diff --git a/app/__init__.py b/app/__init__.py
new file mode 100644
index 0000000..6451a34
--- /dev/null
+++ b/app/__init__.py
@@ -0,0 +1 @@
+"""SillyHome Next application package."""
diff --git a/app/api/__init__.py b/app/api/__init__.py
new file mode 100644
index 0000000..dff53e5
--- /dev/null
+++ b/app/api/__init__.py
@@ -0,0 +1 @@
+"""API package."""
diff --git a/app/api/v1/__init__.py b/app/api/v1/__init__.py
new file mode 100644
index 0000000..7939870
--- /dev/null
+++ b/app/api/v1/__init__.py
@@ -0,0 +1 @@
+"""Version 1 API package."""
diff --git a/app/api/v1/entities.py b/app/api/v1/entities.py
index 8b6f293..34c15c3 100644
--- a/app/api/v1/entities.py
+++ b/app/api/v1/entities.py
@@ -1,10 +1,12 @@
 from __future__ import annotations
 
-from typing import List, Sequence
+from collections.abc import Sequence
 
-from fastapi import APIRouter
+from fastapi import APIRouter, Depends
 
+from app.dependencies import get_ha_reader
 from app.ha.models import HaEntitySummary
+from app.ha.reader import HaReader
 
 router = APIRouter(prefix="/v1", tags=["entities"])
 
@@ -13,7 +15,7 @@ router = APIRouter(prefix="/v1", tags=["entities"])
     "/entities",
     summary="Home-Assistant-Entities auflisten",
     description="Gibt eine kompakte Zusammenfassung aller erreichbaren HA-Entitäten zurück.",
-    response_model=List[HaEntitySummary],
+    response_model=list[HaEntitySummary],
 )
-def list_entities() -> Sequence[HaEntitySummary]:
-    raise NotImplementedError("Integration mit dem HA-Client folgt in separatem Issue.")
+def list_entities(reader: HaReader = Depends(get_ha_reader)) -> Sequence[HaEntitySummary]:
+    return reader.read_entities()
diff --git a/app/config.py b/app/config.py
new file mode 100644
index 0000000..fd9d619
--- /dev/null
+++ b/app/config.py
@@ -0,0 +1,21 @@
+from __future__ import annotations
+
+import os
+from dataclasses import dataclass
+
+
+@dataclass(frozen=True)
+class Settings:
+    ha_url: str | None = None
+    ha_token: str | None = None
+
+    @property
+    def ha_configured(self) -> bool:
+        return bool(self.ha_url and self.ha_token)
+
+
+def load_settings() -> Settings:
+    return Settings(
+        ha_url=os.getenv("SILLYHOME_HA_URL") or os.getenv("HA_URL"),
+        ha_token=os.getenv("SILLYHOME_HA_TOKEN") or os.getenv("HA_TOKEN"),
+    )
diff --git a/app/dependencies.py b/app/dependencies.py
new file mode 100644
index 0000000..de4207c
--- /dev/null
+++ b/app/dependencies.py
@@ -0,0 +1,15 @@
+from __future__ import annotations
+
+from fastapi import HTTPException, Request, status
+
+from app.ha.reader import HaReader
+
+
+def get_ha_reader(request: Request) -> HaReader:
+    reader = getattr(request.app.state, "ha_reader", None)
+    if not isinstance(reader, HaReader):
+        raise HTTPException(
+            status_code=status.HTTP_503_SERVICE_UNAVAILABLE,
+            detail="Home Assistant is not configured.",
+        )
+    return reader
diff --git a/app/ha/client.py b/app/ha/client.py
index 5f64ef9..e952a4f 100644
--- a/app/ha/client.py
+++ b/app/ha/client.py
@@ -2,6 +2,7 @@ from __future__ import annotations
 
 import logging
 from dataclasses import dataclass
+from typing import Any
 
 import requests
 
@@ -24,10 +25,14 @@ class HaClient:
             "Content-Type": "application/json",
         })
 
