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

The annotations already declare readOnlyHint=true, idempotentHint=true, destructiveHint=false, and openWorldHint=false, so safety is covered. Beyond that, the description adds real behavioral content: the tool renders an interactive chart and defines the calculation semantics (period-by-period purchases versus a single first-period purchase, both marked to the final price). It does not cover edge behavior such as what happens with a single price or how the two results are labeled, which keeps it below 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?

Three sentences, front-loaded with the comparison mechanic and closed with the usage trigger, with no filler. Minor deduction because the opening clause largely restates the tool name and title, and the 'interactive chart' remark is dropped in without being tied to a return behavior.

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 two-parameter, fully documented, read-only calculator with no output schema, the description supplies the one thing structured fields cannot: what is actually being compared and how each side is valued. An agent can select and call it correctly from this. The remaining gap is that, with no output schema, it does not say what the response contains besides a chart.

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 100%, and the schema already states that prices are oldest-first with 1-120 entries and that amount_per_period is dollars invested at each price. The description's phrasing ('a fixed amount at each price in a list', 'valued at the last price') reinforces the mapping of both parameters but adds no syntax, ordering, or unit detail beyond what the schema supplies. Baseline 3 for a fully documented two-parameter schema 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 states a precise verb and resource ('Dollar-cost averaging vs lump sum calculator') and then defines the exact comparison: a fixed amount invested at each price in a list versus the same total invested at the first price, both valued at the last price. That mechanism-level detail is enough for an agent to distinguish it from look-alike siblings such as replay_past_investing or plan_recurring_investment 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?

It gives an explicit trigger condition: 'Use it when the person provides the price series.' That is a clear positive-use signal tied to having a concrete data prerequisite. It stops short of the 5 bar because it names no exclusions (e.g. when there is no historical price list) and points to no alternative sibling for those cases.

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