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

Annotations already declare readOnlyHint and idempotentHint, so the description's 'Read-only' is largely redundant. However, it does add a real behavioral constraint the annotations do not carry: the tool requires a connected account. Returned-field expectations are also previewed, which helps the agent interpret the empty-parameter result.

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 compact sentences, front-loaded with the resource and its returned fields, with the routing hint and prerequisite last. The trailing 'Read-only' restates the readOnlyHint annotation and is the one phrase that does not earn its place.

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 and zero input parameters, the description carries the whole informational burden and handles it: it lists what the listing returns, states the practical meaning (can you send from your own domain), names the prerequisite, and points to the tool for the mutation path. Nothing needed to call it correctly is missing.

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 the schema-description baseline is 4. The description adds no parameter meaning (there is none to add) but does describe the fields the call surfaces, which is useful orientation even though it technically belongs to output rather than input semantics.

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 and resource ('List your custom email-sending domains') and enumerates exactly what each entry contains (domain, verification status, verified-at, DNS record count, default flag, sender-identity completeness). It also names the sibling tool responsible for the mutation side (manage_sending_domain), so an agent can separate this read from that write without opening either 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?

Gives the decision context ('Shows whether you can send from your own domain') and routes the write/verify case to manage_sending_domain, plus a prerequisite ('Requires a connected account'). It stops short of an explicit when-not-to-use statement, but the routing intent is unambiguous.

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