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

Annotations already declare read-only, non-destructive, non-idempotent, open-world, but the description adds context the annotations cannot: malformed addresses are rejected with 400 and uncharged, pricing is $0.01/call with the first 25 free, failed calls are free, and the explicit note that 'each call is charged, so repeats are not free' explains the idempotentHint=false in operational terms. It also confirms 'reads or computes only and changes nothing,' reinforcing the safety profile with the billing model an agent needs before retrying.

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 is front-loaded in the first sentence, with usage guidance, error behavior, example, and pricing following in a logical order; nothing is padding. It is slightly overpacked—pricing, example, and billing rules share a single dense sentence—but every clause carries information an agent needs for a paid, non-idempotent call.

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 one required param and no output schema, the description carries the return-value burden and does so by naming the three outputs (bytecode size, proxy flag, implementation). The only gap is the exact response shape or how a plain wallet is represented, which an agent can infer but not confirm.

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% (the single 'address' param is described), so baseline is 3. The description adds value beyond the schema by pinning the address to Arbitrum One semantics, supplying a concrete example value, and stating the failure mode for malformed input (400, uncharged), which tells the agent how to pre-validate.

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 and resource ('check whether a 0x address on Arbitrum One is a smart contract or a plain wallet') and enumerates what the answer contains: bytecode size, EIP-1167 minimal-proxy status, and implementation address. That detail distinguishes it from arb_proxy and the parallel *_is_contract siblings without the agent 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?

Gives an explicit when-to-use ('before sending funds or approving a spender') and names an alternative with the condition that selects it ('for a fuller risk verdict use check_counterparty'). This is the when/when-not/alternative pattern at full strength.

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