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

Annotations already establish readOnlyHint, idempotentHint, and non-destructive behavior. The description adds meaningful behavioral context by stating the message is returned 'in full' and by clarifying that this tool returns attachment metadata rather than raw MIME or attachment download URLs. It does not cover auth semantics, but the schema's Authorization parameter and annotations sufficiently cover side-effect concerns.

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 main action is front-loaded in a single sentence, and the sibling routing is appended compactly. The enumeration of result fields is slightly redundant given the output schema, which keeps this from a 5, but the definition is still focused and without 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 read-only single-message fetch with three fully documented parameters, an output schema, and annotations covering side effects, the description supplies the remaining selection context: full message scope, raw alternative, and attachment alternative. Nothing an agent needs to choose and invoke it correctly 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?

Input schema has 100% description coverage for all three parameters, so the description does not need to add parameter detail. It adds no new input semantics, matching the baseline of 3 for high schema coverage.

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 opens with a specific verb and resource ('Fetches one message in full') and enumerates the exact fields returned. It also differentiates itself from two closely related siblings by pointing to get_agentmail_inbox_message_raw for MIME source and get_agentmail_inbox_message_attachment for attachment URLs.

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?

The description gives explicit routing: if the caller needs the original MIME source, use the raw sibling; if they need an attachment download URL, use the attachment sibling. This tells the agent when this tool is appropriate and when it is not, which is exactly the needed guidance for this cluster of message tools.

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