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

Annotations already declare readOnly, idempotent, non-destructive and closed-world, so the safety profile is covered. The description adds genuinely useful behavior beyond that: the instruction to 'ALWAYS read basis' because figures before the freeze date are hypothetical, and the limitation that only published benchmarks are served. It stops short of describing error behavior for an unknown id.

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?

Long but front-loaded: purpose and returned payload come first, then the critical basis caveat, then the id format. The parenthetical enumeration of statistics is dense but each item earns its place since no output schema exists. Only minor tightening is possible.

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 must convey return values, and it does so thoroughly: methodology rule, fund/weight composition with SEC series ids and expense-ratio dates, and the full statistic set per window with dates and basis. Combined with annotations and the id-format hint, nothing needed to call it correctly is 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 coverage is 0% and there is a single 'id' parameter, so the description carries the burden. It compensates by giving two concrete id forms (prefixed 'etfiq-strategic-60' and bare 'strategic-60'), which tells the agent the prefix is optional. It does not say where ids come from, but the format examples are a real addition over the bare string 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 states a specific verb (retrieve one benchmark in full) plus the resource and enumerates exactly what comes back: the rule, constituent funds with weights and expense ratios, and every computed statistic per window. It is clearly the single-item counterpart to the sibling list_benchmarks, so an agent can distinguish it without reading the 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?

It gives input guidance ('Takes an id such as etfiq-strategic-60 or strategic-60') and a scope exclusion ('Serves the published benchmark only; it cannot be customised'), which implies when not to use it. But it never names an alternative (list_benchmarks, compare_to_benchmark) or states an explicit condition for selecting this tool over them, so usage is only implied.

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