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

Annotations already cover the read-only/idempotent safety profile, so the bar is lower, yet the description adds substantial domain behavior: the 6th-to-5th month rule, the 19th non-electronic due date, the 22nd electronic date moving back to the prior banking day on weekends or England-and-Wales bank holidays, and the limitation that bank holidays are only known up to the bank_holidays_known_until date. That is exactly the kind of edge-case context an agent cannot infer.

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?

A single dense sentence, front-loaded with 'For a date, gives the CIS tax month...', then building outward through each derived date. Despite its length, every clause carries distinct information (month boundary, year, two due dates, holiday-shift rule, known-holiday limit) with no padding.

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 burden of explaining return values and does so fully — four named outputs plus the bank-holiday shift caveat. An agent has everything needed to call the tool and interpret the result.

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?

There is a single parameter at 100% schema description coverage, so the schema already documents the 'date' field including its YYYY-MM-DD format and example values. The description restates that a date is supplied without adding format or constraint detail beyond the schema, which is the expected baseline 3.

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?

Names a specific verb and resource ('for a date, gives the CIS tax month it falls in, the tax year, the CIS300 due date, the electronic payment date') and enumerates the exact outputs. The scope is unmistakable and clearly differs from siblings like cis_late_penalties or cis_deduction.

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 use case evident — supply a date (payment date or today) to obtain CIS period and deadline information — which is clear context for a pure calculation tool. It does not, however, explicitly state when to prefer this over sibling tools such as cis_late_penalties, so it falls short of full when/when-not guidance.

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