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

Annotations already flag destructive and idempotent, and the description adds substantive behavior beyond them: the countdown page and link stop working, an already-open room continues until everyone leaves, the organiser credential is required for authorization, and the action is irreversible. That is exactly the context an agent cannot infer from hints alone.

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 short sentences, front-loaded with the core action and constraining facts (irreversibility, prerequisites) last but still tight. No filler or repetition of the title.

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 destructive, low-parameter mutation with an output schema available, the description covers scope, authorization, edge-case behavior for open rooms, and irreversibility. Nothing material is left for the agent to guess.

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 baseline is 3, but the description adds provenance the schema lacks: the organiser_key/organiser_link must be the values returned by create_watch_room, and they function as the authorization mechanism rather than mere identifiers.

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 ('Cancels a watch party') and immediately ties it to the sibling that creates it ('a room made with create_watch_room, same room_code'), so the agent can distinguish it from create_watch_room and update_watch_party without opening schemas.

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 the condition for use (canceling a scheduled party before it opens) and an explicit safety instruction ('confirm with the user first'), plus the authorization prerequisite. It does not name update_watch_party as the alternative for changing rather than cancelling a party, which is the one missing routing cue.

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