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

Annotations already declare readOnly, idempotent, non-destructive and openWorld, so safety is covered. The description adds value the annotations don't: pricing ($0.001 USDC via x402 on Solana) and an equivalent HTTPS endpoint, which is important context for an agent deciding how to call this. It does not contradict the read-only/idempotent 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?

The core definition is front-loaded in the first sentence, followed by verification examples, the domain constraint, and the payment/endpoint note. Four example values is close to more than needed, but they double as correctness checks and no sentence is filler.

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-parameter arithmetic tool with no output schema, the description covers the input domain, the returned quantity, sample outputs, and cost/transport. An agent has enough to invoke it correctly; only explicit sibling guidance and out-of-range handling are missing.

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 description coverage is 0%, so the description must carry the burden, and it does: 'n runs from 1 to 5000' plus 'at most n' establishes n as an inclusive upper bound with a bounded domain. The worked examples further pin the semantics of n by showing input/output pairs, going beyond the bare min/max in 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 gives a precise verb+resource ('sum of the Duffinian numbers that are at most n') and defines the scope as a prefix/summatory quantity rather than a membership test. Siblings like duffinian, duffinian_count, duffinian_gcd and duffinian_next make the 'sum' variant easy to isolate, though the description never names those alternatives to contrast itself against.

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?

Usage is only implied: the worked values (through 4 = 4, through 9 = 21, etc.) signal that n is an inclusive upper bound, which is genuinely helpful for verification. There is no explicit statement of when to pick this over duffinian_count or duffinian_next, and no note on failure behavior when n exceeds 5000.

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