diff --git a/conda-env.yml b/conda-env.yml
index 48aec619..81c056e1 100644
--- a/conda-env.yml
+++ b/conda-env.yml
@@ -2,7 +2,7 @@ name: dolphin-env
 channels:
   - conda-forge
 dependencies:
-  - python>=3.8
+  - python==3.9
   - pip>=21.3  # https://pip.pypa.io/en/stable/reference/build-system/pyproject-toml/#editable-installation
   - git  # for pip install, due to setuptools_scm
   - gdal>=3.3
diff --git a/pyproject.toml b/pyproject.toml
index f7bd9b80..2fe33dea 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -6,7 +6,7 @@ build-backend = "setuptools.build_meta"
 name = "dolphin"
 description = "Workflows for generating surface displacement maps using InSAR"
 readme = { file = "README.md", content-type = "text/markdown" }
-requires-python = ">=3.8"
+requires-python = ">=3.9"
 
 classifiers = [
   "Development Status :: 2 - Pre-Alpha",
diff --git a/src/dolphin/_cli_unwrap.py b/src/dolphin/_cli_unwrap.py
index 2fc4a485..7005020e 100644
--- a/src/dolphin/_cli_unwrap.py
+++ b/src/dolphin/_cli_unwrap.py
@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 import argparse
 from pathlib import Path
 from typing import TYPE_CHECKING, Any
diff --git a/src/dolphin/_log.py b/src/dolphin/_log.py
index a173b60c..03ff939d 100644
--- a/src/dolphin/_log.py
+++ b/src/dolphin/_log.py
@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 import json
 import logging
 import logging.config
diff --git a/src/dolphin/_overviews.py b/src/dolphin/_overviews.py
index 4bbb46a0..feb22809 100644
--- a/src/dolphin/_overviews.py
+++ b/src/dolphin/_overviews.py
@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 import argparse
 import logging
 from enum import Enum
diff --git a/src/dolphin/filtering.py b/src/dolphin/filtering.py
index c307f722..19039a38 100644
--- a/src/dolphin/filtering.py
+++ b/src/dolphin/filtering.py
@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 import multiprocessing as mp
 from concurrent.futures import ProcessPoolExecutor
 from itertools import repeat
diff --git a/src/dolphin/goldstein.py b/src/dolphin/goldstein.py
index c092bc03..a6735aef 100644
--- a/src/dolphin/goldstein.py
+++ b/src/dolphin/goldstein.py
@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 import numpy as np
 from numpy.typing import NDArray
 
diff --git a/src/dolphin/interpolation.py b/src/dolphin/interpolation.py
index 99cfc0e9..476c1cdd 100644
--- a/src/dolphin/interpolation.py
+++ b/src/dolphin/interpolation.py
@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 import logging
 
 import numba
diff --git a/src/dolphin/io/_process.py b/src/dolphin/io/_process.py
index 0fc9ecce..51ffc448 100644
--- a/src/dolphin/io/_process.py
+++ b/src/dolphin/io/_process.py
@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 from concurrent.futures import FIRST_EXCEPTION, Future, ThreadPoolExecutor, wait
 from typing import Protocol, Sequence
 
diff --git a/src/dolphin/phase_link/_compress.py b/src/dolphin/phase_link/_compress.py
index 2df1e38f..33b737d1 100644
--- a/src/dolphin/phase_link/_compress.py
+++ b/src/dolphin/phase_link/_compress.py
@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 import warnings
 
 import numpy as np
diff --git a/src/dolphin/phase_link/metrics.py b/src/dolphin/phase_link/metrics.py
index a12819a8..56f92b14 100644
--- a/src/dolphin/phase_link/metrics.py
+++ b/src/dolphin/phase_link/metrics.py
@@ -1,5 +1,7 @@
 """Module for computing quality metrics of estimated solutions."""
 
+from __future__ import annotations
+
 import jax.numpy as jnp
 from jax import Array, jit, vmap
 from jax.typing import ArrayLike
diff --git a/src/dolphin/phase_link/simulate.py b/src/dolphin/phase_link/simulate.py
index cdbadd52..e9f7f1f6 100644
--- a/src/dolphin/phase_link/simulate.py
+++ b/src/dolphin/phase_link/simulate.py
@@ -4,6 +4,8 @@
 full CPU/GPU stack implementations.
 """
 
+from __future__ import annotations
+
 import numpy as np
 import numpy.linalg as la
 import scipy.ndimage as ndi
diff --git a/src/dolphin/similarity.py b/src/dolphin/similarity.py
index 12fbe008..7df1a3d3 100644
--- a/src/dolphin/similarity.py
+++ b/src/dolphin/similarity.py
@@ -3,6 +3,8 @@
 Uses metric from [@Wang2022AccuratePersistentScatterer] for similarity.
 """
 
+from __future__ import annotations
+
 import logging
 from pathlib import Path
 from typing import Callable, Literal, Sequence
diff --git a/src/dolphin/stack.py b/src/dolphin/stack.py
index 98e78f80..66c433c7 100755
--- a/src/dolphin/stack.py
+++ b/src/dolphin/stack.py
@@ -151,18 +151,18 @@ class CompressedSlcInfo(BaseModel):
         ..., description="Datetime of the last real SLC used in the compression."
     )
 
-    real_slc_file_list: list[Filename] | None = Field(
+    real_slc_file_list: Optional[list[Filename]] = Field(
         None,
         description="List of real SLC filenames in the ministack.",
     )
-    real_slc_dates: list[datetime] | None = Field(
+    real_slc_dates: Optional[list[datetime]] = Field(
         None,
         description=(
             "List of date sequences, one for each SLC in the ministack. "
             "Each item is a list/tuple of datetime.date or datetime.datetime objects."
         ),
     )
-    compressed_slc_file_list: list[Filename] | None = Field(
+    compressed_slc_file_list: Optional[list[Filename]] = Field(
         None,
         description="List of compressed SLC filenames in the ministack.",
     )
diff --git a/src/dolphin/unwrap/_post_process.py b/src/dolphin/unwrap/_post_process.py
index 64bdfd16..bc068cf9 100644
--- a/src/dolphin/unwrap/_post_process.py
+++ b/src/dolphin/unwrap/_post_process.py
@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 import logging
 
 import numpy as np
diff --git a/src/dolphin/unwrap/_unwrap_3d.py b/src/dolphin/unwrap/_unwrap_3d.py
index 49e9005e..da076ab1 100644
--- a/src/dolphin/unwrap/_unwrap_3d.py
+++ b/src/dolphin/unwrap/_unwrap_3d.py
@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 import logging
 import shutil
 import subprocess
diff --git a/src/dolphin/workflows/_cli_run.py b/src/dolphin/workflows/_cli_run.py
index f0b1a138..29489d31 100644
--- a/src/dolphin/workflows/_cli_run.py
+++ b/src/dolphin/workflows/_cli_run.py
@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 import argparse
 from typing import TYPE_CHECKING, Any, Optional, Sequence
 
diff --git a/src/dolphin/workflows/config/_yaml_model.py b/src/dolphin/workflows/config/_yaml_model.py
index 50a36b70..aaba92fe 100644
--- a/src/dolphin/workflows/config/_yaml_model.py
+++ b/src/dolphin/workflows/config/_yaml_model.py
@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 import json
 import sys
 import textwrap