diff --git a/ai-service/app/api/admin/__init__.py b/ai-service/app/api/admin/__init__.py
index 40b96bb..5bc4b10 100644
--- a/ai-service/app/api/admin/__init__.py
+++ b/ai-service/app/api/admin/__init__.py
@@ -1,8 +1,9 @@
 """
 Admin API routes for AI Service management.
-[AC-ASA-01, AC-ASA-02, AC-ASA-05, AC-ASA-07, AC-ASA-08] Admin management endpoints.
+[AC-ASA-01, AC-ASA-02, AC-ASA-05, AC-ASA-07, AC-ASA-08, AC-AISVC-50] Admin management endpoints.
 """
 
+from app.api.admin.api_key import router as api_key_router
 from app.api.admin.dashboard import router as dashboard_router
 from app.api.admin.embedding import router as embedding_router
 from app.api.admin.kb import router as kb_router
@@ -11,4 +12,4 @@ from app.api.admin.rag import router as rag_router
 from app.api.admin.sessions import router as sessions_router
 from app.api.admin.tenants import router as tenants_router
 
-__all__ = ["dashboard_router", "embedding_router", "kb_router", "llm_router", "rag_router", "sessions_router", "tenants_router"]
+__all__ = ["api_key_router", "dashboard_router", "embedding_router", "kb_router", "llm_router", "rag_router", "sessions_router", "tenants_router"]
diff --git a/ai-service/app/api/admin/api_key.py b/ai-service/app/api/admin/api_key.py
new file mode 100644
index 0000000..b73b78b
--- /dev/null
+++ b/ai-service/app/api/admin/api_key.py
@@ -0,0 +1,154 @@
+"""
+API Key management endpoints.
+[AC-AISVC-50] CRUD operations for API keys.
+"""
+
+import logging
+from typing import Annotated
+
+from fastapi import APIRouter, Depends, HTTPException, status
+from pydantic import BaseModel, Field
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.core.database import get_session
+from app.models.entities import ApiKey, ApiKeyCreate
+from app.services.api_key import get_api_key_service
+
+logger = logging.getLogger(__name__)
+
+router = APIRouter(prefix="/admin/api-keys", tags=["API Keys"])
+
+
+class ApiKeyResponse(BaseModel):
+    """Response model for API key."""
+
+    id: str = Field(..., description="API key ID")
+    key: str = Field(..., description="API key value")
+    name: str = Field(..., description="API key name")
+    is_active: bool = Field(..., description="Whether the key is active")
+    created_at: str = Field(..., description="Creation time")
+    updated_at: str = Field(..., description="Last update time")
+
+
+class ApiKeyListResponse(BaseModel):
+    """Response model for API key list."""
+
+    keys: list[ApiKeyResponse] = Field(..., description="List of API keys")
+    total: int = Field(..., description="Total count")
+
+
+class CreateApiKeyRequest(BaseModel):
+    """Request model for creating API key."""
+
+    name: str = Field(..., description="API key name/description")
+    key: str | None = Field(default=None, description="Custom API key (auto-generated if not provided)")
+
+
+class ToggleApiKeyRequest(BaseModel):
+    """Request model for toggling API key status."""
+
+    is_active: bool = Field(..., description="New active status")
+
+
+def api_key_to_response(api_key: ApiKey) -> ApiKeyResponse:
+    """Convert ApiKey entity to response model."""
+    return ApiKeyResponse(
+        id=str(api_key.id),
+        key=api_key.key,
+        name=api_key.name,
+        is_active=api_key.is_active,
+        created_at=api_key.created_at.isoformat(),
+        updated_at=api_key.updated_at.isoformat(),
+    )
+
+
+@router.get("", response_model=ApiKeyListResponse)
+async def list_api_keys(
+    session: Annotated[AsyncSession, Depends(get_session)],
+):
+    """
+    [AC-AISVC-50] List all API keys.
+    """
+    service = get_api_key_service()
+    keys = await service.list_keys(session)
+    
+    return ApiKeyListResponse(
+        keys=[api_key_to_response(k) for k in keys],
+        total=len(keys),
+    )
+
+
+@router.post("", response_model=ApiKeyResponse, status_code=status.HTTP_201_CREATED)
+async def create_api_key(
+    request: CreateApiKeyRequest,
+    session: Annotated[AsyncSession, Depends(get_session)],
+):
+    """
+    [AC-AISVC-50] Create a new API key.
+    """
+    service = get_api_key_service()
+    
+    key_value = request.key or service.generate_key()
+    
+    key_create = ApiKeyCreate(
+        key=key_value,
+        name=request.name,
+        is_active=True,
+    )
+    
+    api_key = await service.create_key(session, key_create)
+    logger.info(f"[AC-AISVC-50] Created API key: {api_key.name}")
+    
+    return api_key_to_response(api_key)
+
+
+@router.delete("/{key_id}", status_code=status.HTTP_204_NO_CONTENT)
+async def delete_api_key(
+    key_id: str,
+    session: Annotated[AsyncSession, Depends(get_session)],
+):
+    """
+    [AC-AISVC-50] Delete an API key.
+    """
+    service = get_api_key_service()
+    
+    deleted = await service.delete_key(session, key_id)
+    
+    if not deleted:
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND,
+            detail="API key not found",
+        )
+
+
+@router.patch("/{key_id}/toggle", response_model=ApiKeyResponse)
+async def toggle_api_key(
+    key_id: str,
+    request: ToggleApiKeyRequest,
+    session: Annotated[AsyncSession, Depends(get_session)],
+):
+    """
+    [AC-AISVC-50] Toggle API key active status.
+    """
+    service = get_api_key_service()
+    
+    api_key = await service.toggle_key(session, key_id, request.is_active)
+    
+    if not api_key:
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND,
+            detail="API key not found",
+        )
+    
+    return api_key_to_response(api_key)
+
+
+@router.post("/reload-cache", status_code=status.HTTP_204_NO_CONTENT)
+async def reload_api_key_cache(
+    session: Annotated[AsyncSession, Depends(get_session)],
+):
+    """
+    [AC-AISVC-50] Reload API key cache from database.
+    """
+    service = get_api_key_service()
+    await service.reload_cache(session)
diff --git a/ai-service/app/main.py b/ai-service/app/main.py
index c18afba..56d7ccc 100644
--- a/ai-service/app/main.py
+++ b/ai-service/app/main.py
@@ -12,7 +12,7 @@ from fastapi.middleware.cors import CORSMiddleware
 from fastapi.responses import JSONResponse
 
