Configuration¶

Overview¶

Watchtower uses Shoutrrr to provide notification functionality. Notifications are sent via hooks in the logrus logging system.

Enabling Notifications¶

To send notifications, use the NOTIFICATION URL configuration option to specify the Shoutrrr service URL.

The Shoutrrr URL follows the format:

<service>://<required-credentials>[:<optional-credentials>]@<required-service>/<required-path>?<key>=<value>&...

The format is the same for all services, but the parameters, path, and credentials vary between them.

The NOTIFICATION URL configuration option can also reference a file, in which case the contents of the file are used.

Using Multiple Notification Services¶

The NOTIFICATION URL configuration option can also be used multiple times or use a comma-separated list in the WATCHTOWER_NOTIFICATION_URL environment variable to utilize multiple notification services.

Using multiple notifications with environment variables

There is currently a bug in Viper (Issue), which prevents comma-separated slices to be used when using the environment variable.

A workaround is available where we instead put quotes around the environment variable value and replace the commas with spaces:

WATCHTOWER_NOTIFICATIONS="slack msteams"

If you're a docker-compose user, make sure to specify environment variables' values in your .yml file without double quotes ("). This prevents unexpected errors when Watchtower starts.

Startup Notifications¶

Watchtower will log and send a notification every time it is started.

This behavior can be disabled with the DISABLE STARTUP MESSAGE configuration option.

General Notification Settings¶

Level¶

Controls the log level for notifications.

Possible values: panic, fatal, error, warn, info, debug, trace.

            Argument: --notifications-level
Environment Variable: WATCHTOWER_NOTIFICATIONS_LEVEL
                Type: String
             Default: info

Note

In legacy mode (--notification-report=false), only info-level logs trigger notifications, ensuring a focused step-by-step update summary.

Hostname¶

Custom hostname specified in subject/title. Useful for overriding the operating system hostname.

            Argument: --notifications-hostname
Environment Variable: WATCHTOWER_NOTIFICATIONS_HOSTNAME
                Type: String
             Default: None

Delay¶

Delay before sending notifications expressed in seconds.

            Argument: --notifications-delay
Environment Variable: WATCHTOWER_NOTIFICATIONS_DELAY
                Type: Integer
             Default: None

Title Tag¶

Prefix to include in the title. Useful when running multiple Watchtower instances.

            Argument: --notification-title-tag
Environment Variable: WATCHTOWER_NOTIFICATION_TITLE_TAG
                Type: String
             Default: None

Skip Title¶

Used to not pass the title param to notifications. This will not pass a dynamic title override to notification services. If no title is configured for the service, it will remove the title altogether.

            Argument: --notification-skip-title
Environment Variable: WATCHTOWER_NOTIFICATION_SKIP_TITLE
                Type: Boolean
             Default: false

Log Stdout¶

Enable output from logger:// Shoutrrr service to stdout.

            Argument: --notification-log-stdout
Environment Variable: WATCHTOWER_NOTIFICATION_LOG_STDOUT
                Type: Boolean
             Default: false

Split by Container¶

Send separate notifications for each updated container instead of grouping them.

            Argument: --notification-split-by-container
Environment Variable: WATCHTOWER_NOTIFICATION_SPLIT_BY_CONTAINER
                Type: Boolean
             Default: false

Note

When disabled (default), notifications are grouped for all updated containers in a single session. When enabled, a separate notification is sent for each container update.

Usage Example¶

To enable separate notifications per container:

docker run -d \
  --name watchtower \
  -v /var/run/docker.sock:/var/run/docker.sock \
  -e WATCHTOWER_NOTIFICATIONS=slack \
  -e WATCHTOWER_NOTIFICATION_SLACK_HOOK_URL="https://hooks.slack.com/services/xxx/yyyyyyyyyyyyyyy" \
  -e WATCHTOWER_NOTIFICATION_SPLIT_BY_CONTAINER=true \
  nickfedor/watchtower

Email Notifications¶

Watchtower uses Shoutrrr's smtp service to send email notifications.

Either legacy email notification flags or Shoutrrr URLs can be used; however, directly using URLs is recommended for greater control and clarity, especially for configuring TLS settings (e.g., STARTTLS or Implicit TLS).

