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

Annotations only supply destructiveHint=true and a title, so the description usefully adds the mutation warning ('MUTATES OwnerRez data'), the HTTP verb/endpoint, the account-specific linkage caveat, and the return behavior. It does not cover auth/permission needs or rate limits, but it adds real context beyond the annotation.

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?

Four tight clauses with the mutation warning front-loaded, then endpoint, then the linkage caveat, then return value. No filler, though the endpoint notation is arguably schema-adjacent detail rather than agent-critical guidance.

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?

With no output schema, the 'Returns the created message' line correctly covers the return. The mutation is flagged and the tricky nested `fields` escape hatch is explained, which is the main complexity here. Missing only auth/permission expectations for a write call.

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%, so the baseline is 3, but the description goes further by explaining why `fields` exists and giving concrete keys (thread_id, booking_id) that the schema only alludes to as 'additional documented OwnerRez fields'. That is genuine additive meaning, though it doesn't describe value formats or limits.

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 (posts) and resource (a message into a guest-communication thread) and also names the underlying endpoint POST /v2/messages. An agent can immediately distinguish this from create_booking, create_guest, send-type siblings, and the list_messages reader.

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?

Usage is implied rather than stated: the description explains that thread/booking linkage is account-specific and should be passed through `fields`, which tells the agent how to call it. It never says when to prefer this over list_messages or another write path, and gives no explicit prerequisites or exclusions.

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