Virtnumber

Rent a phone number

buy_number

Destructive

Rent a phone number on a plan from get_pricing. Charges the plan price from the balance and renews automatically each period. Pass idempotency_key so a retry never rents twice. status "pending" means the number is still being set up and "number" is a placeholder with X digits; list_my_numbers shows the real number once it is "active".

Tool call template

Replace each placeholder with a value matching the input schema. Only required parameters are included; add optional parameters as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {
      "plan_id": "<plan_id>"
    },
    "name": "buy_number"
  }
}

Input Schema

Name	Required	Description
`plan_id`	Yes	planId from get_pricing
`webhook`	No	Optional URL that receives a POST for each incoming SMS
`idempotency_key`	No	Any unique string (e.g. a UUID). Repeating a call with the same key returns the first result instead of acting twice.

Output Schema

Name	Required	Description	Default
`days`	Yes
`number`	Yes
`planId`	Yes
`status`	Yes
`priceEur`	Yes

Schema Changelog

Changes observed during successful MCP inspections.

First observedNov 8, 2026

TDQS

A4.3/5.0

Behavior5/5

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

Goes well beyond the annotations by disclosing the billing model (plan price charged from balance, auto-renewal each period), the consequence of retries, and the meaning of the 'pending' status with a placeholder number containing X digits. This tells the agent not to surface the placeholder as real and that the rental is a recurring commitment, which the destructiveHint annotation alone would not convey.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness4/5

Is the description appropriately sized, front-loaded, and free of redundancy?

Four tight sentences covering purpose, billing, idempotency, and pending-state semantics, with the core action front-loaded. Dense but every sentence carries distinct information; nothing is padding.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness4/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

Given an output schema exists, return values need not be explained, and the description still adds the critical pending/placeholder caveat plus renewal behavior. Only the webhook parameter's operational effect is left unaddressed for a tool with real-world side effects.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters3/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 100%, so plan_id and idempotency_key semantics are already fully documented in the schema, and the description mostly restates them. The optional webhook parameter is never mentioned in the description, so no meaning is added beyond structured fields.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

States a specific verb and resource ('Rent a phone number on a plan from get_pricing') and immediately identifies the prerequisite source for the plan. An agent can tell this apart from list_my_numbers and delete_number without opening any schema.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines4/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

The prerequisite chain is explicit: the plan comes from get_pricing, and idempotency_key is prescribed for retries. It does not spell out when to prefer this over alternatives (e.g., reusing an existing number found via list_my_numbers), so it stops short of full when/when-not guidance.

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

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