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

Beyond the readOnlyHint annotation it explicitly confirms it "never suppresses, unsubscribes, or creates anything" and explains the two distinct gate mechanisms (suppression blocks all sends; contact unsubscribe blocks marketing only). This is substantive behavioral context an agent could not get from the annotation alone.

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?

The key question and the both-gates warning are front-loaded, and each sentence carries useful information. The gating explanation is a dense em-dash-heavy sentence, slightly reducing readability, but nothing is wasted.

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, return values need not be detailed, yet the description still previews the result ("which gate would close and why"). Combined with the gate semantics and read-only confirmation, an agent has everything needed to call and interpret it.

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% with a single parameter already documented (normalization of case/space). The description adds no parameter-level detail beyond the schema, so the baseline of 3 is appropriate.

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 opens with the exact question it answers ("can I actually send to this address?") and a specific scope: checking the two send-pipeline gates for one address. It is clearly distinguishable from sibling tools like check_delivery or send_test_email.

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 states the context of use explicitly ("before writing or debugging a send path") and stresses checking both gates rather than one. It does not name alternative tools or give when-not-to-use guidance, so it falls short of a 5.

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