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

Annotations already establish readOnly, idempotent, open-world, non-destructive behavior, so the bar is lower. The description adds material context beyond them: per-call billing of $0.09 USDC, prepaid-credit fallback, and the exact header names (X-Payment / X-Payment-Tx / Authorization: Bearer fg_…) plus the warning not to put payment in parameters — an agent cannot call this correctly without that.

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 payload is delivered in three overlapping layers: an English question-style summary, a Chinese enumeration of the same outputs, then a [zh]/[en] pair repeating billing and args verbatim. '参数同左' and the duplicated payment section are pure waste, and the actual purpose statement is not front-loaded.

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, the description properly enumerates what comes back (surface rows, term-structure curve, active-contract ranking) and covers payment mechanics. It still leaves the caller guessing on expiry string format, the meaning of 'top', and how this differs from the snapshot sibling — gaps that matter for a zero-required-parameter tool.

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% — the three properties carry only type/title/default — so the description must compensate. It only repeats the bare names 'currency, expiry, top' with the phrase 'params same as left' (参数同左), giving no format for expiry, no supported currency list, and no explanation of what 'top' ranks or bounds.

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 names a specific resource and enumerates concrete outputs: per-expiry ATM strike and IV, 25-delta skew, max pain, put/call OI, term-structure curve, and most-active contracts, which an agent can match against a 'vol surface' request. It never explicitly contrasts with the sibling vol_snapshot, so the 'full' wording is the only hint about granularity, and the intent statement is buried in a bilingual question-style phrasing.

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?

Opening with 'Ask "what is the option implied volatility term structure"' implies the triggering question, and the payment-header requirement functions as a precondition for successful invocation. However, there is no guidance on when to pick this versus options-iv.vol_snapshot or expiry_ladder_entry, and no stated behavior when expiry or currency are omitted.

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