 from app.api import chat_router, health_router
-from app.api.admin import dashboard_router, embedding_router, kb_router, llm_router, rag_router, sessions_router, tenants_router
+from app.api.admin import api_key_router, dashboard_router, embedding_router, kb_router, llm_router, rag_router, sessions_router, tenants_router
 from app.api.admin.kb_optimized import router as kb_optimized_router
 from app.core.config import get_settings
 from app.core.database import close_db, init_db
@@ -24,7 +24,7 @@ from app.core.exceptions import (
     generic_exception_handler,
     http_exception_handler,
 )
-from app.core.middleware import TenantContextMiddleware
+from app.core.middleware import ApiKeyMiddleware, TenantContextMiddleware
 from app.core.qdrant_client import close_qdrant_client
 
 settings = get_settings()
@@ -40,7 +40,7 @@ logger = logging.getLogger(__name__)
 @asynccontextmanager
 async def lifespan(app: FastAPI):
     """
-    [AC-AISVC-01, AC-AISVC-11] Application lifespan manager.
+    [AC-AISVC-01, AC-AISVC-11, AC-AISVC-50] Application lifespan manager.
     Handles startup and shutdown of database and external connections.
     """
     logger.info(f"[AC-AISVC-01] Starting {settings.app_name} v{settings.app_version}")
@@ -51,6 +51,19 @@ async def lifespan(app: FastAPI):
     except Exception as e:
         logger.warning(f"[AC-AISVC-11] Database initialization skipped: {e}")
 
