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

Annotations already establish a safe, read-only, idempotent, closed-world call, so the bar is lower. The description adds genuine behavioral context beyond them: the statutory basis, the threshold rules driving the result, and what is returned (start date, reasons, GOV.UK sources, matching Landlord Lookout page).

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 sentences, front-loaded with the core purpose before the input requirements. The second sentence is dense but every clause (income types, per-year figures, share of jointly owned property) carries real instruction, with no 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?

For a single read-only check with no output schema, the description covers the eligibility logic, required inputs, and the shape of the response (start date, reasons, sources, link). Nothing needed to call it correctly is missing.

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% and the per-year fields already spell out the £50k/£30k/£20k thresholds and dates, so the schema does the heavy lifting. The description restates the qualifying-income definition and adds only the mild instruction to include each tax year 'you know', 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?

States a precise verb and resource ('Says whether a UK sole trader or landlord must use Making Tax Digital for Income Tax, from when, and why') and anchors it to a legal test (SI 2026/336). It is clearly distinguishable from the software- and deadline-oriented siblings.

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 tells the agent exactly what to supply ('Give the person's income types and, for each tax year you know, their qualifying income') and how to compute qualifying income, which is clear usage context. It does not explicitly state when to prefer this over get_mtd_deadlines or find_mtd_software, but those siblings cover plainly different questions.

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