---
slug: "sendmux-attachments-x-2"
source_type: "clawhub"
source_url: "https://clawhub.ai/skills/sendmux-attachments"
repo: ""
source_file: "description"
---
---
name: "sendmux-attachments"
description: "Move email attachments through Sendmux without putting file bytes in model context, using file paths, presigned URLs, CLI, SDKs, or MCP."
version: "1.3.0"
metadata:
  openclaw:
    skillKey: "sendmux-attachments"
    homepage: "https://github.com/Sendmux/skills"
    primaryEnv: "SENDMUX_API_KEY"
    envVars:
      - name: "SENDMUX_API_KEY"
        required: false
        description: "Optional Sendmux API key or scoped agent token used by CLI, SDK, HTTP, or MCP examples."
      - name: "SENDMUX_MBX_KEY"
        required: false
        description: "Optional Sendmux mailbox key for Mailbox and send-capable mailbox workflows."
---

# Sendmux attachments

## ClawHub account note

This ClawHub skill connects OpenClaw agents to Sendmux. Some workflows require a Sendmux account and an appropriate Sendmux API key or agent token. Sendmux account usage is external to ClawHub; do not ask users to paste secrets into chat.

Use this skill whenever a Sendmux task involves attachment bytes.

## Core rule

Do not pipe real files through model context as base64 unless the file is tiny and agent-authored. Prefer paths or signed URLs.

| Mode | Use when | Token cost | Limit |
| --- | --- | ---: | --- |
| Local `file_path` | Local stdio MCP can read the user-shared file root. | tiny | Mailbox cap: 7,500,000 bytes; Sending upload cap: 18 MiB. |
| Presigned upload URL | Hosted MCP, shell-capable agents, or large local files. | tiny | Mailbox cap: 7,500,000 bytes; Sending upload cap: 18 MiB; exact size is signed. |
| CLI `--attach` / SDK file helpers | Terminal or application code can read the file. | tiny | Mailbox cap for mailbox sends; Sending upload cap: 18 MiB and final message cap: 25 MB. |
| Inline base64 | Small generated text/files only. | high | MCP inline cap is 32 KiB decoded. |

Approximate base64 cost: 25 KB becomes about 11K generated tokens; 1 MB is impractical. A file path is usually under 100 tokens.

## Security model

- A caller must authenticate to mint upload URLs or upload directly.
- The later presigned `PUT` has no `Authorization` header, but it only works with the unguessable short-lived signed URL and exact headers returned by Sendmux.
- Do not invent file-type allow-lists. Set the best `Content-Type`; let Sendmux return the real validation error if a file is rejected.
- For presigned `PUT`, send the exact `Content-Type` and `Content-Length` returned with the URL.
- Direct Sending API binary uploads require exact `Content-Length`. CLI, SDK, and MCP file helpers calculate it for you.
- Do not try to bypass upload size caps. For mailbox uploads, split or externally host files over 7,500,000 bytes.
- For MCP reads, call `mailbox_read_attachment` first. It returns inline text for text-like attachments and a link for binary or oversized files.
- For direct downloads, use the `download_url` in attachment metadata promptly. If it expires, fetch the message or attachment metadata again.
- Sending API sends use `attachment_id` refs returned by Sending upload endpoints. Mailbox sends use `blob_id` refs returned by mailbox upload endpoints. Do not mix them.

## MCP

### Mailbox upload and read

Use `mailbox_upload_attachment` before `mailbox_send_message`.

Local stdio, cheapest path:

```text
mailbox_upload_attachment
filename: report.pdf
content_type: application/pdf
file_path: /absolute/path/report.pdf
```

The file path must be inside a filesystem root shared by the MCP client. Hosted MCP rejects `file_path`.

Hosted or shell-capable path:

```text
mailbox_upload_attachment
filename: report.pdf
content_type: application/pdf
size_bytes: 5242880
presign_upload_url: true
```

Then upload without an API key:

```bash
curl -X PUT "$UPLOAD_URL" \
  -H "Content-Type: application/pdf" \
  -H "Content-Length: 5242880" \
  --data-binary @./report.pdf
```

Use the returned `blob_id` in `mailbox_send_message`:

```json
{
  "attachments": [
    {
      "blob_id": "blob_...",
      "filename": "report.pdf",
      "content_type": "application/pdf"
    }
  ]
}
```

For tiny generated content only, use `content_base64`. If the tool rejects size, switch to `file_path`, presigned upload, CLI, or SDK file helpers.

To read inbound attachments, call `mailbox_read_attachment` with `message_id` and `attachment_id`.

```text
mailbox_read_attachment
message_id: msg_...
attachment_id: att_...
```

Use returned `text` directly for text-like files. If the tool returns `resource_link` / `download_url`, fetch the link promptly outside model context. Use `mailbox_get_attachment` only when metadata is enough or you need to refresh an expired link. Do not construct attachment URLs manually.

### Sending API upload

Local stdio, cheapest path:

```text
sending_upload_attachment
filename: report.pdf
content_type: application/pdf
file_path: /absolute/path/report.pdf
```

Use the returned `attachment_id` in `sending_send_email` or `sending_send_email_batch`:

```json
{
  "attachments": [{ "attachment_id": "att_..." }]
}
```

Hosted or shell-capable path:

```text
sending_create_attachment_upload
filename: report.pdf
content_type: application/pdf
size_bytes: 5242880
```

Then `PUT` the file bytes to the returned `upload_url` with the returned headers, including `X-Sendmux-Upload-Token`; do not add a Sendmux API key:

```bash
curl -X PUT "$UPLOAD_URL" \
  -H "X-Sendmux-Upload-Token: $UPLOAD_TOKEN" \
  -H "Content-Type: application/pdf" \
  -H "Content-Length: 5242880" \
  --data-binary @./report.pdf
```

Use the `attachment_id` from the upload response in the send request. Use `sending_get_attachment` only for metadata checks.

