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

Annotations already declare the mutation/safety profile (readOnly=false, destructive=false, idempotent=false, openWorld=false), so the bar is lower; the description still adds that the signed-in user is the implicit subject and that non-eligible (paid) events yield an event page rather than a registration. It does not mention behavior on repeat calls (notable given idempotentHint=false) or on already-registered/full events.

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 sentences, zero filler. The action and its scope come first, with the paid-event exclusion and the id source following immediately.

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 one-parameter mutation with no output schema, the description covers who is registered, which events qualify, where the id comes from, and roughly what the response carries. The only real gaps are success semantics (immediate confirmation vs. pending) and duplicate-registration behavior.

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?

With a single parameter at 0% schema description coverage, the description must compensate, and it does: eventId is an event id and its provenance is list_events. It adds no format or constraint guidance (length limits live only in the schema), so it is not a full 5.

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 (register), a specific actor (the signed-in person), and a scoped resource (a free upcoming event identified by id). It also draws the boundary against paid events, which is what separates it from the checkout/booking siblings.

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 says when to use it (free upcoming events) and when not (paid events, where the event page is where a ticket is bought), and it points to list_events as the source of the id. It stops short of naming the sibling tool an agent should call instead for paid events (e.g. create_checkout), leaving that to inference.

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