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

Annotations already declare the write/destructive profile (readOnlyHint=false, destructiveHint=true, openWorldHint=false), and the description adds the traits annotations cannot: 8 MB cap, base64 encoding requirement, and a 24-hour lifetime on the returned attachment_ref. The expiry caveat is exactly the kind of operational context that prevents failed downstream calls.

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, zero filler, and the highest-value facts (what it stores, size limit, return value, lifetime) come before the alternative-path note. Every clause earns its place.

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 3-param write tool with no output schema, the description supplies the return value and its expiry, the size ceiling, and the follow-on call. Only the mime_type parameter and behavior on rejected/oversized uploads are unaddressed, which is minor.

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?

Coverage is 33%, so only 'name' is documented in the schema. The description compensates for data_base64 by stating the encoding and the 8 MB limit, but mime_type semantics (accepted values, whether it drives validation) are left entirely unaddressed.

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?

Specific verb and resource: 'Store a file (PDF, image, document) so a draft can carry it', plus the explicit output (an attachment_ref) and the consumer (draft_reply). It also separates itself from the library path for videos/voice notes, so an agent can route without opening schemas.

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 a clear use condition (attaching to a draft via draft_reply) and an exclusion with an alternative ('For videos and voice notes, use the library instead'). The alternative is named only by concept rather than a specific sibling like list_library/get_library_item, which is the one small gap.

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