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

readOnlyHint and openWorldHint already convey safety and external variability. The description adds valuable behavioral details: the return fields, including the null return when the DIČ is absent, and the fact that the unreliable date is only present when applicable. No contradictions with annotations.

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 two concise sentences, front-loaded with the primary purpose and followed by a crisp enumeration of return values. No redundant wording or 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?

Given no output schema, the description fully specifies the response structure: reliability status, subject type, name, address, bank accounts, unreliable date, and null case. This is comprehensive for a simple lookup tool and leaves little ambiguity about what the agent can expect.

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 describes both parameters (dic and ico) at 100% coverage. The description adds crucial semantic information by stating 'Provide either ico or dic', which establishes a mutual-exclusion requirement not encoded in the schema (both are optional in JSON Schema). This goes 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 uses a specific verb ('Check') and resource ('VAT-payer reliability for a single Czech subject'), clearly distinguishing it from the bulk sibling tool via 'single'. It also contains the full purpose, so it stands alone.

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 phrase 'for a single Czech subject' gives clear context for when to use this tool (single lookup). However, it does not explicitly mention alternatives like 'check_bulk_dph_payer' or state when not to use it, so it lacks explicit exclusion guidance.

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