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

Annotations declare destructive/openWorld/non-idempotent, and the description goes well beyond them: irreversibility, the credit-cost formula, that blocked numbers still cost credits, that template placeholders are rejected, and that non-GSM characters become '?' unless auto_unicode is set. This is exactly the behavioral context an agent needs before spending money.

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?

Dense but front-loaded: the WRITE/irreversible/spends-credits warning leads, and cost and encoding caveats follow. Slightly over-packed and repeats the +447700900000 example already in the schema, but nearly every sentence carries 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 destructive, cost-incurring tool with no output schema, the description covers cost accounting (creditCost on sent messages), irreversibility, validation failures, encoding behavior, and recipient limits. Nothing needed to invoke it safely is missing.

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%, so baseline is 3, but the description adds genuine value beyond the schema: the cost model tied to parts and recipients, the shorter-part/higher-cost tradeoff for auto_unicode, and the ban on {F0###}/{fn###}/{sn###} placeholders in body. Only the recipient-format example duplicates 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?

Opens with a specific verb and resource ('SENDS REAL SMS'), names the write nature and the resource it consumes (credits), plus the underlying REST call. It is easily distinguished from siblings like list_messages, get_message, and block_number.

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 operating guidance ('Only send what the user explicitly asked for, confirm the text and recipients first') and excludes unsupported modes ('No groups, no scheduling'). It does not name sibling tools as alternatives, but there is no competing send tool, so the routing is effectively complete.

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