diff --git a/httpx/__init__.py b/httpx/__init__.py
index fd17441..39b44aa 100644
--- a/httpx/__init__.py
+++ b/httpx/__init__.py
@@ -32,10 +32,10 @@ from .exceptions import (
 )
 from .interfaces import (
     AsyncDispatcher,
-    BaseReader,
-    BaseWriter,
     BaseBackgroundManager,
     BasePoolSemaphore,
+    BaseReader,
+    BaseWriter,
     ConcurrencyBackend,
     Dispatcher,
     Protocol,
diff --git a/httpx/client.py b/httpx/client.py
index e22d1a9..6b49797 100644
--- a/httpx/client.py
+++ b/httpx/client.py
@@ -20,11 +20,11 @@ from .dispatch.connection_pool import ConnectionPool
 from .dispatch.threaded import ThreadedDispatcher
 from .dispatch.wsgi import WSGIDispatch
 from .exceptions import (
+    HTTPError,
     InvalidURL,
     RedirectBodyUnavailable,
     RedirectLoop,
     TooManyRedirects,
-    HTTPError,
 )
 from .interfaces import AsyncDispatcher, ConcurrencyBackend, Dispatcher
 from .models import (
@@ -312,6 +312,7 @@ class BaseClient:
         headers = Headers(request.headers)
         if url.origin != request.url.origin:
             del headers["Authorization"]
+            del headers["host"]
         return headers
 
     def redirect_content(
diff --git a/httpx/exceptions.py b/httpx/exceptions.py
index 21dc7f4..ddccbf8 100644
--- a/httpx/exceptions.py
+++ b/httpx/exceptions.py
@@ -9,7 +9,9 @@ class HTTPError(Exception):
     Base class for Httpx exception
     """
 
-    def __init__(self, request: 'BaseRequest' = None, response: 'BaseResponse' = None, *args) -> None:
+    def __init__(
+        self, request: "BaseRequest" = None, response: "BaseResponse" = None, *args
+    ) -> None:
         self.response = response
         self.request = request or getattr(self.response, "request", None)
         super().__init__(*args)
diff --git a/httpx/models.py b/httpx/models.py
index e7e7a57..f397e78 100644
--- a/httpx/models.py
+++ b/httpx/models.py
@@ -33,7 +33,7 @@ from .utils import (
     is_known_encoding,
     normalize_header_key,
     normalize_header_value,
-    str_query_param
+    str_query_param,
 )
 
 PrimitiveData = typing.Union[str, int, float, bool, type(None)]

Erroneous Infinite Redirect Loop detected
Our redirect loop detection or redirects in general is broken somehow because this shouldn't error out:

```python
from httpx import Client

client = Client()
r = client.request("GET", "https://www.howsmyssl.com/a/check")  # Completes successfully

r = client.request("GET", "https://howsmyssl.com/a/check")  # Sends a redirect to 'www.howsmyssl.com' but then errors?
```
diff --git a/tests/client/test_headers.py b/tests/client/test_headers.py
index bf82a57..2d17c12 100755
--- a/tests/client/test_headers.py
+++ b/tests/client/test_headers.py
@@ -1,15 +1,16 @@
 #!/usr/bin/env python3
 
 import json
+
 from httpx import (
-    __version__,
-    Client,
+    AsyncDispatcher,
     AsyncRequest,
     AsyncResponse,
-    VerifyTypes,
     CertTypes,
+    Client,
     TimeoutTypes,
-    AsyncDispatcher,
+    VerifyTypes,
+    __version__,
 )
 
 
diff --git a/tests/client/test_redirects.py b/tests/client/test_redirects.py
index ff75475..f9c2075 100644
--- a/tests/client/test_redirects.py
+++ b/tests/client/test_redirects.py
@@ -86,6 +86,17 @@ class MockDispatch(AsyncDispatcher):
             body = json.dumps({"body": content.decode()}).encode()
             return AsyncResponse(codes.OK, content=body, request=request)
 