## CLI

Mailbox send with a local attachment in one command:

```bash
SENDMUX_API_KEY="$SENDMUX_MBX_KEY" sendmux mailbox:send-message \
  --idempotency-key "$IDEMPOTENCY_KEY" \
  --attach ./report.pdf \
  --body '{
    "to": [{ "email": "user@example.com", "name": null }],
    "subject": "Report",
    "text_body": "Attached."
  }' \
  --json
```

Sending API with a local attachment:

```bash
SENDMUX_API_KEY="$SENDMUX_MBX_KEY" sendmux sending:send \
  --idempotency-key "$IDEMPOTENCY_KEY" \
  --attach ./report.pdf \
  --body-file ./email.json \
  --json
```

Upload a Sending attachment first, then send by `attachment_id`:

```bash
SENDMUX_API_KEY="$SENDMUX_MBX_KEY" sendmux sending:upload-attachment \
  --body-file ./report.pdf \
  --query filename=report.pdf \
  --query content_type=application/pdf \
  --json
```

Presigned mailbox upload from a local file:

```bash
SENDMUX_API_KEY="$SENDMUX_MBX_KEY" sendmux mailbox:upload-attachment \
  --file ./report.pdf \
  --via-presigned \
  --json
```

Mint only:

```bash
SENDMUX_API_KEY="$SENDMUX_MBX_KEY" sendmux mailbox:create-attachment-upload \
  --file ./report.pdf \
  --json
```

Override MIME type with `--content-type` only when inference is wrong.

## Direct HTTP

Sending API direct upload with an API key:

```bash
SIZE_BYTES="$(wc -c < ./report.pdf | tr -d '[:space:]')"

curl -X POST "https://smtp.sendmux.ai/api/v1/emails/attachments?filename=report.pdf&content_type=application/pdf" \
  -H "Authorization: Bearer $SENDMUX_MBX_KEY" \
  -H "Content-Type: application/pdf" \
  -H "Content-Length: $SIZE_BYTES" \
  --data-binary @./report.pdf
```

Sending API delegated upload:

```bash
curl -X POST "https://smtp.sendmux.ai/api/v1/emails/attachment-uploads" \
  -H "Authorization: Bearer $SENDMUX_MBX_KEY" \
  -H "Content-Type: application/json" \
  -d '{"filename":"report.pdf","content_type":"application/pdf","size_bytes":5242880}'
```

Then `PUT` to the returned `upload_url` with returned headers and no Sendmux API key. Use `GET /emails/attachments/{attachment_id}` only for metadata checks.

## TypeScript

Node file helpers live under Node subpaths so browser bundles stay clean.

Mailbox:

```ts
import { createMailboxClient } from "@sendmux/mailbox";
import {
  readMailboxTextAttachment,
  sendMailboxMessageWithFiles,
} from "@sendmux/mailbox/node";

const client = createMailboxClient({ apiKey: process.env.SENDMUX_API_KEY! });

await sendMailboxMessageWithFiles({
  client,
  files: ["./report.pdf"],
  headers: { "Idempotency-Key": idempotencyKey },
  body: {
    to: [{ email: "user@example.com", name: null }],
    subject: "Report",
    text_body: "Attached.",
  },
});

const text = await readMailboxTextAttachment({
  client,
  messageId: "msg_...",
  attachmentId: "att_...",
});
```

Sending API:

```ts
import { createSendingClient } from "@sendmux/sending";
import { sendEmailWithFiles } from "@sendmux/sending/node";

const client = createSendingClient({ apiKey: process.env.SENDMUX_API_KEY! });

await sendEmailWithFiles({
  client,
  files: ["./report.pdf"],
  headers: { "Idempotency-Key": idempotencyKey },
  body: {
    from: { email: "sender@example.com" },
    to: { email: "user@example.com" },
    subject: "Report",
    html_body: "<p>Attached.</p>",
  },
});
```

The combined package also exposes `@sendmux/sdk/node`.

## Python

Mailbox:

```python
from sendmux_mailbox import create_mailbox_client, read_mailbox_text_attachment, send_mailbox_message_with_files

client = create_mailbox_client(api_key=api_key)

send_mailbox_message_with_files(
    client,
    files=["./report.pdf"],
    body={
        "to": [{"email": "user@example.com", "name": None}],
        "subject": "Report",
        "text_body": "Attached.",
    },
    idempotency_key=idempotency_key,
)

text = read_mailbox_text_attachment(
    client,
    message_id="msg_...",
    attachment_id="att_...",
)
```

Sending API:

```python
from sendmux_sending import create_sending_client, send_email_with_files

client = create_sending_client(api_key=api_key)

send_email_with_files(
    client,
    files=["./report.pdf"],
    body={
        "from": {"email": "sender@example.com"},
        "to": {"email": "user@example.com"},
        "subject": "Report",
        "html_body": "<p>Attached.</p>",
    },
    idempotency_key=idempotency_key,
)
```

## Routing

- Sending content and recipient approval: `sendmux-send-email`.
- Mailbox search, triage, reply flow: `sendmux-mailbox-agent`.
- Exact terminal command mechanics: `sendmux-cli`.
- MCP installation or hosted/local setup: `sendmux-mcp-setup`.
- General token-efficiency decisions: `sendmux-token-efficient-usage`.