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

Annotations already flag this as a non-readonly, non-idempotent, open-world write, and the description adds real context beyond them: delivery is immediate and irreversible ('the customer receives it at once'), a human confirmation step is required, and the action is plan-gated. Those are exactly the facts an agent needs before committing an external side effect.

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 sentences, front-loaded with what the tool does, then the safety gate, then the plan requirement. Nothing is repeated and no sentence could be dropped without losing a decision-relevant fact.

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?

Covers the risk profile, prerequisites, and delivery semantics well for a two-parameter mutation tool. The only gap is the response: with no output schema, the description doesn't say whether it returns a message id or updated thread state, which matters slightly for follow-up calls like mark_email_answered.

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 description coverage is 100%, and both parameter descriptions already say what message and thread_id are, including the 'from list_emails' provenance. The description adds no formatting, length, or content constraints, so the baseline 3 applies.

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 (send an email reply) plus scope: the customer of one email conversation, in the same thread, from the support address. That cleanly separates it from draft_email_reply and reply_to_ticket, which an agent can rule out without opening their schemas.

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 a clear precondition (show the user the exact wording and get agreement first) and an eligibility constraint (Pro plan or higher). It never explicitly names draft_email_reply as the alternative for the pre-approval step, so the routing is implied by 'Send' versus 'draft' rather than spelled out.

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