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

Annotations already declare readOnlyHint, idempotentHint, openWorldHint=false and destructiveHint=false, so safety is covered. The description adds valuable non-structured context: the return payload (result rows in page language, raw engine outputs, sources with lastChecked dates, a disclaimer) and the rules vintage (2026, last updated 2026-09-29).

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?

Compact and front-loaded: purpose first, then rate/legal frame, then returns, then freshness. The trailing sentence about inputs marked 'Only used when' dangles — no such marker appears in this schema — so it costs a little clarity for no benefit.

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 usefully discloses what comes back and that results are page-language, source-attributed, and dated, which is enough for an agent to call a stateless calculator correctly. Missing only error/edge behavior (e.g., what happens at the 1e9 maximum or a zero net amount).

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 both enums (smer, stopa) are fully documented with defaults in the schema, so the baseline is 3. The description's VAT-direction and rate language merely mirrors what the schema already says, adding no new syntax, bounds, or edge-case meaning.

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 and resource ('Serbian VAT in both directions: add VAT to a net amount or extract VAT from a gross amount') plus the applicable rates and legal basis (Article 23). An agent can distinguish it immediately from the company-car, leave, and benefit calculators in the sibling set.

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 gives clear context for both operating modes and the schema elaborates on which direction applies to which input meaning. However, it names no alternatives or exclusions, and no sibling is a genuine substitute, so the routing guidance is implicit rather than explicit.

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