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

Goes well beyond the annotations: it discloses that the returned URL needs no extra API key, that links expire after 15 minutes, that they must not be shared, that read status is left unchanged, and that no attachment analysis is performed. These are exactly the operational facts an agent needs before handing a URL onward.

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?

Four short sentences, each carrying a distinct fact (what you get, auth, expiry/sharing, side effects), with the core purpose front-loaded and zero filler.

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 and annotations already declare the read-only, idempotent, non-destructive profile; the description fills the remaining gaps around URL lifetime, credential needs at the returned URL, and the absence of read-status side effects. Nothing needed to invoke it correctly is missing.

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, but the description adds real semantics by explaining that attachment_id selects a single attachment while its absence yields the original EML. It says nothing about the reason/purpose/account_id parameters, but the schema already covers those.

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+resource (retrieve a temporary download URL) and scopes it precisely to one attachment or the original EML, which cleanly separates it from metadata tools like mailbox_email_get and mailbox_file_read.

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 implied trigger (you want the actual bytes of an attachment or EML rather than metadata) is reasonably clear, but there is no explicit when-to-use/when-not guidance and no named alternative among the many sibling read tools, so the agent must infer routing.

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