diff --git a/.librarian/generator-input/client-post-processing/spanner-integration.yaml b/.librarian/generator-input/client-post-processing/spanner-integration.yaml
index 7b1b268ed15a..90e6a312e3b5 100644
--- a/.librarian/generator-input/client-post-processing/spanner-integration.yaml
+++ b/.librarian/generator-input/client-post-processing/spanner-integration.yaml
@@ -659,6 +659,7 @@ replacements:
       import pathlib
       import re
       import shutil
+      import sys
       import warnings
       from typing import Dict, List
 
@@ -669,7 +670,17 @@ replacements:
 
       DEFAULT_PYTHON_VERSION = "3.14"
 
-      DEFAULT_MOCK_SERVER_TESTS_PYTHON_VERSION = "3.12"
+      # Default to Python 3.12 for local development if available. If Python 3.12 is
+      # not installed (such as in CI environments where a different Python version
+      # is provisioned), fall back to the currently running Python interpreter to
+      # avoid InterpreterNotFound errors. Can also be overridden via the
+      # MOCK_SERVER_TESTS_PYTHON_VERSION environment variable.
+      DEFAULT_MOCK_SERVER_TESTS_PYTHON_VERSION = os.environ.get(
+          "MOCK_SERVER_TESTS_PYTHON_VERSION",
+          "3.12"
+          if shutil.which("python3.12")
+          else f"{sys.version_info.major}.{sys.version_info.minor}",
+      )
       SYSTEM_TEST_PYTHON_VERSIONS: List[str] = ["3.12"]
 
       ALL_PYTHON: List[str] = [
diff --git a/packages/google-cloud-spanner/noxfile.py b/packages/google-cloud-spanner/noxfile.py
index e14de4a0bc44..6ea3054a2277 100644
--- a/packages/google-cloud-spanner/noxfile.py
+++ b/packages/google-cloud-spanner/noxfile.py
@@ -17,6 +17,7 @@
 import pathlib
 import re
 import shutil
+import sys
 import warnings
 from typing import Dict, List
 
@@ -27,7 +28,17 @@
 
 DEFAULT_PYTHON_VERSION = "3.14"
 
-DEFAULT_MOCK_SERVER_TESTS_PYTHON_VERSION = "3.12"
+# Default to Python 3.12 for local development if available. If Python 3.12 is
+# not installed (such as in CI environments where a different Python version
+# is provisioned), fall back to the currently running Python interpreter to
+# avoid InterpreterNotFound errors. Can also be overridden via the
+# MOCK_SERVER_TESTS_PYTHON_VERSION environment variable.
+DEFAULT_MOCK_SERVER_TESTS_PYTHON_VERSION = os.environ.get(
+    "MOCK_SERVER_TESTS_PYTHON_VERSION",
+    "3.12"
+    if shutil.which("python3.12")
+    else f"{sys.version_info.major}.{sys.version_info.minor}",
+)
 SYSTEM_TEST_PYTHON_VERSIONS: List[str] = ["3.12"]
 
 ALL_PYTHON: List[str] = [