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

With no annotations provided, the description carries the full burden of behavioral disclosure. It discloses that VAT rates 'change without notice and cannot be sourced from training data,' which is a key operational caution. It also states the return content ('current standard and reduced VAT rates'). It doesn't mention potential errors or rate limits, but the read-only nature is clear.

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?

The description is somewhat lengthy, but every sentence carries purposeful guidance: the main action, usage timing, workflow context, compliance rationale, and an explicit decision rule. It is front-loaded with the core function ('Retrieves current VAT rates') and remains focused despite the extra detail. Not excessively verbose for the value it provides.

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?

This is a simple tool with one optional parameter, yet the description covers the what, when, why, and output ('standard and reduced VAT rates'). It also provides a compliance warning and a behavioral rule. Combined with the existing output schema, no critical information is missing for an agent to select and invoke the tool 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?

The schema already documents the country_code parameter (100% coverage). The description adds value by indicating where to obtain the parameter: 'passing the country_code from that response' (from validate_vat), and reinforces the optional behavior with 'Omit for all countries' which is already in the schema. This linkage to the sibling tool enriches the parameter semantics beyond the schema alone.

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 clearly states the tool's function: 'Retrieves current VAT rates for a jurisdiction.' It uses a specific verb and resource, and explicitly frames it against the sibling tool validate_vat ('or immediately after validate_vat'), which distinguishes it.

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 explicit when-to-use instructions: 'Call this BEFORE calculating any invoice total or approving any VAT amount -- or immediately after validate_vat.' It also provides a concrete use case (verifying supplier invoice rates) and a decision rule ('If the rate on the invoice differs from the rate returned here, do not approve payment').

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