diff --git a/.github/.OwlBot.lock.yaml b/.github/.OwlBot.lock.yaml
index a3da1b0d..ec696b55 100644
--- a/.github/.OwlBot.lock.yaml
+++ b/.github/.OwlBot.lock.yaml
@@ -13,5 +13,5 @@
 # limitations under the License.
 docker:
   image: gcr.io/cloud-devrel-public-resources/owlbot-python:latest
-  digest: sha256:3e3800bb100af5d7f9e810d48212b37812c1856d20ffeafb99ebe66461b61fc7
-# created: 2023-08-02T10:53:29.114535628Z
+  digest: sha256:30470597773378105e239b59fce8eb27cc97375580d592699206d17d117143d0
+# created: 2023-11-03T00:57:07.335914631Z
diff --git a/.github/workflows/docs.yml b/.github/workflows/docs.yml
index e97d89e4..221806ce 100644
--- a/.github/workflows/docs.yml
+++ b/.github/workflows/docs.yml
@@ -28,7 +28,7 @@ jobs:
     - name: Setup Python
       uses: actions/setup-python@v4
       with:
-        python-version: "3.9"
+        python-version: "3.10"
     - name: Install nox
       run: |
         python -m pip install --upgrade setuptools pip wheel
diff --git a/.gitignore b/.gitignore
index 99c3a144..168b201f 100644
--- a/.gitignore
+++ b/.gitignore
@@ -51,6 +51,7 @@ docs.metadata
 
