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

Annotations already cover the safety profile (readOnlyHint=false, destructiveHint=false, idempotentHint=false, openWorldHint=true), and the description adds genuinely non-obvious behavior: creating a contact may fire the list's 'contact created' automations and send notification emails, plus it states the return value. It stops short of noting permission requirements or duplicate-handling behavior beyond what the schema's remove_unsubscribers enum says.

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?

Three tight sentences with the operation class front-loaded, then the precondition, then side effects and return value. Nothing is redundant with the annotations or schema.

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 9-parameter write tool with a nested 'fields' object and no output schema, the description covers side effects, precondition, and return shape ('Returns the new contact'). A brief pointer to vision6_list_list_fields for field keys would close the remaining gap, though the fields schema already mentions it.

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 89%, so the baseline is 3, but the description adds a cross-parameter constraint not visible in the required list (only list_id is formally required yet an email and/or mobile matching a list field is effectively mandatory). That is real semantic value beyond the schema; the POST path also anchors the list_id parameter.

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 ('add one contact to a list') and the 'one' explicitly contrasts with the bulk sibling vision6_add_contacts, while 'WRITE:' flags the operation class up front. An agent can distinguish this from add_contacts and update_contact without opening any 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?

Gives a clear precondition for use: the contact needs an email and/or mobile that matches a field configured on the list. It does not name the alternative (vision6_add_contacts for bulk inserts) or the vision6_list_list_fields prerequisite step explicitly, so routing is implied rather than spelled out.

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