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

The readOnlyHint annotation already establishes this is a safe, non-mutating read, so the description's main burden is lifted. It adds that the response nests contact persons with names and e-mail addresses, but says nothing about error behavior for a missing id, permissions, or rate limits.

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, front-loaded with the payload description before the endpoint notation. The raw 'GET /contacts/{id}' adds little for an agent but costs almost nothing.

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?

There is no output schema, so the description should carry more of the return shape; it mentions only the nested contact persons collection and leaves the primary contact fields, error cases, and any pagination/expansion behavior unstated. Adequate but with a clear gap for a lookup tool.

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 'id' parameter already documents its pattern and provenance ('from billy_list_contacts'). The description only restates 'by id', adding no format or constraint detail beyond the schema, so the baseline 3 applies.

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?

States a specific verb and resource ('One contact by id') and names the primary payload ('contact persons (names and e-mail addresses)'), which cleanly separates it from the sibling billy_list_contacts. It does not explicitly name that sibling as the alternative, so it stops short of a 5.

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: the phrase 'by id' tells the agent it needs an identifier already in hand, and the singular 'one contact' implies this is the single-record lookup counterpart to billy_list_contacts. No explicit when-to-use, when-not-to-use, or named alternative is given.

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