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

Annotations already declare readOnlyHint=false, idempotentHint=true, and destructiveHint=false, so mutation-safety is covered. The description adds the reversibility note '(or undo it)', which reinforces the non-destructive profile, but says nothing about error behavior or what happens on a duplicate check-in. Modest added value over annotations.

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 short sentences, zero filler, with the core action front-loaded and the ID-source hint trailing as a useful pointer. Every clause earns its place.

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 simple mutation tool with full schema coverage and annotations carrying the safety profile, the description covers purpose, the undo variant, and the ID source. Only minor gaps remain (no note on duplicate/already-checked-in behavior), so it is essentially complete.

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 all four parameters including undo, apiKey, eventId, and registrationId are already documented. The description adds only the source of registrationId, which is a marginal supplement 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 and resource ('Check a registrant in at the door') and clearly distinguishes event check-in from the sibling register_for_event. The parenthetical '(or undo it)' also signals the reversible variant. It falls just short of 5 because it doesn't name a sibling explicitly to route the agent.

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 concrete context ('at the door') and, importantly, tells the agent where the registrationId comes from — the sibling list_event_registrations. It doesn't spell out when check-in is inappropriate (e.g., unregistered attendees) or alternative tools, so not a 5.

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