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

The annotations already declare readOnlyHint=true, so the agent knows it's safe. The description adds a valuable behavioral nuance: 'Never send variant_id as a number,' which prevents a common type error. It doesn't cover rate limits or failure modes, but with annotations present, this is sufficient.

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?

The description is two sentences, front-loaded with the trigger condition, and every sentence adds value. The example is compact and informative. No fluff.

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?

The tool has 8 parameters, but only 3 are required and the description covers those well, including a type warning. It does not detail optional parameters or return value, but the name and context make the output obvious (a shipping estimate). Given the moderate complexity and absence of an output schema, this is reasonably complete but not exhaustive, so a 4.

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 0%, so the description must compensate. It explains the key parameters: destination_postal_code, country (with US|CA), items, and the critical variant_id-as-string rule with an example. However, it does not describe optional parameters like journey_id, selected_sku, or match_type, leaving those undocumented. This is a partial compensation, earning a 3.

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?

The description states a clear, specific use case: 'Use when the buyer asks shipping cost for selected catalog variants.' This includes a verb ('get shipping estimate'), a resource ('shipping cost'), and a scope ('selected catalog variants'), which distinguishes it from sibling tools like get_pricing.

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?

It explicitly states when to use ('Use when the buyer asks shipping cost...') and enumerates the required arguments. It lacks explicit when-not-to-use or alternatives, but the trigger condition is clear and the context is well defined, earning a 4 rather than a 5.

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