diff --git a/.github/workflows/import.yml b/.github/workflows/import.yml
index 8a110616b..45d8c6c83 100644
--- a/.github/workflows/import.yml
+++ b/.github/workflows/import.yml
@@ -12,6 +12,9 @@ jobs:
     runs-on: ubuntu-latest
 
     strategy:
+      # Which modules get imported differs by interpreter version, so a failure
+      # on one says nothing about the others. Run them all.
+      fail-fast: false
       matrix:
         python-version: ['3.10', '3.11', '3.12', '3.13', '3.14']
 
@@ -31,6 +34,6 @@ jobs:
           pip install pytest
           pip install .
 
-      - name: Run import test
+      - name: Run import tests
         run: |
-          pytest google/genai/tests/imports/test_no_optional_imports.py
\ No newline at end of file
+          pytest google/genai/tests/imports/
\ No newline at end of file
diff --git a/google/genai/tests/imports/allowed_imports.txt b/google/genai/tests/imports/allowed_imports.txt
new file mode 100644
index 000000000..347027681
--- /dev/null
+++ b/google/genai/tests/imports/allowed_imports.txt
@@ -0,0 +1,66 @@
+# Modules that `import google.genai` is allowed to load.
+#
+# Every google.genai submodule is listed by name, and that part has to match
+# exactly. Everything else is listed by top-level package only and is a
+# ceiling rather than a requirement, because some dependencies are conditional
+# on the interpreter version. Standard library modules are not listed.
+#
+# Adding a line means importing the SDK got more expensive. Deleting lines is
+# the goal.
+#
+# Regenerate after an intentional change:
+#   GOOGLE_GENAI_UPDATE_IMPORT_ALLOWLIST=1 pytest google/genai/tests/imports/test_imported_modules.py
+_cffi_backend
+_openssl
+annotated_types
+anyio
+certifi
+charset_normalizer
+cryptography
+exceptiongroup
+google.auth
+google.genai
+google.genai._adapters
+google.genai._api_client
+google.genai._api_module
+google.genai._base_transformers
+google.genai._base_url
+google.genai._common
+google.genai._extra_utils
+google.genai._gaos
+google.genai._live_converters
+google.genai._mcp_utils
+google.genai._operations_converters
+google.genai._replay_api_client
+google.genai._tokens_converters
+google.genai._transformers
+google.genai.batches
+google.genai.caches
+google.genai.chats
+google.genai.client
+google.genai.documents
+google.genai.errors
+google.genai.file_search_stores
+google.genai.files
+google.genai.interactions
+google.genai.live
+google.genai.live_music
+google.genai.models
+google.genai.operations
+google.genai.pagers
+google.genai.tokens
+google.genai.tunings
+google.genai.types
+google.genai.version
+google.oauth2
+httpx
+idna
+pydantic
+pydantic_core
+requests
+sniffio
+tenacity
+typing_extensions
+typing_inspection
+urllib3
+websockets
diff --git a/google/genai/tests/imports/test_imported_modules.py b/google/genai/tests/imports/test_imported_modules.py
new file mode 100644
index 000000000..802788cad
--- /dev/null
+++ b/google/genai/tests/imports/test_imported_modules.py
@@ -0,0 +1,297 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+"""Guards the set of modules that ``import google.genai`` loads.
+
+Import cost is invisible. An import accidentally added at module scope still
+works, it only makes every program that touches the SDK start more slowly, so
+nothing fails and nobody notices. These tests run the import in a fresh
+interpreter and compare what it loaded against a checked-in allowlist.
+"""
+
+from __future__ import annotations
+
+import functools
+import json
+import os
+import pathlib
+import subprocess
+import sys
+import tempfile
+from typing import Iterable
+
+import pytest
+
+_OPT_IN_ENV_VAR = 'GOOGLE_GENAI_IMPORT_TESTS'
+
+_UPDATE_ENV_VAR = 'GOOGLE_GENAI_UPDATE_IMPORT_ALLOWLIST'
+
+# The allowlist describes the published package layout, so these only run where
+# that layout is what is on the path: the import workflow, or a developer who
+# opts in from a source checkout. Asking for the allowlist to be regenerated is
+# itself opting in, so that does not need both variables.
+pytestmark = pytest.mark.skipif(
+    os.getenv('GITHUB_ACTIONS') != 'true'
+    and os.getenv(_OPT_IN_ENV_VAR) != '1'
+    and not os.getenv(_UPDATE_ENV_VAR),
+    reason=(
+        'Checks the published package layout. Runs on GitHub Actions, or set'
+        f' {_OPT_IN_ENV_VAR}=1.'