 # Virtual environment
 env/
+venv/
 
 # Test logs
 coverage.xml
diff --git a/.kokoro/requirements.txt b/.kokoro/requirements.txt
index 029bd342..16170d0c 100644
--- a/.kokoro/requirements.txt
+++ b/.kokoro/requirements.txt
@@ -113,30 +113,30 @@ commonmark==0.9.1 \
     --hash=sha256:452f9dc859be7f06631ddcb328b6919c67984aca654e5fefb3914d54691aed60 \
     --hash=sha256:da2f38c92590f83de410ba1a3cbceafbc74fee9def35f9251ba9a971d6d66fd9
     # via rich
-cryptography==41.0.3 \
-    --hash=sha256:0d09fb5356f975974dbcb595ad2d178305e5050656affb7890a1583f5e02a306 \
-    --hash=sha256:23c2d778cf829f7d0ae180600b17e9fceea3c2ef8b31a99e3c694cbbf3a24b84 \
-    --hash=sha256:3fb248989b6363906827284cd20cca63bb1a757e0a2864d4c1682a985e3dca47 \
-    --hash=sha256:41d7aa7cdfded09b3d73a47f429c298e80796c8e825ddfadc84c8a7f12df212d \
-    --hash=sha256:42cb413e01a5d36da9929baa9d70ca90d90b969269e5a12d39c1e0d475010116 \
-    --hash=sha256:4c2f0d35703d61002a2bbdcf15548ebb701cfdd83cdc12471d2bae80878a4207 \
-    --hash=sha256:4fd871184321100fb400d759ad0cddddf284c4b696568204d281c902fc7b0d81 \
-    --hash=sha256:5259cb659aa43005eb55a0e4ff2c825ca111a0da1814202c64d28a985d33b087 \
-    --hash=sha256:57a51b89f954f216a81c9d057bf1a24e2f36e764a1ca9a501a6964eb4a6800dd \
-    --hash=sha256:652627a055cb52a84f8c448185922241dd5217443ca194d5739b44612c5e6507 \
-    --hash=sha256:67e120e9a577c64fe1f611e53b30b3e69744e5910ff3b6e97e935aeb96005858 \
-    --hash=sha256:6af1c6387c531cd364b72c28daa29232162010d952ceb7e5ca8e2827526aceae \
-    --hash=sha256:6d192741113ef5e30d89dcb5b956ef4e1578f304708701b8b73d38e3e1461f34 \
-    --hash=sha256:7efe8041897fe7a50863e51b77789b657a133c75c3b094e51b5e4b5cec7bf906 \
-    --hash=sha256:84537453d57f55a50a5b6835622ee405816999a7113267739a1b4581f83535bd \
-    --hash=sha256:8f09daa483aedea50d249ef98ed500569841d6498aa9c9f4b0531b9964658922 \
-    --hash=sha256:95dd7f261bb76948b52a5330ba5202b91a26fbac13ad0e9fc8a3ac04752058c7 \
-    --hash=sha256:a74fbcdb2a0d46fe00504f571a2a540532f4c188e6ccf26f1f178480117b33c4 \
-    --hash=sha256:a983e441a00a9d57a4d7c91b3116a37ae602907a7618b882c8013b5762e80574 \
-    --hash=sha256:ab8de0d091acbf778f74286f4989cf3d1528336af1b59f3e5d2ebca8b5fe49e1 \
-    --hash=sha256:aeb57c421b34af8f9fe830e1955bf493a86a7996cc1338fe41b30047d16e962c \
-    --hash=sha256:ce785cf81a7bdade534297ef9e490ddff800d956625020ab2ec2780a556c313e \
-    --hash=sha256:d0d651aa754ef58d75cec6edfbd21259d93810b73f6ec246436a21b7841908de
+cryptography==41.0.4 \
+    --hash=sha256:004b6ccc95943f6a9ad3142cfabcc769d7ee38a3f60fb0dddbfb431f818c3a67 \
+    --hash=sha256:047c4603aeb4bbd8db2756e38f5b8bd7e94318c047cfe4efeb5d715e08b49311 \
+    --hash=sha256:0d9409894f495d465fe6fda92cb70e8323e9648af912d5b9141d616df40a87b8 \
+    --hash=sha256:23a25c09dfd0d9f28da2352503b23e086f8e78096b9fd585d1d14eca01613e13 \
+    --hash=sha256:2ed09183922d66c4ec5fdaa59b4d14e105c084dd0febd27452de8f6f74704143 \
+    --hash=sha256:35c00f637cd0b9d5b6c6bd11b6c3359194a8eba9c46d4e875a3660e3b400005f \
+    --hash=sha256:37480760ae08065437e6573d14be973112c9e6dcaf5f11d00147ee74f37a3829 \
+    --hash=sha256:3b224890962a2d7b57cf5eeb16ccaafba6083f7b811829f00476309bce2fe0fd \
+    --hash=sha256:5a0f09cefded00e648a127048119f77bc2b2ec61e736660b5789e638f43cc397 \
+    --hash=sha256:5b72205a360f3b6176485a333256b9bcd48700fc755fef51c8e7e67c4b63e3ac \
+    --hash=sha256:7e53db173370dea832190870e975a1e09c86a879b613948f09eb49324218c14d \
+    --hash=sha256:7febc3094125fc126a7f6fb1f420d0da639f3f32cb15c8ff0dc3997c4549f51a \
+    --hash=sha256:80907d3faa55dc5434a16579952ac6da800935cd98d14dbd62f6f042c7f5e839 \
+    --hash=sha256:86defa8d248c3fa029da68ce61fe735432b047e32179883bdb1e79ed9bb8195e \
+    --hash=sha256:8ac4f9ead4bbd0bc8ab2d318f97d85147167a488be0e08814a37eb2f439d5cf6 \
+    --hash=sha256:93530900d14c37a46ce3d6c9e6fd35dbe5f5601bf6b3a5c325c7bffc030344d9 \
+    --hash=sha256:9eeb77214afae972a00dee47382d2591abe77bdae166bda672fb1e24702a3860 \
+    --hash=sha256:b5f4dfe950ff0479f1f00eda09c18798d4f49b98f4e2006d644b3301682ebdca \
+    --hash=sha256:c3391bd8e6de35f6f1140e50aaeb3e2b3d6a9012536ca23ab0d9c35ec18c8a91 \
+    --hash=sha256:c880eba5175f4307129784eca96f4e70b88e57aa3f680aeba3bab0e980b0f37d \
+    --hash=sha256:cecfefa17042941f94ab54f769c8ce0fe14beff2694e9ac684176a2535bf9714 \
+    --hash=sha256:e40211b4923ba5a6dc9769eab704bdb3fbb58d56c5b336d30996c24fcf12aadb \
+    --hash=sha256:efc8ad4e6fc4f1752ebfb58aefece8b4e3c4cae940b0994d43649bdfce8d0d4f
     # via
     #   gcp-releasetool
     #   secretstorage
@@ -382,6 +382,7 @@ protobuf==3.20.3 \
     #   gcp-docuploader
     #   gcp-releasetool
     #   google-api-core
+    #   googleapis-common-protos
 pyasn1==0.4.8 \
     --hash=sha256:39c7e2ec30515947ff4e87fb6f456dfc6e84857d34be479c9d4a4ba4bf46aa5d \
     --hash=sha256:aef77c9fb94a3ac588e87841208bdec464471d9871bd5050a287cc9a475cd0ba
@@ -466,9 +467,9 @@ typing-extensions==4.4.0 \
     --hash=sha256:1511434bb92bf8dd198c12b1cc812e800d4181cfcb867674e0f8279cc93087aa \
     --hash=sha256:16fa4864408f655d35ec496218b85f79b3437c829e93320c7c9215ccfd92489e
     # via -r requirements.in
-urllib3==1.26.12 \
-    --hash=sha256:3fa96cf423e6987997fc326ae8df396db2a8b7c667747d47ddd8ecba91f4a74e \
-    --hash=sha256:b930dd878d5a8afb066a637fbb35144fe7901e3b209d1cd4f524bd0e9deee997
+urllib3==1.26.18 \
+    --hash=sha256:34b97092d7e0a3a8cf7cd10e386f401b3737364026c45e622aa02903dffe0f07 \
+    --hash=sha256:f8ecc1bba5667413457c529ab955bf8c67b45db799d159066261719e328580a0
     # via
     #   requests
     #   twine
diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index 19409cbd..6a8e1695 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -22,7 +22,7 @@ repos:
     -   id: end-of-file-fixer
     -   id: check-yaml
 -   repo: https://github.com/psf/black
-    rev: 22.3.0
+    rev: 23.7.0
     hooks:
     - id: black
 -   repo: https://github.com/pycqa/flake8
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 93c3848c..7b0dacfe 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,20 @@
 
