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

Annotations already provide readOnlyHint=true and openWorldHint=true, so the agent knows this is a safe read against live external data. The description adds the content shape (stock plus estimated-delivery status) but says nothing about freshness, destination scoping behavior, or limits — some added value, but not rich.

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?

Two sentences, zero filler, with the purpose and required inputs front-loaded and the sibling routing placed second. Every clause 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?

With an output schema present, the description needn't explain return values, and it correctly covers the what/where inputs and sibling boundaries. Minor gaps remain around availability semantics (freshness, regional scope) but they are not essential for correct invocation.

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 100% with all three parameters described in the schema, so the baseline is 3. The description only reinforces that postal_code and state define the 'destination' and id identifies the product, adding marginal meaning beyond the schema's own descriptions.

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?

Specific verb (Fetch) plus resource (Costco availability record) with the payload named ('stock and estimated-delivery status') and the required inputs ('product `id` at a destination, using `postal_code` and `state`'). It also names sibling tools, so an agent can tell it apart from costco_product and costco_search without opening a 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?

Explicitly routes the agent to alternatives: 'Use costco_search to find products or costco_product for item details,' which establishes when this tool is the right pick versus lookups. It stops short of stating when-not conditions (e.g. product unavailable or non-US destinations), so it is clear context rather than full exclusions.

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