diff --git a/.github/workflows/nightly.yaml b/.github/workflows/nightly.yaml
index c60825bc0..cf224c368 100644
--- a/.github/workflows/nightly.yaml
+++ b/.github/workflows/nightly.yaml
@@ -24,7 +24,7 @@ env:
   # Oldest version that can reliably be downloaded
   gams-version: 48.6.1
   os: ubuntu-latest
-  python-version: "3.13"
+  python-version: "3.14"
 
 permissions: {contents: read}
 
diff --git a/.github/workflows/pytest.yaml b/.github/workflows/pytest.yaml
index c65e72b04..66cbcfaec 100644
--- a/.github/workflows/pytest.yaml
+++ b/.github/workflows/pytest.yaml
@@ -24,7 +24,7 @@ env:
   GAMS_VERSION: 48.6.1 # Oldest version of GAMS that can reliably be downloaded
   depth: 100  # Must be large enough to include the most recent release
   label: "safe to test"  # Label that must be applied to run on PRs from forks
-  python-version: "3.13"  # For non-matrixed jobs
+  python-version: "3.14"  # For non-matrixed jobs
   # Install:
   # - dask: to work around https://github.com/khaeru/genno/issues/171
   # - ixmp: from its `main` branch.
@@ -57,16 +57,15 @@ jobs:
     strategy:
       matrix:
         os:
-        - macos-13
         - macos-latest
         - ubuntu-latest
         - windows-latest
         python-version:
-        - "3.9"   # Earliest version supported by message_ix
-        - "3.10"
+        - "3.10"  # Earliest version supported by message_ix
         - "3.11"
         - "3.12"
-        - "3.13"  # Latest version supported by message_ix
+        - "3.13"
+        - "3.14"  # Latest version supported by message_ix
 
         # Below this comment are newly released or development versions of
         # Python. For these versions, binary wheels are not available for some
@@ -74,17 +73,7 @@ jobs:
         # these on the job runner requires a more elaborate build environment,
         # currently out of scope for the message_ix project.
 
-        # - "3.14.0-alpha.1"  # Development version
-
-        exclude:
-        # Specific version combinations that are invalid / not to be used
-        # No arm64 distributions of JPype for these Pythons
-        - { os: macos-latest, python-version: "3.9" }
-        # Redundant with macos-latest
-        - { os: macos-13, python-version: "3.10" }
-        - { os: macos-13, python-version: "3.11" }
-        - { os: macos-13, python-version: "3.12" }
-        - { os: macos-13, python-version: "3.13" }
+        # - "3.15.0-alpha.1"  # Development version
 
       fail-fast: false
 
diff --git a/RELEASE_NOTES.rst b/RELEASE_NOTES.rst
index e62255058..1a9710a36 100644
--- a/RELEASE_NOTES.rst
+++ b/RELEASE_NOTES.rst
@@ -22,6 +22,8 @@ Adjust any imports like the following:
 All changes
 -----------
 
+- :mod:`message_ix` is tested and compatible with `Python 3.14 <https://www.python.org/downloads/release/python-3140/>`__ (:pull:`985`).
+- Support for Python 3.9 is dropped (:pull:`985`), as it has reached end-of-life.
 - Add representation of commodity flows associated with construction and retirement of technology capacity (:pull:`451`).
 
   - New parameters
@@ -48,6 +50,7 @@ All changes
 - Document the :ref:`minimum version of Java <install-java>` required for :class:`ixmp.JDBCBackend <ixmp.backend.jdbc.JDBCBackend>` (:pull:`962`).
 - Improve type hinting (:pull:`963`).
 - Fix capitalization in auxiliary_settings.gms to enable GDX output file compression on MacOS and Linux. (:pull:`965`) 
+
 All changes
 -----------
 
diff --git a/doc/conf.py b/doc/conf.py
index 2fc56a29f..95e083c63 100644
--- a/doc/conf.py
+++ b/doc/conf.py
@@ -7,7 +7,6 @@
 import os
 from importlib.metadata import version as get_version
 from pathlib import Path
-from typing import Optional
 
 # -- Project information ---------------------------------------------------------------
 
@@ -193,7 +192,7 @@
 # -- Options for sphinx.ext.intersphinx ------------------------------------------------
 
 
-def local_inv(name: str, *parts: str) -> Optional[str]:
+def local_inv(name: str, *parts: str) -> str | None:
     """Construct the path to a local intersphinx inventory."""
 
     from importlib.util import find_spec
diff --git a/message_ix/common.py b/message_ix/common.py
index 3ec1abe3a..0f32c3ce3 100644
--- a/message_ix/common.py
+++ b/message_ix/common.py
@@ -6,7 +6,7 @@
 from copy import copy
 from dataclasses import InitVar, dataclass, field
 from pathlib import Path
-from typing import TYPE_CHECKING, Any, Optional
+from typing import TYPE_CHECKING, Any
 
 import ixmp.model.gams
 from ixmp import config
@@ -96,7 +96,7 @@ class Item:
     dims: tuple[str, ...] = field(default_factory=tuple)
 
     #: Text description of the item.
-    description: Optional[str] = None
+    description: str | None = None
 
     def __post_init__(self, expr):
         if expr == "":
@@ -169,12 +169,12 @@ class GAMSModel(ixmp.model.gams.GAMSModel):
     model_dir: Path
 
     #: Optional minimum version of GAMS.
-    GAMS_min_version: Optional[str] = None
+    GAMS_min_version: str | None = None
 
     #: Keyword arguments to map to GAMS `solve_args`.
     keyword_to_solve_arg: list[tuple[str, type, str]]
 
-    def __init__(self, name: Optional[str] = None, **model_options) -> None:
+    def __init__(self, name: str | None = None, **model_options) -> None:
         if gmv := self.GAMS_min_version:
             # Check the minimum GAMS version.
             version = ixmp.model.gams.gams_version() or ""
diff --git a/message_ix/core.py b/message_ix/core.py
index 007c1d097..e3b269f6e 100644
--- a/message_ix/core.py
+++ b/message_ix/core.py
@@ -3,7 +3,7 @@
 from collections.abc import Iterable, Mapping, Sequence
 from functools import lru_cache
 from itertools import chain, product, zip_longest
-from typing import Optional, TypeVar, Union
+from typing import TypeVar
 from warnings import warn
 
 import ixmp
@@ -255,12 +255,15 @@ def cat(self, name, cat):
     def add_par(
         self,
         name: str,
-        key_or_data: Optional[
-            Union[int, str, Sequence[Union[int, str]], dict, pd.DataFrame]
-        ] = None,
-        value: Union[float, Iterable[float], None] = None,
-        unit: Union[str, Iterable[str], None] = None,
-        comment: Union[str, Iterable[str], None] = None,
+        key_or_data: int
+        | str
+        | Sequence[int | str]
+        | dict
+        | pd.DataFrame
+        | None = None,
+        value: float | Iterable[float] | None = None,
+        unit: str | Iterable[str] | None = None,
+        comment: str | Iterable[str] | None = None,
     ) -> None:
         # ixmp.Scenario.add_par() is typed as accepting only str, but this method also
         # accepts int for "year"-like dimensions. Proxy the call to avoid type check
@@ -295,14 +298,12 @@ def add_par(
     def add_set(
         self,
         name: str,
-        key: Union[
-            int,
-            str,
-            Iterable[object],
-            dict[str, Union[Sequence[int], Sequence[str]]],
-            pd.DataFrame,
-        ],
-        comment: Union[str, Sequence[str], None] = None,
+        key: int
+        | str
+        | Iterable[object]
+        | dict[str, Sequence[int] | Sequence[str]]
+        | pd.DataFrame,
+        comment: str | Sequence[str] | None = None,
     ) -> None:
         # ixmp.Scenario.add_par() is typed as accepting only str, but this method also
         # accepts int for "year"-like dimensions. Proxy the call to avoid type check
@@ -373,8 +374,8 @@ def recurse(k, v, parent="World"):
     def add_horizon(
         self,
         year: Iterable[int] = [],
-        firstmodelyear: Optional[int] = None,
-        data: Optional[dict] = None,
+        firstmodelyear: int | None = None,
+        data: dict | None = None,
     ) -> None:
         """Set the scenario time horizon via ``year`` and related categories.
 
@@ -502,7 +503,7 @@ def add_horizon(
 
     def vintage_and_active_years(
         self,
-        ya_args: Union[tuple[str, str], tuple[str, str, Union[int, str]], None] = None,
+        ya_args: tuple[str, str] | tuple[str, str, int | str] | None = None,
         tl_only: bool = True,
         **kwargs,
     ) -> pd.DataFrame:
@@ -653,7 +654,7 @@ def _valid(elem):
     #: Alias for :meth:`vintage_and_active_years`.
     yv_ya = vintage_and_active_years
 
-    def years_active(self, node: str, tec: str, yr_vtg: Union[int, str]) -> list[int]:
+    def years_active(self, node: str, tec: str, yr_vtg: int | str) -> list[int]:
         """Return periods in which `tec` hnology of `yr_vtg` can be active in `node`.
 
