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

Annotations already declare readOnly, idempotent, non-destructive, and closed-world, so the safety profile is covered. The description adds value beyond them by disclosing the 'Works without signing in' auth condition, which an agent needs to know before choosing this tool for an unauthenticated flow. It stops short of describing pagination or caching, but neither is relevant for a fixed four-period result.

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?

Three short sentences with zero padding: what it returns first, then when to use it, then the exclusion. The structure is front-loaded and the labeled 'Use this when' / 'Not for' pattern is easy to scan.

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?

There is no output schema, so the description carries the return-value burden and does so by enumerating periods, deadlines, final declaration date, and days-until-next-deadline. Combined with the single optional parameter and fully covered annotations, an agent has everything it needs to call this correctly.

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% and the single parameter is fully documented in the schema, including the example format "2026-27" and the default of the current tax year. The description only says 'for a tax year' without adding format or default detail, 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 specific verb ('Returns') and a precisely scoped resource: the four MTD for Income Tax quarterly update periods for a tax year, plus deadlines, final declaration date, and days remaining. The parenthetical '(standard and calendar period ends)' distinguishes the two period conventions, so the agent knows exactly what comes back.

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?

Has an explicit 'Use this when' clause covering both the deadline question and the 'what a Quarter covers' question, and a 'Not for' clause that names the correct sibling (whos_outstanding) for the adjacent record-tracking question. This is the strongest possible routing guidance.

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