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

Annotations already cover safety (readOnly, idempotent, non-destructive), and the description adds genuinely non-obvious behaviour: the $0.001 USDC x402/Solana payment per successful call and the equivalent HTTPS endpoint. What's still absent is the shape of the outcome — whether an invalid number returns false, an error, or a failed payment.

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 purpose, then the accepted format, then the algorithm, then the lookup caveat, then price/alternate endpoint. No filler sentences; every clause either constrains the input or prevents a misread of the result.

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 single-string, no-output-schema, annotated read-only tool this is nearly complete: format, algorithm, cost, and the non-lookup caveat are all present. The remaining gap is return/error semantics, which an agent must discover by calling.

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?

With one required parameter at 0% schema coverage, the description does the heavy lifting: two-to-seven digits, hyphen, two digits, hyphen, check digit, no leading zero, and hyphens may be omitted and are split the same way. That is strong compensation, though it never names the parameter (`value`) or says whether surrounding whitespace or lowercase is tolerated.

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 operation (check-digit validation) against a specific resource (CAS Registry Number), and explicitly rules out a different interpretation: "This does not look the substance up." That negative scope separates it from lookup-flavoured siblings such as openfda_drug or pubmed_search.

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?

Clear about the use case (verify a CAS RN's check digit) and explicitly states what the tool is not for, plus offers the HTTPS alternative for non-x402 callers. It stops short of pointing at a sibling (e.g. validate_batch) when many numbers must be checked.

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