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

Annotations already declare readOnlyHint=true, idempotentHint=true, and openWorldHint=false, so the safety profile is fully covered and the bar is lower. The description adds the fixed-term income framing and 'level or rising yearly' growth behavior, but does not disclose return format, whether payments are real or nominal, or any other behavior beyond annotations and schema.

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?

A single tightly-packed sentence front-loads the core purpose and mode distinction, followed by a web page reference. It is efficient with little waste, though the URL adds marginal value for an agent that already has the schema.

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 an 8-parameter calculator with 100% schema coverage, strong read-only annotations, and no output schema, the description supplies the two-mode mental model and the income scenario an agent needs to route correctly. It is largely complete; only explicit sibling differentiation 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 description coverage is 100%, and every parameter (mode, rate, years, amount, growth, timing, currency, frequency) is documented in the schema itself. The description echoes concepts already in the schema ('lump sum', 'target payment', 'level or rising yearly') without adding format or constraint detail, so the baseline of 3 applies.

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 (fixed-term annuity income) and clearly articulates both operating modes — 'what a lump sum pays monthly' (payout) versus 'the pot needed for a target payment' (needed) — which map directly to the mode enum. An agent can distinguish this from siblings like retirement_calculator or 401k_calculator without opening 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?

The description implies usage through its scenario framing (fixed-term income over 10–30 years, level or rising payments), which gives useful context. However, it never states when to choose this tool over similar siblings such as retirement_calculator or savings_goal_calculator, and offers no explicit exclusions or prerequisites.

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