         The :ref:`parameters <params-tech>` ``duration_period`` and
@@ -762,7 +763,7 @@ def solve(self, model="MESSAGE", solve_options={}, **kwargs):
 
     def add_macro(
         self,
-        data: Union[Mapping, os.PathLike],
+        data: Mapping | os.PathLike,
         scenario=None,
         check_convergence=True,
         **kwargs,
diff --git a/message_ix/macro/calibrate.py b/message_ix/macro/calibrate.py
index 7b0a27ce9..edc476793 100644
--- a/message_ix/macro/calibrate.py
+++ b/message_ix/macro/calibrate.py
@@ -5,7 +5,7 @@
 from functools import partial
 from operator import itemgetter, mul
 from pathlib import Path
-from typing import TYPE_CHECKING, Optional, Union
+from typing import TYPE_CHECKING
 
 import numpy as np
 import pandas as pd
@@ -460,7 +460,7 @@ def validate_transform(
     return df.set_index(idx)["value"]
 
 
-def _validate_data(name: Optional[str], df: "DataFrame", s: Structures) -> list:
+def _validate_data(name: str | None, df: "DataFrame", s: Structures) -> list:
     """Validate input `df` against `s` for MACRO parameter `name` calibration .
 
     Parameters
@@ -553,7 +553,7 @@ def ym1(df: "Series", macro_periods: Collection[int]) -> int:
 
 
 def add_model_data(
-    base: "Scenario", clone: "Scenario", data: Union[Mapping, os.PathLike]
+    base: "Scenario", clone: "Scenario", data: Mapping | os.PathLike
 ) -> None:
     """Calculate and add MACRO structure and data to `clone`.
 
@@ -643,8 +643,8 @@ def calibrate(s, check_convergence: bool = True, **kwargs):
 
 def prepare_computer(
     base: "Scenario",
-    target: Optional["Scenario"] = None,
-    data: Union[Mapping, os.PathLike, None] = None,
+    target: "Scenario | None" = None,
+    data: Mapping | os.PathLike | None = None,
 ) -> "genno.Computer":
     """Prepare a :class:`.Reporter` to perform MACRO calibration calculations.
 
diff --git a/message_ix/report/__init__.py b/message_ix/report/__init__.py
index 931106fda..90816ea54 100644
--- a/message_ix/report/__init__.py
+++ b/message_ix/report/__init__.py
@@ -2,7 +2,7 @@
 from collections.abc import Mapping
 from functools import lru_cache, partial
 from operator import itemgetter
-from typing import TYPE_CHECKING, Union, cast
+from typing import TYPE_CHECKING, cast
 
 from genno.operator import broadcast_map
 from ixmp.report import (
@@ -220,7 +220,7 @@ def from_scenario(cls, scenario, **kwargs) -> "Reporter":
                 f'Scenario "{scenario.model}/{scenario.scenario}" has no solution'
             )
             log.warning("Some reporting may not function as expected")
-            fail_action: Union[int, str] = logging.DEBUG
+            fail_action: int | str = logging.DEBUG
         else:
             fail_action = "raise"
 
@@ -288,7 +288,7 @@ def add_sankey(
         # Generate the plotly.Figure object; return the key
         return str(self.add(f"sankey figure {unique}", sankey, k[1], k[2]))
 
-    def add_tasks(self, fail_action: Union[int, str] = "raise") -> None:
+    def add_tasks(self, fail_action: int | str = "raise") -> None:
         """Add the pre-defined MESSAGEix reporting tasks to the Reporter.
 
         Parameters
diff --git a/message_ix/report/pyam.py b/message_ix/report/pyam.py
index 716154671..91c148627 100644
--- a/message_ix/report/pyam.py
+++ b/message_ix/report/pyam.py
@@ -1,4 +1,4 @@
-from typing import TYPE_CHECKING, Optional, TypedDict
+from typing import TYPE_CHECKING, TypedDict
 
 if TYPE_CHECKING:
     import pandas
@@ -8,15 +8,15 @@ class CollapseMessageColsKw(TypedDict, total=False):
     """Type hint for :class:`dict` of keyword args to :func:`collapse_message_cols`."""
 
     df: "pandas.DataFrame"
-    var: Optional[str]
-    kind: Optional[str]
+    var: str | None
+    kind: str | None
     var_cols: list[str]
 
 
 def collapse_message_cols(
     df: "pandas.DataFrame",
-    var: Optional[str] = None,
-    kind: Optional[str] = None,
+    var: str | None = None,
+    kind: str | None = None,
     var_cols=[],
 ) -> "pandas.DataFrame":
     """:mod:`genno.compat.pyam` `collapse=...` callback for MESSAGEix quantities.
diff --git a/message_ix/testing/__init__.py b/message_ix/testing/__init__.py
index aaba66bd4..e7a1f7783 100644
--- a/message_ix/testing/__init__.py
+++ b/message_ix/testing/__init__.py
@@ -1,9 +1,10 @@
 import io
 import os
+import platform
 from collections.abc import Callable, Generator
 from itertools import product
 from pathlib import Path
-from typing import TYPE_CHECKING, Any, Optional, Union
+from typing import TYPE_CHECKING, Any
 
 import ixmp
 import numpy as np
@@ -28,12 +29,21 @@
 def pytest_configure(config: pytest.Config) -> None:
     """Force iam-units to use a distinct cache for each worker.
 
-    Work around for https://github.com/hgrecco/flexcache/issues/6 /
-    https://github.com/IAMconsortium/units/issues/54.
+    Work arounds for:
+
+    1. https://github.com/hgrecco/flexcache/issues/6 /
+       https://github.com/IAMconsortium/units/issues/54.
+    2. https://github.com/python/cpython/issues/125235,
+       https://github.com/astral-sh/uv/issues/7036, or similar.
     """
     name = f"iam-units-{os.environ.get('PYTEST_XDIST_WORKER', '')}".rstrip("-")
     os.environ["IAM_UNITS_CACHE"] = str(config.cache.mkdir(name))
 
+    if GHA and platform.system() == "Windows":
+        import matplotlib
+
+        matplotlib.use("agg")
+
 
 def pytest_report_header(config: pytest.Config, start_path: Path) -> str:
     """Add the message_ix import path to the pytest report header."""
@@ -90,7 +100,7 @@ def pytest_sessionstart() -> None:
 
 # Create and populate ixmp databases
 
-_ms: list[Union[str, float]] = [
+_ms: list[str | float] = [
     SCENARIO["dantzig"]["model"],
     SCENARIO["dantzig"]["scenario"],
 ]
@@ -165,7 +175,7 @@ def make_austria(  # noqa: C901
     solve: bool = False,
     quiet: bool = True,
     *,
-    request: Optional["pytest.FixtureRequest"] = None,
+    request: "pytest.FixtureRequest | None" = None,
 ) -> Scenario:
     """Return an :class:`message_ix.Scenario` for the Austrian energy system.
 
@@ -352,7 +362,7 @@ def make_dantzig(
     solve: bool = False,
     multi_year: bool = False,
     *,
-    request: Optional["pytest.FixtureRequest"] = None,
+    request: "pytest.FixtureRequest | None" = None,
     **solve_opts,
 ) -> Scenario:
     """Return an :class:`message_ix.Scenario` for Dantzig's canning problem.
@@ -521,7 +531,7 @@ def make_westeros(
     quiet: bool = True,
     model_horizon: list[int] = [700, 710, 720],
     *,
-    request: Optional["pytest.FixtureRequest"] = None,
+    request: "pytest.FixtureRequest | None" = None,
 ) -> Scenario:
     """Return a new :class:`message_ix.Scenario` containing the ‘Westeros’ model.
 
