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

Annotations already flag destructiveHint=true and openWorldHint=true, and the description adds substantial context beyond them: which mailbox it sends from, that it sets In-Reply-To/References, that it addresses whoever wrote last, that the prior message is auto-quoted, and that the first call only produces a preview plus a confirm_token. That confirmation protocol is critical behavior an agent would otherwise get wrong.

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?

Four short sentences, each carrying distinct information — recipient, threading headers, quoting, and the confirmation handshake — with the core purpose front-loaded. Nothing is redundant.

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 a destructive external side effect, the description covers the important ground: sender, recipients, threading, quoting, and the preview/confirm_token gate. Minor gaps remain — what the preview contains, reply-all vs. single-recipient behavior, and failure modes.

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 only 33%, and the one documented parameter (confirm_token) is described thoroughly in the schema itself. The description compensates partially by implying text is the new body with the prior message quoted automatically, but it never clarifies that 'id' is a thread identifier or what the text constraints are, leaving two undocumented parameters only loosely inferable.

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 ('Answer a thread') and immediately scopes it to a reply within an existing conversation, which cleanly separates it from sibling tools like get_thread and label_thread. An agent knows this sends a message, not reads or labels one.

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?

Usage context is clear: it is for answering an existing thread, and the two-step confirmation flow tells the agent exactly how to proceed on first vs. second call. It does not name explicit alternatives (e.g., get_thread for reading, or a new-message tool), so it stops short of 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.