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

Goes well beyond the annotations: discloses the credential fields returned, that credentials must be kept private, that creation happens once with repeat calls returning the same address, that SIP calls draw on the normal balance with no separate charge, the required scope lines:manage, and per-account rate limiting. readOnlyHint=false and idempotentHint=true are consistent with 'create on first call, repeat calls return the same one'. Only the exact error shape is left unstated.

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 the create/read duality and the resource, followed by tight sentences covering return fields, idempotency, billing, permissions, and rate limits. The opening sentence is long and packs several ideas, but no sentence is 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?

No output schema exists, and the description compensates by enumerating the return object's fields. Preconditions, permissions, billing implications, idempotency, and rate limits are all covered, which is more than enough for an agent to call it correctly.

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 the single parameter is already documented. The description still adds meaning by tying the value's origin to get_answering_line and by stating that a line still being set up causes an error.

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 specific verbs ('Create (first call) or read'), a precise resource ('SIP address and private trunk credentials'), and scope ('one of the signed-in owner's answering lines'), then explains the end goal (phone system sends calls over SIP with no number). It explicitly separates itself from get_answering_line, which handles the number and forwarding codes.

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?

Explicitly names the alternative (get_answering_line) and what it is for, tells the agent to pass the line_id obtained from it, and states the precondition failure ('a line that is still being set up returns an error'). When-to-use, prerequisite, and error condition are all given.

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