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

Annotations already declare this as a safe read operation (readOnlyHint=true, destructiveHint=false), so the description's job is to add context. It does so by enumerating the returned fields (number, E.164, name, site, carrier, activation state, routing id) and noting the underlying GET endpoint, which is useful for a tool with no output schema. It does not mention error behavior for a missing id.

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, front-loaded fragments with no filler. The 'GET /numbers/{id}' suffix is somewhat redundant with the tool name and annotations, but the field list is compact and informative.

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 single-resource read tool with full parameter schema coverage and read-only annotations, the description provides enough context: what it retrieves, by what key, and what fields come back. Error behavior and pagination are not relevant, though a note on missing ids could improve completeness.

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 description coverage is 100% and the single parameter's schema already says 'Placetel number id (from placetel_list_numbers)'. The description only implies the id lookup and adds no syntax or format detail beyond the schema, so 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 states a specific resource (one phone number) and the lookup key (id), and enumerates the returned fields. It implicitly distinguishes itself from placetel_list_numbers by saying 'One phone number by id', though it does not name the list sibling explicitly.

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: retrieve a single number when you have its id. There is no explicit when-to-use, when-not-to-use, or named alternative such as placetel_list_numbers for discovering ids.

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