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

Annotations already cover the safety profile (readOnly, non-destructive, closed-world). The description adds real behavioral context beyond them: include_images returns up to 4 largest-first attachments as viewable images, and an attachment with downloaded=false is not yet local, triggering background fetch that read_attachment/save_attachment then wait on.

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?

Front-loaded with the core purpose, then identifiers, then the include_images rule, then the download caveat. Dense and mostly waste-free, though the final attachment/background-fetch sentence is long and could be tighter.

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?

An output schema exists so return shape needn't be described, and the either/or identifier semantics are made explicit despite zero required parameters. The gap is the undocumented include_body and max_body_chars knobs, which matter for controlling response size.

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 0%, so the description must carry the load. It explains mail_id (source: search_mail), message_id (RFC header) and include_images behavior, but leaves include_body and max_body_chars entirely unexplained — two of five parameters undocumented anywhere.

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 and resource with scope: 'Fetch one message in full: headers, recipients, attachments and body text.' This clearly separates it from search_mail (finding) and read_attachment (one image) without needing the schema.

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?

Gives concrete routing: identify by mail_id 'from search_mail' or by RFC Message-ID, and set include_images only 'when the pictures are the content,' directing single-image needs to read_attachment. It does not, however, compare against get_thread or show_mail, which an agent might also consider.

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