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

Annotations already cover safety (readOnly, idempotent, non-destructive, closed-world), so the bar is lower; the description still adds meaningful behavior beyond them: results come from a frozen dated archive, so values are stable and reproducible unlike the mutable 'today' tools. It does not address archive start dates, missing dates, or error behavior, keeping it short of a 5.

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-loaded with the purpose, then usage conditions, then the format requirement. Three sentences carry little waste; the parenthetical 'today moves overnight' is slightly colorful but reinforces the stability rationale that justifies using this tool.

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 one-parameter read-only lookup with no output schema, the description conveys what the return represents ('the pieces written that night') and the temporal contract. It stops short of enumerating what fields a 'piece' contains or how an out-of-archive date behaves, which would be needed for a perfect score.

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 0% — the single parameter is an unformatted string — so the description must compensate, and it does by specifying the accepted format ('Takes YYYY-MM-DD'). It adds no timezone, range, or validity constraints, so it is not complete, but it closes the most important gap.

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 and resource: returns what ETFIQ published on a given date from a frozen archive. It also explicitly differentiates itself from every sibling by noting the other tools answer about 'today' while this one is historical, which lets an agent route correctly without opening any schema.

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?

Gives two concrete triggers ('a question is about a past date' and 'when a number needs to stay true after it is quoted') and names the generalized alternative ('every other tool here answers about today and today moves overnight'). It also states a negative boundary — 'not an arbitrary per-fund number on an arbitrary day' — which prevents a plausible misuse.

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