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

Annotations already cover the safety profile (readOnlyHint=true), but the description adds real behavioral context beyond annotations: the x402 two-step payment flow (call once with no payment to receive terms, then retry with the signed payment at _meta["x402/payment"]). It also discloses that the value returned is the raw token balance rather than a decimal-adjusted amount.

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 purpose and parameters are front-loaded, followed by the HTTP route and payment mechanics. It is somewhat dense with the URL and measured-output block, but those lines carry genuinely useful information; nothing is redundant.

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 compensates by showing a measured output example and clarifying that 'result' is the raw balance (not human-scaled). Combined with the payment-flow explanation, an agent has what it needs to call and interpret this correctly, though it says nothing about error or failure cases.

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 schema already documents both parameters fully; the description merely restates them (0x + 40 hex). Baseline 3 is appropriate, as it adds no syntax or format detail 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+resource (ERC-20 balanceOf) scoped explicitly to 'Ethereum mainnet', which cleanly separates it from the base_ and gnosis_ siblings. An agent can distinguish this from eth_getbalance (native ETH) and from the other chain reads without opening any 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?

The chain qualifier implies when this tool is the right one, but the description never explicitly states when to prefer it over siblings like eth_getbalance or the base/gnosis ERC-20 variants. No exclusions or alternatives are named, leaving routing to inference.

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