@@ -715,7 +725,7 @@ def make_subannual(
     var_cost={},
     operation_factor={},
     *,
-    request: Optional["pytest.FixtureRequest"] = None,
+    request: "pytest.FixtureRequest | None" = None,
 ) -> Scenario:
     """Return an :class:`message_ix.Scenario` with subannual time resolution.
 
diff --git a/message_ix/tests/model/message/test_cap_comm.py b/message_ix/tests/model/message/test_cap_comm.py
index 21b85278a..a86813a06 100644
--- a/message_ix/tests/model/message/test_cap_comm.py
+++ b/message_ix/tests/model/message/test_cap_comm.py
@@ -1,4 +1,4 @@
-from typing import TYPE_CHECKING, Union
+from typing import TYPE_CHECKING
 
 import numpy as np
 import pandas as pd
@@ -11,8 +11,9 @@
 if TYPE_CHECKING:
     from ixmp import Platform
 
+pytestmark = pytest.mark.ixmp4_209
 
-COMMON: dict[str, Union[int, str]] = dict(
+COMMON: dict[str, int | str] = dict(
     commodity="coal",
     level="end_of_life",
     node_dest="World",
diff --git a/message_ix/tests/model/test_message.py b/message_ix/tests/model/test_message.py
index 8dc1ad5d7..9424959a4 100644
--- a/message_ix/tests/model/test_message.py
+++ b/message_ix/tests/model/test_message.py
@@ -13,6 +13,8 @@
 if TYPE_CHECKING:
     from ixmp import Platform
 
+pytestmark = pytest.mark.ixmp4_209
+
 #: Periods: one historical, two in the time horizon
 Y = [0, 10, 20]
 
diff --git a/message_ix/tests/report/test_operator.py b/message_ix/tests/report/test_operator.py
index cd754b65f..1c40550eb 100644
--- a/message_ix/tests/report/test_operator.py
+++ b/message_ix/tests/report/test_operator.py
@@ -5,6 +5,7 @@
 import matplotlib
 import pandas as pd
 import pyam
+import pytest
 from dask.core import literal
 
 try:
@@ -25,6 +26,7 @@
 # NOTE These tests likely don't need to be parametrized
 
 
+@pytest.mark.ixmp4_209
 def test_as_message_df(test_mp: "Platform") -> None:
     q = random_qty(dict(c=3, h=2, nl=5))
     q.units = "kg"
@@ -84,6 +86,7 @@ def test_as_message_df(test_mp: "Platform") -> None:
     assert q.size == len(s.par("demand"))
 
 
+@pytest.mark.ixmp4_209
 def test_as_pyam(message_test_mp: "Platform") -> None:
     scen = Scenario(message_test_mp, **SCENARIO["dantzig"])
     if not scen.has_solution():
@@ -101,6 +104,7 @@ def test_as_pyam(message_test_mp: "Platform") -> None:
     assert isinstance(p, pyam.IamDataFrame)
 
 
+@pytest.mark.ixmp4_209
 def test_concat(dantzig_reporter: Reporter) -> None:
     """pyam.concat() correctly passes through to ixmp…concat()."""
     rep = dantzig_reporter
diff --git a/message_ix/tests/test_core.py b/message_ix/tests/test_core.py
index fcf03d72b..1003c3967 100644
--- a/message_ix/tests/test_core.py
+++ b/message_ix/tests/test_core.py
@@ -1,9 +1,8 @@
-import sys
 from collections.abc import Generator
 from copy import deepcopy
 from pathlib import Path
 from subprocess import run
-from typing import Any, Optional
+from typing import Any
 
 import ixmp
 import numpy as np
@@ -17,6 +16,8 @@
 from message_ix import Scenario
 from message_ix.testing import GHA, SCENARIO, make_dantzig, make_westeros
 
+pytestmark = pytest.mark.ixmp4_209
+
 
 @pytest.fixture
 def dantzig_message_scenario(
@@ -153,8 +154,7 @@ def test_backends_available() -> None:
     """Check that the expected set of backends are available within GHA workflows."""
     from ixmp.backend import available
 
-    exp = {"ixmp4", "jdbc"} if sys.version_info >= (3, 10) else {"jdbc"}
-    assert exp <= set(available())
+    assert {"ixmp4", "jdbc"} <= set(available())
 
 
 def test_year_int(test_mp: ixmp.Platform, request: pytest.FixtureRequest) -> None:
@@ -301,7 +301,7 @@ def test_add_spatial_hierarchy(test_mp: ixmp.Platform) -> None:
     ],
 )
 def test_add_horizon(
-    test_mp: ixmp.Platform, args, kwargs, exp: Optional[dict[str, list[int]]]
+    test_mp: ixmp.Platform, args, kwargs, exp: dict[str, list[int]] | None
 ) -> None:
     scen = Scenario(test_mp, **SCENARIO["dantzig"], version="new")
 
@@ -393,7 +393,7 @@ def test_add_cat_unique(message_test_mp: ixmp.Platform) -> None:
     assert [1963] == scen2.cat("year", "firstmodelyear")
 
 
-def test_years_active(test_mp: ixmp.Platform) -> None:
+def test_years_active0(test_mp: ixmp.Platform) -> None:
     test_mp.add_unit("year")
     scen = Scenario(test_mp, **SCENARIO["dantzig"], version="new")
     scen.add_set("node", "foo")
@@ -439,7 +439,7 @@ def test_years_active(test_mp: ixmp.Platform) -> None:
     npt.assert_array_equal(result, years[1:-1])
 
 
-def test_years_active_extend(message_test_mp: ixmp.Platform) -> None:
+def test_years_active1(message_test_mp: ixmp.Platform) -> None:
     scen = Scenario(message_test_mp, **SCENARIO["dantzig multi-year"])
 
     # Existing time horizon
@@ -464,7 +464,7 @@ def test_years_active_extend(message_test_mp: ixmp.Platform) -> None:
     npt.assert_array_equal(result, years[1:-1])
 
 
-def test_years_active_extended2(test_mp: ixmp.Platform) -> None:
+def test_years_active2(test_mp: ixmp.Platform) -> None:
     test_mp.add_unit("year")
     scen = Scenario(test_mp, **SCENARIO["dantzig"], version="new")
     scen.add_set("node", "foo")
@@ -511,7 +511,7 @@ def test_years_active_extended2(test_mp: ixmp.Platform) -> None:
     npt.assert_array_equal(result, years[-2])
 
 
-def test_years_active_extend3(test_mp: ixmp.Platform) -> None:
+def test_years_active3(test_mp: ixmp.Platform) -> None:
     test_mp.add_unit("year")
     scen = Scenario(test_mp, **SCENARIO["dantzig"], version="new")
     scen.add_set("node", "foo")
diff --git a/message_ix/tests/test_feature_addon.py b/message_ix/tests/test_feature_addon.py
index 06763b527..2448cda67 100644
--- a/message_ix/tests/test_feature_addon.py
+++ b/message_ix/tests/test_feature_addon.py
@@ -1,12 +1,13 @@
-from typing import Union
-
 import numpy as np
 import pandas as pd
+import pytest
 from ixmp import Platform
 
 from message_ix import Scenario
 from message_ix.testing import SCENARIO
 
+pytestmark = pytest.mark.ixmp4_209
+
 # First model year of the Dantzig scenario
 _year = 1963
 
@@ -29,7 +30,7 @@
 
 
 def add_addon(
-    s: Scenario, costs: Union[bool, int] = False, zero_output: bool = False
+    s: Scenario, costs: bool | int = False, zero_output: bool = False
 ) -> None:
     s.check_out()
     s.add_set("technology", "canning_addon")
diff --git a/message_ix/tests/test_feature_bound_activity_shares.py b/message_ix/tests/test_feature_bound_activity_shares.py
index 49fce690a..d81457ead 100644
--- a/message_ix/tests/test_feature_bound_activity_shares.py
+++ b/message_ix/tests/test_feature_bound_activity_shares.py
@@ -1,5 +1,4 @@
 from pathlib import Path
-from typing import Union
 
 import numpy as np
 import numpy.testing as npt
@@ -12,6 +11,8 @@
 from message_ix import ModelError, Scenario, make_df
 from message_ix.testing import make_dantzig
 
+pytestmark = pytest.mark.ixmp4_209
+
 #: First model year of the :func:`.make_dantzig` scenario.
 Y0 = 1963
 
