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

Annotations already declare readOnlyHint=true, so the safety profile is covered. The description adds a genuine behavioral rule beyond the schema: when both identifiers are supplied, Virtuous prioritises the reference. It does not disclose not-found behavior or the shape of the returned contact.

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 sentences plus an endpoint reference, with the primary use case front-loaded and no filler. Every clause carries operational information.

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?

With no output schema and an empty required-parameter list, the description usefully compensates by specifying the valid parameter combinations. What an agent still cannot infer is the failure mode when neither identifier matches a contact.

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%, so the baseline is 3, but the description adds real semantics the schema lacks: the either/or requirement between email and the reference pair, and the precedence rule when both are provided. That cross-field logic is not expressible in the flat property descriptions.

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 ('Look up a single contact') and names the two lookup keys, which distinguishes it from bulk siblings like virtuous_search_contacts. It never names a sibling explicitly, so the differentiation is inferred rather than stated.

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?

Gives an explicit invocation rule: supply either email, or both reference_source and reference_id, with reference taking precedence when both are present. It does not say when to prefer this over virtuous_search_contacts or virtuous_get_contact, so no exclusions or alternative routing are offered.

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