+    ),
+)
+
+_ALLOWLIST_PATH = pathlib.Path(__file__).with_name('allowed_imports.txt')
+
+_UPDATE_COMMAND = (
+    f'{_UPDATE_ENV_VAR}=1 pytest'
+    ' google/genai/tests/imports/test_imported_modules.py'
+)
+
+# Packages that importing the SDK must never pull in, whether or not they are
+# installed. Each is reachable only from a feature the caller has to ask for:
+# the MCP integration, and the local tokenizer. Naming them here, rather than
+# relying on their absence from the allowlist, keeps the reason visible and
+# gives a failure message that says what went wrong.
+_MUST_NOT_LOAD = (
+    'mcp',
+    'sentencepiece',
+    'torch',
+    'transformers',
+)
+
+# Generated subtrees are pinned by their root only. There are hundreds of them
+# and their individual names are not a contract.
+_COLLAPSED = ('google.genai._gaos',)
+
+# Names that cannot be pinned because they are not stable across machines or
+# builds. The standard library's build configuration carries the platform
+# triple, and mypyc-compiled and cython-compiled wheels ship helper modules
+# named after a build hash.
+_UNPINNABLE_PREFIXES = ('_sysconfigdata_', '_cython_')
+_UNPINNABLE_SUFFIXES = ('__mypyc',)
+
+_SNAPSHOT_SOURCE = """
+import json
+import sys
+
+before = frozenset(sys.modules)
+import google.genai  # noqa: F401
+json.dump(sorted(frozenset(sys.modules) - before), sys.stdout)
+"""
+
+
+@functools.lru_cache(maxsize=None)
+def _modules_loaded_by_import() -> frozenset[str]:
+  """Returns the modules ``import google.genai`` adds to a fresh interpreter.
+
+  A subprocess is the only honest way to measure this. The test session has
+  already imported the SDK by the time any test runs, and an import cannot be
+  undone: deleting entries from ``sys.modules`` leaves C extension state and
+  class identities behind.
+  """
+  # .../google/genai/tests/imports/<this file> -> the directory holding the
+  # ``google`` package. Prepending it puts the sources under test ahead of any
+  # installed copy. Computed here rather than at module scope: other layouts
+  # nest this file less deeply, and importing the module must not fail there.
+  import_root = pathlib.Path(__file__).resolve().parents[4]
+  env = dict(os.environ)
+  env['PYTHONPATH'] = os.pathsep.join(
+      part for part in (str(import_root), env.get('PYTHONPATH')) if part
+  )
+  # Run from an empty directory. The implicit ``sys.path[0]`` entry would
+  # otherwise let this package's own ``types.py`` shadow the standard library.
+  with tempfile.TemporaryDirectory() as empty_cwd:
+    result = subprocess.run(
+        [sys.executable, '-c', _SNAPSHOT_SOURCE],
+        cwd=empty_cwd,
+        env=env,
+        capture_output=True,
+        text=True,
+        check=False,
+    )
+  if result.returncode != 0:
+    pytest.fail(
+        'Could not import google.genai in a fresh interpreter:\n'
+        + result.stderr,
+        pytrace=False,
+    )
+  return frozenset(json.loads(result.stdout))
+
+
+def _tracked(module_names: Iterable[str]) -> frozenset[str]:
+  """Reduces raw module names to the units the allowlist pins.
+
+  Every ``google.genai`` submodule is pinned by name, because those are the
+  ones this repository controls. Everything else is pinned by its top-level
+  package only, because a library's internal module layout changes between
+  releases and is not this repository's business. Standard library modules are
+  ignored: which of them exist depends on the interpreter version, and
+  importing them is close to free.
+
+  Args:
+    module_names: Raw module names, as they appear in ``sys.modules``.
+
+  Returns:
+    The names the allowlist pins, which is a much smaller set.
+  """
+  tracked = set()
+  for name in module_names:
+    parts = name.split('.')
+    if parts[0] in sys.stdlib_module_names:
+      continue
+    if name == 'google':
+      continue  # The namespace package itself, not a dependency.
+    if name.startswith(_UNPINNABLE_PREFIXES) or name.endswith(
+        _UNPINNABLE_SUFFIXES
+    ):
+      continue
+    collapsed = next(
+        (
+            root
+            for root in _COLLAPSED
+            if name == root or name.startswith(root + '.')