To send notifications via e-mail, add email to the --notifications option or the WATCHTOWER_NOTIFICATIONS environment variable.

Email notification flags (e.g., WATCHTOWER_NOTIFICATION_EMAIL_SERVER, WATCHTOWER_NOTIFICATION_EMAIL_FROM) are automatically converted to a Shoutrrr SMTP URL internally.

Common SMTP Configurations¶

GmailAWS SESMicrosoft 365Generic (SSL)Generic (Plain)

Property	Value
Port	`587`
Encryption	`ExplicitTLS`
UseStartTLS	`Yes`

smtp://${USER}:${PASSWORD}@smtp.gmail.com:587/?fromaddress=${FROM}&toaddresses=${TO}&encryption=ExplicitTLS&usestarttls=yes&timeout=30s

Note

For Gmail, use an App Password if two-factor authentication is enabled.

Property	Value
Port	`587`
Encryption	`ExplicitTLS`
UseStartTLS	`Yes`

smtp://${USER}:${PASSWORD}@email-smtp.us-east-1.amazonaws.com:587/?fromaddress=${FROM}&toaddresses=${TO}&encryption=ExplicitTLS&usestarttls=yes&timeout=30s

Property	Value
Port	`587`
Encryption	`ExplicitTLS`
UseStartTLS	`Yes`

smtp://${USER}:${PASSWORD}@smtp.office365.com:587/?fromaddress=${FROM}&toaddresses=${TO}&encryption=ExplicitTLS&usestarttls=yes&timeout=30s

Property	Value
Port	`465`
Encryption	`ImplicitTLS`
UseStartTLS	`No`

smtp://${USER}:${PASSWORD}@smtp.example.com:465/?fromaddress=${FROM}&toaddresses=${TO}&encryption=ImplicitTLS&usestarttls=no&timeout=30s

Property	Value
Port	`25`
Encryption	`None`
UseStartTLS	`No`

smtp://${USER}:${PASSWORD}@smtp.example.com:25/?fromaddress=${FROM}&toaddresses=${TO}&encryption=None&usestarttls=no&timeout=30s

Notes¶

Timeout:
- The default SMTP timeout is 10 seconds.
- If you experience timeouts (e.g., failed to send: timed out: using smtp), add &timeout=30s to the URL to allow more time for server responses, especially with proxies or slow networks.
Authentication:
- Use &auth=Plain for username/password authentication (default if credentials provided).
- For OAuth2 (e.g., Gmail with app-specific passwords), use &auth=OAuth2.
Testing:
- Install Shoutrrr using one of the various installation methods.
- Test your URL with the Shoutrrr CLI:
```
shoutrrr send -u <URL> -m "Test message"
```
Proxy Issues:
- If using a Docker proxy (e.g., tcp://dockerproxy:2375), ensure it allows outbound connections to ${SMTP_HOST}:${SMTP_PORT}.
- Test connectivity with telnet ${SMTP_HOST} ${SMTP_PORT} inside the container.

Example Legacy Email Configuration¶

Docker CLIDocker CLI (SMTP Relay)Docker ComposeDocker Compose (SMTP Relay)

docker run -d \
  --name watchtower \
  -v /var/run/docker.sock:/var/run/docker.sock \
  -e WATCHTOWER_NOTIFICATIONS=email \
  -e WATCHTOWER_NOTIFICATION_EMAIL_FROM=[email protected] \
  -e WATCHTOWER_NOTIFICATION_EMAIL_TO=[email protected] \
  -e WATCHTOWER_NOTIFICATION_EMAIL_SERVER=smtp.example.com \
  -e WATCHTOWER_NOTIFICATION_EMAIL_SERVER_PORT=587 \
  -e WATCHTOWER_NOTIFICATION_EMAIL_SERVER_USER=user \
  -e WATCHTOWER_NOTIFICATION_EMAIL_SERVER_PASSWORD=secret \
  -e WATCHTOWER_NOTIFICATION_EMAIL_DELAY=10 \
  nickfedor/watchtower

docker run -d \
  --name watchtower \
  -v /var/run/docker.sock:/var/run/docker.sock \
  -e WATCHTOWER_NOTIFICATIONS=email \
  -e WATCHTOWER_NOTIFICATION_EMAIL_FROM=[email protected] \
  -e WATCHTOWER_NOTIFICATION_EMAIL_TO=[email protected] \
  -e WATCHTOWER_NOTIFICATION_EMAIL_SERVER=relay.example.com \
  nickfedor/watchtower

