diff --git a/pyproject.toml b/pyproject.toml
new file mode 100644
index 0000000..6243923
--- /dev/null
+++ b/pyproject.toml
@@ -0,0 +1,13 @@
+[project]
+name = "lib-send-notifications"
+version = "0.1.0"
+description = "Shared library to send notifications via the internal backend API"
+authors = [{ name = "Florian Gänsejunge" }]
+dependencies = ["lib-secret-manager","requests"]
+
+[build-system]
+requires = ["setuptools", "wheel"]
+build-backend = "setuptools.build_meta"
+
+[project.urls]
+Homepage = "https://git.gansejunge.com/notifier/lib-send-notifications"
diff --git a/send_notifications/__init__.py b/send_notifications/__init__.py
new file mode 100644
index 0000000..ac23189
--- /dev/null
+++ b/send_notifications/__init__.py
@@ -0,0 +1,5 @@
+import logging
+from .send_notification import send_notification, NotificationError,NotificationUnavailableError
+
+logging.getLogger(__name__).addHandler(logging.NullHandler())
+__all__ = ["send_notification", "NotificationError", "NotificationUnavailableError"]
diff --git a/send_notifications/send_notification.py b/send_notifications/send_notification.py
new file mode 100644
index 0000000..2821de4
--- /dev/null
+++ b/send_notifications/send_notification.py
@@ -0,0 +1,172 @@
+"""Client for sending notifications to the internal notification backend.
+
+Usage:
+	notifier = NotificationClient()   # resolves the API key once, fails fast if missing
+	notifier.send("Title", "Body text", link="https://example.com")
+"""
+
+import logging
+import os
+import time
+
+import requests
+
+
+from secret_manager import get_secret
+
+__all__ = [
+	"NotificationClient",
+	"NotificationError",
+	"NotificationUnavailableError",
+]
+
+logger = logging.getLogger(__name__)
+
+DEFAULT_BACKEND_URL = "http://localhost:8101/internal/receive-notifications"
+DEFAULT_KEY_ENV = "NOTIFY_API_KEY"
+DEFAULT_KEY_PATH = "/etc/secrets/api_key"
+RETRYABLE_STATUS = {429, 502, 503, 504}
+
+
+class NotificationError(Exception):
+	"""The notification could not be delivered."""
+
+	def __init__(self, message: str, status_code: int | None = None):
+		super().__init__(message)
+		self.status_code = status_code
+
+
+class NotificationUnavailableError(NotificationError):
+	"""The backend was unreachable or kept failing after all retries."""
+
+
+class NotificationClient:
+	"""Sends notifications to the internal backend service.
+
+	The API key is resolved once at construction time and kept in memory, so it
+	keeps working after secret files are deleted. Construction raises if no key
+	can be found, so misconfiguration surfaces at startup rather than on the
+	first notification.
+
+	Args:
+		api_key: Internal API key. If omitted, it is read from the environment
+			variable ``key_env`` first, then from the file at ``key_path``.
+		url: Backend endpoint. Defaults to the BACKEND_API_URL environment
+			variable, then to DEFAULT_BACKEND_URL.
+		key_env: Name of the environment variable holding the API key.
+		key_path: Path of the secret file holding the API key.
+
+	Raises:
+		MissingCredentialError: No API key was passed, set in the environment,
+			or found in the secret file.
+	"""
+
+	def __init__(
+		self,
+		api_key: str | None = None,
+		url: str | None = None,
+		*,
+		key_env: str = DEFAULT_KEY_ENV,
+		key_path: str = DEFAULT_KEY_PATH,
+	):
+		self._api_key = api_key or get_secret(key_env, key_path)
+		self.url = url or os.getenv("BACKEND_API_URL", DEFAULT_BACKEND_URL)
+
+	def __repr__(self) -> str:
+		# Never include the API key in logs or tracebacks.
+		return f"{type(self).__name__}(url={self.url!r})"
+
+	def send(
+		self,
+		title: str,
+		body: str,
+		link: str | None = None,
+		category: str = "utility",
+		*,
+		user_id: int = 1,
+		max_attempts: int = 5,
+		timeout: float = 5.0,
+	) -> str:
+		"""Send a notification to the backend.
+
+		Delivery is retried with exponential backoff (1s, 2s, 4s, ...) on
+		timeouts, connection errors, and HTTP 429/502/503/504. Other HTTP
+		errors fail immediately.
+
+		Args:
+			title: Short headline shown for the notification.
+			body: Main notification text.
+			link: Optional URL the notification should open.
+			category: Notification category used by the backend for routing
+				and display. Defaults to "utility".
+			user_id: ID of the user who receives the notification.
+			max_attempts: Total number of delivery attempts, including the
+				first.
+			timeout: Per-request timeout in seconds.
+
+		Returns:
+			The backend's response body as text.
+
+		Raises:
+			NotificationUnavailableError: The backend was unreachable or kept
+				returning transient errors after all attempts.
+			NotificationError: The backend rejected the request (non-retryable
+				HTTP error) or the request could not be sent. The HTTP status
+				is available as ``status_code`` when there is one.
+		"""
+		headers = {
+			"X-API-Key-Internal": self._api_key,
+			"Content-Type": "application/json",
+		}
+		payload = {
+			"user_id": user_id,
+			"message": {
+				"title": title,
+				"body": body,
+				"link": link,
+				"category": category,
+				"timestamp": int(time.time()),
+			},
+		}
+
+		logger.debug("Sending notification: title=%r link=%r", title, link)
+		last_error = "unknown error"
+
+		with requests.Session() as session:
+			for attempt in range(1, max_attempts + 1):
+				try:
+					response = session.post(
+						self.url, headers=headers, json=payload, timeout=timeout
+					)
+				except (requests.Timeout, requests.ConnectionError) as e:
+					last_error = type(e).__name__
+				except requests.RequestException as e:
+					raise NotificationError(f"Request failed: {e}") from e
+				else:
+					if response.ok:
+						logger.info("Notification sent: %r", title)
+						return response.text
+
+					logger.warning(
+						"Backend returned HTTP %s: %s",
+						response.status_code,
+						response.text,
+					)
+					if response.status_code not in RETRYABLE_STATUS:
+						raise NotificationError(
+							f"Backend rejected notification "
+							f"(HTTP {response.status_code})",
+							status_code=response.status_code,
+						)
+					last_error = f"HTTP {response.status_code}"
+
+				logger.warning(
+					"Attempt %d/%d failed: %s", attempt, max_attempts, last_error
+				)
+				if attempt < max_attempts:
+					time.sleep(2 ** (attempt - 1))
+
+		raise NotificationUnavailableError(
+			f"Notification service unavailable after {max_attempts} attempts: "
+			f"{last_error}"
+		)
\ No newline at end of file