diff --git a/api_service.py b/api_service.py
index 319ab83..6694efe 100644
--- a/api_service.py
+++ b/api_service.py
@@ -1,16 +1,64 @@
-"""Lightweight Python API service for signature validation
-This can run independently to support the PHP application
+"""Lightweight Python API service for signature validation.
+
+This can run independently to support the PHP application.
 """
 
+from __future__ import annotations
+import asyncio
+from contextlib import asynccontextmanager
+import json
+import logging
 import os
 import re
+import time
+from typing import Any, TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from collections.abc import AsyncIterator, Callable, Awaitable
 
 # Avoid importing heavy modules at top-level to keep `import api_service` lightweight
-from fastapi import FastAPI, Query
+from fastapi import FastAPI, Query, Request, Response
 from fastapi.middleware.cors import CORSMiddleware
 from fastapi.responses import JSONResponse
 
-app = FastAPI(title="Signature Validation API")
+CACHE_TTL_SECONDS = int(os.getenv("CACHE_TTL", str(72 * 3600)))
+REDIS_URL = os.getenv("REDIS_URL", "")
+redis_client = None
+
+
+@asynccontextmanager
+async def _lifespan(_app: FastAPI) -> AsyncIterator[None]:
+    """Lifespan handler: connect to Redis on startup and close on shutdown."""
+    global redis_client
+    if REDIS_URL:
+        try:
+            import redis.asyncio as aioredis
+
+            redis_client = aioredis.from_url(REDIS_URL)
+            try:
+                pong = redis_client.ping()
+                if asyncio.iscoroutine(pong) or asyncio.isfuture(pong):
+                    pong = await pong
+                if not pong:
+                    logging.exception("redis ping failed")
+                    redis_client = None
+            except Exception:
+                logging.exception("redis ping failed")
+                redis_client = None
+        except Exception:
+            logging.exception("failed to create redis client")
+            redis_client = None
+
+    yield
+
+    if redis_client is not None:
+        try:
+            await redis_client.close()
+        except Exception:
+            logging.exception("failed to close redis client")
+
+
+app = FastAPI(title="Signature Validation API", lifespan=_lifespan)
 
 # Optional path prefix support: when behind a reverse-proxy that uses a
 # URL prefix (eg. `https://api.example.tld/library/...`) set `API_PREFIX` to
@@ -26,7 +74,10 @@ if _api_prefix_raw:
 
 
 @app.middleware("http")
-async def _strip_api_prefix(request, call_next):
+async def _strip_api_prefix(
+    request: Request,
+    call_next: Callable[[Request], Awaitable[Response]],
+) -> Response:
     if api_prefix and request.url.path.startswith(api_prefix):
         new_path = request.url.path[len(api_prefix) :]
         request.scope["path"] = new_path or "/"
@@ -47,7 +98,7 @@ app.add_middleware(
 cat = None
 
 
-def _get_catalogue():
+def _get_catalogue() -> Any:
     global cat
     if cat is None:
         # import inside function to avoid expensive work during module import
@@ -57,9 +108,62 @@ def _get_catalogue():
     return cat
 
 
+# ---- Caching support ----------------------------------------------
+# Uses an async Redis client when `REDIS_URL` is set, otherwise falls
+# back to a small in-memory store with TTL. Cache TTL defaults to 72h.
+CacheValue = dict[str, Any]
+
+_in_memory_cache: dict[str, tuple[float, CacheValue]] = {}
+_in_memory_lock = asyncio.Lock()
+
+
+async def _cache_get(key: str) -> CacheValue | None:
+    if redis_client:
+        try:
+            val = await redis_client.get(key)
+            if val is None:
+                return None
+            return json.loads(val)
+        except Exception:
+            logging.exception("redis get failed")
+            return None
+
+    # fallback in-memory
+    async with _in_memory_lock:
+        entry = _in_memory_cache.get(key)
+        if not entry:
+            return None
+        expires_at, value = entry
+        if time.time() >= expires_at:
+            del _in_memory_cache[key]
+            return None
+        return value
+
+
+async def _cache_set(key: str, value: CacheValue, ttl: int = CACHE_TTL_SECONDS) -> None:
+    if redis_client:
+        try:
+            await redis_client.set(key, json.dumps(value), ex=ttl)
+            return
+        except Exception:
+            logging.exception("redis set failed")
+
+    async with _in_memory_lock:
+        _in_memory_cache[key] = (time.time() + ttl, value)
+
+
+# Redis lifecycle is handled by the lifespan context manager defined earlier
+
+
 @app.get("/api/validate-signature")
-async def validate_signature(signature: str = Query(...)):
-    """Validate a book signature and return total pages"""
+async def validate_signature(signature: str = Query(...)) -> JSONResponse:
+    """Validate a book signature and return total pages."""
+    # check cache first
+    cache_key = f"signature:{signature}"
+    cached = await _cache_get(cache_key)
+    if cached is not None:
+        return JSONResponse(cached)
+
     try:
         book_result = _get_catalogue().get_book_with_data(signature)
         if book_result and hasattr(book_result, "pages") and book_result.pages:
@@ -69,31 +173,36 @@ async def validate_signature(signature: str = Query(...)):
             match = re.search(r"(\d+)", pages_str)
             if match:
                 total_pages = int(match.group(1))
-                return JSONResponse(
-                    {"valid": True, "total_pages": total_pages, "signature": signature},
-                )
+                result: CacheValue = {
+                    "valid": True,
+                    "total_pages": total_pages,
+                    "signature": signature,
+                }
+                await _cache_set(cache_key, result)
+                return JSONResponse(result)
 
-        return JSONResponse(
-            {
-                "valid": False,
-                "error": "Signatur nicht gefunden oder keine Seitenzahl verfügbar",
-                "signature": signature,
-            },
-        )
+        result: CacheValue = {
+            "valid": False,
+            "error": "Signatur nicht gefunden oder keine Seitenzahl verfügbar",
+            "signature": signature,
+        }
+        await _cache_set(cache_key, result)
+        return JSONResponse(result)
     except Exception as e:
-        return JSONResponse(
-            {
-                "valid": False,
-                "error": f"Fehler bei der Validierung: {e!s}",
-                "signature": signature,
-            },
-            status_code=500,
-        )
+        logging.exception("validate_signature failure")
+        result: CacheValue = {
+            "valid": False,
+            "error": f"Fehler bei der Validierung: {e!s}",
+            "signature": signature,
+        }
+        # store a failed response in cache as well so we avoid replaying errors
+        await _cache_set(cache_key, result)
+        return JSONResponse(result, status_code=500)
 
 
 @app.get("/health")
-async def health_check():
-    """Health check endpoint"""
+async def health_check() -> dict[str, str]:
+    """Health check endpoint."""
     return {"status": "ok", "service": "signature-validation"}
 
 
diff --git a/pyproject.toml b/pyproject.toml
index b81dd65..c9afc27 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -8,6 +8,7 @@ dependencies = [
     "bibapi[catalogue]>=0.0.6",
     "fastapi>=0.122.0",
     "uvicorn>=0.38.0",
+    "redis>=4.6.0",
 ]
 [[tool.uv.index]]
 name = "gitea"