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

Annotations only say destructiveHint=false and openWorldHint=false; the description adds real behavioral detail beyond that: the batch cap, that the call is additive ('append'), the exact breakdown of the response (accepted, suppressed, duplicates, per-row rejections), and the idempotency guarantee on retry. It does not cover what happens above the 10000 limit or why a row would be suppressed, 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?

Three short sentences, zero filler: capability and limit first, return shape second, safety idiom last. Every clause carries information an agent can act on.

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 no output schema, the description sensibly describes the return fields itself, and it discloses the batch ceiling and idempotency mechanism for what is a write operation. Remaining gaps are modest: no mention of what suppresses a contact or how partial rejections affect the overall response status.

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 schema already documents phone, fields, priority, timezone, callback_url, external_ref, campaign_id and idempotency_key. The description restates E.164 phones plus custom fields and the idempotency purpose but adds no new constraints or formats beyond what the schema supplies, which matches the baseline of 3.

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 ('Append ... contacts ... to a campaign') plus a hard batch ceiling of 10000, so an agent knows precisely what the tool does. The 'campaign' scoping implicitly separates it from add_broadcast_contacts, but that sibling is never named, so differentiation is left to inference.

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?

The only explicit guidance is retry behavior ('Pass idempotency_key so a retry is not appended twice'), which is genuinely useful. There is no statement of when to use this rather than add_broadcast_contacts, add_dialer_suppression, or list_dialer_contacts, and no prerequisites such as the campaign needing to exist or be paused.

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