@@ -259,7 +260,7 @@ def calc_share(s):
     VALUE = 0.45
 
     # common operations for both subtests
-    def add_data(s: Scenario, modes: Union[str, list[str]]) -> None:
+    def add_data(s: Scenario, modes: str | list[str]) -> None:
         with s.transact("Add share_commodity_up"):
             s.add_set("shares", SHARES)
             s.add_cat("technology", tt_share, "canning_plant")
diff --git a/message_ix/tests/test_feature_bound_emission.py b/message_ix/tests/test_feature_bound_emission.py
index 6a1554007..28b4671b7 100644
--- a/message_ix/tests/test_feature_bound_emission.py
+++ b/message_ix/tests/test_feature_bound_emission.py
@@ -1,9 +1,10 @@
-from typing import Union
-
+import pytest
 from ixmp import Platform
 
 from message_ix import Scenario
 
+pytestmark = pytest.mark.ixmp4_209
+
 
 def model_setup(scen: Scenario, years: list[int]) -> None:
     scen.add_set("node", "node")
@@ -18,14 +19,14 @@ def model_setup(scen: Scenario, years: list[int]) -> None:
 
     scen.add_set("technology", ["tec1", "tec2"])
     scen.add_set("mode", "mode")
-    output_specs: list[Union[int, str]] = ["node", "comm", "level", "year", "year"]
+    output_specs: list[int | str] = ["node", "comm", "level", "year", "year"]
     dict_var_cost = {"tec1": 1, "tec2": 2}
     dict_em_factor = {"tec1": 1.5, "tec2": 1}
 
     for yr in years:
         scen.add_par("demand", ["node", "comm", "level", yr, "year"], 1, "GWa")
         for t in dict_var_cost.keys():
-            tec_specs: list[Union[int, str]] = ["node", t, yr, yr, "mode"]
+            tec_specs: list[int | str] = ["node", t, yr, yr, "mode"]
             scen.add_par("output", tec_specs + output_specs, 1, "GWa")
             scen.add_par("var_cost", tec_specs + ["year"], dict_var_cost[t], "USD/GWa")
             scen.add_par(
@@ -34,14 +35,14 @@ def model_setup(scen: Scenario, years: list[int]) -> None:
 
 
 def add_bound_emission(
-    scen: Scenario, bound: float, year: Union[int, str] = "cumulative"
+    scen: Scenario, bound: float, year: int | str = "cumulative"
 ) -> None:
     scen.check_out()
     scen.add_par("bound_emission", ["node", "emiss_type", "all", year], bound, "kg")
     scen.commit("Emission bound added")
 
 
-def assert_function(scen: Scenario, year: Union[int, str]) -> None:
+def assert_function(scen: Scenario, year: int | str) -> None:
     var_em = scen.var("EMISS", {"node": "node"}).set_index(["year"])["lvl"]
     bound_em = scen.par("bound_emission", {"type_year": year}).at[0, "value"]
 
diff --git a/message_ix/tests/test_feature_capacity_factor.py b/message_ix/tests/test_feature_capacity_factor.py
index 510003854..bad03a3bf 100644
--- a/message_ix/tests/test_feature_capacity_factor.py
+++ b/message_ix/tests/test_feature_capacity_factor.py
@@ -6,6 +6,8 @@
 from message_ix import ModelError, Scenario
 from message_ix.testing import make_subannual
 
+pytestmark = pytest.mark.ixmp4_209
+
 
 def check_solution(scen: Scenario) -> None:
     """Perform several assertions about the solution of `scen`."""
diff --git a/message_ix/tests/test_feature_duration_time.py b/message_ix/tests/test_feature_duration_time.py
index 415fbba4c..bccbaf212 100644
--- a/message_ix/tests/test_feature_duration_time.py
+++ b/message_ix/tests/test_feature_duration_time.py
@@ -7,12 +7,14 @@
 """
 
 from itertools import product
-from typing import Union
 
+import pytest
 from ixmp import Platform
 
 from message_ix import Scenario
 
+pytestmark = pytest.mark.ixmp4_209
+
 
 # A function for generating a simple model with sub-annual time slices
 def model_generator(
@@ -126,7 +128,7 @@ def model_generator(
         # "output"
         for h in times_out:
             out = com_dict[tec]["output"]
-            out_spec: list[Union[int, str]] = [
+            out_spec: list[int | str] = [
                 yr,
                 yr,
                 "standard",
diff --git a/message_ix/tests/test_feature_price_commodity.py b/message_ix/tests/test_feature_price_commodity.py
index 8ee9c3ef2..fbab9db58 100644
--- a/message_ix/tests/test_feature_price_commodity.py
+++ b/message_ix/tests/test_feature_price_commodity.py
@@ -1,12 +1,12 @@
-from typing import Optional, Union
-
 import pytest
 from ixmp import Platform
 
 from message_ix import ModelError, Scenario
 
+pytestmark = pytest.mark.ixmp4_209
+
 
-def model_setup(scen: Scenario, var_cost: Optional[int] = 1) -> None:
+def model_setup(scen: Scenario, var_cost: int | None = 1) -> None:
     scen.add_set("node", "node")
     scen.add_set("commodity", "comm")
     scen.add_set("level", "level")
@@ -16,8 +16,8 @@ def model_setup(scen: Scenario, var_cost: Optional[int] = 1) -> None:
 
     scen.add_set("technology", "tec")
     scen.add_set("mode", "mode")
-    tec_specs: list[Union[int, str]] = ["node", "tec", 2020, 2020, "mode"]
-    output_specs: list[Union[int, str]] = ["node", "comm", "level", "year", "year"]
+    tec_specs: list[int | str] = ["node", "tec", 2020, 2020, "mode"]
+    output_specs: list[int | str] = ["node", "comm", "level", "year", "year"]
     scen.add_par("output", tec_specs + output_specs, 1, "GWa")
     scen.add_par("var_cost", tec_specs + ["year"], var_cost, "USD/GWa")
 
diff --git a/message_ix/tests/test_feature_price_emission.py b/message_ix/tests/test_feature_price_emission.py
index cf3158498..4a90994ca 100644
--- a/message_ix/tests/test_feature_price_emission.py
+++ b/message_ix/tests/test_feature_price_emission.py
@@ -1,11 +1,11 @@
-from typing import Union
-
 import numpy.testing as npt
 import pytest
 from ixmp import Platform
 
 from message_ix import Scenario, make_df
 
+pytestmark = pytest.mark.ixmp4_209
+
 MODEL = "test_emissions_price"
 
 solve_args = {"equ_list": ["EMISSION_EQUIVALENCE"]}
@@ -94,13 +94,13 @@ def add_two_tecs(scen: Scenario, years: list[int]) -> None:
 
 def add_many_tecs(scen: Scenario, years: list[int], n: int = 50) -> None:
     """add a range of dirty-to-clean technologies to the scenario"""
-    output_specs: list[Union[int, str]] = ["node", "comm", "level", "year", "year"]
+    output_specs: list[int | str] = ["node", "comm", "level", "year", "year"]
 
     for i in range(n + 1):
         t = "tec{}".format(i)
         scen.add_set("technology", t)
         for y in years:
-            tec_specs: list[Union[int, str]] = ["node", t, y, y, "mode"]
+            tec_specs: list[int | str] = ["node", t, y, y, "mode"]
             # variable costs grow quadratically over technologies
             # to get rid of the curse of linearity
             c = (10 * i / n) ** 2 * (1.045) ** (y - years[0])
diff --git a/message_ix/tests/test_feature_storage.py b/message_ix/tests/test_feature_storage.py
index 86357d53a..20e62eeb0 100644
--- a/message_ix/tests/test_feature_storage.py
+++ b/message_ix/tests/test_feature_storage.py
@@ -11,7 +11,6 @@
 
 import logging
 from itertools import product
-from typing import Union
 
 import pandas as pd
 import pandas.testing as pdt
@@ -24,6 +23,8 @@
 from message_ix.testing import make_dantzig
 from message_ix.util import expand_dims
 
+pytestmark = pytest.mark.ixmp4_209
+
 
 # A function for generating a simple MESSAGEix model with two technologies
 def model_setup(scen: Scenario, years: list[int]) -> None:
@@ -34,12 +35,12 @@ def model_setup(scen: Scenario, years: list[int]) -> None:
     scen.add_set("type_year", years)
     scen.add_set("technology", ["wind_ppl", "gas_ppl"])
     scen.add_set("mode", "M1")
-    output_specs: list[Union[int, str]] = ["node", "electr", "level", "year", "year"]
+    output_specs: list[int | str] = ["node", "electr", "level", "year", "year"]
     # Two technologies, one cheaper than the other
     var_cost = {"wind_ppl": 0, "gas_ppl": 2}
     for year, (tec, cost) in product(years, var_cost.items()):
         scen.add_par("demand", ["node", "electr", "level", year, "year"], 1, "GWa")
-        tec_specs: list[Union[int, str]] = ["node", tec, year, year, "M1"]
+        tec_specs: list[int | str] = ["node", tec, year, year, "M1"]
         scen.add_par("output", tec_specs + output_specs, 1, "GWa")
         scen.add_par("var_cost", tec_specs + ["year"], cost, "USD/GWa")
 
@@ -55,7 +56,7 @@ def add_seasonality(scen: Scenario, time_duration: dict[str, float]) -> None:
 
 # A function for modifying model parameters after adding sub-annual time steps
 def year_to_time(
-    scen: Scenario, parname: str, time_share: Union[dict[str, float], dict[str, int]]
+    scen: Scenario, parname: str, time_share: dict[str, float] | dict[str, int]
 ) -> None:
     old = scen.par(parname)
     scen.remove_par(parname, old)
diff --git a/message_ix/tests/test_feature_temporal_level.py b/message_ix/tests/test_feature_temporal_level.py
index c54512d67..66a24ef94 100644
--- a/message_ix/tests/test_feature_temporal_level.py
+++ b/message_ix/tests/test_feature_temporal_level.py
@@ -15,6 +15,8 @@
 from message_ix import ModelError, Scenario
 from message_ix.testing import make_subannual
 
+pytestmark = pytest.mark.ixmp4_209
+
 # Values for the com_dict argument to make_subannual
 COM_DICT = {
     "gas_ppl": {"input": "fuel", "output": "electr"},
diff --git a/message_ix/tests/test_feature_vintage_and_active_years.py b/message_ix/tests/test_feature_vintage_and_active_years.py
index 244a7d79c..c19ef0617 100644
--- a/message_ix/tests/test_feature_vintage_and_active_years.py
+++ b/message_ix/tests/test_feature_vintage_and_active_years.py
@@ -1,6 +1,5 @@
 from collections.abc import Sequence
 from functools import lru_cache
-from typing import Optional
 
 import numpy as np
 import pandas as pd
@@ -16,6 +15,9 @@
 # tested here (and not just in ixmp).
 
