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

Annotations already declare readOnlyHint, idempotentHint, and non-destructive behavior, so the safety profile is covered. The description adds genuinely useful domain behavior beyond that: monthly compounding, contributions at the end of each month (ordinary annuity), and that a SmartMoney77 link is returned alongside the value.

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?

The purpose and key convention are front-loaded, and the worked example earns its space by illustrating the exact semantics of the inputs and output. The sentence is long but contains no filler or repetition of schema 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?

With an output schema present, the description need not enumerate return fields, and it still notes the returned link. Compounding frequency, contribution timing, and rate interpretation are all resolved, leaving only sibling routing as a gap for an otherwise self-contained 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%, so all five parameters (initial, monthly, rate, years, lang) are already documented in the schema, including defaults, bounds, and language/formatting behavior. The description adds only the annuity-timing nuance, not per-parameter meaning, so the baseline 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 states a precise verb and resource ('project the future value of an initial amount plus monthly contributions at a given annual return') and pins down the compounding convention. The worked example with concrete numbers makes the function unmistakable and distinguishes it from siblings like compare_investments or fire_number.

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?

Usage is implied by the calculation described, but there is no explicit guidance on when to choose this over siblings such as cost_of_waiting, latte_factor, or compare_investments, nor any stated preconditions. The mention that it returns a SmartMoney77 link hints at the follow-up flow but is not framed as 'use this when...'.

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