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

With annotations present the bar is lower, but this description adds real behavioral context: malformed addresses return 400 and are uncharged, failed calls are free, pricing is $0.003/call with 25 free calls, and it proactively explains the otherwise puzzling readOnlyHint=false ('not marked read-only because each call is charged'). That annotation rationale is exactly the kind of disclosure that prevents an agent from misreading the structured hints.

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-loads the core question and the returned data before usage, pricing and annotation rationale. Dense but every clause carries information; the payment/free-tier sentence is the only slightly tangential element, though still decision-relevant for an agent weighing a paid call.

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?

There is no output schema, yet the description enumerates what comes back (bytecode size, proxy flag, implementation), plus error semantics and cost. For a one-parameter tool this leaves nothing an agent needs 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?

Schema coverage is 100% for the single 'address' parameter, so the baseline is 3; the description goes beyond it by supplying a concrete example value and defining the malformed-input behavior (400, uncharged). It does not describe accepted address formats beyond the 0x prefix, so it stops short of a 5.

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 on Arbitrum One is a smart contract or a plain wallet') and even enumerates the returned facts (bytecode size, EIP-1167 minimal proxy status, implementation address). The chain scoping ('on Arbitrum One') separates it from the sibling eth_is_contract, base_is_contract, op_is_contract, etc., which share the same verb but a different network.

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?

Explicitly says when to use it ('before sending funds or approving a spender') and names the better alternative for a different need ('for a fuller risk verdict use check_counterparty'). This is the when/why/alternative triad done properly.

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