 
+pytestmark = pytest.mark.ixmp4_209
+
+
 @lru_cache()
 def _generate_yv_ya(periods: tuple[int, ...]) -> pd.DataFrame:
     """All meaningful combinations of (vintage year, active year) given `periods`."""
@@ -41,7 +43,7 @@ def _setup(
     mp: Platform,
     years: Sequence[int],
     firstmodelyear: int,
-    tl_years: Optional[filter] = None,
+    tl_years: filter | None = None,
 ) -> tuple[Scenario, pd.DataFrame]:
     """Common setup for test of :meth:`.vintage_and_active_years`.
 
@@ -75,7 +77,7 @@ def _setup(
 
 
 def _q(
-    df: pd.DataFrame, query: str, append: Optional[pd.DataFrame] = None
+    df: pd.DataFrame, query: str, append: pd.DataFrame | None = None
 ) -> pd.DataFrame:
     """Shorthand to query the results of :func:`_generate_yv_ya`.
 
diff --git a/message_ix/tests/test_macro.py b/message_ix/tests/test_macro.py
index e3b198b02..705c65051 100644
--- a/message_ix/tests/test_macro.py
+++ b/message_ix/tests/test_macro.py
@@ -1,6 +1,6 @@
 from collections.abc import Generator
 from pathlib import Path
-from typing import Any, Literal, Union
+from typing import Any, Literal
 
 import numpy as np
 import numpy.testing as npt
@@ -14,6 +14,8 @@
 from message_ix.report import Quantity
 from message_ix.testing import SCENARIO, make_westeros
 
+pytestmark = pytest.mark.ixmp4_209
+
 # NOTE These tests maybe don't need to be parametrized
 # Do the following depend on otherwise untested Scenario functions?
 # Scenario.add_macro()
@@ -227,7 +229,7 @@ def test_calc(
     w_data_path: Path,
     key: str,
     test: Literal["allclose", "equal"],
-    expected: Union[list[float], list[int]],
+    expected: list[float] | list[int],
 ) -> None:
     """Test calculation of intermediate values on a solved Westeros scenario."""
     c = prepare_computer(westeros_solved, data=w_data_path)
diff --git a/message_ix/tests/test_message.py b/message_ix/tests/test_message.py
index e3e65b310..5226aaeb8 100644
--- a/message_ix/tests/test_message.py
+++ b/message_ix/tests/test_message.py
@@ -3,6 +3,7 @@
 from typing import TYPE_CHECKING
 
 import ixmp
+import pytest
 from ixmp.backend.jdbc import JDBCBackend
 
 from message_ix.message import MESSAGE
@@ -11,6 +12,9 @@
     from ixmp import Platform
 
 
+pytestmark = pytest.mark.ixmp4_209
+
+
 class TestMESSAGE:
     """Tests of :class:`.MESSAGE`."""
 
diff --git a/message_ix/tests/test_message_macro.py b/message_ix/tests/test_message_macro.py
index f606f8ea0..5aa207b7e 100644
--- a/message_ix/tests/test_message_macro.py
+++ b/message_ix/tests/test_message_macro.py
@@ -41,6 +41,7 @@ class _MM(MESSAGE_MACRO):
         ):
             _MM()
 
+    @pytest.mark.ixmp4_209
     def test_initialize(self, test_mp: Platform) -> None:
         # MESSAGE_MACRO.initialize() runs
         Scenario(
diff --git a/message_ix/tests/test_report.py b/message_ix/tests/test_report.py
index bc62cd5c6..057711fc6 100644
--- a/message_ix/tests/test_report.py
+++ b/message_ix/tests/test_report.py
@@ -1,6 +1,5 @@
 import logging
 import re
-import sys
 from collections import defaultdict
 from collections.abc import Generator
 from functools import partial
@@ -25,6 +24,8 @@
 from message_ix.report import Reporter, configure
 from message_ix.testing import SCENARIO, make_dantzig, make_westeros
 
+pytestmark = pytest.mark.ixmp4_209
+
 # NOTE These tests maybe don't need to be parametrized.
 # Does `Reporter.from_scenario()` depend on otherwise untested Scenario functions?
 
@@ -353,34 +354,18 @@ def add_tm(df, name="Activity"):
     assert not any(c in df2.columns for c in ["h", "m", "t"])
 
     # Variable names were formatted by the callback
-    reg_var = (
-        pd.DataFrame(
-            [
-                ["seattle", "Activity|canning_plant|production"],
-                ["seattle", "Activity|transport_from_seattle|to_new-york"],
-                ["seattle", "Activity|transport_from_seattle|to_chicago"],
-                ["seattle", "Activity|transport_from_seattle|to_topeka"],
-                ["san-diego", "Activity|canning_plant|production"],
-                ["san-diego", "Activity|transport_from_san-diego|to_new-york"],
-                ["san-diego", "Activity|transport_from_san-diego|to_chicago"],
-                ["san-diego", "Activity|transport_from_san-diego|to_topeka"],
-            ],
-            columns=["region", "variable"],
-        )
-        if sys.version_info >= (3, 10)
-        else pd.DataFrame(
-            [
-                ["san-diego", "Activity|canning_plant|production"],
-                ["san-diego", "Activity|transport_from_san-diego|to_chicago"],
-                ["san-diego", "Activity|transport_from_san-diego|to_new-york"],
-                ["san-diego", "Activity|transport_from_san-diego|to_topeka"],
-                ["seattle", "Activity|canning_plant|production"],
-                ["seattle", "Activity|transport_from_seattle|to_chicago"],
-                ["seattle", "Activity|transport_from_seattle|to_new-york"],
-                ["seattle", "Activity|transport_from_seattle|to_topeka"],
-            ],
-            columns=["region", "variable"],
-        )
+    reg_var = pd.DataFrame(
+        [
+            ["seattle", "Activity|canning_plant|production"],
+            ["seattle", "Activity|transport_from_seattle|to_new-york"],
+            ["seattle", "Activity|transport_from_seattle|to_chicago"],
+            ["seattle", "Activity|transport_from_seattle|to_topeka"],
+            ["san-diego", "Activity|canning_plant|production"],
+            ["san-diego", "Activity|transport_from_san-diego|to_new-york"],
+            ["san-diego", "Activity|transport_from_san-diego|to_chicago"],
+            ["san-diego", "Activity|transport_from_san-diego|to_topeka"],
+        ],
+        columns=["region", "variable"],
     )
     assert_frame_equal(df2[["region", "variable"]], reg_var)
 
diff --git a/message_ix/tests/test_testing.py b/message_ix/tests/test_testing.py
index e9371a14d..cff6fa532 100644
--- a/message_ix/tests/test_testing.py
+++ b/message_ix/tests/test_testing.py
@@ -4,6 +4,8 @@
 from message_ix import ModelError, Scenario
 from message_ix.testing import make_austria, make_dantzig, make_westeros
 
+pytestmark = pytest.mark.ixmp4_209
+
 
 @pytest.mark.parametrize(
     "kwargs",
@@ -24,6 +26,7 @@ def test_make_austria(
     assert isinstance(s, Scenario)
 
 
+@pytest.mark.ixmp4_209
 @pytest.mark.parametrize(
     "kwargs",
     (
diff --git a/message_ix/tests/test_tutorials.py b/message_ix/tests/test_tutorials.py
index e1150a368..59bf39655 100644
--- a/message_ix/tests/test_tutorials.py
+++ b/message_ix/tests/test_tutorials.py
@@ -4,7 +4,7 @@
 from collections.abc import Generator
 from pathlib import Path
 from shutil import copyfile
-from typing import Any, Optional, Union
+from typing import Any
 
 import ixmp.backend
 import numpy.testing as npt
@@ -47,11 +47,11 @@
 
 # NOTE Pytest.param returns ParamSet, which is private and hard to type hint
 def _t(
-    group: Union[str, None],
+    group: str | None,
     basename: str,
     *,
-    check: Optional[list[tuple]] = None,
-    marks: Optional[list[pytest.MarkDecorator]] = None,
+    check: list[tuple] | None = None,
+    marks: list[pytest.MarkDecorator] | None = None,
 ) -> tuple:
     """Shorthand for defining tutorial test cases.
 
