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

Annotations already indicate read-only and non-destructive, so the bar is lower. The description adds valuable context: it states the tool is illustrative, will 'state every omission (taxes, penalties, rate changes, tiers, availability),' and includes defaults for the APY values. This goes beyond the annotations by explaining the nature of the output and its limitations, though it does not detail the exact return format or edge-case behavior.

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 description is two sentences long and immediately states the core purpose in the first sentence. The second sentence packs in defaults, omissions, and a disclaimer without redundancy. Every sentence contributes essential information, making it highly concise and front-loaded.

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?

Given the tool is a simple calculation with no output schema, the description is fairly complete. It explains what is computed, the default rates, and that all omissions will be stated. It lacks an explicit mention of the output format (e.g., dollar amount vs. percentage), but given the name and the context of an 'earnings difference,' this is inferable. The absence of return-value docs is acceptable because the tool's purpose is clear.

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?

The input schema already covers all parameters with descriptions, including defaults for current_apy_percent and comparison_apy_percent. The tool description mentions the defaults again but adds no new parameter-level semantics beyond what the schema provides. Baseline of 3 is appropriate because schema coverage is 100%.

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 clearly states the tool computes an 'illustrative annualized earnings difference between two savings APYs for a given balance and horizon,' which is a specific calculation task. It distinguishes from sibling tools (get_cd_rates, get_mortgage_rates, get_savings_rates) by focusing on comparing APYs rather than simply fetching rates. The title and name align, and the description adds value by specifying defaults and the illustrative nature.

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 the use case: when you have a balance and horizon and want to compare two savings APYs. It includes an explicit exclusion: 'never use the result to recommend an investment allocation' and notes 'not financial advice.' However, it does not name alternative tools or explicitly contrast with sibling rate-lookup tools, so it stops short of a full 5.

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