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

Annotations already declare readOnlyHint=true, idempotentHint=true and openWorldHint=false, so the safety profile is fully covered. The description adds useful scope context (which loan plans are modeled, that interest and write-off dates are computed, and the 2026/27 vintage), but says nothing about output shape or units beyond what the schema notes. A 3 is appropriate given the annotations carry the behavioral load.

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?

Two tight sentences, front-loaded with the purpose and output list. The trailing web-page URL is minor promotional filler but does not materially bloat the text.

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?

For a single-purpose, zero-required-parameter calculator with full schema coverage and no output schema, the description tells the agent enough to call it correctly. It is complete except for the absence of explicit routing guidance against lookalike loan tools.

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 every parameter (plan, growth, salary, balance, yearsRepaying) is already documented in the schema, including the enum values and the note that growth keeps thresholds at 2026/27 levels. The description adds no parameter syntax or format detail beyond this, 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 gives a specific verb+resource ("UK student loan repayment calculator") and enumerates what it computes: monthly repayments, interest, years to repay and write-off across Plans 1, 2, 4, 5 and Postgraduate. That scope clearly separates it from generic siblings like loan_calculator or simple_interest_calculator, though it never names an alternative explicitly.

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?

There is no when-to-use or when-not-to-use guidance and no mention of alternatives (e.g. loan_calculator for non-UK/non-student loans). Usage is only implied by the domain, so an agent must infer the trigger conditions itself.

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