 [1]: https://pypi.org/project/google-api-core/#history
 
+## [2.13.0](https://github.com/googleapis/python-api-core/compare/v2.12.0...v2.13.0) (2023-11-03)
+
+
+### Features
+
+* Add caching to routing header calculation ([#526](https://github.com/googleapis/python-api-core/issues/526)) ([6251eab](https://github.com/googleapis/python-api-core/commit/6251eab3fca5f7e509cb9b6e476ce1184094b711))
+
+
+### Bug Fixes
+
+* Add warning to retry target to avoid incorrect usage ([#543](https://github.com/googleapis/python-api-core/issues/543)) ([bfb40e6](https://github.com/googleapis/python-api-core/commit/bfb40e6929ef47be7a6464d2f1e0d06595736b8d))
+* Drop usage of distutils ([#541](https://github.com/googleapis/python-api-core/issues/541)) ([4bd9e10](https://github.com/googleapis/python-api-core/commit/4bd9e10f20eea227c88e3e1496010cca6dd8a270))
+* Ensure exception is available when BackgroundConsumer open stream fails ([#357](https://github.com/googleapis/python-api-core/issues/357)) ([405272c](https://github.com/googleapis/python-api-core/commit/405272c05f8c6d20e242c6172b01f78f0fd3bf32))
+
 ## [2.12.0](https://github.com/googleapis/python-api-core/compare/v2.11.1...v2.12.0) (2023-09-07)
 
 
diff --git a/google/api_core/bidi.py b/google/api_core/bidi.py
index 57f5f9dd..74abc495 100644
--- a/google/api_core/bidi.py
+++ b/google/api_core/bidi.py
@@ -92,10 +92,7 @@ def _is_active(self):
         # Note: there is a possibility that this starts *before* the call
         # property is set. So we have to check if self.call is set before
         # seeing if it's active.
-        if self.call is not None and not self.call.is_active():
-            return False
-        else:
-            return True
+        return self.call is not None and self.call.is_active()
 