@@ -144,7 +144,7 @@ def nb_path(
     yield tutorial_path.joinpath(*request.param).with_suffix(".ipynb")
 
 
-def default_args() -> dict[str, Union[int, str]]:
+def default_args() -> dict[str, int | str]:
     """Default arguments for :func:`.run_notebook."""
     if GHA:
         # Use a longer timeout
diff --git a/message_ix/tests/tools/test_add_year.py b/message_ix/tests/tools/test_add_year.py
index e35bc6251..907c676b2 100644
--- a/message_ix/tests/tools/test_add_year.py
+++ b/message_ix/tests/tools/test_add_year.py
@@ -1,5 +1,5 @@
 from collections.abc import Callable, Generator
-from typing import Any, Union
+from typing import Any
 
 import pytest
 from click.testing import Result
@@ -27,12 +27,12 @@ def base_scen_mp(
         scen.add_set("year", years)
         scen.add_set("technology", "tec")
         scen.add_set("mode", "mode")
-        output_specs: list[Union[int, str]] = ["node", "comm", "level", "year", "year"]
+        output_specs: list[int | str] = ["node", "comm", "level", "year", "year"]
 
         for yr, value in data.items():
             scen.add_par("demand", ["node", "comm", "level", yr, "year"], 1, "GWa")
             scen.add_par("technical_lifetime", ["node", "tec", yr], 10, "y")
-            tec_specs: list[Union[int, str]] = ["node", "tec", yr, yr, "mode"]
+            tec_specs: list[int | str] = ["node", "tec", yr, yr, "mode"]
             scen.add_par("output", tec_specs + output_specs, 1, "-")
             scen.add_par("var_cost", tec_specs + ["year"], value, "USD/GWa")
 
diff --git a/message_ix/tests/tools/test_migrate.py b/message_ix/tests/tools/test_migrate.py
index bfc0fb514..843e94f0e 100644
--- a/message_ix/tests/tools/test_migrate.py
+++ b/message_ix/tests/tools/test_migrate.py
@@ -1,7 +1,5 @@
 """Tests of :mod:`message_ix.tools.migrate`."""
 
-import sys
-
 import pytest
 from ixmp import Platform
 
@@ -10,11 +8,6 @@
 
 
 @pytest.mark.jdbc
-@pytest.mark.xfail(
-    condition=sys.version_info < (3, 10),
-    raises=ImportError,
-    reason="Uses itertools.pairwise, added in Python 3.10",
-)
 def test_v311(
     caplog: pytest.LogCaptureFixture, request: pytest.FixtureRequest, test_mp: Platform
 ) -> None:
diff --git a/message_ix/tests/tools/test_sankey.py b/message_ix/tests/tools/test_sankey.py
index 84dd3e9c7..0e5baf17d 100644
--- a/message_ix/tests/tools/test_sankey.py
+++ b/message_ix/tests/tools/test_sankey.py
@@ -11,6 +11,8 @@
 if TYPE_CHECKING:
     import pyam
 
+pytestmark = pytest.mark.ixmp4_209
+
 # NOTE This test likely doesn't need to be parametrized
 
 
diff --git a/message_ix/tests/util/test_gams_io.py b/message_ix/tests/util/test_gams_io.py
index 545773837..3f4dd21a6 100644
--- a/message_ix/tests/util/test_gams_io.py
+++ b/message_ix/tests/util/test_gams_io.py
@@ -1,14 +1,11 @@
 import pandas as pd
 import pytest
 from ixmp import Platform
-from ixmp.testing import min_ixmp4_version
 
 from message_ix import Scenario
 from message_ix.testing import make_westeros
 from message_ix.util.scenario_data import HELPER_INDEXSETS, HELPER_TABLES
 
-pytestmark = min_ixmp4_version
-
 
 def test_store_message_version() -> None:
     from ixmp.util.ixmp4 import ContainerData
@@ -23,6 +20,7 @@ def test_store_message_version() -> None:
 
 
 @pytest.mark.ixmp4
+@pytest.mark.ixmp4_209
 def test_add_default_data_to_container_data_list(test_mp: Platform) -> None:
     from ixmp.util.ixmp4 import ContainerData
 
@@ -66,6 +64,7 @@ def test_add_default_data_to_container_data_list(test_mp: Platform) -> None:
 
 
 @pytest.mark.ixmp4
+@pytest.mark.ixmp4_209
 def test_add_auxiliary_items_to_container_data_list(
     test_mp: Platform, request: pytest.FixtureRequest
 ) -> None:
diff --git a/message_ix/tests/util/test_tutorial.py b/message_ix/tests/util/test_tutorial.py
index 608e50a04..c03dd7886 100644
--- a/message_ix/tests/util/test_tutorial.py
+++ b/message_ix/tests/util/test_tutorial.py
@@ -1,5 +1,6 @@
 from typing import TYPE_CHECKING
 
+import pytest
 from ixmp.report import Key
 
 from message_ix import Reporter, Scenario, make_df
@@ -10,6 +11,8 @@
     import pytest
     from ixmp import Platform
 
+pytestmark = pytest.mark.ixmp4_209
+
 
 # NOTE This test likely doesn't need to be parametrized
 def test_prepare_plots(dantzig_reporter: Reporter) -> None:
diff --git a/message_ix/tools/add_year/__init__.py b/message_ix/tools/add_year/__init__.py
index e5ad7094b..92c50d7f9 100644
--- a/message_ix/tools/add_year/__init__.py
+++ b/message_ix/tools/add_year/__init__.py
@@ -12,7 +12,7 @@
 
 # %% I) Importing required packages
 import logging
-from typing import Literal, Optional, Union
+from typing import Literal
 
 import numpy as np
 import pandas as pd
@@ -24,12 +24,12 @@
 
 # %% II) Utility functions for dataframe manupulation
 def intpol(
-    y1: Union[float, pd.Series, pd.DataFrame],
-    y2: Union[float, pd.Series, pd.DataFrame],
+    y1: float | pd.Series | pd.DataFrame,
+    y2: float | pd.Series | pd.DataFrame,
     x1: int,
     x2: int,
     x: int,
-) -> Union[float, pd.Series, pd.DataFrame]:
+) -> float | pd.Series | pd.DataFrame:
     """Interpolate between (*x1*, *y1*) and (*x2*, *y2*) at *x*.
 
     Parameters
@@ -52,7 +52,7 @@ def slice_df(
     idx: list[str],
     level: str,
     locator: list,
-    value: Union[int, str, None],
+    value: int | str | None,
 ) -> pd.DataFrame:
     """Slice a MultiIndex DataFrame and set a value to a specific level.
 
@@ -76,9 +76,7 @@ def slice_df(
     return df.set_index(idx)
 
 
-def mask_df(
-    df: pd.DataFrame, index: tuple[Union[int, str], ...], count: int, value
-) -> None:
+def mask_df(df: pd.DataFrame, index: tuple[int | str, ...], count: int, value) -> None:
     """Create a mask for removing extra values from *df*."""
     df.loc[
         index,
@@ -110,15 +108,15 @@ def add_year(
     sc_ref: Scenario,
     sc_new: Scenario,
     years_new: list[int],
-    firstyear_new: Optional[int] = None,
-    lastyear_new: Optional[int] = None,
+    firstyear_new: int | None = None,
+    lastyear_new: int | None = None,
     macro: bool = False,
-    baseyear_macro: Optional[int] = None,
-    parameter: Union[list[str], Literal["all"]] = "all",
-    region: Union[list[str], Literal["all"]] = "all",
+    baseyear_macro: int | None = None,
+    parameter: list[str] | Literal["all"] = "all",
+    region: list[str] | Literal["all"] = "all",
     rewrite: bool = True,
     unit_check: bool = True,
-    extrapol_neg: Optional[float] = None,
+    extrapol_neg: float | None = None,
     bound_extend: bool = True,
 ) -> None:
     """Add years to *sc_ref* to produce *sc_new*.
@@ -292,9 +290,9 @@ def add_year_set(  # noqa: C901
     sc_ref: Scenario,
     sc_new: Scenario,
     years_new: list[int],
-    firstyear_new: Optional[int] = None,
-    lastyear_new: Optional[int] = None,
-    baseyear_macro: Optional[int] = None,
+    firstyear_new: int | None = None,
+    lastyear_new: int | None = None,
+    baseyear_macro: int | None = None,
 ) -> None:
     """Add new years to sets.
 
