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

Goes well beyond the annotations: it discloses real-world side effects (the recipient receives it immediately), monetary cost ('SMS charges apply'), irreversibility ('cannot be recalled'), and an authorization precondition. The annotations only mark it non-readonly, non-idempotent and open-world, so this description is what actually conveys the stakes.

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?

Three short sentences, front-loaded with the operation class and the verb, then cost/irreversibility, then the usage gate, then endpoint. No filler and every clause carries decision-relevant information.

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 two-parameter, no-output-schema write tool with annotations present, the description covers preconditions, cost, and irreversibility well. It doesn't say what the call returns (message ID, delivery status), which is the only notable omission since no output schema exists to cover 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 description coverage is 100%, so both parameters are already documented with pattern and length bounds. The description reinforces 'exact text' and the phone-number recipient, but adds no format, encoding, or segmentation detail beyond the schema, so baseline 3 applies.

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 ('send a real SMS to a phone number from the Placetel account') and prefixes the mode ('WRITE:'), making it instantly distinguishable from the read-oriented list/get siblings and from placetel_initiate_call. An agent can pick this tool without opening the 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?

Gives explicit when-not guidance: 'Do not use without the user's explicit request and exact text', which is a concrete gating condition rather than a vague hint. It doesn't point to an alternative channel (e.g. placetel_initiate_call for voice) but none of the siblings offer an SMS substitute, so the gap is minor.

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