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

Annotations already declare destructiveHint=true and idempotentHint=true; the description adds context the annotations cannot, namely that the link and already-sent invitations survive the change and that guests with the old time in their calendars must be notified. It does not explain why the operation is flagged destructive (e.g. whether prior times are overwritten or history lost), which is the one gap.

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 sentences, front-loaded with what changes, then why to prefer it, then what is required. The final sentence about telling guests to update calendars is advisory rather than operational but is short and genuinely useful context.

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?

An output schema exists, so return values need not be explained, and all parameters are documented. The one omission is that no parameter is required, yet the tool only makes sense when at least one of title or start_time is supplied, and the description does not say that. Otherwise complete for a five-parameter mutation.

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 the schema already documents every parameter, including the ISO 8601 format, the 60-day limit and the organiser_key/organiser_link provenance. The description reinforces that room_code stays the same and where the organiser credentials come from, but adds little beyond the schema. 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 ('Changes') plus the exact mutable fields ('start time and/or title') and the resource, and ties the room back to create_watch_room via room_code. An agent can distinguish it from cancel_watch_party and create_watch_room without opening any 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?

Explicitly routes the agent away from the cancel-and-recreate pattern ('prefer this over cancelling and creating a new one') and states the prerequisite credential (organiser_key or organiser_link from create_watch_room). It stops short of stating what to do if the party has already started or which fields are mandatory, so no full when-not coverage.

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