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

Annotations already declare readOnlyHint=true, destructiveHint=false, openWorldHint=true, so safety is covered. The description adds value by disclosing what comes back (including verification status, which is behaviorally important since only verified senders are usable) — a meaningful detail with no output schema to convey it. It doesn't mention result volume or pagination, keeping it below a 5.

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 short sentences: what it returns, why it matters, and the endpoint. Nothing is repeated from the title or annotations, and the resource description is front-loaded.

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 zero-parameter read-only list tool with no output schema, the description covers everything an agent needs: the returned fields, the verification-status signal, and the use case (obtaining a sender id). Nothing material is left ambiguous.

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, which is the baseline-4 case, and the description correctly implies a plain unfiltered GET /senders with no filtering or paging arguments to explain.

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?

Specific verb (list) plus specific resource (sender identities) and an enumeration of the returned fields (name, email, verification status, postal address). No sibling tool covers senders, and the endpoint reference reinforces scope, so an agent can distinguish this from the many list_* siblings immediately.

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?

"Campaigns and emails need a verified sender id" gives a concrete reason to call this and implicitly points to send_campaign/send_email as the downstream consumers. It stops short of explicit exclusions or a named alternative, so it's clear context rather than full when/when-not guidance.

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