FastAPI + Flutter: Full-Stack Mobile Architecture

March 20, 2026·4 min read

FastAPI and Flutter are a strong pairing — both are typed, both have excellent tooling, and FastAPI's OpenAPI output means you can generate a type-safe Flutter client automatically. Here's the architecture I use on production projects.

Contract-First Development

The single most impactful practice is defining Pydantic schemas before writing implementation. These become the contract between backend and mobile:

from pydantic import BaseModel
from datetime import datetime
 
class TaskCreate(BaseModel):
    title: str
    description: str | None = None
 
class TaskResponse(BaseModel):
    id: str
    title:

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class AuthInterceptor extends Interceptor {
  final TokenStorage _storage;
  final Dio _dio;
  bool _refreshing = false;
  final _queue = <({RequestOptions options, ErrorInterceptorHandler handler})>[];
 
  @override
  Future<void> onError(DioException err, ErrorInterceptorHandler handler) async {
    if (err.response?.statusCode != 401) return handler.next(err);
 
    if (_refreshing) {
      _queue.add((options: err.requestOptions, handler: handler));
      return;
    }
 
    _refreshing = true;
    try {
      final refreshToken = await _storage.getRefreshToken();
      final res = await _dio.post('/auth/refresh',
        data: {'refresh_token': refreshToken});
      await _storage.saveTokens(
        access: res.data['access_token'],
        refresh: res.data['refresh_token'],
      );
      // Retry original request and all queued ones
      handler.resolve(await _dio.fetch(err.requestOptions));
      for (final q in _queue) {
        q.handler.resolve(await _dio.fetch(q.options));
      }
    } catch (_) {
      await _storage.clear();
    } finally {
      _refreshing = false;
      _queue.clear();
    }
  }
}

class APIError(Exception):
    def __init__(self, code: str, message: str, status: int = 400):
        self.code = code
        self.message = message
        self.status = status
 
@app.exception_handler(APIError)
async def api_error_handler(request, exc: APIError):
    return JSONResponse(
        status_code=exc.status,
        content={"error": {"code": exc.code, "message": exc.message}},
    )

class ApiException implements Exception {
  final String code;
  final String message;
  final int statusCode;
}
 
try {
  await taskService.createTask(title);
} on ApiException catch (e) {
  if (e.code == 'QUOTA_EXCEEDED') showUpgradeDialog();
  else showSnackBar(e.message);
}

from fastapi import WebSocket, WebSocketDisconnect
 
class ConnectionManager:
    def __init__(self):
        self.connections: dict[str, WebSocket] = {}
 
    async def connect(self, user_id: str, ws: WebSocket):
        await ws.accept()
        self.connections[user_id] = ws
 
    async def send(self, user_id: str, data: dict):
        if ws := self.connections.get(user_id):
            await ws.send_json(data)

FastAPI + Flutter: Full-Stack Mobile Architecture

Contract-First Development

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