diff --git a/app/dto/UserDTO.py b/app/dto/UserDTO.py
new file mode 100644
index 0000000..0a705a3
--- /dev/null
+++ b/app/dto/UserDTO.py
@@ -0,0 +1,5 @@
+from pydantic import BaseModel
+
+class UserDTO(BaseModel):
+    uid: str
+    username: str
\ No newline at end of file
diff --git a/app/dto/__init__.py b/app/dto/__init__.py
index b8e753a..a6f30f0 100644
--- a/app/dto/__init__.py
+++ b/app/dto/__init__.py
@@ -1,3 +1,4 @@
 from .UserRegisterDTO import UserRegisterDTO
 from .FriendAddDTO import FriendAddDTO
-from .FriendListDTO import FriendListDTO
\ No newline at end of file
+from .FriendListDTO import FriendListDTO
+from .UserDTO import UserDTO
\ No newline at end of file
diff --git a/app/main.py b/app/main.py
index 8c60859..807cd7a 100644
--- a/app/main.py
+++ b/app/main.py
@@ -1,8 +1,8 @@
 from fastapi import FastAPI, Depends, HTTPException, status
+from fastapi.responses import JSONResponse
 from fastapi.security import OAuth2PasswordBearer, OAuth2PasswordRequestForm
 from typing import Optional
 from datetime import datetime, timedelta
-from pymongo import MongoClient
 from jose import JWTError, jwt
 from bson.objectid import ObjectId
 from app.utils import get_password_hash, verify_password
@@ -20,8 +20,10 @@ from app.dto import *
 # Contains all constants
 from app.config import *
 
+import pymongo
+
 # Database setup
-client = MongoClient(MONGODB_URL, username=MONGODB_USERNAME, password=MONGODB_PASSWORD)
+client = pymongo.MongoClient(MONGODB_URL, username=MONGODB_USERNAME, password=MONGODB_PASSWORD)
 db = client[MONGODB_DATABASE]
 
 # FastAPI app instance
@@ -73,7 +75,11 @@ async def get_current_user(token: str = Depends(oauth2_scheme)) -> User:
 
 
 # Routes - TODO: find workaround to display 401/409/... HTTP error codes in openapi.json 
-@app.post("/register", response_model=Token)
+@app.post(
+    path="/register",
+    response_model=Token,
+    responses={409: {"model": Message}}
+)
 async def register(user: UserRegisterDTO):
     user_exists = users_collection.find_one({"username": user.username})
     if user_exists:
@@ -90,7 +96,11 @@ async def register(user: UserRegisterDTO):
 
     return {"access_token": access_token, "token_type": "bearer", "user_id": str(user["_id"])}
 
-@app.post("/login", response_model=Token)
+@app.post(
+    path="/login", 
+    response_model=Token,
+    responses={401: {"model": Message}}
+)
 async def login(form_data: OAuth2PasswordRequestForm = Depends()):
     user = users_collection.find_one({"username": form_data.username})
     if not user or not verify_password(form_data.password, user["password"]):
@@ -105,12 +115,18 @@ async def login(form_data: OAuth2PasswordRequestForm = Depends()):
 
     return {"access_token": access_token, "token_type": "bearer", "user_id": str(user["_id"])}
 
-@app.get("/logout")
+@app.get(
+    path="/logout",
+    responses={401: {"model": Message}}
+)
 async def logout(current_user: User = Depends(get_current_user)):
     # TODO: find usecase / what to do ??
     return {"message": "Logged out"}
 
-@app.get("/pin/{id}")
+@app.get(
+    path="/pin/{id}",
+    responses={401: {"model": Message}, 404: {"model": Message}}
+)
 async def get_pin(id: str, current_user: User = Depends(get_current_user)):
     pin = pins_collection.find_one({"_id": ObjectId(id)})
     if pin is None:
@@ -118,7 +134,10 @@ async def get_pin(id: str, current_user: User = Depends(get_current_user)):
     
     return pin
 
-@app.patch("/pin/{id}")
+@app.patch(
+    path="/pin/{id}",
+    responses={401: {"model": Message}, 404: {"model": Message}}
+)
 async def update_pin(id: str, pin: Pin, current_user: User = Depends(get_current_user)):
     result = pins_collection.update_one({"_id": ObjectId(id)}, {"$set": pin.model_dump()})
     if result.matched_count == 0:
@@ -126,17 +145,26 @@ async def update_pin(id: str, pin: Pin, current_user: User = Depends(get_current
     
     return {"message": "Pin updated"}
 
-@app.post("/pin/add")
+@app.post(
+    path="/pin/add",
+    responses={401: {"model": Message}}    
+)
 async def add_pin(pin: Pin, current_user: User = Depends(get_current_user)):
     pin_id = pins_collection.insert_one(pin.model_dump()).inserted_id
     return {"id": str(pin_id)}
 
-@app.get("/pins")
+@app.get(
+    path="/pins",
+    responses={401: {"model": Message}}
+)
 async def list_pins(current_user: User = Depends(get_current_user)):
     pins = pins_serialize(pins_collection.find().to_list())
     return pins
 
