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

Goes well beyond the annotations: it discloses the billing model (consumes a pre-paid add-on slot), the provider dependency (operator-enabled Telnyx only), and a critical operational protocol for ambiguous outcomes. Rather than contradicting idempotentHint=false, it explains how to compensate for a non-idempotent purchase using idempotencyKey, which is exactly the context an agent needs before spending money.

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?

Four short sentences, front-loaded with purpose, then prerequisites, then the safety rule. No filler, and the most consequential instruction (never retry with a new key) is placed last for emphasis.

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?

Covers the prerequisites, cost source, provider restriction, and reconciliation policy that matter for a money-spending mutation with no output schema. It omits what the call returns on success (a number record? a purchase id?) and any rate or quota limits, which leaves a small gap for a tool whose result the agent will likely need to reference later.

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%, so the baseline is 3; the description still adds value by tying both parameters to a workflow — the number must be sourced from inventory, and the idempotencyKey must be preserved and never regenerated on retry. It stops short of describing key format or lifetime, which the schema also omits.

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 ("Buy a US number") plus the funding mechanism ("an already-paid phone add-on slot"), which cleanly separates it from list_phone_numbers and release_phone_number in the sibling set. The Telnyx-only qualifier further narrows the operation.

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?

Gives an explicit prerequisite ("Obtain a number from the phone inventory API first") and a failure-handling rule (uncertain purchases must be reconciled, never retried with a new key). It does not name the concrete sibling tools (list_phone_numbers / release_phone_number), leaving the agent to map "phone inventory API" itself, but the when-to-use context is unambiguous.

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