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

Annotations already declare readOnlyHint=true, destructiveHint=false, and idempotentHint=false, so the safety profile is covered. The description adds genuinely useful context beyond that — pricing, free failures, and the 'each call is charged, so repeats are not free' caveat that explains the idempotentHint=false. It stops short of describing the response shape (e.g. whether an is_contract boolean is returned alongside bytecode size and proxy info), which is left implicit.

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 the key caveat (proxy detection) are front-loaded, and most sentences carry information an agent needs. The pricing/payment sentence is dense (x402, card key, first 25 free, failed calls free) and slightly crowds out the return-value detail, costing one point.

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 and one parameter, the description carries most of the burden and mostly does: it explains what is computed, the error case, and the cost model. It never spells out the returned field names or the is-contract boolean explicitly, so an agent still infers the response shape.

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% with a single 'address' field, so the baseline would be 3. The description goes a step further by supplying a concrete example address and specifying the accepted form (0x-prefixed) and the rejection behaviour for malformed input, which is real meaning beyond the schema.

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?

The description opens with a specific verb and resource ('Check whether a 0x address on Avalanche is a smart contract or a plain wallet') and then names the actual payload: bytecode size, EIP-1167 minimal-proxy detection, and implementation address. That detail separates it from the many sibling *_proxy and *_balance tools.

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?

It states the concrete trigger ('Use before sending funds or approving a spender') and routes heavier needs elsewhere ('for a fuller risk verdict use check_counterparty'). It also covers the failure path ('malformed address is rejected with 400, uncharged'), so an agent knows what a bad call costs.

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