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

Annotations already declare readOnlyHint=true, idempotentHint=true, etc. The description adds value by listing the exact profile fields returned (id, display name, view count, etc.) and showing the expected input structure with an example. This helps the agent understand what to expect beyond the annotations, though it does not discuss rate limits or authentication details already implied by the _apiKey parameter.

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?

The description is two sentences plus a useful example. It is front-loaded with the purpose, immediately tells what the tool does, and the example shows concrete usage. Every sentence is necessary and there is no fluff.

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 simple lookup tool with high schema coverage and no output schema, the description covers the key inputs and outputs. It lists the return fields and provides an example. Minor missing details: error handling if user not found, or clarification that the lookup is by exact login only. Overall, sufficient for an agent to use the tool correctly in most scenarios.

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?

Input schema coverage is 100% with descriptions for both login and _apiKey. The description's mention of 'login (username)' and the example adds minor context but does not provide significant new semantics beyond what is already in the schema per-property descriptions. With full coverage, the baseline is 3.

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 description clearly states the tool looks up a Twitch user by login (username) and lists the profile fields returned. This distinguishes it from sibling tools like search_channels which likely do fuzzy searches. The verb 'Look up' and the resource 'Twitch user/channel' are specific, and the example reinforces the exact match intent.

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 description does not explicitly state when to use this tool versus alternatives like search_channels or entity_profile. It implies exact lookup by login, but provides no guidance on when to choose different tools or what preconditions exist (e.g., must have known username). This is adequate but lacks proactive direction.

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