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

Annotations only declare readOnlyHint=true and openWorldHint=false; the description adds the crucial non-obvious trait that the server does not retain the selection between calls, which directly affects how an agent must call it. It also notes the template-clearing side effect. It stops short of 5 because it doesn't explain what the 'active site' affects downstream or what the call returns.

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 short sentences, front-loaded with the primary action and followed by the critical calling constraint. Every clause earns its place with no filler.

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 tool with no output schema and an annotation-covered safety profile, the description covers the essential behavior and the persistence caveat. The remaining gap is what 'active site' actually influences for subsequent calls, which is left implicit.

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 100% for the single `id` parameter, so the schema already carries the semantics and baseline 3 applies. The description only reinforces that `id` must be supplied on every call, adding no format or value-level detail beyond the 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?

States a specific verb+resource ('Set the active site for this session') and adds scope ('session') plus a side effect ('clear any active template'), which implicitly separates it from select_template. It does not explicitly name a sibling for routing, so it falls just short of the top of the range.

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 gives one concrete usage rule ('still pass `id` on every call') because state is not persisted, which is genuinely useful. However, it never states when to use this versus select_template or other session-affecting tools, and gives no exclusions, so guidance is implied rather than explicit.

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