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

readOnlyHint=true already covers the safety profile. The description goes further by naming the concrete data returned (phone numbers, contact groups, ticket counts, custom fields) and the backing endpoint GET /api/1.1/json/user/<id or email>/, which helps an agent anticipate payload shape in the absence of an output schema.

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 tight sentences with the identity/scope constraint front-loaded before the returned-fields list. The raw endpoint tail is slightly redundant but compact, so it doesn't materially dilute the definition.

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 read tool with annotations covering safety and a fully documented parameter, the description is complete enough: it discloses returned fields despite no output schema. Minor gap in not describing error/not-found behavior keeps it from a 5.

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 single parameter's description already documents the id-or-email format, so the description adds no meaning beyond the schema. Baseline 3 is appropriate 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?

Specific verb (Fetch) plus resource (a contact) and it explicitly enumerates returned artifacts — phones, contact groups, ticket counts, custom fields. The singular scope plus the id-or-email identification distinguishes it from the sibling happyfox_list_contacts.

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?

"by id or by email address" implies the appropriate usage context for a single-record lookup, and the sibling list_contacts makes the single/list split inferable. However, it never explicitly states when to prefer this over list_contacts or what happens on a not-found id.

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