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

Discloses a genuinely non-obvious implementation behavior: the tool performs read-modify-write because 'Emailchef blanks any omitted field', so partial updates are safe despite the destructiveHint=true annotation. It also states the DRAFT-only refusal condition, which annotations cannot express. This is exactly the kind of context that changes how an agent calls the tool.

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 tight sentences, front-loaded with the write action and field scope, then the critical blanking/read-modify-write caveat, then the endpoint. No filler and nothing buried.

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 9-parameter mutation with no output schema, the description covers the mutation model, the partial-update safety mechanism, and the draft constraint. The only modest gap is that it doesn't say what comes back (updated campaign object vs. error payload), which matters given there is no output schema.

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 56%, and the description compensates by enumerating the updatable fields (name, subject, body, sender, reply-to, recipient lists) and by asserting 'Only the fields you pass change', which clarifies the partial-update semantics of all eight optional params. It does not add detail on the id-typed params or the list_ids/segment_id interaction, which the schema partly covers.

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?

Opens with an explicit verb and mode ('WRITE: change a DRAFT campaign') followed by the exact field set it can modify. An agent can distinguish it from create_campaign, clone_campaign, or schedule_campaign without opening the schema.

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?

States the hard precondition clearly: the campaign must be a DRAFT, and the tool 'Refuses a campaign that is not a DRAFT' — which steers the agent away from using it on scheduled/sent campaigns. It does not name the specific alternative sibling (e.g. clone or create) when the campaign is not a draft, so it stops short of explicit routing.

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