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

Annotations already declare read-only, idempotent, non-destructive behavior, and the description adds the crucial non-obvious trait: the call is metered at $0.05 USDC via x402 on Base mainnet, with a free validation path. It also discloses the methodology constraint (only data available that day), which materially affects results. It stops short of covering failure modes, whether payment is settled before or after the call, or what happens with date=null.

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?

Three tight sentences, each earning its place: purpose and methodology first, then the cost model, then the free alternative. The core definition is front-loaded and nothing is redundant.

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 and four parameters at 25% description coverage, the description should say more about what a snapshot entry contains and what null date resolves to. Cost and methodology are well covered, but return shape and default semantics are not.

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 only 25%, so the description must carry more weight; it explains the ranking metric (trailing 20-day dollar volume) and the point-in-time semantics that give 'date' meaning, which is real added value. However, min_age_days and min_avg_dollar_volume are never mentioned, and the behavior when 'date' is null (presumably today) is left unstated.

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 resource and scope: a top-N coin list for past dates, constructed from Coinbase USD pairs ranked by trailing 20-day dollar volume. The survivorship-bias-free framing and the "as of a past date" qualifier make the output unambiguous, and naming validate_universe as the pre-check separates it from the closest sibling.

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 places usage in backtests ("survivorship-bias-free universe for backtests") and names the alternative path ("Free pre-check: validate_universe"). It doesn't say when NOT to use it, e.g. that it is inappropriate for live/current universe construction where market_signals or token_check might fit.

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