Preview message cost

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

Annotations already declare readOnlyHint=true, so 'Read-only' and 'Sends nothing' are somewhat redundant, but the explicit no-side-effect guarantee for a messaging-adjacent tool is worth reinforcing. More valuable is the disclosure of return content (segment count, unit cost, total, currency), which no output schema provides.

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?

One dense sentence with the outcome stated first, the inputs second and the safety caveat last. Every clause carries information; nothing to trim.

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?

Because there is no output schema, describing the returned fields is exactly what the description needed to do, and it does so. It leaves the channel (sms vs mms) dimension unaddressed, which matters since pricing differs by channel, but the schema covers it.

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% and the schema already documents E.164 formats, the 1600-char limit and the sms/mms enum. The description only restates sender/recipient/text and omits the channel parameter entirely, so it adds little beyond the schema.

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 ('Price one message') followed by an exact enumeration of what it returns (segment count, unit cost, total cost, currency). The endpoint path and the 'Sends nothing' clause make it unambiguously distinct from the sibling send_message.

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?

'before sending' gives a clear temporal context that implicitly routes the agent here ahead of send_message, and the inputs are scoped to sender/recipient/text. There is no explicit naming of the alternative tool or of when-not to use it (e.g. for bulk or broadcast costing), so it stops short of a 5.

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