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

readOnlyHint=true already declares the safe-read profile, so the bar is lower. The description adds useful specifics — the API endpoint and the 3-digit minimum on `q` — but says nothing about result volume, pagination behavior, or what the listing contains beyond phone numbers.

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?

Two sentences, front-loaded with the purpose and followed by the one non-obvious parameter constraint and endpoint. Nothing is wasted and nothing is buried.

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 simple three-parameter read-only list with no output schema, the agent has enough to call it correctly: purpose, the key search constraint, and the endpoint. The return shape (what fields accompany each phone number) is not described, but it is minimally inferable from the name.

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. The description does add one constraint not in the schema — `q` requires 3 or more digits for a partial search — but cursor and account semantics are left entirely to 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+resource ("List phone numbers that opted out of messages for an account") and names the underlying endpoint, which pins down the entity precisely. It does not explicitly contrast with siblings like textus_list_contacts, but the opt-out subject matter is distinctive enough that confusion is unlikely.

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?

Usage is only implied: the reader infers that calling without `q` lists all opt-outs and calling with `q` narrows the result. There is no explicit when-to-use, when-not-to-use, or alternative-tool guidance (e.g. pointing to textus_list_contacts for non-opted-out numbers).

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