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

The annotations already mark the tool as readOnly, idempotent, and non-destructive, so the description only needs to add behavioral context, which it does thoroughly. It discloses each plan's rate and threshold, the optional Postgraduate Loan stacking behavior, that yearly and monthly repayments are returned, that amounts are in GBP, and the source URL.

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 dense but every sentence earns its place: purpose, plan breakdown, optional stacking, output format, currency, and source. It front-loads the main purpose and keeps all necessary qualifying details in a compact form without filler.

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's multi-plan complexity, the description covers everything an agent needs to invoke it correctly: all plan identifiers, thresholds, percentages, the optional PG combination, and the output type. The presence of an output schema means return-value structure does not need to be explained here, and the description even provides the authoritative source.

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?

Although the schema covers all parameters, the description adds substantial meaning beyond the schema's terse definitions. Most importantly, it expands the plan enum values with exact rates and thresholds, and it clarifies how addPostgraduate interacts with undergraduate plans, which the schema only hints at.

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 opens with a specific verb and resource: 'Work out UK student loan repayments from an annual salary using 2026/27 thresholds.' This clearly distinguishes it from the generic hopi_loan_calculator and other finance calculators by naming the exact domain, input, and calculation year.

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 makes the intended context clear — use this tool whenever UK student loan repayments need to be calculated from an annual salary. It does not explicitly name alternative tools or state when not to use it, but the specificity of 'UK student loan repayments' leaves little ambiguity about appropriate use.

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