Note

This assumes that you already have an SMTP server up and running that you can connect to. If you don't or you want to bring up Watchtower with your own simple SMTP relay, then check out the Docker Compose example.

services:
  watchtower:
    image: nickfedor/watchtower:latest
    environment:
      WATCHTOWER_NOTIFICATIONS: email
      WATCHTOWER_NOTIFICATION_EMAIL_FROM: [email protected]
      WATCHTOWER_NOTIFICATION_EMAIL_TO: [email protected]
      WATCHTOWER_NOTIFICATION_EMAIL_SERVER: smtp.example.com
      WATCHTOWER_NOTIFICATION_EMAIL_SERVER_PORT: 587
      WATCHTOWER_NOTIFICATION_EMAIL_SERVER_USER: user
      WATCHTOWER_NOTIFICATION_EMAIL_SERVER_PASSWORD: secret
      WATCHTOWER_NOTIFICATION_EMAIL_DELAY: 10
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock

services:
  watchtower:
    image: nickfedor/watchtower:latest
    environment:
      WATCHTOWER_NOTIFICATIONS: email
      WATCHTOWER_NOTIFICATION_EMAIL_FROM: [email protected]
      WATCHTOWER_NOTIFICATION_EMAIL_TO: [email protected]
      WATCHTOWER_NOTIFICATION_EMAIL_SERVER: relay.example.com
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock

Note

The example assumes that your domain is called example.com and that you are going to use a valid certificate for smtp.example.com.

This hostname has to be used as WATCHTOWER_NOTIFICATION_EMAIL_SERVER, otherwise the TLS connection will fail with Failed to send notification email or connection: connection refused errors.

We also have to add a network for this setup in order to add an alias to it.

If you also want to enable DKIM or other features on the SMTP server, then you will find more information at freinet/postfix-relay

Legacy Notification Flags¶

Email From¶

The e-mail address from which notifications will be sent.

            Argument: --notification-email-from
Environment Variable: WATCHTOWER_NOTIFICATION_EMAIL_FROM
                Type: String
             Default: None

Email To¶

The e-mail address to which notifications will be sent.

            Argument: --notification-email-to
Environment Variable: WATCHTOWER_NOTIFICATION_EMAIL_TO
                Type: String
             Default: None

Email Server¶

The SMTP server (IP or FQDN) to send notifications through.

            Argument: --notification-email-server
Environment Variable: WATCHTOWER_NOTIFICATION_EMAIL_SERVER
                Type: String
             Default: None

Email Server TLS Skip Verify¶

Skip verification of the server certificate when using TLS.

            Argument: --notification-email-server-tls-skip-verify
Environment Variable: WATCHTOWER_NOTIFICATION_EMAIL_SERVER_TLS_SKIP_VERIFY
                Type: Boolean
             Default: false

Email Server User¶

The username for the SMTP server if it requires authentication.

            Argument: --notification-email-server-user
Environment Variable: WATCHTOWER_NOTIFICATION_EMAIL_SERVER_USER
                Type: String
             Default: None

Email Server Password¶

The password for the SMTP server if it requires authentication.

            Argument: --notification-email-server-password
Environment Variable: WATCHTOWER_NOTIFICATION_EMAIL_SERVER_PASSWORD
                Type: String
             Default: None

Note

This option can also reference a file, in which case the contents of the file are used.

Email Subject Tag¶

Subject prefix tag for notifications via mail.

            Argument: --notification-email-subjecttag
Environment Variable: WATCHTOWER_NOTIFICATION_EMAIL_SUBJECTTAG
                Type: String
             Default: ""

Email Server Port¶

The port the SMTP server listens on.

            Argument: --notification-email-server-port
Environment Variable: WATCHTOWER_NOTIFICATION_EMAIL_SERVER_PORT
                Type: Integer
             Default: 25

Email Delay¶

The delay (in seconds) between sending notifications if multiple containers are updated at once.

