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

Annotations establish that this is a non-read-only, non-destructive, closed-world write, but the description does not confirm or elaborate: it never says the supplied number is stored/overwritten, whether an existing number is replaced, or what happens on the next inbound call hand-off. The one useful nugget — that this number is dialed when the agent has a caller on the line — is buried in a figure of speech.

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?

It is a single short sentence with no wasted words, which is good, but the brevity comes at the cost of clarity: the front-loaded content is atmospheric rather than informative, so the sentence does not earn its place as the primary specification.

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 write tool with no output schema, the description should at minimum state the mutation and its effect. Instead it leaves the core action and its consequences unstated, so an agent has to infer the operation from the parameter name alone.

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% and the single 'mobile' parameter already documents the E.164 / 10-digit US format, so the schema carries parameter meaning. The description adds nothing about the parameter — it does not even clearly tie the value being supplied to the ringing phone it describes — so the baseline of 3 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?

The description is a metaphor ("The phone that rings when the agent has someone on the line") rather than a verb+resource statement. It hints at the domain — a mobile number used for live hand-off — but never says whether the tool sets, registers, or retrieves that number, so an agent cannot tell from the text alone what calling it does.

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?

There is no when-to-use guidance, no prerequisites, and no mention of any alternative despite close siblings such as my_number, who_can_reach_me, and call. The agent is left to guess which of these tools configures a phone number.

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