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

Annotations already establish readOnlyHint=true and openWorldHint=false, so the safety profile is covered. The description adds genuinely useful behavior beyond that: the freemium gating rule (free only for S&P 500 in the current month), the degraded fallback response, and the presence of a pro_required signal, which tells the agent to expect a paywall marker rather than real data.

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?

Two tightly packed sentences with the core purpose front-loaded and the access caveat second. Dense but every clause (outputs, free tier, fallback, pro_required) carries information; no filler or repetition.

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 describe the return shape, and it does list the concrete fields (average return, share of years up, rank among all days) plus the pro_required marker. What remains unclear is the response envelope for the paywalled case, but for a two-parameter read tool this is close to sufficient.

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 50%: the date parameter is documented as 'MM-DD or YYYY-MM-DD' and the asset parameter is a constrained enum, so the schema carries most of the burden. The description adds no extra parameter meaning (e.g. whether a single date or a range is expected), so baseline 3 is appropriate.

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 scope: performance of an asset on one calendar date aggregated across all years, with named outputs (average return, share of years up, rank among all days). The word 'day' plus 'across all years' implicitly separates it from monthly_seasonality and window_seasonality, though it never names those siblings outright.

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?

The description conveys the access condition ('Free for the S&P 500 in the current month') and what happens otherwise, which implies when the tool is fully usable, but it never states when to pick this tool over monthly_seasonality or window_seasonality. Usage is inferable from the day-scoped framing rather than explicitly guided.

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