diff --git a/backend/app/core/exceptions.py b/backend/app/core/exceptions.py
new file mode 100644
index 0000000..e643c6e
--- /dev/null
+++ b/backend/app/core/exceptions.py
@@ -0,0 +1,52 @@
+"""Custom application exceptions."""
+from fastapi import HTTPException, status
+
+
+class AppException(HTTPException):
+    """Base application exception."""
+
+    def __init__(
+        self,
+        detail: str,
+        status_code: int = status.HTTP_400_BAD_REQUEST,
+    ):
+        super().__init__(status_code=status_code, detail=detail)
+
+
+class NotFoundError(AppException):
+    def __init__(self, resource: str, resource_id=None):
+        msg = f"{resource} not found"
+        if resource_id is not None:
+            msg = f"{resource} #{resource_id} not found"
+        super().__init__(msg, status.HTTP_404_NOT_FOUND)
+
+
+class PermissionDeniedError(AppException):
+    def __init__(self, detail: str = "Permission denied"):
+        super().__init__(detail, status.HTTP_403_FORBIDDEN)
+
+
+class TenantMismatchError(AppException):
+    def __init__(self):
+        super().__init__(
+            "Tenant mismatch: resource does not belong to your tenant",
+            status.HTTP_403_FORBIDDEN,
+        )
+
+
+class WorkerOfflineError(AppException):
+    def __init__(self, worker_name: str):
+        super().__init__(
+            f"AI Worker '{worker_name}' is offline or misconfigured",
+            status.HTTP_503_SERVICE_UNAVAILABLE,
+        )
+
+
+class BudgetExceededError(AppException):
+    def __init__(self, detail: str = "Budget limit exceeded"):
+        super().__init__(detail, status.HTTP_402_PAYMENT_REQUIRED)
+
+
+class TaskNotExecutableError(AppException):
+    def __init__(self, detail: str = "Task cannot be executed in current state"):
+        super().__init__(detail, status.HTTP_409_CONFLICT)