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

Goes well beyond the annotations: the parenthetical '(existing group memberships are kept)' confirms additive, non-replacing semantics, and the mailings consequence discloses the downstream effect of the write. It also discloses the return value ('the member's groups after the update'), which matters since there is no output schema, and this is consistent with idempotentHint=true and destructiveHint=false.

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 short sentences, front-loaded with the WRITE marker, the scope, and the non-destructive guarantee; the endpoint is placed last where it is least needed. No sentence is redundant with 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 two-parameter write with no output schema, the description covers the precondition, the additive semantics, the business consequence, and the return value. Nothing an agent needs in order to call this correctly is missing.

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 schema documents both 'key' (email or numeric id) and 'group_ids' with min/max item bounds, so the schema carries the burden. The description adds only the implicit constraint that the member must already exist; it adds no format or batching 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 ('add an existing member to more groups') and tags it WRITE, so it is immediately distinguishable from the read-only sibling get_member_groups and from add_member, which creates a new member. The endpoint POST /members/{key}/groups reinforces the resource scope.

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?

'add an existing member to more groups' makes the precondition (member must already exist) explicit, which is the key when-to-use signal versus add_member. It explains the consequence ('being in a group makes the member a recipient of mailings'), but never names an alternative tool such as get_member_groups for reading memberships or list_group_members for the inverse view.

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