     def __iter__(self):
         if self._initial_request is not None:
@@ -265,6 +262,10 @@ def add_done_callback(self, callback):
         self._callbacks.append(callback)
 
     def _on_call_done(self, future):
+        # This occurs when the RPC errors or is successfully terminated.
+        # Note that grpc's "future" here can also be a grpc.RpcError.
+        # See note in https://github.com/grpc/grpc/issues/10885#issuecomment-302651331
+        # that `grpc.RpcError` is also `grpc.call`.
         for callback in self._callbacks:
             callback(future)
 
@@ -276,7 +277,13 @@ def open(self):
         request_generator = _RequestQueueGenerator(
             self._request_queue, initial_request=self._initial_request
         )
-        call = self._start_rpc(iter(request_generator), metadata=self._rpc_metadata)
+        try:
+            call = self._start_rpc(iter(request_generator), metadata=self._rpc_metadata)
+        except exceptions.GoogleAPICallError as exc:
+            # The original `grpc.RpcError` (which is usually also a `grpc.Call`) is
+            # available from the ``response`` property on the mapped exception.
+            self._on_call_done(exc.response)
+            raise
 
         request_generator.call = call
 
diff --git a/google/api_core/gapic_v1/routing_header.py b/google/api_core/gapic_v1/routing_header.py
index 28b13abf..c0c6f648 100644
--- a/google/api_core/gapic_v1/routing_header.py
+++ b/google/api_core/gapic_v1/routing_header.py
@@ -20,17 +20,22 @@
 Generally, these headers are specified as gRPC metadata.
 """
 
+import functools
 from enum import Enum
 from urllib.parse import urlencode
 
 ROUTING_METADATA_KEY = "x-goog-request-params"
+# This is the value for the `maxsize` argument of @functools.lru_cache
+# https://docs.python.org/3/library/functools.html#functools.lru_cache
+# This represents the number of recent function calls to store.
+ROUTING_PARAM_CACHE_SIZE = 32
 
 
 def to_routing_header(params, qualified_enums=True):
     """Returns a routing header string for the given request parameters.
 
     Args:
-        params (Mapping[str, Any]): A dictionary containing the request
+        params (Mapping[str, str | bytes | Enum]): A dictionary containing the request
             parameters used for routing.
         qualified_enums (bool): Whether to represent enum values
             as their type-qualified symbol names instead of as their
@@ -38,19 +43,11 @@ def to_routing_header(params, qualified_enums=True):
 
     Returns:
         str: The routing header string.
-
     """
+    tuples = params.items() if isinstance(params, dict) else params
     if not qualified_enums:
-        if isinstance(params, dict):
-            tuples = params.items()
-        else:
-            tuples = params
-        params = [(x[0], x[1].name) if isinstance(x[1], Enum) else x for x in tuples]
-    return urlencode(
-        params,
-        # Per Google API policy (go/api-url-encoding), / is not encoded.
-        safe="/",
-    )
+        tuples = [(x[0], x[1].name) if isinstance(x[1], Enum) else x for x in tuples]
+    return "&".join([_urlencode_param(*t) for t in tuples])
 
 
 def to_grpc_metadata(params, qualified_enums=True):
@@ -58,7 +55,7 @@ def to_grpc_metadata(params, qualified_enums=True):
     request parameters.
 
