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

The description adds real context beyond the annotations: it explains the 400/uncharged path for malformed addresses, the per-call price and free tier, and notes that repeats are billed again (which explains the idempotentHint=false). Combined with readOnlyHint=true and destructiveHint=false, that gives a clear safety and cost picture. It stops short of describing the response payload shape, so a 3 rather than higher.

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?

Effectively three sentences that front-load the purpose, then usage, then billing. The pricing and free-tier detail is relevant for an x402 paid tool but is denser than strictly necessary; nothing is truly wasted.

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?

For a one-parameter read-only tool with no output schema, the description covers what the call returns (bytecode size, proxy detection, implementation), failure behavior, cost, and the sibling alternative. An agent has everything needed to decide and invoke 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?

Schema coverage is 100% for the single 'address' parameter, so the baseline is 3. The description adds a concrete sample address and constrains the format to 0x-prefixed, but does not mention chain-specific or checksum requirements beyond what the schema implies.

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 Sonic is a smart contract or a plain wallet') and enumerates exactly what is returned: bytecode size, EIP-1167 minimal-proxy status, and implementation address. Naming check_counterparty as the fuller-verdict sibling makes it distinguishable without 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 trigger ('before sending funds or approving a spender') and an explicit alternative for the adjacent need ('for a fuller risk verdict use check_counterparty'). Edge behavior for bad input is also covered (malformed address → 400, uncharged).

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