            Argument: --notification-email-delay
Environment Variable: WATCHTOWER_NOTIFICATION_EMAIL_DELAY
                Type: Integer
             Default: None

Transitioning from Legacy Email Notifications to Shoutrrr¶

Watchtower includes a watchtower notify-upgrade command to convert legacy flags to a Shoutrrr URL.

The output is written to a temporary file, which you can copy using:

docker cp <CONTAINER>:<FILE_PATH> ./watchtower-notifications.env

Example Walkthrough¶

Example Legacy Configuration:

Docker CLIDocker Compose

docker run -d \
  --name watchtower \
  -v /var/run/docker.sock:/var/run/docker.sock \
  -e WATCHTOWER_NOTIFICATIONS=email \
  -e WATCHTOWER_NOTIFICATION_EMAIL_SERVER=smtp.example.com \
  -e WATCHTOWER_NOTIFICATION_EMAIL_SERVER_PORT=587 \
  -e WATCHTOWER_NOTIFICATION_EMAIL_SERVER_USER=[email protected] \
  -e WATCHTOWER_NOTIFICATION_EMAIL_SERVER_PASSWORD=secret \
  -e WATCHTOWER_NOTIFICATION_EMAIL_FROM=[email protected] \
  -e WATCHTOWER_NOTIFICATION_EMAIL_TO=[email protected] \
  nickfedor/watchtower

services:
  watchtower:
    image: nickfedor/watchtower:latest
    environment:
      WATCHTOWER_NOTIFICATIONS: email
      WATCHTOWER_NOTIFICATION_EMAIL_SERVER: smtp.example.com
      WATCHTOWER_NOTIFICATION_EMAIL_SERVER_PORT: 587
      WATCHTOWER_NOTIFICATION_EMAIL_SERVER_USER: [email protected]
      WATCHTOWER_NOTIFICATION_EMAIL_SERVER_PASSWORD: secret
      WATCHTOWER_NOTIFICATION_EMAIL_FROM: [email protected]
      WATCHTOWER_NOTIFICATION_EMAIL_TO: [email protected]
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock

Run the following CLI command:

docker compose exec watchtower watchtower notify-upgrade

Converted Shoutrrr URL:

smtp://[email protected]:[email protected]:587/[email protected]&[email protected]&encryption=ExplicitTLS&usestarttls=yes

Replace the legacy flags with:

Docker CLIDocker Compose

docker run -d \
  --name watchtower \
  -v /var/run/docker.sock:/var/run/docker.sock \
  -e WATCHTOWER_NOTIFICATIONS=shoutrrr \
  -e WATCHTOWER_NOTIFICATION_URL=smtp://[email protected]:[email protected]:587/?fromaddress=[email protected]&toaddresses=[email protected]&encryption=ExplicitTLS&usestarttls=yes \
  -e WATCHTOWER_NOTIFICATION_EMAIL_DELAY=10 \
  -e WATCHTOWER_NOTIFICATION_EMAIL_SUBJECTTAG=Watchtower \
  nickfedor/watchtower

services:
  watchtower:
    image: nickfedor/watchtower:latest
    environment:
      WATCHTOWER_NOTIFICATIONS: shoutrrr
      WATCHTOWER_NOTIFICATION_URL: smtp://[email protected]:[email protected]:587/[email protected]&[email protected]&encryption=ExplicitTLS&usestarttls=yes
      WATCHTOWER_NOTIFICATION_EMAIL_DELAY: "10"
      WATCHTOWER_NOTIFICATION_EMAIL_SUBJECTTAG: Watchtower
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock

Note

Avoid using unrecognized flags like WATCHTOWER_NOTIFICATION_EMAIL_SERVER_SSL, as they are ignored and may cause confusion.

Use WATCHTOWER_NOTIFICATION_EMAIL_SERVER_TLS_SKIP_VERIFY to disable TLS verification if needed (not recommended for production).

