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

Beyond the annotations (readOnly=false, destructive=false), it discloses the required write scope and the activation prerequisite, which are genuinely useful behavioral facts. It does not say whether additions are additive vs. replacing, how partial failures across the contacts array are handled, or what the response contains.

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?

Two short sentences, front-loaded with the core action, with the dual-mode and the write-scope prerequisite each stated once and without filler. Nothing could be removed without losing 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?

For a four-parameter mutation tool with annotations and no output schema, the description covers authentication, the prerequisite for activation, and the two input modes. Only edge-case behavior (partial failure, duplicate handling beyond the schema default) is left unstated.

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 phrase 'by id or by creating them inline' adds meaning by framing contact_ids and contacts as the two mutually exclusive input paths. It does not add detail on the deduplicate flag or the expected contact shape for inline creation beyond pointing at create_contact.

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?

The description uses a specific verb+resource ('add contacts to a campaign') and distinguishes two operating modes: linking existing contacts by id or creating them inline. It is clearly not a read tool like list_campaign_contacts or search_campaign_contacts, though it never names a sibling explicitly.

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 supplies useful context ('a campaign needs at least one contact before it can be activated'), which implies when this call is necessary, but it gives no explicit when-not condition and does not point at alternatives such as bulk_import_contacts or attach_audience_to_campaign.

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