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

Annotations already declare readOnlyHint, idempotentHint and openWorldHint=false, so safety is covered. The description adds real behavioral context the annotations cannot: the calculation includes money already saved, growth is applied at a yearly rate, and a worked example ('$1,000,000 in 30 years at 8% needs about $671 a month') gives the agent a sanity check on output magnitude. Return shape and rounding are still unstated, 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?

Front-loaded with the usage trigger, then the computation, then a concrete example — a sensible order. The two user-phrasing examples and the numeric example overlap slightly, so one of them is arguably redundant, but nothing is padded or vague.

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 carries the burden of signalling the return value, and the '$671 a month' example does exactly that. Required vs optional inputs are handled by the schema, and the only notable omission (rounding/currency formatting) is minor for a deterministic calculator.

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 each parameter already carries a description and example, so the baseline of 3 applies. The description adds only light reinforcement — 'counting money already saved' maps to current_savings and 'yearly rate' to annual_rate — without new format, unit, or edge-case guidance (e.g. what a 0% rate or sub-year horizon yields).

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 specific verb and resource — the monthly amount to save to reach a target over N years at a given rate, including existing savings. That computation is clearly distinct from siblings like calculate_compound_interest or calculate_fire_number, so an agent can route correctly 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?

Opens with an explicit trigger ('Use when a user asks how much to save each month for a goal') and reinforces it with two natural-language user phrasings. It does not name a competing sibling or state when not to use it, which keeps it 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.