Send SMS — World (EU + APAC + LATAM core, Telnyx)

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

The description reveals behavioral traits beyond annotations: it states the cost ($0.10/message) and that it returns HTTP 400 for wrong tiers. Annotations indicate a write operation (readOnlyHint=false) and non-idempotence, which aligns with sending a message. The description adds value by disclosing specific error conditions and pricing, which are not in annotations.

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?

The description is concise (70 words) and front-loaded with the action and destination scope. It uses a clear format with country code examples and a note on error handling. Every sentence adds value, though the list could be slightly more tabular. No wasted words.

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 tool with 4 parameters and an output schema (present), the description covers purpose, geographical scope, pricing, and error conditions. It lacks explicit mention of success response format, but the output schema likely provides that. It also omits rate limits or delivery guarantees, but given the sibling differentiation and schema richness, it is fairly complete.

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 input schema already describes all parameters well (E.164 format, required vs optional). The description adds a list of example country codes, which indirectly clarifies the 'to' parameter's allowed values but does not add new semantic meaning beyond the schema. With 100% schema coverage, the baseline is 3, and the description does not elevate it further.

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 states it sends SMS to international destinations with a verb-action pairing ('Send SMS') and specifies the resource (world destinations). It lists example country codes and differentiates from siblings by explicitly naming the alternative tools for NA and premium tiers.

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?

The description provides explicit guidance on when not to use this tool: when the destination is in NA or premium tiers, instructing the agent to use send_sms_na or send_sms_premium. It also mentions the cost per message. However, it does not elaborate on prerequisites beyond what the schema parameters cover (e.g., owning a Telnyx number).

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