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

Goes well beyond the annotations (readOnlyHint=false, idempotentHint=true) by disclosing atomicity ('changing nothing' on refusal), the name-based matching limitation imposed by the API, the merge/de-duplicate save semantics, and the specific failure modes. This is exactly the extra context an agent needs before mutating a campaign.

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?

Four sentences, front-loaded with the action, then the matching caveat, then refusal conditions, then the safe-preview advice. 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?

Covers mutation scope, refusal behavior, and side effects for a tool with no output schema and full annotation coverage. Minor gap: it doesn't say what a successful non-dry-run response contains (ids vs. names), though the dry_run sentence hints at the shape.

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 baseline is 3; the description adds real value by explaining how list_ids are resolved (exact-name matching to current lists) and what dry_run surfaces ('resulting lists and approximate audience'). It also implicitly corrects the schema's list_ids text ('add or remove'), since this tool only adds and retains.

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 (Add), resource (lists), and target scope (a DRAFT campaign's recipients) while explicitly noting existing lists are retained — which separates it from remove_lists_from_campaign at a glance. No schema inspection needed to know what it does.

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 concrete when-not conditions ('refuses if the campaign is not a draft, or if it sends to a segment') and recommends dry_run: true to preview results. It never names remove_lists_from_campaign as the counterpart for removal, so the alternative-routing is only implied by the 'keeping the lists it already has' phrasing.

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