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

Annotations already declare readOnlyHint=false, idempotentHint=false and openWorldHint=true, so the agent knows this is a non-idempotent external write. The description adds an anti-fabrication guard on the email source, which is real value beyond the annotations, but it says nothing about whether guests are emailed/notified, how duplicate guests are handled, or whether existing guests are preserved.

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?

Three short sentences, front-loaded with the action, followed by the safety constraint. Every sentence carries a distinct instruction and there is no padding.

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?

For a two-parameter mutation with no output schema, the description covers the action and the main agent failure mode (inventing emails). The remaining gap — no distinction from `add_booking_attendee` and no note on guest notification side effects — is notable but secondary given openWorldHint already signals external effects.

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 50%: `bookingUid` is documented in the schema and the nested `language` field is described, while `name`, `timeZone` and `phoneNumber` carry no explanation anywhere. The description adds meaning for the critical `email` field (must come from the user, not be invented), which partially compensates for the gap but leaves most guest sub-fields unexplained.

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?

States a specific verb and resource ('Add guests to a booking'), so the operation is immediately clear. However, the sibling tool `add_booking_attendee` is nearly synonymous and the description does nothing to distinguish the two, leaving the agent to guess which one applies.

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 gives a concrete operating constraint ('Ask the user — never choose for them. Each guest's email comes from the user'), which tells the agent not to fabricate emails. But it offers no when-to-use guidance versus `add_booking_attendee` or `list_booking_attendees`, and no prerequisites such as needing an existing booking UID.

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