio for all holdings, so an agent can select it without opening the 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?

It says exactly when to use the tool: checking whether a wallet can pay gas or a transfer. It also names the alternatives for ERC-20 balances and aggregate holdings, and clarifies that malformed addresses are rejected with 400 and uncharged.

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