+        ),
+        None,
+    )
+    if collapsed is not None:
+      tracked.add(collapsed)
+    elif name.startswith('google.genai'):
+      tracked.add(name)
+    elif parts[0] == 'google':
+      tracked.add('.'.join(parts[:2]))
+    else:
+      tracked.add(parts[0])
+  return frozenset(tracked)
+
+
+def _read_allowlist() -> frozenset[str]:
+  return frozenset(
+      line.strip()
+      for line in _ALLOWLIST_PATH.read_text().splitlines()
+      if line.strip() and not line.startswith('#')
+  )
+
+
+def _write_allowlist(loaded: frozenset[str]) -> None:
+  """Rewrites the allowlist from what this interpreter loaded."""
+  names = set(loaded)
+  if _ALLOWLIST_PATH.exists():
+    # A third-party dependency can be conditional on the interpreter version,
+    # so keep entries this interpreter did not happen to load. Regenerating on
+    # a recent interpreter would otherwise drop the backports that older ones
+    # still need. Only google.genai entries are dropped; those are the ratchet.
+    names |= {
+        name
+        for name in _read_allowlist() - loaded
+        if not name.startswith('google.genai')
+    }
+  header = f"""\
+# Modules that `import google.genai` is allowed to load.
+#
+# Every google.genai submodule is listed by name, and that part has to match
+# exactly. Everything else is listed by top-level package only and is a
+# ceiling rather than a requirement, because some dependencies are conditional
+# on the interpreter version. Standard library modules are not listed.
+#
+# Adding a line means importing the SDK got more expensive. Deleting lines is
+# the goal.
+#
+# Regenerate after an intentional change:
+#   {_UPDATE_COMMAND}
+"""
+  _ALLOWLIST_PATH.write_text(header + '\n'.join(sorted(names)) + '\n')
+
+
+def test_import_does_not_load_optional_features() -> None:
+  """Importing the SDK leaves the opt-in feature dependencies alone."""
+  loaded = _modules_loaded_by_import()
+  offenders = sorted(
+      package
+      for package in _MUST_NOT_LOAD
+      if any(
+          name == package or name.startswith(package + '.') for name in loaded
+      )
+  )
+  if not offenders:
+    return
+  pytest.fail(
+      '`import google.genai` now loads packages that belong to an opt-in'
+      ' feature:\n'
+      + '\n'.join(f'  {name}' for name in offenders)
+      + '\n\nSomething in your change imports one of these while the SDK is'
+      ' being imported, so everyone pays for a feature they may not use.'
+      ' Move the import into the function that needs it, or behind the'
+      ' module __getattr__.\n\nTo see what pulled it in:\n'
+      "  python -X importtime -c 'import google.genai' 2>&1 | grep "
+      f"'{offenders[0]}'",
+      pytrace=False,
+  )
+
+
+def test_imported_modules_match_allowlist() -> None:
+  """The import loads exactly the modules the allowlist permits."""
+  loaded = _tracked(_modules_loaded_by_import())
+
+  if os.environ.get(_UPDATE_ENV_VAR):
+    _write_allowlist(loaded)
+    pytest.skip(f'Rewrote {_ALLOWLIST_PATH.name} from this interpreter.')
+
+  allowed = _read_allowlist()
+  added = sorted(loaded - allowed)
+  # Only google.genai removals are enforced. A third-party package that stops
+  # being imported on one interpreter version is not a regression, but one of
+  # our own modules dropping out is a win worth recording so it cannot quietly
+  # come back.
+  removed = sorted(
+      name for name in allowed - loaded if name.startswith('google.genai')
+  )
+
+  if not added and not removed:
+    return
+
+  report = ['`import google.genai` no longer loads what the allowlist says.']
+  if added:
+    report += [
+        '',
+        f'{len(added)} module(s) are now loaded that were not before:',
+        *(f'  + {name}' for name in added[:40]),
+    ]
+    if len(added) > 40:
+      report.append(f'  ... and {len(added) - 40} more')
+    report += [
+        '',
+        (
+            'If you did not mean to make importing the SDK more expensive, find'
+            ' the new module-scope import and move it into the function that'
+            " needs it. `python -X importtime -c 'import google.genai'` shows"
+            ' what pulled it in.'
+        ),
+    ]
+  if removed:
+    report += [
+        '',
+        f'{len(removed)} module(s) are no longer loaded:',
+        *(f'  - {name}' for name in removed[:40]),
+    ]
+    if len(removed) > 40:
+      report.append(f'  ... and {len(removed) - 40} more')
+    report.append('')
+    report.append('That is an improvement. Record it so it cannot regress.')
+  report += [
+      '',
+      'Either way, update the allowlist and include it in your change:',
+      f'  {_UPDATE_COMMAND}',
+  ]
+  pytest.fail('\n'.join(report), pytrace=False)