diff --git a/.env.example b/.env.example
index 2d016a7..d93a09e 100644
--- a/.env.example
+++ b/.env.example
@@ -15,6 +15,11 @@ DATABASE_URL=postgresql+psycopg2://evsy:evsy@db:5432/evsy
 # For local dev use http://localhost:3000
 FRONTEND_URL=http://localhost:3000
 
+# Public URL of the backend (used for building OAuth callback URLs)
+# For local dev use http://localhost:8000
+# IMPORTANT: In production, this must match the actual backend domain.
+BACKEND_URL=http://localhost:8000
+
 
 # ========================
 # Frontend Configuration
diff --git a/backend/app/api/v1/routes/fields.py b/backend/app/api/v1/routes/fields.py
index 51e4c19..c206e2c 100644
--- a/backend/app/api/v1/routes/fields.py
+++ b/backend/app/api/v1/routes/fields.py
@@ -19,11 +19,20 @@
     description="Create a new field that can be associated with events.",
     responses={
         201: {"description": "Field created successfully"},
-        400: {"description": "Validation error"},
+        400: {"description": "Validation error or field already exists"},
     },
 )
 def create_field_route(field: FieldCreate, db: Session = Depends(get_db)):
-    return field_crud.create_field(db=db, field=field)
+    try:
+        return field_crud.create_field(db=db, field=field)
+    except ValueError as e:
+        raise HTTPException(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            detail={
+                "code": "duplicate_resource",
+                "message": str(e),
+            },
+        ) from e
 
 
 @router.get(
@@ -79,22 +88,31 @@ def get_field_route(
     responses={
         200: {"description": "Field updated successfully"},
         404: {"description": "Field not found"},
-        400: {"description": "Validation error"},
+        400: {"description": "Validation error or field already exists"},
     },
 )
 def update_field_route(
     field_id: int, field: FieldCreate, db: Session = Depends(get_db)
 ):
-    db_field = field_crud.update_field(db=db, field_id=field_id, field=field)
-    if db_field is None:
+    try:
+        db_field = field_crud.update_field(db=db, field_id=field_id, field=field)
+        if db_field is None:
+            raise HTTPException(
+                status_code=status.HTTP_404_NOT_FOUND,
+                detail={
+                    "code": "resource_not_found",
+                    "message": f"Field with id {field_id} not found",
+                },
+            )
+        return db_field
+    except ValueError as e:
         raise HTTPException(
-            status_code=status.HTTP_404_NOT_FOUND,
+            status_code=status.HTTP_400_BAD_REQUEST,
             detail={
-                "code": "resource_not_found",
-                "message": f"Field with id {field_id} not found",
+                "code": "duplicate_resource",
+                "message": str(e),
             },
-        )
-    return db_field
+        ) from e
 
 
 @router.delete(
diff --git a/backend/app/api/v1/routes/tags.py b/backend/app/api/v1/routes/tags.py
index bec3349..e69dd85 100644
--- a/backend/app/api/v1/routes/tags.py
+++ b/backend/app/api/v1/routes/tags.py
@@ -19,11 +19,20 @@
     description="Create a tag manually. Typically, tags are created automatically when creating or updating an event.",
     responses={
         201: {"description": "Tag created successfully"},
-        400: {"description": "Validation error"},
+        400: {"description": "Validation error or tag already exists"},
     },
 )
 def create_tag_route(tag: TagCreate, db: Session = Depends(get_db)):
-    return tag_crud.create_tag(db=db, tag=tag)
+    try:
+        return tag_crud.create_tag(db=db, tag=tag)
+    except ValueError as e:
+        raise HTTPException(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            detail={
+                "code": "duplicate_resource",
+                "message": str(e),
+            },
+        ) from e
 
 
 @router.get(
diff --git a/backend/app/modules/auth/oauth.py b/backend/app/modules/auth/oauth.py
index f4037dd..44c29c7 100644
--- a/backend/app/modules/auth/oauth.py
+++ b/backend/app/modules/auth/oauth.py
@@ -129,5 +129,5 @@ def exchange_code_for_email(provider: str, code: str, redirect_uri: str) -> str:
 
 
 def get_email_from_oauth(login: OAuthLogin) -> str:
-    redirect_uri = "http://localhost:8000/api/v1/auth/oauth/callback"
+    redirect_uri = f"{settings.backend_url}/api/v1/auth/oauth/callback"
     return exchange_code_for_email(login.provider, login.token, redirect_uri)
diff --git a/backend/app/modules/auth/router.py b/backend/app/modules/auth/router.py
index 5731988..298bb25 100644
--- a/backend/app/modules/auth/router.py
+++ b/backend/app/modules/auth/router.py
@@ -1,5 +1,6 @@
 import base64
 import json
+import secrets
 
 from fastapi import APIRouter, Depends, HTTPException, Query, Request, status
 from fastapi.responses import RedirectResponse
@@ -143,6 +144,7 @@ def start_oauth_login(
     state_payload = {
         "provider": provider,
         "redirect": redirect,
+        "nonce": secrets.token_urlsafe(16),
     }
     state = base64.urlsafe_b64encode(json.dumps(state_payload).encode()).decode()
 
diff --git a/backend/app/modules/auth/schemas.py b/backend/app/modules/auth/schemas.py
index 5ec752a..3a78b0e 100644
--- a/backend/app/modules/auth/schemas.py
+++ b/backend/app/modules/auth/schemas.py
@@ -1,6 +1,6 @@
 from typing import Annotated, Literal, Optional
 
