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

Annotations already declare readOnlyHint=true, destructiveHint=false, and openWorldHint=false, so the safety profile is covered. The description adds useful behavior context: it fires only when no number is available and it returns instructions plus a clickable download link, telling the agent what it gets back for a tool with no output schema.

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 return value and closes with the triggering condition. No filler, no restatement of the title.

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 zero-parameter, read-only setup helper with no output schema, the description tells the agent when it applies and what it returns, which is enough to invoke it correctly. Minor gaps remain around what happens if a number is already configured, but nothing critical is missing.

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?

The tool takes zero parameters and the empty schema already makes that unambiguous. With no parameters to document, the baseline is 4; the description correctly does not invent parameter semantics that don't exist.

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 gives a concrete verb+resource: returns first-setup instructions and an App Store download link. It doesn't name or differentiate itself from siblings like call or text, but the purpose is unmistakable for a tool named 'setup' with no parameters.

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?

It states a condition — 'when no number is available' — which implies the agent should call this before a number is configured. However, it doesn't say what to do instead once a number exists, or name the alternative tools (call/text) that become usable. 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.