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

Annotations indicate destructiveHint true and readOnlyHint false, and the description adds valuable behavioral details: the server appends the original message with quoted history, and if the combined body exceeds the limit, REPLY_TOO_LARGE is returned without sending or truncating. This goes beyond the annotations and clarifies side effects and error handling.

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 three sentences, tightly written, and front-loaded with the core purpose and recipient logic. Every sentence adds critical information without redundancy. It is concise and well-structured for quick agent comprehension.

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?

For a tool with 6 parameters and an output schema, the description covers the essential behavioral aspects: recipient selection, body handling, size limit behavior, and the requirement to check recipients. It does not detail the return value, but the output schema likely covers that. It omits explicit mention of attachments, though attachmentIds are in the schema. Overall, it provides sufficient context for correct invocation.

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%, so parameters are already documented. The description adds some meaning to replyAll (includes original To/Cc) and clarifies that the reply uses Reply-To or sender, which helps with recipient semantics. However, it does not explain idempotencyKey or attachmentIds beyond the schema, so it adds marginal value overall.

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's function: replying to an email using the original Reply-To or sender, with an option to reply all. It distinguishes itself from siblings like forward_email and send_email by specifying the recipient logic. The verb 'reply' and resource 'email' are explicit.

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 provides clear guidance on what to provide (only the new reply) and warns to check recipients before sending. While it doesn't explicitly compare to alternatives like send_email, the context implies this is for replying to an existing message. The mention of size limit and error behavior adds practical usage context.

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