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

Annotations already declare readOnly, idempotent, non-destructive, closed-world, so safety is covered. The description goes beyond that by defining the applicable model precisely (fixed-rate, level-payment), which tells the agent where results stop being valid — useful behavioral context. It still omits computational conventions such as rounding or how a 0% rate is handled.

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 loan model and the outputs, then a short usage clause. The output enumeration is dense but every element (payment, interest, total repaid, split, progress) maps to a distinct returned value, so it earns its space; it is slightly long for a single run-on sentence.

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 return-value burden and does so by listing each computed figure. Combined with 100% schema coverage on inputs, an agent has enough to call it correctly; only edge-case behavior (rate=0, rounding) is unaddressed.

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 adds meaning by explaining what the optional payments_made parameter yields ('how much has been paid and is still owed after a number of payments'), which is semantics not evident from the parameter's one-line schema description.

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 precise verb+resource set: monthly payment, total interest, total repaid, first-payment split, and optional progress. The scoping phrase 'fixed-rate installment loan (personal, auto, any level-payment loan)' also implicitly separates it from siblings like payoff_time or refinance_check.

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 two concrete triggering questions ('what will this loan cost', 'where did my payments go') and bounds the loan type it applies to. It stops short of naming an alternative sibling or stating when NOT to use it (e.g. variable-rate or interest-only loans), so it is clear context but not full routing guidance.

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