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

Annotations already declare readOnly/idempotent/non-destructive, and the description adds real behavioral context beyond them: the snapshot-not-history semantics for guests who change their mind, the specific error code for wrong message types, and the reassurance that zero counts with an empty list is a normal answer rather than a failure.

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?

Front-loads the purpose, then layers the error condition, the return shape, and the empty-result caveat in a logical order. Mild redundancy in describing `counts` and `responses` fields when an output schema exists, which costs some words.

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?

Covers everything an agent needs: applicability constraint, error identifier, return semantics including the snapshot caveat, and the empty-result non-error case. Any remaining field-level detail is handled by the output schema.

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% and both parameters are documented in the schema with formats and defaults, so the description does not need to explain them. It adds no meaning about `message_id` or `number_alias` beyond what the schema already says; baseline 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 (see how guests answered a calendar event) and enumerates the answer space. It is clearly distinguishable from neighboring read tools like get_message_reactions and get_message_receipts, since it is scoped to calendar events.

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?

Explicitly restricts applicability: 'Only works on a calendar event' with the exact error returned otherwise, and points to `list_events` plus the `message.calendar_responded` event for live tracking. It stops short of naming which sibling to use for non-calendar messages, leaving that inference to the agent.

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