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

Annotations already declare readOnly/idempotent/non-destructive/closed-world, but the description adds real behavior: a 256 KiB ceiling, paginated base64 plus text, size and SHA-256 in the response, and room scoping. The line "Fetching does not attest reading" usefully signals that a separate acknowledgment action exists. It stops short of saying whether the 256 KiB cap truncates or errors and gives no auth/rate detail beyond "room-scoped."

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?

Three dense sentences with the core operation, the scope caveat and the payload shape front-loaded in that order. Nothing is repeated or padded, and the closing caveat earns its place by preventing a mis-assumption about acknowledgment.

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?

Because there is no output schema, the description correctly carries the return shape (base64, text, size, SHA-256). However, it calls the payload "paginated" while the schema exposes no cursor or page parameter, and it never says what happens at the 256 KiB boundary, leaving an agent without a way to reason about continuation or truncation.

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 both room_id (OAuth-authorized) and attachment_id (uuid from message metadata) are already documented in the schema. The description only reinforces that the ID must be "exact" and that scoping is room-based, which is marginal added meaning over the structured fields.

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?

"Returns original UTF-8 chat-attachment bytes by exact attachment_id" names a specific verb, resource and lookup key, and the clause "not a URL or project file" carves it away from file-oriented siblings such as read_file and list_files. The 256 KiB bound and returned fields further pin down what kind of read this is.

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?

Usage is only implied: the agent must already hold an exact attachment_id from room message metadata, and the note "Fetching does not attest reading" hints that ack-style tools are a separate step. No sibling is named as an alternative, and there is no explicit when-not-to-use condition (e.g. large files, non-attachment content).

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