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

Annotations already declare the write profile (readOnlyHint=false, destructiveHint=false, idempotentHint=false), so the bar is lower, and the description still adds genuine context: the address must belong to a verified domain and the first sender silently becomes the default unless is_default is false. It doesn't say what happens on handle collisions or whether creation is rejected as a duplicate, which matters given 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 sentences, zero filler, with the most decision-relevant constraint (verified domain, resolved via list_domains) placed before the secondary defaulting rule.

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 an output schema present the return shape needn't be explained, and the description covers prerequisites and the defaulting side effect for a 5-parameter write tool. The remaining gap is failure behavior - duplicate handle or unverified address - which an agent would want before calling.

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 baseline is 3, but the description adds semantics the schema lacks: the default-sender rule for the first created sender and the fact that the handle is what sends reference (sender: "billing"). Those cross-field behaviors are not visible from the individual property descriptions.

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 concrete verb (save/create) and resource (sender) and enumerates the parts being created: display name, address on a verified domain, optional reply-to, and handle. It is instantly distinguishable from list_senders or send_email, though it never names a sibling to contrast with explicitly.

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?

It gives the key prerequisite - the address must be on a verified domain, pointing the agent at list_domains to find one - and explains the defaulting rule. It stops short of saying when not to use this tool (e.g. for updating an existing sender) or what to do if a sender already exists.

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