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

Annotations already declare readOnlyHint, idempotentHint, non-destructive and closed-world, so the safety profile needs no restating. The description adds genuinely new behavioral context: it requires OAuth authentication and a trial or paid plan, and it describes the shape of each returned item (a count plus a slug/code). Minor gap: nothing about result size or rate limits.

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?

Three sentences, front-loaded with the resource and its contents before the routing advice and the auth/plan caveat. Every clause carries information, though the middle sentence packs two extra facet families (buyer_types, kinds) densely enough that it reads slightly list-heavy.

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 parameters and no output schema, the description must carry the return-value burden, and it does — enumerating the facet families and stating each entry has a count and a slug/code. It also covers the auth/plan prerequisite. What "active tender set" means and whether the facets are user-scoped is left slightly ambiguous.

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 tool takes zero parameters, so the baseline is 4 and there is nothing the description could add or omit. The description correctly implies no input is needed, focusing entirely on what the call returns.

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?

Opens with a specific verb and resource ("List the browsable facets of the active tender set") and enumerates exactly what is returned: CPV categories, voivodeships, cities, buyer_types and kinds. It also states the payload usefulness (count plus the slug/code to pass to list_tenders), which distinguishes it clearly from siblings like cpv_tree or search_entities.

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?

"Call this to discover valid filter values" is an explicit when-to-use instruction, and the description names the downstream consumers (list_tenders, search_tenders) so the agent understands the discovery-then-filter workflow. It lacks an explicit when-not or a pointer to a competing sibling, which keeps it below a 5.

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