Get a message

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

Beyond the readOnlyHint and idempotentHint annotations, the description discloses detailed behavioral traits: it enumerates the returned message fields, explains custom-id name resolution, and explicitly states that deleted messages return deletionMetadata instead of content. This gives an agent accurate expectations about response shape and edge cases without an output schema.

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 front-loaded with the core operation and then adds compact, high-value details. Every sentence earns its place: the field list summarizes the return shape, the custom-id note clarifies an important input variant, and the deletion behavior warns about a key edge case. There is no fluff.

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?

Given the simple single-parameter schema, rich annotations, and absence of an output schema, the description is complete. It covers what the tool returns, the accepted input forms, and the deletion edge case. Nothing essential is missing for an agent to invoke this tool correctly.

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 already documents the message parameter with a pattern and example, providing 100% coverage. The description goes further by explaining that custom-id names are also resolved, which is a meaningful semantic addition that tells the agent this parameter accepts more than just canonical names returned by list_messages/send_message.

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 by its full resource name,' which clearly distinguishes it from list_messages and other siblings. It also lists the message fields returned and special behaviors like custom-id resolution and deletion metadata, making the tool's scope unambiguous.

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 implies the correct usage context: use this tool when you have a full message resource name, typically from list_messages or send_message, and it notes that custom-id names are accepted for messages sent with message_id. It does not explicitly contrast with alternatives like get_attachment or list_messages, but the context is clear enough for an agent to select it appropriately.

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