Slack Notifications¶

Example Slack Configuration¶

Docker CLIDocker Compose

docker run -d \
  --name watchtower \
  -v /var/run/docker.sock:/var/run/docker.sock \
  -e WATCHTOWER_NOTIFICATIONS=slack \
  -e WATCHTOWER_NOTIFICATION_SLACK_HOOK_URL="https://hooks.slack.com/services/xxx/yyyyyyyyyyyyyyy" \
  -e WATCHTOWER_NOTIFICATION_SLACK_IDENTIFIER=watchtower-server-1 \
  -e WATCHTOWER_NOTIFICATION_SLACK_CHANNEL=#my-custom-channel \
  nickfedor/watchtower

services:
  watchtower:
    image: nickfedor/watchtower:latest
    environment:
      WATCHTOWER_NOTIFICATIONS: slack
      WATCHTOWER_NOTIFICATION_SLACK_HOOK_URL: "https://hooks.slack.com/services/xxx/yyyyyyyyyyyyyyy"
      WATCHTOWER_NOTIFICATION_SLACK_IDENTIFIER: watchtower-server-1
      WATCHTOWER_NOTIFICATION_SLACK_CHANNEL: "#my-custom-channel"
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock

Slack Configuration Options¶

To receive notifications in Slack, add slack to the --notifications option or the WATCHTOWER_NOTIFICATIONS environment variable.

Watchtower supports the following Slack-related options:

Slack Hook URL¶

The Slack webhook URL for notifications.

            Argument: --notification-slack-hook-url
Environment Variable: WATCHTOWER_NOTIFICATION_SLACK_HOOK_URL
                Type: String
             Default: None

Note

This option can also reference a file, in which case the contents of the file are used.

Slack Identifier¶

Custom name under which messages are sent.

            Argument: --notification-slack-identifier
Environment Variable: WATCHTOWER_NOTIFICATION_SLACK_IDENTIFIER
                Type: String
             Default: watchtower

Slack Channel¶

A string which overrides the webhook's default channel (optional).

            Argument: --notification-slack-channel
Environment Variable: WATCHTOWER_NOTIFICATION_SLACK_CHANNEL
                Type: String
             Default: None

Microsoft Teams Notifications¶

Docker CLIDocker Compose

docker run -d \
  --name watchtower \
  -v /var/run/docker.sock:/var/run/docker.sock \
  -e WATCHTOWER_NOTIFICATIONS=msteams \
  -e WATCHTOWER_NOTIFICATION_MSTEAMS_HOOK_URL="https://outlook.office.com/webhook/xxxxxxxx@xxxxxxx/IncomingWebhook/yyyyyyyy/zzzzzzzzzz" \
  -e WATCHTOWER_NOTIFICATION_MSTEAMS_USE_LOG_DATA=true \
  nickfedor/watchtower

services:
  watchtower:
    image: nickfedor/watchtower:latest
    environment:
      WATCHTOWER_NOTIFICATIONS: msteams
      WATCHTOWER_NOTIFICATION_MSTEAMS_HOOK_URL: "https://outlook.office.com/webhook/xxxxxxxx@xxxxxxx/IncomingWebhook/yyyyyyyy/zzzzzzzzzz"
      WATCHTOWER_NOTIFICATION_MSTEAMS_USE_LOG_DATA: true
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock

Microsoft Teams Configuration Options¶

To receive notifications in Microsoft Teams, add msteams to the --notifications option or the WATCHTOWER_NOTIFICATIONS environment variable.

Watchtower supports the following Microsoft Teams-related options:

MSTeams Hook URL¶

The Microsoft Teams webhook URL for notifications.

            Argument: --notification-msteams-hook
Environment Variable: WATCHTOWER_NOTIFICATION_MSTEAMS_HOOK_URL
                Type: String
             Default: None

Note

This option can also reference a file, in which case the contents of the file are used.

MSTeams Use Log Data¶

Enable sending keys/values filled by log.WithField or log.WithFields as Microsoft Teams message facts.

            Argument: --notification-msteams-data
Environment Variable: WATCHTOWER_NOTIFICATION_MSTEAMS_USE_LOG_DATA
                Type: Boolean
             Default: false