-@app.get("/friend/{id}")
+@app.get(
+    path="/friend/{id}",
+    responses={401: {"model": Message}, 404: {"model": Message}}
+)
 async def get_friend(id: str, current_user: User = Depends(get_current_user)):
     friend = friends_collection.find_one({"_id": ObjectId(id)})
     if friend is None:
@@ -144,7 +172,10 @@ async def get_friend(id: str, current_user: User = Depends(get_current_user)):
     
     return friend
 
-@app.post("/friend/add")
+@app.post(
+    path="/friend/add", 
+    responses={401: {"model": Message}, 409: {"model": Message}}
+)
 async def add_friend(friendAdd: FriendAddDTO, current_user: User = Depends(get_current_user)):
     # TODO: test if exists
     friend: Friend = friendAdd.model_dump()
@@ -161,7 +192,10 @@ async def add_friend(friendAdd: FriendAddDTO, current_user: User = Depends(get_c
     friend_id = friends_collection.insert_one(friend).inserted_id
     return {"id": str(friend_id)}
 
-@app.delete("/friend/{id}/delete")
+@app.delete(
+    path="/friend/{id}/delete",
+    responses={401: {"model": Message}, 404: {"model": Message}}
+)
 async def delete_friend(id: str, current_user: User = Depends(get_current_user)):
     result = friends_collection.delete_one({"_id": ObjectId(id)})
     if result.deleted_count == 0:
@@ -169,7 +203,10 @@ async def delete_friend(id: str, current_user: User = Depends(get_current_user))
     
     return {"message": "Friend deleted"}
 
-@app.patch("/friend/{id}/accept")
+@app.patch(
+    path="/friend/{id}/accept",
+    responses={401: {"model": Message}, 404: {"model": Message}}
+)
 async def accept_friend(id: str, current_user: User = Depends(get_current_user)):
     result = friends_collection.update_one({"_id": ObjectId(id)}, {"$set": {"status": "accepted"}})
     if result.matched_count == 0:
@@ -177,7 +214,10 @@ async def accept_friend(id: str, current_user: User = Depends(get_current_user))
     
     return {"message": "Friend request accepted"}
 
-@app.post("/friend/{id}/deny")
+@app.post(
+    path="/friend/{id}/deny",    
+    responses={401: {"model": Message}, 404: {"model": Message}}
+)
 async def deny_friend(id: str, current_user: User = Depends(get_current_user)):
     result = friends_collection.update_one({"_id": ObjectId(id)}, {"$set": {"status": "denied"}})
     if result.matched_count == 0:
@@ -185,12 +225,23 @@ async def deny_friend(id: str, current_user: User = Depends(get_current_user)):
     
     return {"message": "Friend request denied"}
 
-@app.get("/friends", response_model=FriendListDTO)
+@app.get(
+    path="/friends",
+    response_model=FriendListDTO,
+    responses={401: {"model": Message}}
+)
 async def list_friends(current_user: User = Depends(get_current_user)):
     return FriendListDTO(**friends_serialize(friends_collection.find({"user_id": current_user.uid}).to_list(), friends_collection.find({"friend_user_id": current_user.uid})))
 
-@app.get("/users")
+@app.get(
+    path="/users",
+    responses={401: {"model": Message}, 422: {"model": Message}},
+    response_model=list[UserDTO]
+)
 async def search_users(name: str, current_user: User = Depends(get_current_user)):
-    # TODO: /!\ pymongo.errors.OperationFailure if regex is poop
-    users = users_serialize(users_collection.find({"username": {"$regex": name, "$options": "i"}}).to_list())
+    try:
+        users = users_serialize(users_collection.find({"username": {"$regex": name, "$options": "i"}}).to_list())
+    except pymongo.errors.OperationFailure:
+        raise HTTPException(status_code=status.HTTP_422_UNPROCESSABLE_ENTITY, detail="Regex may be wrongly formatted")
+    
     return users
\ No newline at end of file
diff --git a/app/models/__init__.py b/app/models/__init__.py
index 957a693..eae06aa 100644
--- a/app/models/__init__.py
+++ b/app/models/__init__.py
@@ -2,4 +2,5 @@ from .friend import Friend
 from .pin import Pin
 from .token_data import TokenData
 from .token import Token
-from .user import User
\ No newline at end of file
+from .user import User
+from .message import Message
\ No newline at end of file
diff --git a/app/models/message.py b/app/models/message.py
new file mode 100644
index 0000000..e9c82e1
--- /dev/null
+++ b/app/models/message.py
@@ -0,0 +1,5 @@
+from pydantic import BaseModel
+
+
+class Message(BaseModel):
+    detail: str
diff --git a/app/serializers/user_serializer.py b/app/serializers/user_serializer.py
index e1b01ee..1a29231 100644
--- a/app/serializers/user_serializer.py
+++ b/app/serializers/user_serializer.py
@@ -1,11 +1,12 @@
+from app.dto.UserDTO import UserDTO
 from app.models.user import User
 
 
-def users_serialize(users: list) -> list:
+def users_serialize(users: list) -> list[UserDTO]:
     serialized_users: list = []
     for user in users:
         serialized_users.append({
-            "id": str(user["_id"]),
+            "uid": str(user["_id"]),
             "username": user["username"]
         })
     return serialized_users