Backend & API

The FastAPI + LangGraph + Celery backbone of the FCA support agent. Each sub-page covers one architectural pattern verbatim from the fca project context:

• Specialist Agent Deep Dives & LangGraph Flow — intent classification, specialist routing, compliance gating, SSE streaming
• FastAPI SSE Streaming — per-request ContextVar queue + SSELogHandler
• LangGraph Multi-Agent Streaming — node-by-node stream_message tuples
• LangGraph Checkpointing (AsyncPostgresSaver) — deadlock-safe DDL on first init, graph persist across restarts
• LangGraph Human-in-the-Loop — interrupt graph execution for manual approval
• Multi-Agent Supervisor Routing — IntentClassifier + conditional edge dispatch
• Async Repository Pattern — generic AsyncRepository[ModelT] with create, get, filter
• Async Service Layer (Context Manager) — async context manager for session lifecycle
• FastAPI Lifespan Startup/Shutdown — @asynccontextmanager lifespan for init_db / close_db
• Structured Logging with JSONFormatter — ContextVar[asyncio.Queue] for live SSE log piping
• FastAPI Depends() for Auth/RBAC — scope-checking Depends() guard on every protected route
• Langfuse LLM Tracing (@observe) — @observe() decorator for end-to-end LLM call observability
• Redis Cache Service & Normalized Keys — CacheService with TTL, normalized key format, optional circuit breaker
• Celery Async Worker Bridge — sync Celery wrapping an asyncio coroutine
• Pydantic-Settings Validation — typed BaseSettings with validators and computed properties
• BaseAgent (Circuit Breaker + Tenacity) — fail-fast + exponential backoff for every specialist agent
• HumanAgent Escalation Priority — priority-queue escalation with EscalationService
• ComplianceChecker Hybrid Short-Circuit — redact / block / warn triage before any agent response
• WorkflowState Pydantic Schema — typed WorkflowState with TypedDict for LangGraph node I/O
• Message API Route (IDOR + Scopes) — resource-level IDOR check + read:messages / write:messages scopes
• Streamlit Chat UI (Session State + SSE) — st.session_state message history + requests.get(stream=True) consumption
• Per-Resource Repository Specialization — domain-specific CustomerRepository / AccountRepository extending AsyncRepository
• JWT Login Route with Scopes — POST /auth/login returning access token with embedded scope claims
• SecurityService Sanitization Pipeline — PII redaction → Lakera injection check → allow/block triage