diff --git a/gh-actions/common/notify-mm-on-failure/README.md b/gh-actions/common/notify-mm-on-failure/README.md
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+# Notify Mattermost on failure
+
+Posts a workflow failure notification to a Mattermost channel using a bot account.
+
+![Example Mattermost notification](example-notification.png)
+
+## Requirements
+
+The bot must be a member of the target channel and its token must be able to create posts. Store the token as a GitHub Actions secret. The Mattermost server URL and channel ID can be stored as repository or organization variables.
+
+## Usage
+
+```yaml
+- name: Notify Mattermost on failure
+  if: ${{ failure() }}
+  uses: canonical/desktop-engineering/gh-actions/common/notify-mm-on-failure@main
+  with:
+    server-url: ${{ vars.MATTERMOST_SERVER_URL }}
+    channel-id: ${{ vars.MATTERMOST_CHANNEL_ID }}
+    bot-token: ${{ secrets.MATTERMOST_BOT_TOKEN }}
+    ping-users: alice,bob
+    additional-info: The package build did not complete.
+    request-ack: true
+    silent: true
+    priority: important
+```
+
+The action does not check the workflow result itself. Callers must use an appropriate condition, such as `failure()`.
+
+## Inputs
+
+| Input             | Required | Default      | Description                                                                                  |
+| ----------------- | -------- | ------------ | -------------------------------------------------------------------------------------------- |
+| `server-url`      | Yes      |              | Base URL of the Mattermost server.                                                           |
+| `channel-id`      | Yes      |              | ID of the Mattermost channel to notify.                                                      |
+| `bot-token`       | Yes      |              | Access token for the Mattermost bot account.                                                 |
+| `ping-users`      | No       | `""`         | Comma-separated Mattermost usernames to mention in the notified users field.                 |
+| `additional-info` | No       | `""`         | Markdown-formatted information to include in the notification.                               |
+| `color`           | No       | `#FF0000`    | Color of the notification attachment.                                                        |
+| `author-icon`     | No       | `repo-owner` | URL of the attachment author icon. See [Author icon](#author-icon).                          |
+| `request-ack`     | No       | `"true"`     | Whether users must acknowledge the notification.                                             |
+| `silent`          | No       | `"false"`    | Whether to deliver the post without user notifications or unread counts.                     |
+| `priority`        | No       | `""`         | Mattermost post priority. See [Priority and acknowledgement](#priority-and-acknowledgement). |
+
+## Notification Behavior
+
+### Priority and acknowledgement
+
+Acknowledgement is requested by default, which marks the post as important. Set `request-ack` to `"false"` to send a notification without requiring acknowledgement.
+
+Set `priority` to override the post priority. When it is empty, the action uses `important` if acknowledgement is requested and otherwise omits priority metadata.
+
+### Silent posts
+
+Set `silent` to `"true"` to post without desktop, push, or email notifications and without incrementing unread or mention counts.
+
+### Author icon
+
+The attachment author icon defaults to the calling repository owner's GitHub icon. Set `author-icon` to a URL to override it, or to an empty string (`author-icon: ""`) to omit the icon.
+
+## API
+
+The action calls the [Mattermost `POST /api/v4/posts` endpoint](https://developers.mattermost.com/api-documentation/#/operations/CreatePost).
diff --git a/gh-actions/common/notify-mm-on-failure/action.yaml b/gh-actions/common/notify-mm-on-failure/action.yaml
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+++ b/gh-actions/common/notify-mm-on-failure/action.yaml
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+name: Notify Mattermost on failure
+description: Notify a Mattermost channel of a workflow failure using a bot account
+
+inputs:
+  server-url:
+    description: The base URL of the Mattermost server.
+    required: true
+  channel-id:
+    description: The ID of the Mattermost channel to notify.
+    required: true
+  bot-token:
+    description: The access token for the Mattermost bot account.
+    required: true
+  ping-users:
+    description: A comma-separated list of Mattermost usernames to mention.
+    required: false
+    default: ""
+  additional-info:
+    description: Markdown-formatted additional information to include in the notification.
+    required: false
+    default: ""
+  color:
+    description: The color of the notification attachment.
+    required: false
+    default: "#FF0000"
+  author-icon:
+    description: >-
+      The URL of the notification attachment author icon. Use repo-owner to use
+      the calling repository owner's GitHub icon, or an empty string to omit it.
+    required: false
+    default: repo-owner
+  request-ack:
+    description: Whether users must acknowledge the notification.
+    required: false
+    default: "true"
+  silent:
+    description: Whether to deliver the notification without user notifications or unread counts.
+    required: false
+    default: "false"
+  priority:
+    description: The Mattermost post priority. Leave empty to infer it from request-ack.
