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

Annotations already indicate non-read-only (readOnlyHint false) and non-destructive (destructiveHint false). The description adds meaningful behavioral context: it 'binds the rate to the cart so the total includes shipping', explaining the side effect beyond just selecting. This clarifies the impact on cart state and the necessity of ordering with execute_checkout.

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: the first states the core purpose, the second adds behavioral and timing details. It is front-loaded with the action and entirely free of fluff. Every sentence adds value and is necessary for correct invocation.

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 description covers prerequisites (set_shipping_address called), the effect (rate binding, total update), and the appropriate time to call (before execute_checkout). Given the presence of an output schema and the tool's simplicity, this is nearly complete. One minor gap: it does not explain what happens if the option_id is invalid, but the error handling can be inferred from typical API behavior.

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 basic descriptions for each parameter. The description adds specific context for option_id by noting it comes from 'cart.shipping_options', which is a helpful cross-reference. However, cart_id and site_id receive no additional explanation beyond the schema. Since schema carries most of the burden, a score of 3 is appropriate.

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 clearly specifies the action: selecting one of the shipping options returned by set_shipping_address. It explicitly ties the tool to a specific context (shipping selection within checkout flow) and distinguishes it from siblings by referencing the sequential dependency on set_shipping_address and the precondition for execute_checkout.

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?

The description gives explicit usage context: call after set_shipping_address and before execute_checkout. It states the effect (binds rate, updates total). It does not explicitly list alternatives, but the sequential instruction is clear and helpful for an agent navigating the checkout flow.

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