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

Annotations already declare readOnlyHint=true and openWorldHint=false, so safety is covered. The description usefully lists the return payload (including device city and usual hour) which annotations cannot convey, but it says nothing about not-found behavior, lookback windows for "recent events", or result size limits.

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 sentence that front-loads the resource and then enumerates return fields, with no filler. The identifier parenthetical is slightly dense but each clause carries distinct information.

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 two-parameter read tool with no output schema, the description does the important work: it defines accepted identifier types and sketches the response shape. Remaining gaps (empty-result behavior, recency definition) are minor rather than call-blocking.

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 50% — externalId has no schema description at all. The description compensates well by explaining that externalId accepts your user id, a Superwall user id, a Superwall alias id, or a RevenueCat id, which is meaning an agent could not get from the schema. It leaves appId semantics entirely to the schema, which already documents it.

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?

"One user by your user id" states a specific verb+resource and enumerates the payload returned (tags, devices, recent events, tapped topics). The singular framing implicitly separates it from list_subscribers, but no sibling is named 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?

Usage is only implied: fetch a single user profile when you have one identifier. There is no statement of when to prefer it over list_subscribers or get_account, and no prerequisites or failure conditions are given, though the parenthetical does clarify which identifier flavors are accepted.

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