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

Annotations already declare readOnlyHint=true and destructiveHint=false, so the description does not need to restate safety. It adds useful behavioral detail: the scope is the signed-in user for exactly one event, and the response summarizes multiple aspects of membership. The 'cheaper' note adds performance/cost context beyond what annotations provide.

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?

The description is two purposeful sentences with no filler. The first sentence front-loads the core action and result contents; the second justifies when to use this tool. Every clause earns its place, and the structure makes it easy for an agent to scan.

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?

With no output schema, the description does the necessary work of explaining what the tool returns: joined status, registration answers, free-agent status, and roles. It also includes the key selection context and cost trade-off. It could name the sibling alternatives explicitly, but for a single-parameter read-only tool it is substantially 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?

The only parameter, event_slug, is already fully documented in the schema with 'Slug of the event.' Schema description coverage is 100%, so the description's mention of 'one event' adds little beyond the schema. Baseline 3 is appropriate because the schema carries the parameter documentation burden.

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?

The description uses a specific verb ('Summarise') and a clear resource ('the signed-in user's relationship to one event'). It enumerates the exact facets covered: joined status, registration answers, free-agent listing, and roles. It also distinguishes itself from sibling tools by framing itself as a cheaper aggregate than combining several other tools.

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?

The description gives an explicit use case: 'when you just need "is this user in, and how" for one event.' It also provides comparative guidance by saying it is cheaper than combining several other tools. It does not name those alternatives explicitly, but the context is clear enough for an agent to select appropriately.

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