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

Annotations already declare readOnlyHint=true, so the safety profile is covered by structured data. The description adds nothing beyond the endpoint string – no return shape, no error behavior for a missing id, no mention of what fields a number has. With the lower bar set by annotations, this still under-delivers.

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 short sentences, zero waste, purpose front-loaded in the first clause. The endpoint reference is compact and informative. Exemplary economy.

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 trivial single-resource getter with one fully documented parameter and readOnlyHint, this is adequate. It lacks any note on 404 behavior or what the response contains, but the tool's simplicity keeps the gap modest.

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% and the sole parameter 'id' is documented there as 'The number id.' The description adds only the type context ('phone number'), which is marginal. Baseline 3 applies when the schema does the heavy lifting.

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?

Clear verb+resource: 'Get a single phone number by id,' and it even names the underlying REST endpoint. It distinguishes itself from aircall_list_numbers implicitly via 'single...by id,' though it doesn't explicitly name the sibling. Strong but not maximally differentiated.

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?

The phrase 'by id' implies this is for when you already have a number id, which suggests the list-based alternative. However, no explicit when-to-use or when-not-to-use guidance is given, and aircall_list_numbers is never named. Implied usage only.

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