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

The description discloses that it loads the original message and sets threading headers, which are meaningful side effects. It does not mention idempotency, delivery outcomes, or potential failures, but annotations already indicate non-read-only, non-idempotent behavior.

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 description is concise, using two sentences with no redundant or irrelevant content. It communicates the core purpose and key behavior efficiently.

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?

The description is sufficient to invoke the tool given the schema and sibling context. It could mention the return value or error scenarios, but the presence of an output schema and required parameters makes the current description adequately complete.

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 schema provides full parameter descriptions, so the tool description adds little beyond referencing the reply context. The schema's per-field descriptions are thorough, including attachment semantics, which covers the parameter understanding.

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 clearly states the tool replies to an existing email thread, and distinguishes it by noting it loads the original message and sets threading headers. This differentiates it from related tools like email_send or email_forward.

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 explicitly says 'Use this when you need a proper in-thread response,' which gives a clear usage condition. It does not explicitly contrast alternatives, but the guidance is sufficient for the common case.

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