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

Annotations already declare read-only/non-destructive/openWorld, but the description adds genuinely new behavioral context: 404 on missing contract and 422 on non-ERC-20, both uncharged; $0.003/call pricing, 25 free calls, failed calls free; and the warning that each call is charged so repeats are not free. That is exactly the kind of cost and failure-mode detail annotations cannot express.

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 core capability, return fields and sibling routing are front-loaded in the first two clauses; error codes and pricing follow. It is information-dense rather than padded, though the pricing/free-tier sentence is a long run-on that could be split for readability.

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 carries the return-value burden and does so by enumerating raw amount, decimals, symbol and human-readable amount. Combined with error semantics, pricing and the explicit read-only guarantee, an agent has everything needed to call it correctly and interpret failure.

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 format detail beyond the schema by specifying both inputs are 0x addresses of 40 hex characters. It stops short of explaining address normalization or token-vs-wallet validation beyond that, so it is a modest increment rather than a full one.

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 (read), a precisely scoped resource (one wallet's balance of one ERC-20 token on Arbitrum One), and enumerates the returned fields (raw amount, decimals, symbol, human-readable amount). It also distinguishes itself from the sibling arb_balance by naming the native-coin case, so an agent can route without opening another 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 or trade" gives an explicit use context, and "for the native coin use arb_balance" names the sibling and the condition that selects it. Both the when and the alternative are stated, not inferred.

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