Sendmux Email Inbox API + Sending

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Upload Attachment

qubrics_mailbox_upload_attachment

Uploads email attachments for sending by returning a short-lived presigned PUT URL for real files or accepting tiny inline base64 content, plus a blob ID to include in the send request.

Instructions

Use this before sending a mailbox attachment. For real files set presign_upload_url=true with filename, content_type, and size_bytes, then PUT the file to the returned short-lived URL promptly and send the returned blob_id. Inline content_base64 is a last resort for tiny agent-authored files only and is capped at 32 KiB decoded.

Input Schema

Name	Required	Description	Default
`filename`	Yes	Filename to use when sending the uploaded attachment.
`mailbox_id`	No	Mailbox public ID when the credential can access more than one mailbox.
`size_bytes`	No	Exact byte size required when presign_upload_url=true.
`content_type`	No	MIME type to store with the upload, for example application/pdf.	application/octet-stream
`content_base64`	No	Last-resort inline base64 for tiny agent-authored files only. Decoded content must be at most 32 KiB; use presign_upload_url for real files.
`presign_upload_url`	No	When true, return a short-lived signed PUT URL instead of reading bytes. Provide size_bytes and upload with a shell/client promptly.

Output Schema

Name	Required	Description	Default
No arguments

Schema Changelog

Changes observed during successful MCP inspections.

Addedv1.0.17Nov 7, 2026

TDQS

A4.4/5.0

Behavior5/5

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

Beyond the annotations, the description discloses the presigned-upload workflow, the short-lived URL, the need to PUT promptly, and the requirement to use the returned blob_id when sending. It also states the 32 KiB decoded cap for inline content and explains the intended ordering of operations.

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?

The description is compact and front-loaded: it first states the purpose, then explains the preferred presigned workflow, then the inline fallback. Every sentence carries actionable information without repetition.

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?

Given the moderate parameter count, full schema coverage, annotations, and available output schema, the description provides enough workflow and constraint detail for an agent to call the tool correctly. The key sequencing and size-limit behaviors are explicitly covered.

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

Parameters4/5

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

Schema coverage is 100%, so the schema already documents each parameter. The description adds useful meaning by connecting presign_upload_url=true to filename, content_type, and size_bytes, and by clarifying the inline content_base64 fallback and size cap.

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

Purpose4/5

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

The description states a specific action and resource: uploading a mailbox attachment before sending. It clearly describes the two upload modes. However, it does not distinguish this tool from similarly named sibling upload tools such as sending_upload_attachment or qubrics_sending_upload_attachment.

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

Usage Guidelines4/5

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

It gives clear when-to-use guidance: use before sending an attachment, use presigned upload for real files, and use inline base64 only as a last resort for tiny files. It does not explicitly compare this tool against the other attachment-upload siblings, so the alternative-selection guidance is incomplete.

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

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