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

Annotations already cover the read-only safety profile, and the description adds real context beyond them: the answer shape differs by the `as` parameter (plain text vs base64 file), and it discloses that a GET 'carrying this same credential' is required. It could say more about unsupported attachment types or failure modes, but the added auth and format context is valuable.

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?

Three sentences, mostly front-loaded, but the final sentence ('Either answer names the address the original is served at whole, to a GET carrying this same credential') is syntactically tangled and harder to parse than its content warrants.

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?

With an output schema, full param coverage, and annotations, the description only needs to add what the structured data cannot: the format contract and the credential requirement, which it does. For a 5-param read tool it is nearly complete, missing only edge-case guidance.

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 coverage is 100%, so the baseline is 3. The description restates the `as` distinction ('what it says' vs 'the file itself, base64') and adds the continuation nuance ('The original always comes whole'), which the schema already conveys, so it adds little beyond the structured fields.

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 ('Read') and resource ('one attachment of a message'), and explicitly routes enumeration to the siblings by noting 'messages_get and threads_get name a message's attachments and their sizes.' An agent can distinguish this from messages_get/threads_get without opening a 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?

It clarifies the relationship to alternatives by explaining that messages_get/threads_get only name attachments and sizes, implying this tool retrieves the content itself. It gives no explicit exclusions or prerequisite conditions, but the routing intent is clear.

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