-    def list_entities(self) -> list[dict[str, object]]:
+    def list_entities(self) -> list[dict[str, Any]]:
         response = self._session.get(
             f"{self._settings.url}/api/states",
             timeout=self._settings.timeout_seconds,
         )
         response.raise_for_status()
-        return response.json()
+        payload = response.json()
+        if not isinstance(payload, list):
+            msg = "Home Assistant states response must be a list."
+            raise TypeError(msg)
+        return payload
diff --git a/app/ha/reader.py b/app/ha/reader.py
index ccb72c0..828d28b 100644
--- a/app/ha/reader.py
+++ b/app/ha/reader.py
@@ -1,9 +1,10 @@
 from __future__ import annotations
 
 from collections.abc import Sequence
+from typing import Any
 
 from app.ha.client import HaClient
-from app.ha.models import HaEntitySummary, HaState
+from app.ha.models import HaEntitySummary
 
 
 class HaReader:
@@ -18,14 +19,21 @@ class HaReader:
             if "." not in entity_id:
                 continue
             domain = entity_id.split(".", 1)[0]
-            attributes = item.get("attributes") or {}
+            raw_attributes = item.get("attributes") or {}
+            attributes: dict[str, Any] = raw_attributes if isinstance(raw_attributes, dict) else {}
             summaries.append(
                 HaEntitySummary(
                     entity_id=entity_id,
                     domain=domain,
-                    state_class=str(attributes.get("state_class") or ""),
-                    device_class=str(attributes.get("device_class") or ""),
-                    unit_of_measurement=str(attributes.get("unit_of_measurement") or ""),
+                    state_class=_optional_str(attributes.get("state_class")),
+                    device_class=_optional_str(attributes.get("device_class")),
+                    unit_of_measurement=_optional_str(attributes.get("unit_of_measurement")),
                 )
             )
         return summaries
+
+
+def _optional_str(value: object) -> str | None:
+    if value is None or value == "":
+        return None
+    return str(value)
diff --git a/app/main.py b/app/main.py
index 6826c1c..72d6839 100644
--- a/app/main.py
+++ b/app/main.py
@@ -1,20 +1,26 @@
+from collections.abc import AsyncIterator
 from contextlib import asynccontextmanager
 
-from fastapi import FastAPI, Depends
+from fastapi import FastAPI
 
 from app.api.v1.entities import router as entities_router
+from app.config import load_settings
 from app.ha.client import HaClient, HaClientSettings
 from app.ha.reader import HaReader
 
 
 @asynccontextmanager
-async def lifespan(app: FastAPI):
-    settings = HaClientSettings(
-        url=app.state.settings.ha_url,
-        token=app.state.settings.ha_token,
-    )
-    client = HaClient(settings=settings)
-    app.state.ha_reader = HaReader(client=client)
+async def lifespan(app: FastAPI) -> AsyncIterator[None]:
+    settings = load_settings()
+    app.state.settings = settings
+    if settings.ha_configured:
+        client = HaClient(
+            settings=HaClientSettings(
+                url=settings.ha_url or "",
+                token=settings.ha_token or "",
+            )
+        )
+        app.state.ha_reader = HaReader(client=client)
     yield
 
 
@@ -26,19 +32,7 @@ app = FastAPI(
 )
 
 
-class Settings:
-    ha_url: str
-    ha_token: str
-
-
-app.state.settings = Settings()
-
-
-def get_ha_reader() -> HaReader:
-    return app.state.ha_reader
-
-
-app.include_router(entities_router, dependencies=[Depends(get_ha_reader)])
+app.include_router(entities_router)
 
 
 @app.get("/health")
@@ -48,4 +42,4 @@ def health() -> dict[str, str]:
 
