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

Annotations already signal read-only/idempotent/safe, and the description adds beyond them: bytes are not inlined, URL is short-lived and must be fetched before expires_at. This prevents a likely misuse where an agent assumes the raw bytes are returned directly.

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?

Two sentences, front-loaded with the result shape, then the operational caveat and routing guidance. No filler.

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 simple read-only retrieval with an output schema and full param schema, the description covers result format, expiry behavior, and the sibling alternative. Nothing essential is missing.

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 covers all three parameters (inbox_id, message_id, Authorization) at 100%, so the baseline applies. The description does not add parameter-specific semantics beyond what the schema already documents.

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 ('Returns') and exact resource: the original RFC 822 source of one message. Clearly distinguishes from sibling get_agentmail_inbox_message by mentioning parsed text/HTML, and enumerates return fields.

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?

Explicitly names use cases (headers, DKIM, exact MIME structure) and names the alternative for parsed text/HTML. Also warns to fetch download_url before expiry, guiding operational use.

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