+        elif request.url.path == "/cross_subdomain":
+            if request.headers["host"] != "www.example.org":
+                headers = {"location": "https://www.example.org/cross_subdomain"}
+                return AsyncResponse(
+                    codes.PERMANENT_REDIRECT, headers=headers, request=request
+                )
+            else:
+                return AsyncResponse(
+                    codes.OK, content=b"Hello, world!", request=request
+                )
+
         return AsyncResponse(codes.OK, content=b"Hello, world!", request=request)
 
 
@@ -250,3 +261,11 @@ async def test_cannot_redirect_streaming_body():
 
     with pytest.raises(RedirectBodyUnavailable):
         await client.post(url, data=streaming_body())
+
+
+@pytest.mark.asyncio
+async def test_cross_dubdomain_redirect():
+    client = AsyncClient(dispatch=MockDispatch())
+    url = "https://example.com/cross_subdomain"
+    response = await client.get(url)
+    assert response.url == URL("https://www.example.org/cross_subdomain")

pip install -e .
parse_log_pytest
requirements.txt
pytest
3.9
requirements.txt
pytest --no-header -rA --tb=line --color=no -p no:cacheprovider -W ignore::DeprecationWarning
attrs==25.3.0
autoflake==2.3.1
babel==2.17.0
backrefs==5.8
black==25.1.0
brotlipy==0.7.0
certifi==2025.1.31
cffi==1.17.1
chardet==3.0.4
charset-normalizer==3.4.1
click==8.1.8
colorama==0.4.6
coverage==7.8.0
cryptography==44.0.2
exceptiongroup==1.2.2
flake8==7.2.0
flake8-bugbear==24.12.12
flake8-comprehensions==3.16.0
ghp-import==2.1.0
h11==0.8.1
h2==3.2.0
hpack==3.0.0
hstspreload==2025.1.1
-e git+https://github.com/encode/httpx.git@ebbc003c55c5a0280208c4d7502bd6038520f29b#egg=httpx
hyperframe==5.2.0
idna==2.10
importlib_metadata==8.6.1
iniconfig==2.1.0
isort==6.0.1
Jinja2==3.1.6
Markdown==3.7
MarkupSafe==3.0.2
mccabe==0.7.0
mergedeep==1.3.4
mkdocs==1.6.1
mkdocs-get-deps==0.2.0
mkdocs-material==9.6.10
mkdocs-material-extensions==1.3.1
mypy==1.15.0
mypy-extensions==1.0.0
packaging==24.2
paginate==0.5.7
pathspec==0.12.1
platformdirs==4.3.7
pluggy==1.5.0
pycodestyle==2.13.0
pycparser==2.22
pyflakes==3.3.2
Pygments==2.19.1
pymdown-extensions==10.14.3
pytest==8.3.5
pytest-asyncio==0.26.0
pytest-cov==6.0.0
python-dateutil==2.9.0.post0
PyYAML==6.0.2
pyyaml_env_tag==0.1
requests==2.32.3
rfc3986==1.5.0
six==1.17.0
tomli==2.2.1
trustme==1.2.1
typing_extensions==4.13.0
urllib3==2.3.0
uvicorn==0.34.0
watchdog==6.0.0
zipp==3.21.0

