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

Annotations declare readOnlyHint=false with destructiveHint=false, and the description adds meaningful context beyond them: selecting a site can create an API key if none exists (a real write side effect), and it explicitly reassures that site content is not changed. That resolves the tension between 'not destructive' and 'not read-only'. It does not cover failure cases (invalid site_id) or permission requirements.

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?

Three compact sentences, each earning its place: scope of effect, API-key side effect, and the content-unchanged reassurance, with the primary purpose front-loaded. 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 session-state tool with no output schema and non-destructive annotations, the description covers purpose, scope, persistence, and side effects adequately. Minor gaps remain around behavior for an unknown/invalid site_id and whether the selection survives beyond the session boundary.

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 the single parameter already documents 'Numeric site id from list_sites', so the schema carries the parameter meaning. The description implies which site is targeted but adds no format, range, or validation detail beyond the schema — the baseline 3 for high coverage applies.

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 ('Sets the active site') and immediately scopes it ('for the following calls in this session'), which distinguishes it cleanly from siblings like list_sites and create_site. An agent can tell this is a session-state setter without opening the schema.

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 clear context for when this tool matters: it configures the site that subsequent site tools will read and change, and the schema points to list_sites as the id source. It does not explicitly name alternatives or state when-not-to-call, so it falls short of a 5.

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