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

Annotations already establish readOnly/idempotent/non-destructive/closed-world, so safety needs no restating; the description instead adds real behavioral context: which 2026 IRS limits are applied ($24,500 employee, catch-up at 50+ and 60–63, $72,000 overall, $360,000 pay cap) and that a two-tier formula is supported. It doesn't say what happens when contribution exceeds the cap or how unclaimed-match is derived, so not a 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?

Trigger conditions are front-loaded and every clause carries information, but the second sentence is a verbless fragment listing outputs and the first is long; tightening and adding the missing verb would improve readability without losing content.

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 9-parameter, no-output-schema tool this covers the essentials: triggers, applied limits, formula shape, defaults, and the returned figures. A brief note on cap/limit edge cases (contribution above the $24,500 deferral limit) would close the remaining gap.

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 earns above that by explaining the tiered formula ('100% up to 3% then 50% of the next 2%'), which is the only way an agent will correctly map match_percent/match_up_to_percent/tier2_* to a real plan, and by tying age to catch-up limits and years/annual_return to projection.

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?

Names a specific verb+resource (calculate the 401(k) employer match) and enumerates concrete user phrasings it answers, which cleanly separates it from siblings like calculate_compound_interest or calculate_savings_goal. An agent can distinguish its job 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?

Gives three explicit trigger questions ('am I getting my full match?', 'how much is my employer match worth?', 'what percent should I put in?'), so when-to-use is unambiguous. It stops short of naming alternatives or exclusion conditions (e.g., don't use for pure growth projections — use calculate_compound_interest), so it falls short of a 5.

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