-from pydantic import BaseModel, ConfigDict, EmailStr, StringConstraints
+from pydantic import BaseModel, ConfigDict, EmailStr, Field, StringConstraints
 
 PasswordStr = Annotated[str, StringConstraints(min_length=6)]
 ProviderName = Literal["github", "google"]
@@ -21,7 +21,10 @@ class UserLogin(UserBase):
 
 class OAuthLogin(BaseModel):
     provider: ProviderName
-    token: str
+    token: str = Field(
+        ...,
+        description="The authorization code received from the OAuth provider callback.",
+    )
 
 
 class UserOut(UserBase):
diff --git a/backend/app/modules/events/crud.py b/backend/app/modules/events/crud.py
index bcd3494..3509455 100644
--- a/backend/app/modules/events/crud.py
+++ b/backend/app/modules/events/crud.py
@@ -24,10 +24,6 @@ def create_event(db: Session, event: schemas.EventCreate):
     return db_event
 
 
-def get_event(db: Session, event_id: int):
-    return db.query(models.Event).filter(models.Event.id == event_id).first()
-
-
 def get_events(db: Session):
     return (
         db.query(models.Event)
@@ -40,6 +36,10 @@ def get_events(db: Session):
     )
 
 
+def get_event(db: Session, event_id: int):
+    return db.query(models.Event).filter(models.Event.id == event_id).first()
+
+
 def update_event(db: Session, event_id: int, event: schemas.EventCreate):
     db_event = db.query(models.Event).filter(models.Event.id == event_id).first()
     if db_event is None:
diff --git a/backend/app/modules/events/schemas.py b/backend/app/modules/events/schemas.py
index 39dfb59..a14072b 100644
--- a/backend/app/modules/events/schemas.py
+++ b/backend/app/modules/events/schemas.py
@@ -42,11 +42,8 @@ class EventCreate(EventBase):
     pass
 
 
-class EventOut(BaseModel):
+class EventOut(EventBase):
     id: int
-    name: str
-    description: Optional[str] = None
-    links: Optional[List[EventLink]] = None
     tags: list[TagOut]
     fields: list[FieldOut]
     created_at: datetime
diff --git a/backend/app/modules/fields/crud.py b/backend/app/modules/fields/crud.py
index 7891d1f..e735c7f 100644
--- a/backend/app/modules/fields/crud.py
+++ b/backend/app/modules/fields/crud.py
@@ -1,4 +1,3 @@
-from fastapi import HTTPException
 from sqlalchemy import func
 from sqlalchemy.exc import IntegrityError
 from sqlalchemy.orm import Session
@@ -19,9 +18,7 @@ def create_field(db: Session, field: schemas.FieldCreate):
         return db_field
     except IntegrityError:
         db.rollback()
-        raise HTTPException(
-            status_code=400, detail=f"Field with name {field.name!r} already exists."
-        ) from None
+        raise ValueError(f"Field with name {field.name!r} already exists.") from None
 
 
 def get_fields(db: Session):
@@ -38,9 +35,15 @@ def update_field(db: Session, field_id: int, field: schemas.FieldCreate):
         db_field.name = field.name
         db_field.description = field.description
         db_field.field_type = field.field_type
-        db.commit()
-        db.refresh(db_field)
-        return db_field
+        try:
+            db.commit()
+            db.refresh(db_field)
+            return db_field
+        except IntegrityError:
+            db.rollback()
+            raise ValueError(
+                f"Field with name {field.name!r} already exists."
+            ) from None
     return None
 
 
diff --git a/backend/app/modules/tags/crud.py b/backend/app/modules/tags/crud.py
index 1c56d77..7120534 100644
--- a/backend/app/modules/tags/crud.py
+++ b/backend/app/modules/tags/crud.py
@@ -1,3 +1,4 @@
+from sqlalchemy.exc import IntegrityError
 from sqlalchemy.orm import Session
 
 from app.shared.models import EventTag
@@ -8,9 +9,13 @@
 def create_tag(db: Session, tag: schemas.TagCreate):
     db_tag = models.Tag(id=tag.id, description=tag.description)
     db.add(db_tag)
-    db.commit()
-    db.refresh(db_tag)
-    return db_tag
+    try:
+        db.commit()
+        db.refresh(db_tag)
+        return db_tag
+    except IntegrityError:
+        db.rollback()
+        raise ValueError(f"Tag with id {tag.id!r} already exists.") from None
 
 
 def get_tags(db: Session):
diff --git a/backend/app/modules/tags/schemas.py b/backend/app/modules/tags/schemas.py
index 2893a0e..b38c047 100644
--- a/backend/app/modules/tags/schemas.py
+++ b/backend/app/modules/tags/schemas.py
@@ -1,10 +1,14 @@
 from datetime import datetime
 
-from pydantic import BaseModel, ConfigDict
+from pydantic import BaseModel, ConfigDict, Field
 
 
 class TagBase(BaseModel):
-    id: str
+    id: str = Field(
+        min_length=1,
+        max_length=50,
+        description="Tag ID must be between 1 and 50 characters",
+    )
     description: str | None = None
 
 
diff --git a/backend/app/settings.py b/backend/app/settings.py
index 3711d7c..6c02062 100644
--- a/backend/app/settings.py
+++ b/backend/app/settings.py
@@ -51,6 +51,7 @@ def __init__(self, _env_file: Optional[str] = None, **kwargs):
     env: Literal["dev", "prod", "demo", "test"] = Field(default="dev", alias="ENV")
     database_url: str = "sqlite:///./test.db"
     frontend_url: Optional[str] = None
+    backend_url: str = "http://localhost:8000"
 
     secret_key: str = "your_secret_key_here"