+    try:
+        from app.core.database import async_session_maker
+        from app.services.api_key import get_api_key_service
+
+        async with async_session_maker() as session:
+            api_key_service = get_api_key_service()
+            await api_key_service.initialize(session)
+            default_key = await api_key_service.create_default_key(session)
+            if default_key:
+                logger.info(f"[AC-AISVC-50] Default API key created: {default_key.key}")
+    except Exception as e:
+        logger.warning(f"[AC-AISVC-50] API key initialization skipped: {e}")
+
     yield
 
     await close_db()
@@ -87,6 +100,7 @@ app.add_middleware(
 )
 
 app.add_middleware(TenantContextMiddleware)
+app.add_middleware(ApiKeyMiddleware)
 
 app.add_exception_handler(AIServiceException, ai_service_exception_handler)
 app.add_exception_handler(HTTPException, http_exception_handler)
@@ -113,6 +127,7 @@ async def validation_exception_handler(request: Request, exc: RequestValidationE
 app.include_router(health_router)
 app.include_router(chat_router)
 
+app.include_router(api_key_router)
 app.include_router(dashboard_router)
 app.include_router(embedding_router)
 app.include_router(kb_router)
diff --git a/ai-service/app/models/__init__.py b/ai-service/app/models/__init__.py
index cbbe9e7..f30da8b 100644
--- a/ai-service/app/models/__init__.py
+++ b/ai-service/app/models/__init__.py
@@ -50,6 +50,7 @@ class ErrorCode(str, Enum):
     INVALID_REQUEST = "INVALID_REQUEST"
     MISSING_TENANT_ID = "MISSING_TENANT_ID"
     INVALID_TENANT_ID = "INVALID_TENANT_ID"
+    UNAUTHORIZED = "UNAUTHORIZED"
     INTERNAL_ERROR = "INTERNAL_ERROR"
     SERVICE_UNAVAILABLE = "SERVICE_UNAVAILABLE"
     TIMEOUT = "TIMEOUT"
diff --git a/ai-service/app/models/entities.py b/ai-service/app/models/entities.py
index cf41595..dd9363a 100644
--- a/ai-service/app/models/entities.py
+++ b/ai-service/app/models/entities.py
@@ -198,3 +198,27 @@ class DocumentCreate(SQLModel):
     file_path: str | None = None
     file_size: int | None = None
     file_type: str | None = None
+
+
+class ApiKey(SQLModel, table=True):
+    """
+    [AC-AISVC-50] API Key entity for lightweight authentication.
+    Keys are loaded into memory on startup for fast validation.
+    """
+
+    __tablename__ = "api_keys"
+
+    id: uuid.UUID = Field(default_factory=uuid.uuid4, primary_key=True)
+    key: str = Field(..., description="API Key (unique)", unique=True, index=True)
+    name: str = Field(..., description="Key name/description for identification")
+    is_active: bool = Field(default=True, description="Whether the key is active")
+    created_at: datetime = Field(default_factory=datetime.utcnow, description="Creation time")
+    updated_at: datetime = Field(default_factory=datetime.utcnow, description="Last update time")
+
+
+class ApiKeyCreate(SQLModel):
+    """Schema for creating a new API key."""
+
+    key: str
+    name: str
+    is_active: bool = True
diff --git a/ai-service/app/services/api_key.py b/ai-service/app/services/api_key.py
new file mode 100644
index 0000000..a4c14a6
--- /dev/null
+++ b/ai-service/app/services/api_key.py
@@ -0,0 +1,249 @@
+"""
+API Key management service.
+[AC-AISVC-50] Lightweight authentication with in-memory cache.
+"""
+
+import logging
+import secrets
+from datetime import datetime
+from typing import Optional
+
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.models.entities import ApiKey, ApiKeyCreate
+
+logger = logging.getLogger(__name__)
+
+
+class ApiKeyService:
+    """
+    [AC-AISVC-50] API Key management service.
+    
+    Features:
+    - In-memory cache for fast validation
+    - Database persistence
+    - Hot-reload support
+    """
+
+    def __init__(self):
+        self._keys_cache: set[str] = set()
+        self._initialized: bool = False
+
+    async def initialize(self, session: AsyncSession) -> None:
+        """
+        Load all active API keys from database into memory.
+        Should be called on application startup.
+        """
+        result = await session.execute(
+            select(ApiKey).where(ApiKey.is_active == True)
+        )
+        keys = result.scalars().all()
+        
+        self._keys_cache = {key.key for key in keys}
+        self._initialized = True
+        
+        logger.info(f"[AC-AISVC-50] Loaded {len(self._keys_cache)} API keys into memory")
+
+    def validate_key(self, key: str) -> bool:
+        """
+        Validate an API key against the in-memory cache.
+        
+        Args:
+            key: The API key to validate
+            
+        Returns:
+            True if the key is valid, False otherwise
+        """
+        if not self._initialized:
+            logger.warning("[AC-AISVC-50] API key service not initialized")
+            return False
+        
+        return key in self._keys_cache
+
+    def generate_key(self) -> str:
+        """
+        Generate a new secure API key.
+        
+        Returns:
+            A URL-safe random string
+        """
+        return secrets.token_urlsafe(32)
+
+    async def create_key(
+        self, 
+        session: AsyncSession, 
