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

Annotations only declare destructiveHint=true; the description goes well beyond that by disclosing that the message is delivered IMMEDIATELY, cannot be unsent, and is routed to the booking channel. This is exactly the irreversibility and side-effect context an agent needs before triggering an irreversible send.

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 tight sentences plus a REST endpoint reference; the irreversible-delivery warning is front-loaded where it matters. The endpoint citation is mildly redundant with the title and description but small and arguably useful for callers.

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 3-parameter send action with no output schema, the description covers the essentials: what is sent, where it goes, and that it is irreversible. It does not describe what the call returns, but a minimal confirmation response is a reasonable assumption and not a blocking gap.

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 100%, so reservation_id, message_body, and subject are already documented in the schema. The description only restates the reservation id via the endpoint path and does not add format or content guidance beyond what the schema provides, so the baseline of 3 applies.

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 (send a message to the guest of a reservation) and clarifies the delivery channel (Airbnb/Booking.com inbox or email), which distinguishes it from smoobu_send_message_to_host and from the read-only smoobu_list_reservation_messages. An agent can identify the correct tool without opening a schema.

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?

Gives a clear precondition for use — confirm the text with the user first because the message is delivered immediately and cannot be unsent. It does not explicitly contrast with sibling tools such as send_message_to_host or note when not to use it, but the operative guardrail is stated plainly.

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