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

Annotations only declare readOnlyHint=true; the description adds substantive behavior beyond that — the full whenKey classification rule (day = Mo–Fr 08:00:00–17:59:59 Berlin and not a Baden-Württemberg statutory holiday, else night) plus the holiday-detection logic. That is real business behavior an agent cannot infer from structured fields.

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?

Front-loaded with WHAT/RULE/RETURNS/USE/NEXT labels, which makes the density navigable, and every rule sentence earns its place. It loses a point because the long RETURNS field list duplicates an output schema that already exists, which is redundant per the stated guideline.

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 timezone+holiday clock tool, the description covers the classification rule, the tool-routing, and the sibling distinction, while the output schema carries return shape. An agent has everything needed to call it and chain it into invoicing.

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%, so both parameters (at, locale) are already fully documented in the schema. The description adds essentially nothing about at/locale semantics, so the baseline 3 applies; its whenKey discussion concerns an output field, not inputs.

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+resource+precision: 'Europe/Berlin wall clock to the second + whenKey for prices.' It also explicitly distinguishes itself from a similarly-named sibling ('THIS www tool does NOT look up a place (no slug/plz). maps.ikeytz.com has a different geo clock tool'), so an agent can route without opening either schema.

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?

Gives an explicit when ('USE before invoicing'), a when-not ('does NOT look up a place'), and concrete next steps ('NEXT: get_invoice_line_items({situationKey, whenKey: now})'). The alternative clock tool is named, so the selection condition is unambiguous.

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