     Args:
-        params (Mapping[str, Any]): A dictionary containing the request
+        params (Mapping[str, str | bytes | Enum]): A dictionary containing the request
             parameters used for routing.
         qualified_enums (bool): Whether to represent enum values
             as their type-qualified symbol names instead of as their
@@ -69,3 +66,22 @@ def to_grpc_metadata(params, qualified_enums=True):
             and value.
     """
     return (ROUTING_METADATA_KEY, to_routing_header(params, qualified_enums))
+
+
+# use caching to avoid repeated computation
+@functools.lru_cache(maxsize=ROUTING_PARAM_CACHE_SIZE)
+def _urlencode_param(key, value):
+    """Cacheable wrapper over urlencode
+
+    Args:
+        key (str): The key of the parameter to encode.
+        value (str | bytes | Enum): The value of the parameter to encode.
+
+    Returns:
+        str: The encoded parameter.
+    """
+    return urlencode(
+        {key: value},
+        # Per Google API policy (go/api-url-encoding), / is not encoded.
+        safe="/",
+    )
diff --git a/google/api_core/operations_v1/abstract_operations_client.py b/google/api_core/operations_v1/abstract_operations_client.py
index 714c2aae..38f532af 100644
--- a/google/api_core/operations_v1/abstract_operations_client.py
+++ b/google/api_core/operations_v1/abstract_operations_client.py
@@ -14,7 +14,6 @@
 # limitations under the License.
 #
 from collections import OrderedDict
-from distutils import util
 import os
 import re
 from typing import Dict, Optional, Sequence, Tuple, Type, Union
@@ -294,13 +293,16 @@ def __init__(
             client_options = client_options_lib.ClientOptions()
 
         # Create SSL credentials for mutual TLS if needed.
-        use_client_cert = bool(
-            util.strtobool(os.getenv("GOOGLE_API_USE_CLIENT_CERTIFICATE", "false"))
-        )
-
+        use_client_cert = os.getenv(
+            "GOOGLE_API_USE_CLIENT_CERTIFICATE", "false"
+        ).lower()
+        if use_client_cert not in ("true", "false"):
+            raise ValueError(
+                "Environment variable `GOOGLE_API_USE_CLIENT_CERTIFICATE` must be either `true` or `false`"
+            )
         client_cert_source_func = None
         is_mtls = False
-        if use_client_cert:
+        if use_client_cert == "true":
             if client_options.client_cert_source:
                 is_mtls = True
                 client_cert_source_func = client_options.client_cert_source
diff --git a/google/api_core/retry.py b/google/api_core/retry.py
index 84b5d0fe..08f8209e 100644
--- a/google/api_core/retry.py
+++ b/google/api_core/retry.py
@@ -62,6 +62,8 @@ def check_if_exists():
 import random
 import sys
 import time
+import inspect
+import warnings
 from typing import Any, Callable, TypeVar, TYPE_CHECKING
 
 import requests.exceptions
@@ -84,6 +86,7 @@ def check_if_exists():
 _DEFAULT_MAXIMUM_DELAY = 60.0  # seconds
 _DEFAULT_DELAY_MULTIPLIER = 2.0
 _DEFAULT_DEADLINE = 60.0 * 2.0  # seconds
+_ASYNC_RETRY_WARNING = "Using the synchronous google.api_core.retry.Retry with asynchronous calls may lead to unexpected results. Please use google.api_core.retry_async.AsyncRetry instead."
 
 
 def if_exception_type(
@@ -201,7 +204,10 @@ def retry_target(
 
     for sleep in sleep_generator:
         try:
-            return target()
+            result = target()
+            if inspect.isawaitable(result):
+                warnings.warn(_ASYNC_RETRY_WARNING)
+            return result
 
         # pylint: disable=broad-except
         # This function explicitly must deal with broad exceptions.
diff --git a/google/api_core/version.py b/google/api_core/version.py
index 67e043bd..b6000e20 100644
--- a/google/api_core/version.py
+++ b/google/api_core/version.py
@@ -12,4 +12,4 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-__version__ = "2.12.0"
+__version__ = "2.13.0"
diff --git a/noxfile.py b/noxfile.py
index 84abb498..82ea2669 100644
--- a/noxfile.py
+++ b/noxfile.py
@@ -232,7 +232,7 @@ def docs(session):
     )
 
 
-@nox.session(python="3.9")
+@nox.session(python="3.10")
 def docfx(session):
     """Build the docfx yaml files for this library."""
 
