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

Annotations already declare readOnly/idempotent/non-destructive, so the safety profile is covered. The description adds value beyond them by stating the result is an estimate rather than a charge or live quote, and by clarifying the token-unit semantics that would otherwise be misread.

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 tight sentences, zero filler, with the core purpose front-loaded and the unit caveat and negative scoping following in priority order.

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?

No output schema exists, so the description should ideally state the return shape (currency, whether it itemizes input/output cost). Combined with an unexplained locale parameter and no pointer to where valid priceIds come from, the picture is adequate but not complete.

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 coverage is 0%, so the description carries the burden. It resolves the most dangerous ambiguity by explaining inputMillion/outputMillion are millions of tokens, not token counts, but it says nothing about locale (which the schema enum ties to shipped locales) or the priceId pattern/'listed' constraint.

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 (Estimate) and resource (USD cost) and bounds the scope to ONE listed priceId with its exact billing scenario. It doesn't name a sibling, but the wording 'one listed priceId' implicitly points at the price-reference listing rather than any of the sibling tools, so an agent can place it without ambiguity.

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?

Explicitly excludes three tempting misuses: live quote, free quota calculation, and charge. That is genuine when-not guidance. It stops short of naming the alternative source for valid priceIds (e.g. list_price_references) or stating prerequisites, so it is strong but not complete.

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