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

Annotations already cover read-only/idempotent/non-destructive/open-world, but the description adds material context they cannot: the $0.001 per-call cost, the x402/MPP payment flow needed to retrieve data, and the failure policy ('Bad input is refused before any charge'). It also discloses the return shape (wei, decimal, block number, as_of).

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 'Use when' condition and the sibling routing, which is what an agent needs first. The body is a dense, comma-chained run-on packing pricing, URL, response schema reference and client instructions into one sentence, so it is slightly overstuffed but every clause carries real 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?

For a two-param read tool with no output schema, the description supplies the missing return detail (wei + decimal, block number, as_of) and the payment prerequisite. It is complete enough to call correctly, though the 'returns the paid x402 request' phrasing makes the two-step retrieval flow slightly ambiguous.

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 description coverage is 100%, so baseline is 3. The description restates the chain enum and single-chain constraint but adds no format or syntax detail beyond what the schema already documents for address (pattern) and chain (enum + default).

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 ('native coin balance (ETH or POL) of an address') and the supported chains, and explicitly distinguishes itself from siblings by naming evm_erc20_balance's domain ('stablecoins') and wallet_balance ('several chains'). An agent can tell it apart from evm_erc20_balance and wallet_balance without opening schemas.

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?

Opens with 'Use when you need...' and states the inverse condition explicitly: 'Native coin on one chain only; for stablecoins or several chains use wallet_balance.' Both the when-to-use and the alternative paths are named, leaving nothing to inference.

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