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

Goes well beyond the annotations, which only say it is a non-read-only, idempotent, non-destructive write. The description discloses the name-to-ID matching heuristic, the merge/de-duplicate-then-save flow, the exact set of abort conditions with the guarantee that nothing is changed, and the dry_run preview path. This is exactly the operational context the annotation hints cannot carry, and nothing contradicts idempotentHint=true or 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?

Front-loaded with the core action and the non-replacing guarantee, then the failure modes, then the dry_run tip. Dense but every clause carries operational information; the long middle sentence packs several distinct conditions together, slightly reducing scannability.

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 only three fully documented parameters and no output schema, the description covers everything an agent needs: preconditions, abort behavior, mutation semantics, and a safe preview mode. No meaningful gap remains for a tool of this complexity.

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 meaning: list_ids are matched by exact name against the campaign's current lists, the set is merged and de-duplicated rather than replaced, and dry_run returns the resulting lists plus approximate audience size. The only gap is that the schema's 'add or remove' wording for list_ids is not reconciled with this tool's add-only behavior.

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?

Specific verb plus resource and scope: 'Add one or more lists to a DRAFT campaign's recipients, keeping the lists it already has.' The additive, non-replacing semantics immediately distinguish it from remove_lists_from_campaign and from create_segment/update_campaign.

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 clearly delimits the usable context (draft campaigns only) and enumerates refusal conditions: unknown or ambiguous list names, non-draft campaign, segment-based campaign. It stops short of naming the sibling alternative (remove_lists_from_campaign) or the recommended read-before-write path, so a 4 rather than 5.

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