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

Annotations already cover safety (readOnly, idempotent, non-destructive), and the description adds substantial non-obvious context: the backend does not flag which candidate currency is the country's own, the agent must infer it via world knowledge, and the choice must be remembered and reused for the rest of the conversation. That is real behavioral disclosure beyond the 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?

Front-loaded with the core purpose, then usage, then the tricky currency-selection nuance. The parenthetical examples (JP→JPY, CH→CHF, GB→GBP) are illustrative but slightly verbose; overall it is dense and every section earns its place.

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?

Although an output schema exists, the description adds interpretation guidance the schema cannot: the candidate currency list is a broad superset, and the agent must pick the country's own currency and reuse it. Nothing needed to call or act on this tool correctly is missing.

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. The description does useful work by explaining what the (param-less) call returns and how to interpret the candidate currency set, though strictly there are no parameters whose semantics need clarifying.

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 precise verb and resource ('Fetch the user's marketplace region from their profile') and explicitly distinguishes the region from other region-related operations by naming the siblings set_user_region and list_available_regions. An agent can immediately tell this apart from its siblings.

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 trigger ('Call this before questioning'), names the alternatives for the mutation path (list_available_regions, set_user_region), and states the exclusion condition ('only on the user's explicit request, never on your own initiative'). It also prescribes downstream behavior (pass the currency as search_offers's price_currency).

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