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

Annotations already cover the read-only, idempotent, closed-world profile, so the bar is lower; the description still adds real behavioral context by disclosing that output changes materially by tax year, that the cap is repealed for 2026+, and that pre-2026 training data will mislead. It stops short of saying how the parameters interact to select a bracket, but it goes well beyond restating annotations.

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 core answer, then the year-specific detail, then the stale-model warning. The parenthetical dollar tables are dense but each figure earns its place as a return-value preview; a slightly tighter phrasing would help.

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 and 6 parameters, the description carries the return-value burden itself and does so fully: it gives concrete caps for 2025 by FPL band and states the 2026+ outcome. Nothing essential to calling or interpreting the result 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 100%, so the baseline is 3; the description still adds meaning by tying filing_status and household_income/household_size to the Table 5 columns and FPL brackets, and by flagging the 400% FPL boundary behavior. It does not explain how state-specific poverty lines alter the result beyond what the schema already says.

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 precise verb and resource: returns the cap on repaying excess APTC. It is immediately distinguishable from siblings (credit calculation, subsidy cliff, poverty level) because it names the exact quantity and the tax-time repayment context.

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 makes clear this is the tool for repayment-cap questions and explains the year split (2025 caps vs 2026+ no cap), but it never explicitly routes to or away from siblings like calculate_premium_tax_credit or get_federal_poverty_level, leaving the agent to infer the boundary.

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