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

Annotations cover the safety profile, and the description adds genuinely non-structured context: the value is read live from the contract (not cached), and the call costs $0.001 USDC on Base via x402 per successful call. It does not explain the zero-balance/error path, and doesn't reconcile the read-only-looking operation with readOnlyHint=false and idempotentHint=false, though those flags are defensible given the x402 payment side effect.

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?

Three sentences, front-loaded with what is returned, then when to use it, then the cost — each sentence carries distinct information with no filler or repetition of the name.

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 does the work of describing the return shape (raw amount, decimals, symbol, human-readable amount) and discloses the pay-per-call cost model. It stops short of covering failure modes (invalid token address, non-ERC-20 contract) that an agent calling this live contract read might encounter.

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 both parameters ('token' as 0x ERC-20 contract address, 'address' as 0x wallet address) are already fully documented in structured form. The description adds no syntax, format, or edge-case detail beyond what the schema provides, so the baseline 3 applies.

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?

Names a specific verb+resource ('ERC-20 token balance of a wallet on Ethereum') and distinguishes itself from the native-coin sibling eth_balance by scoping to ERC-20 contracts. It also enumerates the returned fields (raw amount, decimals, symbol, human-readable amount), so an agent knows exactly what it gets without opening a 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?

"Use to check stablecoin or token holdings before a transfer, payment or trade" gives concrete triggering scenarios. It does not explicitly name alternatives (e.g., eth_balance for native ETH, eth_token_info for metadata), so routing between siblings is left partly to inference.

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