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

The description discloses key behavioral traits: cost per message (cheapest tier), error condition (400 for non-NANP), and region restriction. Annotations (readOnlyHint=false, destructiveHint=false) are complemented rather than contradicted, as 'send' implies mutation.

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?

Two compact sentences with emoji bullet convey essential details without filler. Includes price, region restriction, error condition, and recommendation for estimation tool. Every word earns its place.

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 an output schema present, the description covers main action, region, cost, error, and alternative tool. Slightly less than 5 because it omits details like character encoding or delivery confirmations, but these are reasonable given schema coverage.

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?

Input schema has 100% description coverage for all parameters, so the description need not add parameter details. It adds context on pricing and errors but no new parameter info beyond schema; baseline 3 is appropriate.

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 clearly specifies the action (Send SMS), target region (North America NANP +1), and distinguishes from siblings by mentioning the cheapest tier and a 400 error for non-NANP destinations. Contrasting with phone.telnyx.sms_premium and phone.telnyx.sms_world reinforces scope.

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?

Explicitly advises using telnyx.estimate_price first if unsure, and alerts that the tool returns 400 for destinations outside +1. This gives clear when-to-use and when-not-to-use guidance, with an alternative tool named.

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