diff --git a/.github/workflows/python-pull-request.yml b/.github/workflows/python-pull-request.yml
index 337a238..c2be6a4 100644
--- a/.github/workflows/python-pull-request.yml
+++ b/.github/workflows/python-pull-request.yml
@@ -17,7 +17,7 @@ jobs:
   checks:
     strategy:
       matrix:
-        python-version: ["3.10", "3.11", "3.12"]
+        python-version: ["3.9", "3.10", "3.11", "3.12"]
     uses: ecmwf-actions/reusable-workflows/.github/workflows/qa-pytest-pyproject.yml@v2
     with:
       python-version: ${{ matrix.python-version }}
diff --git a/src/anemoi/inference/checkpoint.py b/src/anemoi/inference/checkpoint.py
index de09388..bfaaeb1 100644
--- a/src/anemoi/inference/checkpoint.py
+++ b/src/anemoi/inference/checkpoint.py
@@ -13,6 +13,7 @@
 from collections import defaultdict
 from functools import cached_property
 from pathlib import Path
+from typing import Optional
 
 from anemoi.utils.checkpoints import load_metadata
 from earthkit.data.utils.dates import to_datetime
@@ -179,7 +180,7 @@ def validate_environment(
         *,
         all_packages: bool = False,
         on_difference: str = "warn",
-        exempt_packages: list[str] | None = None,
+        exempt_packages: Optional[list[str]] = None,
     ) -> bool:
         return self._metadata.validate_environment(
             all_packages=all_packages, on_difference=on_difference, exempt_packages=exempt_packages
diff --git a/src/anemoi/inference/config.py b/src/anemoi/inference/config.py
index b34e993..0673f5a 100644
--- a/src/anemoi/inference/config.py
+++ b/src/anemoi/inference/config.py
@@ -15,6 +15,8 @@
 from typing import Any
 from typing import Dict
 from typing import Literal
+from typing import Optional
+from typing import Union
 
 import yaml
 from pydantic import BaseModel
@@ -27,12 +29,12 @@ class Configuration(BaseModel):
     class Config:
         extra = "forbid"
 
-    description: str | None = None
+    description: Optional[str] = None
 
     checkpoint: str | Dict[Literal["huggingface"], Dict[str, Any] | str]
     """A path to an Anemoi checkpoint file."""
 
-    date: str | int | datetime.datetime | None = None
+    date: Union[str, int, datetime.datetime, None] = None
     """The starting date for the forecast. If not provided, the date will depend on the selected Input object. If a string, it is parsed by :func:`anemoi.utils.dates.as_datetime`.
     """
 
@@ -41,7 +43,7 @@ class Config:
     If an integer, it represents a number of hours. Otherwise, it is parsed by :func:`anemoi.utils.dates.as_timedelta`.
     """
 
-    name: str | None = None
+    name: Optional[str] = None
     """Used by prepml."""
 
     verbosity: int = 0
@@ -50,19 +52,19 @@ class Config:
     report_error: bool = False
     """If True, the runner list the training versions of the packages in case of error."""
 
-    input: str | Dict | None = "test"
-    output: str | Dict | None = "printer"
+    input: Union[str, Dict, None] = "test"
+    output: Union[str, Dict, None] = "printer"
 
-    forcings: Dict[str, Dict] | None = None
+    forcings: Union[Dict[str, Dict], None] = None
     """Where to find the forcings."""
 
     device: str = "cuda"
     """The device on which the model should run. This can be "cpu", "cuda" or any other value supported by PyTorch."""
 
-    precision: str | None = None
+    precision: Optional[str] = None
     """The precision in which the model should run. If not provided, the model will use the precision used during training."""
 
-    allow_nans: bool | None = None
+    allow_nans: Optional[bool] = None
     """
     - If None (default), the model will check for NaNs in the input. If NaNs are found, the model issue a warning and `allow_nans` to True.
     - If False, the model will raise an exception if NaNs are found in the input and output.
diff --git a/src/anemoi/inference/metadata.py b/src/anemoi/inference/metadata.py
index c12b833..25f29f6 100644
--- a/src/anemoi/inference/metadata.py
+++ b/src/anemoi/inference/metadata.py
@@ -15,6 +15,7 @@
 from functools import cached_property
 from types import MappingProxyType as frozendict
 from typing import Literal
+from typing import Optional
 
 import numpy as np
 from anemoi.transform.variables import Variable
@@ -461,7 +462,7 @@ def validate_environment(
         *,
         all_packages: bool = False,
         on_difference: Literal["warn", "error", "ignore"] = "warn",
-        exempt_packages: list[str] | None = None,
+        exempt_packages: Optional[list[str]] = None,
     ) -> bool:
         """
         Validate environment of the checkpoint against the current environment.
diff --git a/src/anemoi/inference/provenance.py b/src/anemoi/inference/provenance.py
index 47b9a12..9eb607c 100644
--- a/src/anemoi/inference/provenance.py
+++ b/src/anemoi/inference/provenance.py
@@ -11,6 +11,7 @@
 import logging
 from typing import TYPE_CHECKING
 from typing import Literal
+from typing import Optional
 
 from anemoi.utils.provenance import gather_provenance_info
 from packaging.version import Version
@@ -43,7 +44,7 @@ def validate_environment(
     *,
     all_packages: bool = False,
     on_difference: Literal["warn", "error", "ignore"] = "warn",
-    exempt_packages: list[str] | None = None,
+    exempt_packages: Optional[list[str]] = None,
 ) -> bool:
     """
     Validate environment of the checkpoint against the current environment.
diff --git a/src/anemoi/inference/state.py b/src/anemoi/inference/state.py
index 635f7a7..23f18a3 100644
--- a/src/anemoi/inference/state.py
+++ b/src/anemoi/inference/state.py
@@ -15,6 +15,7 @@
 from typing import TYPE_CHECKING
 from typing import Any
 from typing import Callable
+from typing import Optional
 from typing import TypeVar
 
 import numpy as np
@@ -163,7 +164,7 @@ def from_xarray(
         self,
         data: xr.Dataset | xr.DataArray,
         *,
-        flatten: str | None = None,
+        flatten: Optional[str] = None,
         variable_dim: str = "variable",
         private_info: Any = None,
     ) -> State: