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

Annotations already declare destructiveHint and idempotentHint, and the description adds valuable context: authentication requirement, timing constraint, and the scope of modifications. It does not contradict any annotation and provides beyond what the annotations alone would convey.

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 exactly two sentences, front-loaded with the primary action, and every phrase earns its place. It avoids redundancy and effectively communicates the tool's purpose and key constraints.

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?

Given the presence of an output schema, the description does not need to explain return values. It covers the core action, prerequisites (authentication), timing ('before payment is completed'), and update scope, making it complete for a checkout update tool.

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 schema descriptions are comprehensive (100% coverage), but the tool description enriches the 'updates' parameter with concrete examples (shipping method, promo code, customer details), giving semantic meaning beyond the generic schema description.

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 uses a specific verb 'Update' and resource 'existing checkout session,' clearly distinguishing it from siblings like checkout_create, checkout_cancel, or checkout_status. It also lists concrete capabilities (modify shipping method, apply promo codes, update customer details) and adds a temporal constraint ('before payment is completed').

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 provides clear context for when to use the tool: 'before payment is completed' and requires authentication. It does not explicitly name alternatives or state when not to use it, but the context strongly implies it is for updating existing sessions rather than creating or canceling.

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