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

Annotations only declare destructiveHint=true; the description adds the crucial consequences: the operation is permanent ('cannot remove a number again'), future sends are blocked, and blocked sends return error 20. This is exactly the beyond-annotation behavioral context a mutation tool needs.

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-loaded with 'WRITE:' to signal the operation class, then effect, then usage, then irreversibility, then the underlying REST endpoint. Every clause carries information; no filler.

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?

One required parameter, no output schema, and a description that covers the write semantics, irreversibility, and the failure mode. Nothing an agent needs to call this 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 description coverage is 100% and the single parameter already documents the international-format requirement with an example. The description only implicitly reinforces that one number is added per call, adding little beyond the schema. Baseline 3 is appropriate.

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 ('adds one number to the account's opt-out blacklist') plus the concrete downstream effect (no SMS sent, sends fail with error 20). An agent can distinguish this from voodoosms_list_blacklist and the contact-list tools without opening any 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 an explicit trigger: 'Use it for a recipient who asked to stop.' That is clear when-to-use guidance. It does not name an alternative for the opposite need (viewing the blacklist via voodoosms_list_blacklist) or state any prerequisites, so it stops short of the 5 bar.

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