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

Annotations already declare readOnlyHint=true, destructiveHint=false and openWorldHint=true, so safety is covered. The description adds the returned field set and the at-least-one-key constraint, but is silent on not-found behavior and on whether an email lookup can return multiple referrals — meaningful gaps for a lookup-by-key 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?

A single dense sentence front-loads the lookup keys and the cardinality constraint, then lists returned fields. The trailing 'GET /api/referral/single' is mild surplus REST trivia but costs little.

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 right thing by enumerating the returned fields, and it covers the empty-required-list ambiguity. What remains missing is error/not-found behavior and multi-match semantics, which for a three-optional-key lookup tool is a real but modest gap.

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 coverage is 100% and all three keys are documented in the schema, so the baseline would be 3. The description earns an extra point by stating that at least one of the keys is required — a constraint that is NOT expressed in the schema (required list is empty) and that an agent could otherwise miss.

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 ('One referral') and enumerates the fields returned (status, amount, referring member, notes, custom fields, dates), which lets an agent distinguish it from referralrock_list_referrals. It does not name the sibling alternatives explicitly, so it stops short of a 5.

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?

'at least one is required' gives a real usage constraint for the lookup keys, and the singular framing implies single-record retrieval. However, there is no explicit guidance on when to prefer this over referralrock_list_referrals or what to do if a lookup key matches nothing or multiple records.

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