+        key_create: ApiKeyCreate
+    ) -> ApiKey:
+        """
+        Create a new API key.
+        
+        Args:
+            session: Database session
+            key_create: Key creation data
+            
+        Returns:
+            The created ApiKey entity
+        """
+        api_key = ApiKey(
+            key=key_create.key,
+            name=key_create.name,
+            is_active=key_create.is_active,
+        )
+        
+        session.add(api_key)
+        await session.commit()
+        await session.refresh(api_key)
+        
+        if api_key.is_active:
+            self._keys_cache.add(api_key.key)
+        
+        logger.info(f"[AC-AISVC-50] Created API key: {api_key.name}")
+        return api_key
+
+    async def create_default_key(self, session: AsyncSession) -> Optional[ApiKey]:
+        """
+        Create a default API key if none exists.
+        
+        Returns:
+            The created ApiKey or None if keys already exist
+        """
+        result = await session.execute(select(ApiKey).limit(1))
+        existing = result.scalar_one_or_none()
+        
+        if existing:
+            return None
+        
+        default_key = secrets.token_urlsafe(32)
+        api_key = ApiKey(
+            key=default_key,
+            name="Default API Key",
+            is_active=True,
+        )
+        
+        session.add(api_key)
+        await session.commit()
+        await session.refresh(api_key)
+        
+        self._keys_cache.add(api_key.key)
+        
+        logger.info(f"[AC-AISVC-50] Created default API key: {api_key.key}")
+        return api_key
+
+    async def delete_key(
+        self, 
+        session: AsyncSession, 
+        key_id: str
+    ) -> bool:
+        """
+        Delete an API key.
+        
+        Args:
+            session: Database session
+            key_id: The key ID to delete
+            
+        Returns:
+            True if deleted, False if not found
+        """
+        import uuid
+        
+        try:
+            key_uuid = uuid.UUID(key_id)
+        except ValueError:
+            return False
+        
+        result = await session.execute(
+            select(ApiKey).where(ApiKey.id == key_uuid)
+        )
+        api_key = result.scalar_one_or_none()
+        
+        if not api_key:
+            return False
+        
+        key_value = api_key.key
+        await session.delete(api_key)
+        await session.commit()
+        
+        self._keys_cache.discard(key_value)
+        
+        logger.info(f"[AC-AISVC-50] Deleted API key: {api_key.name}")
+        return True
+
+    async def toggle_key(
+        self, 
+        session: AsyncSession, 
+        key_id: str,
+        is_active: bool
+    ) -> Optional[ApiKey]:
+        """
+        Toggle API key active status.
+        
+        Args:
+            session: Database session
+            key_id: The key ID to toggle
+            is_active: New active status
+            
+        Returns:
+            The updated ApiKey or None if not found
+        """
+        import uuid
+        
+        try:
+            key_uuid = uuid.UUID(key_id)
+        except ValueError:
+            return None
+        
+        result = await session.execute(
+            select(ApiKey).where(ApiKey.id == key_uuid)
+        )
+        api_key = result.scalar_one_or_none()
+        
+        if not api_key:
+            return None
+        
+        api_key.is_active = is_active
+        api_key.updated_at = datetime.utcnow()
+        
+        session.add(api_key)
+        await session.commit()
+        await session.refresh(api_key)
+        
+        if is_active:
+            self._keys_cache.add(api_key.key)
+        else:
+            self._keys_cache.discard(api_key.key)
+        
+        logger.info(f"[AC-AISVC-50] Toggled API key {api_key.name}: active={is_active}")
+        return api_key
+
+    async def list_keys(self, session: AsyncSession) -> list[ApiKey]:
+        """
+        List all API keys.
+        
+        Args:
+            session: Database session
+            
+        Returns:
+            List of all ApiKey entities
+        """
+        result = await session.execute(select(ApiKey))
+        return list(result.scalars().all())
+
+    async def reload_cache(self, session: AsyncSession) -> None:
+        """
+        Reload all API keys from database into memory.
+        """
+        self._keys_cache.clear()
+        await self.initialize(session)
+        logger.info("[AC-AISVC-50] API key cache reloaded")
+
+
+_api_key_service: ApiKeyService | None = None
+
+
+def get_api_key_service() -> ApiKeyService:
+    """Get the global API key service instance."""
+    global _api_key_service
+    if _api_key_service is None:
+        _api_key_service = ApiKeyService()
+    return _api_key_service
diff --git a/ai-service/scripts/init_db.sql b/ai-service/scripts/init_db.sql
index 9a68dac..b4690ea 100644
--- a/ai-service/scripts/init_db.sql
+++ b/ai-service/scripts/init_db.sql
@@ -74,6 +74,18 @@ CREATE TABLE IF NOT EXISTS index_jobs (
     updated_at TIMESTAMP WITHOUT TIME ZONE NOT NULL
 );
 
