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

Annotations already declare readOnly, idempotent, non-destructive, closed-world behavior, so the description's job is to add context — and it does: it discloses the projection assumptions (steady returns after plan fees) and that the monthly figure is the amount that would fully fund the goal from today. It stops short of covering failure modes (e.g., invalid school id) or how fees are sourced.

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: purpose first, required inputs second, return values and modeling assumption last. No filler, and the most decision-relevant information is front-loaded.

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 usefully names the return values (per-year cost, total cost, monthly funding amount) and the modeling assumption, which is enough for an agent to invoke and interpret it. The one loose end is 'the plan's fees' — no plan parameter exists, so it is unclear which fee schedule is applied.

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%, so the schema already documents all seven parameters, making 3 the baseline. The description adds a marginal clarification by framing returns as net of 'the plan's fees', which reinforces the schema's 'before fees' note on annual_return_percent, but it adds no syntax or format detail beyond the 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 and resource ('project what college will cost and what to save') and enumerates the outputs (projected cost per year and total, monthly funding amount), so the agent knows exactly what this tool produces. It does not explicitly differentiate itself from siblings like compare_529_plans or get_529_plan, though the cost-projection framing is fairly distinct.

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 input precondition ('Give a US school id and the year the student starts'), which implies the usage context, but it never states when to prefer this over compare_529_plans or how it relates to get_529_plan. Usage is inferable rather than explicit; no exclusions or alternatives are named.

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