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

readOnlyHint=true already tells the agent this is a safe read. The description adds the domain constraint tying senders to draft-campaign validity and the HTTP endpoint, but says nothing about pagination, result size, or ordering.

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, front-loaded with purpose and followed by the key constraint. The trailing 'Moosend: GET /senders/find_all.json' is largely redundant plumbing but costs little.

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 usefully names the fields returned and explains the value of the result for draft campaigns. Combined with the readOnly annotation, an agent has enough to call this correctly without further input.

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?

The tool takes zero parameters, so there is nothing for the description to disambiguate; baseline for a no-param tool is 4. The field list describes the payload rather than inputs, which is a mild bonus.

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?

Starts with a specific verb+resource ('List the account's campaign senders') and enumerates the returned fields (name, email, enabled, SPF/DKIM verification). No sibling tool covers senders, so the unique resource is unambiguous to an agent.

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 sentence 'A draft campaign's sender and reply-to must be one of these' gives a concrete reason to call this tool before creating a draft campaign. It states clear context but offers no explicit exclusion or named alternative tool.

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