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

Annotations already declare readOnly/destructive false, and the description adds substantial context beyond them: error semantics (malformed address returns 400 and is uncharged), pricing model ($0.01/call, first 25 free, failed calls free), and the note that repeats are not free — which explains why idempotentHint is false despite the read-only nature. It also confirms it only reads/computes and mutates nothing.

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?

Front-loaded with the core check, then usage, then error/pricing details — a logical ordering with no filler. The final sentence packs statelessness and per-call charging into two dense clauses; it earns its place but is the tightest place to trim.

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 still enumerates what is returned (bytecode size, minimal-proxy flag, implementation address), plus error and cost behavior. For a single-parameter read tool, an agent has everything needed to call it and interpret the result.

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% and the single parameter is trivially documented, so the baseline is 3. The description goes slightly further by giving a concrete example address and specifying rejection behavior for malformed input, which adds meaning beyond the schema's '0x address'.

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?

A specific verb+resource ('Check whether a 0x address on Optimism is a smart contract or a plain wallet') with the added scope of bytecode size and EIP-1167 proxy/implementation detection. This clearly distinguishes it from the sibling is_contract tools on other chains and from op_proxy.

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?

States the decision it supports ('Use before sending funds or approving a spender') and names a superseding alternative ('for a fuller risk verdict use check_counterparty'). The condition selecting the alternative is explicit, so nothing 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.