Send a one-way message call

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

Annotations only cover openWorldHint and idempotentHint, so the description carries the rest and does so richly: TTS-only with no conversation, billed at one-way rate, values pre-formatted for speech, empty script fields refused by name, and externalId deduplication for safe retries. This adds substantial behavioral context the annotations do not provide.

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?

Front-loads the core action and keeps the comparison to place_call early. Dense but nearly every clause carries new information; sentences are long and packed with parentheticals, costing some readability.

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?

No output schema, but the description explains the send/retry outcome ('answers with the first send instead of ringing again'), billing context, and refusal behavior. It omits error cases beyond the empty-script-field rule and any return payload shape, leaving a minor gap for a 5-param call tool.

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 80% (to, agentId, variables, externalId already documented), yet the description still adds real meaning: how variables' values are formatted for speech, that a valueless script field is refused by name, and what externalId does on retry. Slightly above the baseline because it explains parameter behavior the schema only implies.

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+resource ('Plays a one-way agent's fixed script to one number') and immediately names the sibling it differs from ('far cheaper than place_call'). An agent can distinguish this from place_call without opening either schema.

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?

Makes the selection condition clear via the price comparison to place_call and the 'text-to-speech only, no conversation' constraint, and the schema/description require a one-way agent. No explicit when-not statement, but the alternative is named with the reason to prefer this one.

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