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

Annotations already mark the tool as destructive and idempotent. The description adds critical context: that any PayPal payment for the old total is dropped. This discloses a specific destructive consequence not apparent from the annotations alone, though it does not mention permission requirements or rate limits.

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: the first front-loads the core action, the second adds a key side effect. No wasted words; every sentence 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?

The description conveys the main purpose and a critical destructive side effect, but it omits important contextual details: it does not clarify that the session must be open, does not mention the replacement of all existing items, and does not address the required parameters given sparse schema documentation. For a destructive mutation tool, more guidance would be helpful.

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 only 33%, and the description provides no information about the parameters. It does not explain the session_id, the items structure, or the optional api_key. With such low coverage, the description should compensate but does not.

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 specific verb ('Replace') and resource ('items in an open checkout'), making it clearly distinct from create_checkout or get_checkout. It gives an example of when this might be used, but does not explicitly name a sibling tool to contrast with.

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 parenthetical 'e.g. the user changed their mind' implies a use case, but there is no explicit guidance on when to use this tool versus alternatives like create_checkout or cancel_checkout. The reader must infer that this is for modifying an existing open session.

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