+-- ============================================
+-- API Keys Table [AC-AISVC-50]
+-- ============================================
+CREATE TABLE IF NOT EXISTS api_keys (
+    id UUID NOT NULL PRIMARY KEY,
+    key VARCHAR NOT NULL UNIQUE,
+    name VARCHAR NOT NULL,
+    is_active BOOLEAN NOT NULL DEFAULT TRUE,
+    created_at TIMESTAMP WITHOUT TIME ZONE NOT NULL,
+    updated_at TIMESTAMP WITHOUT TIME ZONE NOT NULL
+);
+
 -- ============================================
 -- Indexes
 -- ============================================
@@ -100,6 +112,10 @@ CREATE INDEX IF NOT EXISTS ix_index_jobs_tenant_id ON index_jobs (tenant_id);
 CREATE INDEX IF NOT EXISTS ix_index_jobs_tenant_doc ON index_jobs (tenant_id, doc_id);
 CREATE INDEX IF NOT EXISTS ix_index_jobs_tenant_status ON index_jobs (tenant_id, status);
 
+-- API Keys Indexes [AC-AISVC-50]
+CREATE INDEX IF NOT EXISTS ix_api_keys_key ON api_keys (key);
+CREATE INDEX IF NOT EXISTS ix_api_keys_is_active ON api_keys (is_active);
+
 -- ============================================
 -- Verification
 -- ============================================