Get a number

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

Annotations already declare readOnly, idempotent and non-destructive, so safety is covered. The description adds behavior beyond that: the return is a PhoneNumberResponse dict or an {"error": "<message>"} envelope, which tells the agent to expect in-band errors rather than exceptions. It could still say more about lookup-miss behavior, but this is solid for a read tool.

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, no filler, with the identity and scope stated before the return-shape detail. Every clause carries information the agent cannot get from structured fields.

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?

An output schema exists, so return values needn't be spelled out, yet the description still flags the error variant, which is the part an agent most needs. For a single-parameter read tool with full annotation coverage, nothing material 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 coverage is 0% for the single number_id parameter, but the baseline for a 1-param tool is high. The description compensates by stating provenance ("number_id comes from list_numbers"), which the bare string schema does not convey.

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?

Specific verb and resource ("Fetch one dedicated number by id") plus a scoping qualifier ("one") that separates it from list_numbers. It also enumerates what the payload contains (state, capabilities, answering agents), so the agent knows exactly what it gets back.

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 ties itself to list_numbers as the source of number_id, which implicitly defines the workflow: list first, then get. It does not explicitly state exclusions or when to prefer it over list_numbers, but the "one by id" framing makes the distinction inferable.

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