Gotify Notifications¶

Docker CLIDocker Compose

docker run -d \
  --name watchtower \
  -v /var/run/docker.sock:/var/run/docker.sock \
  -e WATCHTOWER_NOTIFICATIONS=gotify \
  -e WATCHTOWER_NOTIFICATION_GOTIFY_URL="https://my.gotify.tld/" \
  -e WATCHTOWER_NOTIFICATION_GOTIFY_TOKEN="SuperSecretToken" \
  -e WATCHTOWER_NOTIFICATION_GOTIFY_TLS_SKIP_VERIFY=true \
  nickfedor/watchtower

services:
  watchtower:
    image: nickfedor/watchtower:latest
    environment:
      WATCHTOWER_NOTIFICATIONS: gotify
      WATCHTOWER_NOTIFICATION_GOTIFY_URL: "https://my.gotify.tld/"
      WATCHTOWER_NOTIFICATION_GOTIFY_TOKEN: "SuperSecretToken"
      WATCHTOWER_NOTIFICATION_GOTIFY_TLS_SKIP_VERIFY: true
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock

Gotify Configuration Options¶

To push a notification to your Gotify instance, add gotify to the --notifications option or the WATCHTOWER_NOTIFICATIONS environment variable.

Watchtower supports the following Gotify-related options:

Gotify URL¶

The URL of the Gotify instance.

            Argument: --notification-gotify-url
Environment Variable: WATCHTOWER_NOTIFICATION_GOTIFY_URL
                Type: String
             Default: None

Gotify Token¶

The app token for the Gotify instance.

            Argument: --notification-gotify-token
Environment Variable: WATCHTOWER_NOTIFICATION_GOTIFY_TOKEN
                Type: String
             Default: None

Note

This option can also reference a file, in which case the contents of the file are used.

Gotify TLS Skip Verify¶

Skip verification of the server certificate when using TLS.

            Argument: --notification-gotify-tls-skip-verify
Environment Variable: WATCHTOWER_NOTIFICATION_GOTIFY_TLS_SKIP_VERIFY
                Type: Boolean
             Default: false

Signal Notifications¶

Watchtower uses Shoutrrr's signal service to send Signal notifications.

Signal notifications require a Signal API server that can send messages on behalf of a registered Signal account. This is typically done using signal-cli-rest-api or secured-signal-api.

Setting up Signal API Server¶

Phone Number: A dedicated phone number registered with Signal
API Server: A server running signal-cli with REST API capabilities
Account Linking: Linking the server as a secondary device to your Signal account
Optional Security Layer: Authentication and endpoint restrictions via a proxy

The server must be able to receive SMS verification codes during initial setup and maintain a persistent connection to Signal's servers.

Example Signal Configuration¶

Docker CLIDocker Compose

docker run -d \
  --name watchtower \
  -v /var/run/docker.sock:/var/run/docker.sock \
  -e WATCHTOWER_NOTIFICATION_URL=signal://localhost:8080/+1234567890/+0987654321 \
  nickfedor/watchtower

services:
  watchtower:
    image: nickfedor/watchtower:latest
    environment:
      WATCHTOWER_NOTIFICATION_URL: signal://localhost:8080/+1234567890/+0987654321
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock

Signal URL Format¶

signal://[user:password@]host:port/source_phone/recipient1/recipient2

Parameters¶

host: Signal API server hostname or IP address
port: Signal API server port (default: 8080)
user: Username for HTTP Basic Authentication (optional)
password: Password for HTTP Basic Authentication (optional)
source_phone: Your Signal phone number with country code (e.g., +1234567890)
recipient1, recipient2: Phone numbers or group IDs to send to

TLS Configuration¶

Use signal:// for HTTPS (default, recommended)
Use signal://...?disabletls=yes for HTTP (insecure, for local testing only)

Examples¶

Send to a single phone number:

signal://localhost:8080/+1234567890/+0987654321

Send to multiple recipients:

signal://localhost:8080/+1234567890/+0987654321/+1123456789/group.testgroup

Send to a group:

signal://localhost:8080/+1234567890/group.abcdefghijklmnop=

With authentication:

signal://user:password@localhost:8080/+1234567890/+0987654321

With API token (Bearer auth):

signal://localhost:8080/+1234567890/+0987654321?token=YOUR_API_TOKEN

Using HTTP instead of HTTPS:

signal://localhost:8080/+1234567890/+0987654321?disabletls=yes

Signal Attachments¶

The Signal service supports sending base64-encoded attachments:

shoutrrr send "signal://localhost:8080/+1234567890/+0987654321" \
  "Message with attachment" \
  --attachments "base64data1,base64data2"

Note

Attachments must be provided as base64-encoded data. The API server handles MIME type detection and file handling.