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

Annotations already declare readOnly=false, destructive=false, openWorld=false, idempotent=false, so the safety profile is partly covered; the description adds real value by disclosing the return fields (member id, points balance, tier) and the repeat-enrolment behavior. There is a mild tension with idempotentHint=false, since "returns existing data in that case" reads as repeat-safe, but it is not a flat contradiction of the read/write nature of the tool.

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 tight sentences, front-loaded with purpose, then return values, then safety and prerequisites. The final sentence largely duplicates what the schema already states, which is the only wasted content.

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 correctly compensates by naming the returned fields. It covers required inputs, repeat-call behavior, and the return shape; only error cases and permission details are absent, which is minor for this operation.

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 the schema already documents all four parameters, including the email-or-phone requirement and E.164 formatting. The description only restates the session and email/phone requirement without adding format or validation detail, so the 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?

The opening sentence gives a precise verb and resource: "Enrol the authenticated customer in the restaurant loyalty programme." This clearly separates it from the read-side sibling check_loyalty, though that sibling is never named explicitly, so the differentiation is inferred rather than stated.

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?

It states prerequisites ("Requires an active agent_session_id and the customer email or phone") and gives a useful when-to-call condition ("Safe to call even if the customer is already enrolled"), which tells the agent not to gate on a prior check_loyalty call. No explicit alternative or when-not-to-use guidance is given, so it falls short of a 5.

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