One attachment's decoded bytes, base64-encoded (up to 1 MiB — larger files must be fetched from GET /v1/messages/{id}/attachments/{index}). Index against the attachments array on get_message.
| Name | Required | Description | Default |
|---|---|---|---|
| index | Yes | Zero-based index into the message's attachments array. | |
| message_id | Yes | Message id (uuid). |
Changes observed during successful MCP inspections.
Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
No annotations provided, so description carries full burden. It discloses the base64 encoding, size limit, and alternative endpoint, but does not mention error handling (e.g., out-of-bounds index) or authentication requirements.
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 sentences, front-loaded with key information (bytes, encoding, size limit, alternative). No filler or repetition.
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?
Given no output schema, description explains return format and size limit. Covers main usage but lacks error behavior or rate limits. Adequate for a simple retrieval tool.
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 100%, so baseline is 3. Description adds no additional semantic detail beyond what is already in the parameter descriptions ('uuid', 'zero-based index'). No extra value.
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?
Description clearly states it returns one attachment's decoded bytes base64-encoded, specifies size limit and alternative for larger files, and explains indexing against the attachments array. Distinguished from sibling tools like get_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?
States when to use (for attachments up to 1 MiB) and provides alternative for larger files. Mentions indexing context but lacks explicit when-not-to-use or alternatives beyond the REST endpoint.
Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.