+    required: false
+    default: ""
+
+runs:
+  using: composite
+  steps:
+    - name: Notify Mattermost
+      shell: bash --noprofile --norc -euo pipefail {0}
+      env:
+        MM_SERVER_URL: ${{ inputs.server-url }}
+        MM_CHANNEL_ID: ${{ inputs.channel-id }}
+        MM_BOT_TOKEN: ${{ inputs.bot-token }}
+        MM_PING_USERS: ${{ inputs.ping-users }}
+        MM_ADDITIONAL_INFO: ${{ inputs.additional-info }}
+        MM_COLOR: ${{ inputs.color }}
+        MM_AUTHOR_ICON: ${{ inputs.author-icon }}
+        MM_REQUEST_ACK: ${{ inputs.request-ack }}
+        MM_SILENT: ${{ inputs.silent }}
+        MM_PRIORITY: ${{ inputs.priority }}
+      run: |
+        echo "::add-mask::${MM_BOT_TOKEN}"
+
+        case "${MM_REQUEST_ACK}" in
+          true) request_ack=true ;;
+          false) request_ack=false ;;
+          *)
+            echo "request-ack must be either 'true' or 'false'" >&2
+            exit 1
+            ;;
+        esac
+
+        case "${MM_SILENT}" in
+          true) silent=true ;;
+          false) silent=false ;;
+          *)
+            echo "silent must be either 'true' or 'false'" >&2
+            exit 1
+            ;;
+        esac
+
+        ping_list=''
+        IFS=',' read -ra users <<< "${MM_PING_USERS}"
+        for user in "${users[@]}"; do
+          user="${user#"${user%%[![:space:]]*}"}"
+          user="${user%"${user##*[![:space:]]}"}"
+          if [ -n "${user}" ]; then
+            ping_list="${ping_list} @${user}"
+          fi
+        done
+        ping_list="${ping_list# }"
+
+        author_icon="${MM_AUTHOR_ICON}"
+        if [ "${author_icon}" = "repo-owner" ]; then
+          author_icon="${GITHUB_SERVER_URL}/${GITHUB_REPOSITORY_OWNER}.png?size=16"
+        fi
+
+        # Prepare the JSON payload for the Mattermost post
+        payload=$(jq --null-input \
+          --arg channel_id "${MM_CHANNEL_ID}" \
+          --arg color "${MM_COLOR}" \
+          --arg author_name "${GITHUB_REPOSITORY} Action Failure" \
+          --arg author_link "${GITHUB_SERVER_URL}/${GITHUB_REPOSITORY}" \
+          --arg author_icon "${author_icon}" \
+          --arg title "Workflow '${GITHUB_WORKFLOW}' failed" \
+          --arg title_link "${GITHUB_SERVER_URL}/${GITHUB_REPOSITORY}/actions/runs/${GITHUB_RUN_ID}" \
+          --arg additional_info "${MM_ADDITIONAL_INFO}" \
+          --arg event_name "${GITHUB_EVENT_NAME}" \
+          --arg ref "${GITHUB_REF}" \
+          --arg ref_link "${GITHUB_SERVER_URL}/${GITHUB_REPOSITORY}/tree/${GITHUB_REF}" \
+          --arg run_id "${GITHUB_RUN_ID}" \
+          --arg actor "${GITHUB_ACTOR}" \
+          --arg ping_list "${ping_list}" \
+          --arg priority "${MM_PRIORITY}" \
+          --argjson request_ack "${request_ack}" \
+          '{
+            channel_id: $channel_id,
+            props: {
+              attachments: [{
+                color: $color,
+                author_name: $author_name,
+                author_link: $author_link,
+                title: $title,
+                title_link: $title_link,
+                text: $additional_info,
+                fields: [
+                  {short: true, title: "Event", value: $event_name},
+                  {short: true, title: "Ref", value: "[\($ref)](\($ref_link))"},
+                  {short: true, title: "Run ID", value: "[\($run_id)](\($title_link))"},
+                  {short: true, title: "Actor", value: $actor},
+                  {short: false, title: ":warning: Notified Users", value: $ping_list}
+                ]
+              }]
+            }
+          }
+          | if $author_icon != "" then
+              .props.attachments[0].author_icon = $author_icon
+            else
+              .
+            end
+          | if $priority != "" or $request_ack then
+              .metadata.priority = {
+                priority: (if $priority != "" then $priority else "important" end),
+                requested_ack: $request_ack
+              }
+            else
+              .
+            end')
+
+        # Send the notification to Mattermost
+        response=$(curl --fail-with-body --silent --show-error --location \
+          --request POST \
+          --header "Authorization: Bearer ${MM_BOT_TOKEN}" \
+          --header 'Content-Type: application/json' \
+          --header 'Accept: application/json' \
+          --data-binary "${payload}" \
+          "${MM_SERVER_URL%/}/api/v4/posts?silent=${silent}")
+
+        # Check the response for a valid post ID
+        if ! post_id=$(jq --exit-status --raw-output \
+          'if (.id | type) == "string" and .id != "" then .id else empty end' \
+          <<< "${response}" 2>/dev/null); then
+          echo "::error::Mattermost returned an invalid response. Check that server-url is the Mattermost base URL and is accessible without an interactive login."
+          exit 1
+        fi
+        echo "Mattermost notification posted with ID ${post_id}"
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