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

Annotations already mark the tool read-only and idempotent, so the description does not need to cover safety. It adds useful behavioral context: days are distinct (deduplicated), counted per calendar year, and assessed under the generic 183-day rule, the US SPT weighted formula, and the UK tax-year test; the 'early-warning, not tax advice' qualifier also sets expectations about precision.

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 tightly written sentences; the first packs the core computation and the three tests, and the second adds an important scope disclaimer. Every phrase earns its place and the most important information is front-loaded.

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 read-only calculation tool with an output schema, the description covers the computation, the three tests, the per-calendar-year scope, and the advisory nature. The main gap is the lack of differentiation from overlapping sibling tools, which matters given the long sibling list.

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?

The schema documents referenceDate and the country code format, leaving the stays array only structurally defined. The description supplies the missing semantics: stays represent periods of presence and are counted as distinct days per country per calendar year, which clarifies how entry/exit pairs are interpreted. It does not discuss overlap handling or date parsing, but it adds meaningful value beyond the schema.

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 concrete verb ('Count') and a specific resource ('distinct days of presence per country per calendar year'), then names the exact tax tests evaluated. It is clear what the tool computes, but it never distinguishes itself from similarly named siblings like check_tax_residency_risk or check_travel_residency.

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?

No explicit when-to-use or alternative routing is given; the closest is the 'Early-warning day counter — not tax advice' boundary, which implies use for preliminary counts rather than official determinations. Since it neither names any alternative tool nor gives clear exclusion conditions beyond the disclaimer, guidance is only implied.

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