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

Annotations already mark readOnly, idempotent, and non-destructive. The description adds value by stating 'nothing is sent' and by detailing what the tool returns: whether the send would go out, why not if applicable, recipient count, and address-level exclusion reasons. This goes well beyond the annotations.

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?

The description is compact and front-loaded with the critical read-only guarantee. Every sentence contributes: behavior, return value, optional parameter effect, and usage timing. No filler or redundancy.

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?

There is no output schema, so the description appropriately explains the return semantics: whether the campaign would go out, why not if blocked, recipient count, and per-address reasons. Combined with the annotations covering safety, an agent has enough to use this tool correctly.

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 both parameters. The description adds meaningful behavior context for `to`, explaining that it triggers address-by-address reporting of kept/excluded recipients with reasons. That is beyond what the schema states.

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 states a specific verb and resource: previewing a campaign send. It clearly distinguishes itself from send_campaign by saying it returns exactly what send_campaign would do without actually sending, including whether it would go out and how many recipients would receive it.

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 description gives an explicit usage instruction: 'Call it BEFORE send_campaign', and it explains the workflow of giving the resulting number to the person for confirmation. This makes the intended sequencing and purpose unambiguous.

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