 @app.get("/")
 def root() -> dict[str, str]:
-    return {"service": "sillyhome-next", "docs": "/docs"}
\ No newline at end of file
+    return {"service": "sillyhome-next", "docs": "/docs"}
diff --git a/pyproject.toml b/pyproject.toml
index c1d26ab..cc8d904 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -7,10 +7,12 @@ dependencies = [
   "fastapi>=0.110.0",
   "uvicorn[standard]>=0.29.0",
   "pydantic>=2.6.0",
+  "requests>=2.31.0",
 ]
 
 [project.optional-dependencies]
 dev = [
+  "httpx2>=2.3.0",
   "pytest>=8.0.0",
   "ruff>=0.4.0",
   "mypy>=1.9.0",
@@ -25,4 +27,4 @@ strict = true
 
 [tool.ruff]
 line-length = 100
-target-version = "py311"
\ No newline at end of file
+target-version = "py311"
diff --git a/scripts/gitea_setup.py b/scripts/gitea_setup.py
index f9f46a2..3e37476 100644
--- a/scripts/gitea_setup.py
+++ b/scripts/gitea_setup.py
@@ -1,5 +1,4 @@
 import requests
-import json
 from pathlib import Path
 
 p = Path('/root/.openclaw/secrets/gitea.env')
diff --git a/tests/api/test_entities.py b/tests/api/test_entities.py
index c98d5c3..5db1e29 100644
--- a/tests/api/test_entities.py
+++ b/tests/api/test_entities.py
@@ -1,10 +1,46 @@
+from collections.abc import Sequence
+
 from fastapi.testclient import TestClient
+
+from app.ha.models import HaEntitySummary
+from app.ha.reader import HaReader
 from app.main import app
 
-client = TestClient(app)
+
+class FakeHaReader(HaReader):
+    def __init__(self) -> None:
+        pass
+
+    def read_entities(self) -> Sequence[HaEntitySummary]:
+        return [HaEntitySummary(entity_id="sensor.temperature", domain="sensor")]
 
 
 def test_openapi_docs_are_available() -> None:
-    response = client.get("/docs")
+    with TestClient(app) as client:
+        response = client.get("/docs")
     assert response.status_code == 200
     assert "SillyHome Next API" in response.text
+
+
+def test_entities_returns_reader_data() -> None:
+    with TestClient(app) as client:
+        app.state.ha_reader = FakeHaReader()
+        response = client.get("/v1/entities")
+    assert response.status_code == 200
+    assert response.json() == [
+        {
+            "entity_id": "sensor.temperature",
+            "domain": "sensor",
+            "state_class": None,
+            "device_class": None,
+            "unit_of_measurement": None,
+        }
+    ]
+
+
+def test_entities_returns_503_without_home_assistant_config() -> None:
+    with TestClient(app) as client:
+        if hasattr(app.state, "ha_reader"):
+            delattr(app.state, "ha_reader")
+        response = client.get("/v1/entities")
+    assert response.status_code == 503
diff --git a/tests/ha/test_ha_reader.py b/tests/ha/test_ha_reader.py
index 02bbc40..8740759 100644
--- a/tests/ha/test_ha_reader.py
+++ b/tests/ha/test_ha_reader.py
@@ -1,7 +1,6 @@
 from __future__ import annotations
 
 from app.ha.client import HaClient, HaClientSettings
-from app.ha.models import HaEntitySummary, HaState
 from app.ha.reader import HaReader
 
 
diff --git a/tests/test_health.py b/tests/test_health.py
index 83a4062..0fba9d7 100644
--- a/tests/test_health.py
+++ b/tests/test_health.py
@@ -1,10 +1,10 @@
 from fastapi.testclient import TestClient
-from app.main import app
 
-client = TestClient(app)
+from app.main import app
 
 
 def test_health_returns_ok() -> None:
-    response = client.get("/health")
+    with TestClient(app) as client:
+        response = client.get("/health")
     assert response.status_code == 200
     assert response.json() == {"status": "ok"}