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

The annotation readOnlyHint=true already communicates the safe read-only nature. The description adds the REST method 'GET /numbers/{id}', which aligns with the annotation but does not disclose other behavioral traits such as authentication requirements, error responses, or return format. This adds minimal extra value beyond the annotation.

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?

The description is two short sentences, front-loaded with the core action and object. It includes the REST endpoint as a useful detail without redundancy. Every word earns its place, and it is appropriately concise for a simple tool.

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 single-parameter getter with a read-only annotation, the description is nearly complete. It states the action and target but does not explicitly describe the return value or response format, which would be helpful since no output schema exists. However, the tool's name and sibling context imply a straightforward number object return, so the gap is minor.

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?

The input schema already provides 100% description coverage for the single 'id' parameter, with a clear description 'The number id.' The tool description merely says 'by id' and adds no new meaning or format details beyond what the schema provides. 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?

The description clearly states 'Get a single phone number by id', using a specific verb and resource. It distinguishes from sibling get tools (e.g., aircall_get_call, aircall_get_contact) by specifying 'phone number'. The REST endpoint pattern further reinforces the purpose.

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 description makes usage context clear: use this tool when you have a number id and need that single number. It does not explicitly contrast with list_numbers, but the singular 'a single' implies a targeted lookup, and sibling names fill the alternative use case. No exclusions are stated, but none are necessary for this simple getter.

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