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

Annotations already cover the read-only, non-destructive, open-world profile, and the description adds real behavior beyond them: the not-found case is spelled out ('Returns found: false when Robly has no such contact') and the underlying endpoint is disclosed. It stops short of describing error/auth behavior or pagination, which keeps it from a 5.

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 compact sentences: identity of the tool first, return payload second, edge-case and endpoint last. Every clause carries information and nothing is repeated from the schema or annotations.

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 read-only lookup with full schema coverage and no output schema, the description covers inputs, the meaningful returned fields, and the not-found case, which is enough to call it confidently. It omits auth/rate-limit expectations and whether only one of email/member_id may be supplied, small residual gaps for an open-world API.

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%, with each parameter already documented including the 'this OR that' constraint for email/member_id, so the description need not carry this dimension. Its phrase 'on request, the lists and data fields' loosely maps to include_sub_lists/include_fields but adds no format or precedence detail beyond the schema, so 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?

The description opens with a specific verb and resource ('Look up one contact') and enumerates the returned facts (status, subscription date, sign-up source, lists, data fields). 'One contact' implicitly separates it from the plural robly_list_contacts sibling, so an agent can pick between them without opening a schema.

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?

It states the two lookup keys and that lists/data fields come only 'on request', which guides parameter choice, but never says when to prefer this over robly_list_contacts, robly_check_list_membership, or robly_update_contact, and gives no exclusions. Usage is therefore implied rather than stated.

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