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

Annotations already cover safety (readOnly, idempotent, non-destructive), but the description adds genuinely new behavior: asynchronous readiness signalled by 200 vs 202, and the failure mode where a card index is rejected as invalidRequest. That is real context an agent could not get from the annotations alone, though rate limits and URL expiry are unstated.

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?

Very short, status code semantics are front-loaded, and every clause carries information. The opening repetition of the title 'Get attachment' adds nothing beyond the name, but the rest is efficient.

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, the description partially carries the return burden by describing the URL and the pending state, which is the most important part. It still omits parameter meaning and any indication of how long a 'fresh URL' remains valid, leaving gaps for a two-required-parameter tool with undocumented inputs.

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% for both required parameters. The description never explains what messageId identifies or what index enumerates (attachment position? card position?), only obliquely hinting that index can be a card index in the invalidRequest case. The agent is left guessing at the two inputs it must supply.

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?

The description gives a specific verb and resource (get attachment) and adds non-obvious return semantics: a fresh URL on 200, a pending state on 202. It does not distinguish itself from siblings such as messages_get or threads_get, so an agent must infer that this is specifically the media/blob fetch for a message.

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 200/202 split implicitly tells the agent to retry when the media is still processing, and the note that a card index yields invalidRequest warns against one misuse. Beyond that there is no explicit when-to-use guidance, no prerequisites, and no named alternative for retrieving message content.

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