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

Annotations already declare readOnlyHint=true, idempotentHint=true and destructiveHint=false, so the safety profile is covered. The description adds genuinely useful context — the MCP transport returns base64 and there is a 5 MB ceiling — but it never says what happens above 5 MB (error, truncation, fallback) or anything about auth/access constraints.

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 short sentences, both front-loaded with the operation first and the transport caveat second. No filler and nothing repeated from the schema or annotations.

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 need not be described, and annotations cover the safety profile. However, the undefined parameters and the unresolved boundary behavior at 5 MB leave gaps an agent would hit in practice.

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 carries the full burden and says nothing about either parameter. The agent gets no explanation of `id` (a 64-hex identifier — presumably the message or attachment container) or `index` (0–1000, presumably which attachment within that container), which is a real ambiguity for a two-required-param tool.

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 ("Download an attachment") so an agent knows exactly what it retrieves. There is no competing attachment-downloading sibling, so no differentiation is required, but the description stops short of stating scope (e.g., which object the attachment belongs to).

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 explains transport behavior (REST binary vs MCP base64) but gives no guidance on when to call this tool versus siblings like get_message, nor any prerequisite such as needing a valid attachment id from a message. Usage must be inferred entirely.

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