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

Annotations cover the safety profile (readOnlyHint=false, destructiveHint=false, idempotentHint=false, openWorldHint=true), so the bar is lower. The description adds genuinely non-obvious behavior: the file is a temporary staging artifact that is removed once attached, plus the one-shot return tuple despite there being no 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?

A single front-loaded sentence: purpose, encoding rule, size limit, and return value, followed by a one-clause routing note. Dense but every clause carries information; only the heavy compression of the return tuple is slightly terse.

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 no output schema and only 40% schema coverage, the description still covers the return shape, the size ceiling, the one-shot lifecycle, and the sibling alternative. Remaining gaps are the unexplained account/filename/content_type parameters rather than anything that would cause a wrong invocation.

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 only 40%, so the description must compensate. It clarifies the content parameter (UTF-8 text or base64 for binary) and the implied utf8/base64 encoding enum, but account, filename, and content_type receive no added meaning 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 (stores a file), names the storage location ("mailmcp-uploads"), and gives the return contract {folder, uid, part}. It also names the sibling request_upload as the alternative, so an agent can separate it from other upload-related tools 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?

Explicitly routes large or local files to request_upload and states the size cap (policy.max_upload_bytes), which is real usage guidance. It does not, however, explain when the text-vs-base64 branch should be chosen or any prerequisites tied to the required account parameter.

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