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

Annotations already declare readOnlyHint=true, so the safety profile is covered; the description adds non-obvious behavioral context by disclosing the authentication prerequisite and the session-opens-this-tool nuance. No return format or pagination detail is given, but the auth behavior is the important gap it fills.

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?

Front-loaded with the purpose in one sentence, then a compact ACCESS block. Every sentence carries useful information; the auth recovery instructions are slightly verbose but justified for a tool that requires setup.

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?

With no parameters, a read-only annotation, and no output schema, the definition covers the essentials: what is listed, the scope, and how to satisfy the auth requirement. It stops short of describing the returned phone-number shape, which is a minor gap absent an output schema.

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, so there is nothing for the description to clarify and the baseline is 4. No parameter claims are made or needed.

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 ('List all phone numbers') plus the scope ('associated with the authenticated account'). It is clearly distinguishable from list_emails and list_domains by resource, though it does not explicitly name those siblings.

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?

Provides a concrete precondition (needs a Proof account) and a recovery path (authenticate the client, or call start_login and retry), naming a specific sibling tool. It does not discuss when this tool is preferred over adjacent listing tools, so it stops short of full when/when-not coverage.

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