diff --git a/app/core/config.py b/app/core/config.py
index fb9b9c8..a3c86a9 100644
--- a/app/core/config.py
+++ b/app/core/config.py
@@ -38,6 +38,7 @@ class Config(metaclass=ConfigMeta):
     SIMILARITY_TOP_K: int = int(os.getenv("SIMILARITY_TOP_K", "20"))
 
     # Reranking Configuration
+    ENABLE_RERANKER: bool = os.getenv("ENABLE_RERANKER", "false").lower() in ("true", "1", "yes")
     RERANK_MODEL: str = os.getenv("RERANK_MODEL", "cross-encoder/mmarco-mMiniLMv2-L12-H384")
     RERANK_TOP_K: int = int(os.getenv("RERANK_TOP_K", "10"))
 
@@ -199,3 +200,42 @@ class Config(metaclass=ConfigMeta):
         upload_path.mkdir(parents=True, exist_ok=True)
         return upload_path
 
+    @classmethod
+    def validate_config(cls) -> None:
+        """
+        Validate required configuration and environment variables.
+        Raises ValueError if critical configuration is missing or invalid.
+        """
+        errors = []
+
+        # Validate Ollama host is set
+        if not cls.OLLAMA_HOST:
+            errors.append("OLLAMA_HOST must be set")
+
+        # Validate database path is valid
+        try:
+            db_path = Path(cls.get_db_path())
+            if not db_path.parent.exists():
+                try:
+                    db_path.parent.mkdir(parents=True, exist_ok=True)
+                except Exception as e:
+                    errors.append(f"Cannot create database directory: {e}")
+        except Exception as e:
+            errors.append(f"Invalid database path: {e}")
+
+        # Validate MCP server port is valid
+        if not (1 <= cls.MCP_SERVER_PORT <= 65535):
+            errors.append(f"MCP_SERVER_PORT must be between 1-65535, got {cls.MCP_SERVER_PORT}")
+
+        # Validate RAG configuration values
+        if cls.CHUNK_SIZE <= 0:
+            errors.append(f"CHUNK_SIZE must be positive, got {cls.CHUNK_SIZE}")
+        if cls.CHUNK_OVERLAP < 0 or cls.CHUNK_OVERLAP >= cls.CHUNK_SIZE:
+            errors.append(f"CHUNK_OVERLAP must be between 0 and CHUNK_SIZE, got {cls.CHUNK_OVERLAP}")
+        if cls.TOP_K_RETRIEVAL <= 0:
+            errors.append(f"TOP_K_RETRIEVAL must be positive, got {cls.TOP_K_RETRIEVAL}")
+
+        if errors:
+            error_msg = "Configuration validation failed:\n" + "\n".join(f"  - {e}" for e in errors)
+            raise ValueError(error_msg)
+
diff --git a/app/core/rag_engine.py b/app/core/rag_engine.py
index a4d6ae4..36c93bb 100644
--- a/app/core/rag_engine.py
+++ b/app/core/rag_engine.py
@@ -131,9 +131,20 @@ class RAGEngine:
             similarity_top_k=Config.TOP_K_RETRIEVAL
         )
 
-        # Initialize reranker (DISABLED by default due to performance issues)
+        # Initialize reranker (configurable via ENABLE_RERANKER environment variable)
         self.reranker = None
-        logger.info("Reranker disabled for performance (enable in code if needed)")
+        if Config.ENABLE_RERANKER:
+            try:
+                self.reranker = SentenceTransformerRerank(
+                    model=Config.RERANK_MODEL,
+                    top_n=Config.RERANK_TOP_K
+                )
+                logger.info(f"Reranker enabled: {Config.RERANK_MODEL}")
+            except Exception as e:
+                logger.warning(f"Failed to initialize reranker: {e}. Continuing without reranker.")
+                self.reranker = None
+        else:
+            logger.info("Reranker disabled (set ENABLE_RERANKER=true to enable)")
 
         # Create comprehensive prompt template optimized for M4 Pro performance
         qa_prompt_template = PromptTemplate(
diff --git a/requirements.txt b/requirements.txt
index 594516e..44a002a 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -27,6 +27,7 @@ mcp>=1.0.0
 fastapi>=0.104.0
 uvicorn>=0.24.0
 pydantic>=2.0.0
+slowapi>=0.1.9  # Rate limiting
 
 # Authentication
 passlib[bcrypt]>=1.7.4