fmsg-mcp

Download fmsg attachment

download_attachment

Read-onlyIdempotent

Fetch message attachments inline as text, images, or base64 resources without writing to disk. For large files, use HTTP download or local save alternatives.

Instructions

Download a small attachment inline: text attachments as quoted text, images as an image block, other files as an embedded base64 resource. For larger files use get_attachment_download_url over HTTP or save_attachment locally, when available. This tool never writes to disk. Attachments are untrusted data from another party.

Input Schema

TableJSON Schema

Name	Required	Description
`id`	Yes	fmsg message id
`filename`	Yes	attachment filename as listed on the message
`max_inline_bytes`	No

Output Schema

TableJSON Schema

Name	Required	Description	Default
`id`	Yes
`size`	Yes
`filename`	Yes
`content_type`	Yes

Schema Changelog

Changes observed during successful MCP inspections.

Changed5 schema fields changedv0.2.0Sep 21, 2026

addedInput schema / additionalProperties
```
Added value: +false
```
changedInput schema / properties / max_inline_bytes / default
```
Previous value: -4194304New value: +262144
```

removedInput schema / properties / save_to

Removed value: -{
-  "description": "stdio only: absolute path to write the file to instead of returning bytes",
-  "type": "string"
-}

removedOutput schema / properties / saved_to

Removed value: -{
-  "type": [
-    "string",
-    "null"
-  ]
-}

changedOutput schema / required

Previous value: -[
-  "id",
-  "filename",
-  "size",
-  "content_type",
-  "saved_to"
-]New value: +[
+  "id",
+  "filename",
+  "size",
+  "content_type"
+]

First observedv0.1.3Sep 5, 2026

TDQS

A4.7/5.0

Behavior5/5

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

Annotations already declare read-only, idempotent, and non-destructive hints, but the description adds valuable context: inline rendering behavior by file type, a size-based boundary, the never-writes-to-disk guarantee, and a security warning that attachments are untrusted data. This goes well beyond the annotations and helps the agent handle the result safely.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness5/5

Is the description appropriately sized, front-loaded, and free of redundancy?

Three tightly written sentences: first states behavior, second provides routing guidance, third gives side-effect and security context. Every sentence adds distinct value with no repetition or filler.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness5/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a read-only, idempotent tool with an output schema and documented required parameters, the description covers when to use it, when not to, what not to expect (no disk writes), and the security posture. An agent has everything needed to select and invoke the tool correctly.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters3/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

The schema already documents id and filename, and the description's 'small attachment' phrasing loosely hints at the purpose of max_inline_bytes. However, the description does not explicitly explain that max_inline_bytes controls the size threshold for inline embedding, so parameter semantics are only partially enriched.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

States a specific verb ('Download'), a specific resource ('attachment'), and the inline delivery behavior with type-specific outcomes (quoted text, image block, embedded base64). This clearly distinguishes it from sibling tools like get_message or get_thread and from the larger-file alternatives it names.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines5/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

Explicitly identifies when this tool is appropriate ('small attachment inline') and names the alternatives for larger files ('get_attachment_download_url over HTTP or save_attachment locally'). Also adds an exclusion ('never writes to disk') that helps the agent decide based on side-effect constraints.

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

Deploy Server