@@ -340,7 +338,7 @@ def add_year_set(  # noqa: C901
                 baseyear_macro
             )
 
-    yr_pair: list[list[Union[int, str]]] = []
+    yr_pair: list[list[int | str]] = []
     for yr in years_new:
         yr_pair.append([yr, yr])
         yr_pair.append(["cumulative", yr])
@@ -404,7 +402,7 @@ def add_year_par(
     extrapolate: bool = False,
     rewrite: bool = True,
     unit_check: bool = True,
-    extrapol_neg: Optional[float] = None,
+    extrapol_neg: float | None = None,
     bound_extend: bool = True,
 ) -> None:
     """Add new years to parameters.
@@ -570,7 +568,7 @@ def interpolate_1d(  # noqa: C901
     year_col: str,
     value_col: str = "value",
     extrapolate: bool = False,
-    extrapol_neg: Optional[float] = None,
+    extrapol_neg: float | None = None,
     bound_extend: bool = True,
 ):
     """Interpolate data with one year dimension.
@@ -729,8 +727,8 @@ def interpolate_2d(  # noqa: C901
     par_tec: pd.DataFrame,
     value_col: str = "value",
     extrapolate: bool = False,
-    extrapol_neg: Optional[float] = None,
-    year_diff: Optional[list[int]] = None,
+    extrapol_neg: float | None = None,
+    year_diff: list[int] | None = None,
     bound_extend: bool = True,
 ):
     """Interpolate parameters with two dimensions related year.
diff --git a/message_ix/tools/lp_diag/__init__.py b/message_ix/tools/lp_diag/__init__.py
index b57279ccd..a2a634cd5 100644
--- a/message_ix/tools/lp_diag/__init__.py
+++ b/message_ix/tools/lp_diag/__init__.py
@@ -647,19 +647,18 @@ def row_att(
                 print(f"{sec_name} value {val} ignored for neutral row {row_name}.")
                 return
             attr = self.seq_row.get(row_seq)  # [row_name, lo_bnd, up_bnd, row_type]
-            if sec_name == "rhs":  # process the RHS value
-                if row_type == "G":  # update lo_bnd
+            match sec_name, row_type:
+                case "rhs", "G":  # update lo_bnd
                     attr[1] = val
-                elif row_type == "L":  # update up_bnd
+                case "rhs", "L":  # update up_bnd
                     attr[2] = val
-                elif row_type == "E":  # update both bounds
+                case "rhs", "E":  # update both bounds
                     attr[1] = attr[2] = val
-            else:  # process the ranges value
-                if row_type == "G":  # update up_bnd
+                case "ranges", "G":  # update up_bnd
                     attr[2] = attr[1] + abs(val)
-                elif row_type == "L":  # update lo_bnd
+                case "ranges", "L":  # update lo_bnd
                     attr[1] = attr[2] - abs(val)
-                elif row_type == "E":  # update both bounds
+                case "ranges", "E":  # update both bounds
                     if val > 0:
                         attr[2] = attr[1] + val
                     else:
diff --git a/message_ix/tools/lp_diag/cli.py b/message_ix/tools/lp_diag/cli.py
index 2843d28d1..415b3329f 100644
--- a/message_ix/tools/lp_diag/cli.py
+++ b/message_ix/tools/lp_diag/cli.py
@@ -16,7 +16,6 @@
 import sys  # for redirecting stdout
 from datetime import datetime as dt
 from pathlib import Path
-from typing import Optional
 
 import click
 
@@ -52,7 +51,7 @@
     help="Magnitude order of the upper tail (default: 5).",
 )
 @click.option("--outp", "fn_outp", metavar="PATH", help="Path for file output.")
-def main(w_dir: Path, prob_id: Path, fn_outp: Optional[Path], lo_tail, up_tail) -> None:
+def main(w_dir: Path, prob_id: Path, fn_outp: Path | None, lo_tail, up_tail) -> None:
     """Diagnostics of basic properties of LP problems stored in the MPS format.
 
     \b
diff --git a/message_ix/tools/migrate.py b/message_ix/tools/migrate.py
index d7ba60d50..9209394d2 100644
--- a/message_ix/tools/migrate.py
+++ b/message_ix/tools/migrate.py
@@ -1,6 +1,7 @@
 """Tools for migrating :class:`Scenario` data across versions of :mod:`message_ix`."""
 
 import logging
+from itertools import pairwise
 from typing import TYPE_CHECKING
 
 if TYPE_CHECKING:
@@ -25,8 +26,6 @@ def initial_new_capacity_up_v311(
 
     *All* values in initial_new_capacity_up are rewritten in this way.
     """
-    # TODO Move this to top level when dropping support for Python 3.9
-    from itertools import pairwise
 
     if migration_applied(s, "initial_new_capacity_up_v311"):
         log.info(
diff --git a/message_ix/tools/sankey.py b/message_ix/tools/sankey.py
index f655e1688..12b00ce0c 100644
--- a/message_ix/tools/sankey.py
+++ b/message_ix/tools/sankey.py
@@ -1,11 +1,7 @@
 import logging
 from typing import TYPE_CHECKING
 
-try:
-    from pyam.str import get_variable_components
-except ImportError:  # Python < 3.10 → pyam-iamc < 3
-    from pyam.utils import get_variable_components
-
+from pyam.str import get_variable_components
 
 if TYPE_CHECKING:
     import pyam
diff --git a/message_ix/util/__init__.py b/message_ix/util/__init__.py
index 06683aa5e..1261286d9 100644
--- a/message_ix/util/__init__.py
+++ b/message_ix/util/__init__.py
@@ -4,7 +4,7 @@
 from collections import ChainMap, defaultdict
 from collections.abc import Mapping
 from pathlib import Path
-from typing import TYPE_CHECKING, Optional
+from typing import TYPE_CHECKING
 
 import pandas as pd
 from ixmp.backend import ItemType
@@ -23,7 +23,7 @@ def copy_model(
     set_default: bool = False,
     quiet: bool = False,
     *,
-    source_dir: Optional[Path] = None,
+    source_dir: Path | None = None,
 ) -> None:
     """Copy the MESSAGE GAMS files to a new `path`.
 
diff --git a/message_ix/util/gams_io.py b/message_ix/util/gams_io.py
index fb08d089e..d98ade055 100644
--- a/message_ix/util/gams_io.py
+++ b/message_ix/util/gams_io.py
@@ -1,4 +1,4 @@
-from typing import TYPE_CHECKING, Literal, Optional, Union, cast
+from typing import TYPE_CHECKING, Literal, cast
 
 import pandas as pd
 
@@ -33,7 +33,7 @@
 # 	}
 
 # This must be convertible to pd.DataFrame
-MESSAGE_IX_VERSION: dict[str, Union[list[float], list[int], list[str]]] = {
+MESSAGE_IX_VERSION: dict[str, list[float] | list[int] | list[str]] = {
     "version_part": ["major", "minor"],
     "value": [2, 0],
 }
@@ -115,9 +115,7 @@ def _compose_resource_grade_map(
     return resource_grade_map
 
 
-def _handle_renames(
-    df: pd.DataFrame, renames: Optional[dict[str, str]]
-) -> pd.DataFrame:
+def _handle_renames(df: pd.DataFrame, renames: dict[str, str] | None) -> pd.DataFrame:
     """Rename columns from `df` according to `renames`, if present."""
     return df.rename(columns=renames) if renames else df
 
@@ -125,8 +123,8 @@ def _handle_renames(
 def _handle_empty_parameter(
     scenario: "Scenario",
     source_name: str,
-    columns: Optional[list[str]],
-    renames: Optional[dict[str, str]] = None,
+    columns: list[str] | None,
+    renames: dict[str, str] | None = None,
 ) -> pd.DataFrame:
     """Create an empty pd.DataFrame with correct column names."""
     _columns = columns or scenario.idx_names(name=source_name)
@@ -136,9 +134,9 @@ def _handle_empty_parameter(
 
 def _compose_records(
     scenario: "Scenario",
-    sources: dict[str, Optional[list[str]]],
-    filters: Optional[HelperFilterInfo],
-    renames: Optional[dict[str, str]],
+    sources: dict[str, list[str] | None],
+    filters: HelperFilterInfo | None,
+    renames: dict[str, str] | None,
 ) -> pd.DataFrame:
     """Compose the records for an auxiliary IndexSet/Table.
 
