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

Goes well beyond the annotations: it discloses that the unsubscribe link is injected automatically, that invalid/role/blocked/unsubscribed recipients are skipped at no cost, that each accepted recipient consumes one email of the daily allowance, and that an oversized campaign is refused atomically. That is exactly the context an agent needs before an irreversible send (idempotentHint=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?

Two tight sentences with the core action and the 1,000-recipient bound front-loaded, followed by the billing and refusal rules. No clause is filler.

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 non-idempotent, open-world send with six parameters, the description covers the side effects, skip logic, quota and refusal behavior, which is what matters most. It does not hint at what a successful call returns, a minor gap with no output schema present.

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 50%, so the schema documents from, list_id and recipients while html, text and subject are bare. The description adds the 1,000-recipient cap and the list_id/unsubscribe relationship, but leaves the html/text split and subject semantics unexplained. Baseline 3 is appropriate given the partial coverage.

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+resource (send a marketing email) and bounds it precisely (up to 1,000 recipients), which separates it from the sibling send_email. An agent can identify the operation 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?

The cost/allowance model implies this is the bulk-marketing path, but the description never explicitly says when to use this instead of send_email, nor states prerequisites such as needing a verified sender or purchased allowance. Usage context is inferable rather than stated.

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