diff --git a/tests/unit/gapic/test_routing_header.py b/tests/unit/gapic/test_routing_header.py
index 9d31eb39..2c8c7546 100644
--- a/tests/unit/gapic/test_routing_header.py
+++ b/tests/unit/gapic/test_routing_header.py
@@ -70,3 +70,34 @@ def test_to_grpc_metadata():
     params = [("name", "meep"), ("book.read", "1")]
     metadata = routing_header.to_grpc_metadata(params)
     assert metadata == (routing_header.ROUTING_METADATA_KEY, "name=meep&book.read=1")
+
+
+@pytest.mark.parametrize(
+    "key,value,expected",
+    [
+        ("book.read", "1", "book.read=1"),
+        ("name", "me/ep", "name=me/ep"),
+        ("\\", "=", "%5C=%3D"),
+        (b"hello", "world", "hello=world"),
+        ("✔️", "✌️", "%E2%9C%94%EF%B8%8F=%E2%9C%8C%EF%B8%8F"),
+    ],
+)
+def test__urlencode_param(key, value, expected):
+    result = routing_header._urlencode_param(key, value)
+    assert result == expected
+
+
+def test__urlencode_param_caching_performance():
+    import time
+
+    key = "key" * 100
+    value = "value" * 100
+    # time with empty cache
+    start_time = time.perf_counter()
+    routing_header._urlencode_param(key, value)
+    duration = time.perf_counter() - start_time
+    second_start_time = time.perf_counter()
+    routing_header._urlencode_param(key, value)
+    second_duration = time.perf_counter() - second_start_time
+    # second call should be approximately 10 times faster
+    assert second_duration < duration / 10
diff --git a/tests/unit/test_bidi.py b/tests/unit/test_bidi.py
index f5e2b72b..84ac9dc5 100644
--- a/tests/unit/test_bidi.py
+++ b/tests/unit/test_bidi.py
@@ -804,6 +804,21 @@ def test_wake_on_error(self):
         while consumer.is_active:
             pass
 
+    def test_rpc_callback_fires_when_consumer_start_fails(self):
+        expected_exception = exceptions.InvalidArgument(
+            "test", response=grpc.StatusCode.INVALID_ARGUMENT
+        )
+        callback = mock.Mock(spec=["__call__"])
+
+        rpc, _ = make_rpc()
+        bidi_rpc = bidi.BidiRpc(rpc)
+        bidi_rpc.add_done_callback(callback)
+        bidi_rpc._start_rpc.side_effect = expected_exception
+
+        consumer = bidi.BackgroundConsumer(bidi_rpc, on_response=None)
+        consumer.start()
+        assert callback.call_args.args[0] == grpc.StatusCode.INVALID_ARGUMENT
+
     def test_consumer_expected_error(self, caplog):
         caplog.set_level(logging.DEBUG)
 
diff --git a/tests/unit/test_retry.py b/tests/unit/test_retry.py
index ec27056d..d770f7a2 100644
--- a/tests/unit/test_retry.py
+++ b/tests/unit/test_retry.py
@@ -129,6 +129,26 @@ def test_retry_target_non_retryable_error(utcnow, sleep):
     sleep.assert_not_called()
 
 
+@mock.patch("asyncio.sleep", autospec=True)
+@mock.patch(
+    "google.api_core.datetime_helpers.utcnow",
+    return_value=datetime.datetime.min,
+    autospec=True,
+)
+@pytest.mark.asyncio
+async def test_retry_target_warning_for_retry(utcnow, sleep):
+    predicate = retry.if_exception_type(ValueError)
+    target = mock.AsyncMock(spec=["__call__"])
+
+    with pytest.warns(Warning) as exc_info:
+        # Note: predicate is just a filler and doesn't affect the test
+        retry.retry_target(target, predicate, range(10), None)
+
+    assert len(exc_info) == 2
+    assert str(exc_info[0].message) == retry._ASYNC_RETRY_WARNING
+    sleep.assert_not_called()
+
+
 @mock.patch("time.sleep", autospec=True)
 @mock.patch("google.api_core.datetime_helpers.utcnow", autospec=True)
 def test_retry_target_deadline_exceeded(utcnow, sleep):