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

Annotations set readOnlyHint=false and the description proactively reconciles that apparent contradiction ('reads or computes only; not marked read-only because each call is charged'), which is exactly the disclosure an agent needs. It also adds pricing, the 400-on-malformed behavior, and the free-failed-call policy beyond what annotations carry.

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?

Purpose and scope are front-loaded, and most sentences earn their place. The pricing/credits clause is somewhat dense and runs long, but it is operationally relevant for an x402-charged tool.

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 naming the returned signals (bytecode size, minimal-proxy detection, implementation pointer). Combined with error semantics, cost, and the sibling alternative, an agent has everything needed to call it correctly.

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?

Only one parameter with 100% schema coverage, so the schema carries the burden; the description still adds value with a concrete example address and the rejection behavior for malformed input. It stops short of documenting format variants (e.g. EIP-55 checksum requirements).

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 (check whether a 0x address is a contract vs wallet) and enumerates what it returns: bytecode size, EIP-1167 minimal-proxy status, and implementation target. This clearly differentiates it from the sibling is_contract tools on other chains and from check_counterparty.

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 explicit when-to-use ('before sending funds or approving a spender') and names the alternative for a deeper need ('for a fuller risk verdict use check_counterparty'). The malformed-address handling and free-on-failure condition are stated outright.

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