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

Annotations already declare the safety profile (not read-only, not destructive, not idempotent), so the bar is lower, and the description still adds real behavioral context: the accepted character set and the fact that failures surface as 'the app's own sentence' when a code is taken or disallowed. It omits auth/permission requirements, which keeps it from a 5.

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 tight sentences that front-load the action and follow with format and failure behavior; nothing is padded. The phrasing 'the app's own sentence' is slightly opaque but compact.

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 annotations and no output schema, the description covers purpose, accepted input format, and error behavior adequately. Only auth/prerequisite context 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?

Schema description coverage is 0% for the single 'code' parameter, so the description must compensate, and it does by specifying the allowed character set (letters, digits, dashes) that the schema's minLength/maxLength constraints do not convey. It does not cover case sensitivity or leading-character rules, so it is not exhaustive.

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 ('Set or change the person's own referral code') and clarifies scope with 'own', which implicitly separates it from the sibling claim_referral_code. It stops short of naming that sibling, so the differentiation relies on inference.

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 word 'own' implies this is for setting your personal code rather than claiming someone else's, which is the key distinction from claim_referral_code, but no alternative is named and no when/when-not conditions are stated explicitly. Usage is implied rather than guided.

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