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

Annotations already declare readOnly, non-destructive, open-world, and non-idempotent. The description goes further, explaining the billing model ($0.003/call, first 25 free, failed calls free) and the 400/uncharged response for malformed input, which gives real meaning to idempotentHint=false. It stops short of latency, rate limits, or output shape.

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?

Dense and front-loaded: purpose, then the returned data, then usage guidance, then error and pricing facts. Every sentence carries information, though the billing/free-tier detail is somewhat ancillary to invocation correctness and lengthens the block.

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: bytecode size, proxy status, implementation address, plus error and pricing behavior. An agent has everything needed to call this correctly for a one-parameter read-only tool.

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% for the single address parameter, so the baseline is 3. The description adds genuine value beyond the schema by supplying a concrete example address and specifying the failure behavior (malformed address => 400, uncharged), which tells the agent what valid input looks like and what happens otherwise.

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 (check) and resource (whether a 0x address on Optimism is a smart contract), plus the concrete returned facts: bytecode size, EIP-1167 minimal-proxy detection, and implementation address. It clearly separates itself from the sibling family (eth_is_contract, base_is_contract, etc.) by naming the chain explicitly.

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 the alternative for a different need ('for a fuller risk verdict use check_counterparty'). Nothing about tool selection is left to inference.

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