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

Goes well beyond the annotations: it explicitly warns the call is not read-only and discloses the concrete side effects — lazy provisioning of the referral code and enrolment of a Freemius affiliate plus coupon on the first eligible call. This reconciles neatly with readOnlyHint=false, idempotentHint=true (effects fire only once) and openWorldHint=true (external Freemius service), giving the agent real cost/consent context.

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, both earning their place: the first front-loads the returned fields, the second front-loads the side-effect warning with 'Not read-only'. No filler, no restatement of the tool name or 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?

With no output schema and no parameters, the description carries the burden of describing the return value and does so by naming every field. It also covers the mutation behaviour an agent must know about before invoking it, so nothing material is missing.

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?

Zero parameters and 100% schema coverage, so there is nothing to document; the baseline of 4 applies. The description correctly spends no space restating an empty input schema.

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?

A specific verb-plus-resource with the exact return payload enumerated: referral code, registration URL, friends coupon code, and three named stats. No sibling tool in the list touches referrals, so there is no ambiguity to resolve, and the agent knows precisely what it gets back before calling.

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?

Usage is only implied — an agent can infer this is the tool for surfacing a user's referral/friends programme info, and there are no alternatives to disambiguate against. It does not state prerequisites (e.g. whether the user must be logged in, or what makes a call 'eligible'), which is the only real guidance gap.

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