@@ -149,9 +147,9 @@ def _compose_records(
     sources: dict
         A mapping to specify the sources. Keys are parameter names, values are optional
         columns to limit the data to.
-    filters: Optional[HelperFilterInfo]
+    filters: HelperFilterInfo
         Optionally, specify a filter that limits a single column to specific values.
-    renames: Optional[dict[str, str]]
+    renames: dict[str, str]
         Optionally, rename columns of the records to align them properly.
     """
     # Create a list to collect records
@@ -195,7 +193,7 @@ def _compose_records(
 
 
 def _compose_map_tec_time(
-    scenario: "Scenario", sources: dict[str, Optional[list[str]]]
+    scenario: "Scenario", sources: dict[str, list[str] | None]
 ) -> pd.DataFrame:
     """Compose the records for an auxiliary IndexSet/Table.
 
@@ -235,8 +233,8 @@ def _compose_map_tec_time(
 
 def _compose_map_resource(
     scenario: "Scenario",
-    sources: dict[str, Optional[list[str]]],
-    filters: Optional[HelperFilterInfo],
+    sources: dict[str, list[str] | None],
+    filters: HelperFilterInfo | None,
     resource_grade_map: dict[tuple[str, str], list[str]],
 ) -> pd.DataFrame:
     """Compose the records for an auxiliary IndexSet/Table.
@@ -248,7 +246,7 @@ def _compose_map_resource(
     sources: dict
         A mapping to specify the sources. Keys are parameter names, values are optional
         columns to limit the data to.
-    filters: Optional[HelperFilterInfo]
+    filters: HelperFilterInfo
         Optionally, specify a filter that limits a single column to specific values.
     resource_grade_map: dict[list[str], list[str]]
         An auxiliary mapping from (`node`, `commodity`) to list of `grade` to construct
diff --git a/message_ix/util/scenario_data.py b/message_ix/util/scenario_data.py
index 9b4196143..906e4eeb7 100644
--- a/message_ix/util/scenario_data.py
+++ b/message_ix/util/scenario_data.py
@@ -1,5 +1,5 @@
 from dataclasses import dataclass
-from typing import Literal, Optional, Union
+from typing import Literal
 
 # NOTE on transcribing from ixmp_source:
 # The data in this file is based on
@@ -64,7 +64,7 @@ class DefaultIndexSetData:
 class TableInfo:
     name: str
     indexsets: list[str]
-    column_names: Optional[list[str]] = None
+    column_names: list[str] | None = None
 
 
 # List of required table names and indexsets they are linked to, including column_names
@@ -200,11 +200,11 @@ class ParameterInfo:
     name: str
     gams_name: str  # could default to None  and use name in that case, maybe?
     indexsets: list[str]
-    column_names: Optional[list[str]] = None
+    column_names: list[str] | None = None
     write_to_gdx: bool = True
     is_tec: bool = False
     is_tec_act: bool = False
-    section: Optional[
+    section: (
         Literal[
             "energyforms:",
             "demand:",
@@ -213,7 +213,8 @@ class ParameterInfo:
             "variables:",
             "relationsX:",
         ]
-    ] = None
+        | None
+    ) = None
     # NOTE changing '2LDB' to 'toLDB'
     toLDB: bool = False
     mode: bool = False
@@ -1372,7 +1373,7 @@ class ParameterInfo:
 @dataclass
 class DefaultParameterData:
     name: str
-    data: dict[str, Union[list[int], list[str]]]  # based on current usage
+    data: dict[str, list[int] | list[str]]  # based on current usage
 
 
 DEFAULT_PARAMETER_DATA = [
@@ -1386,8 +1387,8 @@ class DefaultParameterData:
 class VariableInfo:
     name: str
     gams_name: str
-    indexsets: Optional[list[str]] = None
-    column_names: Optional[list[str]] = None
+    indexsets: list[str] | None = None
+    column_names: list[str] | None = None
 
 
 # List of required variable names and indexsets they are linked to, including
@@ -1534,14 +1535,14 @@ class VariableInfo:
 class HelperFilterInfo:
     column_name: str
     target_name: Literal["resource", "renewables", "stocks"]
-    target: Optional[list[str]] = None
+    target: list[str] | None = None
 
 
 @dataclass
 class HelperIndexSetInfo:
     name: str
-    sources: dict[str, Optional[list[str]]]
-    filters: Optional[HelperFilterInfo] = None
+    sources: dict[str, list[str] | None]
+    filters: HelperFilterInfo | None = None
 
 
 HELPER_INDEXSETS = [
@@ -1568,7 +1569,7 @@ class HelperIndexSetInfo:
 
 @dataclass
 class HelperTableInfo(HelperIndexSetInfo):
-    renames: Optional[dict[str, str]] = None
+    renames: dict[str, str] | None = None
 
 
 # NOTE on transcribing from ixmp_source: ixmp.Element.getVector() only returns keys,
diff --git a/message_ix/util/scenario_setup.py b/message_ix/util/scenario_setup.py
index 86aaf78e8..87a513589 100644
--- a/message_ix/util/scenario_setup.py
+++ b/message_ix/util/scenario_setup.py
@@ -1,5 +1,5 @@
 import logging
-from typing import TYPE_CHECKING, Any, Literal, TypeVar, Union, cast
+from typing import TYPE_CHECKING, Any, Literal, TypeVar, cast
 
 import pandas as pd
 from ixmp import Platform
@@ -161,7 +161,7 @@ def ensure_required_indexsets_have_data(scenario: "Scenario") -> None:
 
 def _maybe_add_to_table(
     table: "Table",
-    data: Union[dict[str, Any], pd.DataFrame],
+    data: dict[str, Any] | pd.DataFrame,
     backend: "Backend",
 ) -> None:
     """Add (parts of) `data` to `table` if they are missing."""
@@ -268,7 +268,7 @@ def _find_all_descendants(parent: T) -> list[T]:
 
 
 def _maybe_add_single_item_to_indexset(
-    indexset: "IndexSet", data: Union[float, int, str], backend: "Backend"
+    indexset: "IndexSet", data: float | int | str, backend: "Backend"
 ) -> None:
     """Add `data` to `indexset` if it is missing."""
     if data not in list(indexset.data):
@@ -277,7 +277,7 @@ def _maybe_add_single_item_to_indexset(
 
 def _maybe_add_list_to_indexset(
     indexset: "IndexSet",
-    data: Union[list[float], list[int], list[str]],
+    data: list[float] | list[int] | list[str],
     backend: "Backend",
 ) -> None:
     """Add missing parts of `data` to `indexset`."""
@@ -290,7 +290,7 @@ def _maybe_add_list_to_indexset(
 
 def _maybe_add_to_indexset(
     indexset: "IndexSet",
-    data: Union[float, int, str, list[float], list[int], list[str]],
+    data: float | int | str | list[float] | list[int] | list[str],
     backend: "Backend",
 ) -> None:
     """Add (parts of) `data` to `indexset` if they are missing."""
@@ -307,7 +307,7 @@ def _maybe_add_to_indexset(
 # slower than necessary (though likely not by much). Is the maintenance effort worth it?
 def _maybe_add_to_parameter(
     parameter: "Parameter",
-    data: Union[dict[str, Any], pd.DataFrame],
+    data: dict[str, Any] | pd.DataFrame,
     backend: "Backend",
 ) -> None:
     """Add (parts of) `data` to `parameter` if they are missing."""
diff --git a/pyproject.toml b/pyproject.toml
index 55a10b0f0..d56e45b37 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -20,16 +20,16 @@ classifiers = [
   "Operating System :: OS Independent",
   "Programming Language :: Python",
   "Programming Language :: Python :: 3",
-  "Programming Language :: Python :: 3.9",
   "Programming Language :: Python :: 3.10",
   "Programming Language :: Python :: 3.11",
   "Programming Language :: Python :: 3.12",
   "Programming Language :: Python :: 3.13",
+  "Programming Language :: Python :: 3.14",
   "Programming Language :: R",
   "Topic :: Scientific/Engineering",
   "Topic :: Scientific/Engineering :: Information Analysis",
 ]
-requires-python = ">=3.9"
+requires-python = ">=3.10"
 dependencies = [
   "click",
   "ixmp >= 3.11",
@@ -119,6 +119,7 @@ filterwarnings = [
 ]
 markers = [
   "ixmp4: tests exclusive to IXMP4Backend.",
+  "ixmp4_209: https://github.com/iiasa/ixmp4/pull/209",
   "jdbc: tests exclusive to JDBCBackend.",
   "nightly: Slow-running nightly tests of particular scenarios.",
   "rmessageix: test of the message_ix R interface.",