name: httpx
channels:
  - defaults
  - https://repo.anaconda.com/pkgs/main
  - https://repo.anaconda.com/pkgs/r
  - conda-forge
dependencies:
  - _libgcc_mutex=0.1=main
  - _openmp_mutex=5.1=1_gnu
  - ca-certificates=2025.2.25=h06a4308_0
  - ld_impl_linux-64=2.40=h12ee557_0
  - libffi=3.4.4=h6a678d5_1
  - libgcc-ng=11.2.0=h1234567_1
  - libgomp=11.2.0=h1234567_1
  - libstdcxx-ng=11.2.0=h1234567_1
  - ncurses=6.4=h6a678d5_0
  - openssl=3.0.16=h5eee18b_0
  - pip=25.0=py39h06a4308_0
  - python=3.9.21=he870216_1
  - readline=8.2=h5eee18b_0
  - setuptools=75.8.0=py39h06a4308_0
  - sqlite=3.45.3=h5eee18b_0
  - tk=8.6.14=h39e8969_0
  - tzdata=2025a=h04d1e81_0
  - wheel=0.45.1=py39h06a4308_0
  - xz=5.6.4=h5eee18b_1
  - zlib=1.2.13=h5eee18b_1
  - pip:
      - attrs==25.3.0
      - autoflake==2.3.1
      - babel==2.17.0
      - backrefs==5.8
      - black==25.1.0
      - brotlipy==0.7.0
      - certifi==2025.1.31
      - cffi==1.17.1
      - chardet==3.0.4
      - charset-normalizer==3.4.1
      - click==8.1.8
      - colorama==0.4.6
      - coverage==7.8.0
      - cryptography==44.0.2
      - exceptiongroup==1.2.2
      - flake8==7.2.0
      - flake8-bugbear==24.12.12
      - flake8-comprehensions==3.16.0
      - ghp-import==2.1.0
      - h11==0.8.1
      - h2==3.2.0
      - hpack==3.0.0
      - hstspreload==2025.1.1
      - hyperframe==5.2.0
      - idna==2.10
      - importlib-metadata==8.6.1
      - iniconfig==2.1.0
      - isort==6.0.1
      - jinja2==3.1.6
      - markdown==3.7
      - markupsafe==3.0.2
      - mccabe==0.7.0
      - mergedeep==1.3.4
      - mkdocs==1.6.1
      - mkdocs-get-deps==0.2.0
      - mkdocs-material==9.6.10
      - mkdocs-material-extensions==1.3.1
      - mypy==1.15.0
      - mypy-extensions==1.0.0
      - packaging==24.2
      - paginate==0.5.7
      - pathspec==0.12.1
      - platformdirs==4.3.7
      - pluggy==1.5.0
      - pycodestyle==2.13.0
      - pycparser==2.22
      - pyflakes==3.3.2
      - pygments==2.19.1
      - pymdown-extensions==10.14.3
      - pytest==8.3.5
      - pytest-asyncio==0.26.0
      - pytest-cov==6.0.0
      - python-dateutil==2.9.0.post0
      - pyyaml==6.0.2
      - pyyaml-env-tag==0.1
      - requests==2.32.3
      - rfc3986==1.5.0
      - six==1.17.0
      - tomli==2.2.1
      - trustme==1.2.1
      - typing-extensions==4.13.0
      - urllib3==2.3.0
      - uvicorn==0.34.0
      - watchdog==6.0.0
      - zipp==3.21.0
prefix: /opt/conda/envs/httpx

tests/client/test_redirects.py::test_cross_dubdomain_redirect
tests/client/test_headers.py::test_client_header tests/client/test_headers.py::test_header_merge tests/client/test_headers.py::test_header_merge_conflicting_headers tests/client/test_headers.py::test_header_update tests/client/test_redirects.py::test_redirect_301 tests/client/test_redirects.py::test_redirect_302 tests/client/test_redirects.py::test_redirect_303 tests/client/test_redirects.py::test_disallow_redirects tests/client/test_redirects.py::test_relative_redirect tests/client/test_redirects.py::test_no_scheme_redirect tests/client/test_redirects.py::test_fragment_redirect tests/client/test_redirects.py::test_multiple_redirects tests/client/test_redirects.py::test_too_many_redirects tests/client/test_redirects.py::test_too_many_redirects_calling_next tests/client/test_redirects.py::test_redirect_loop tests/client/test_redirects.py::test_redirect_loop_calling_next tests/client/test_redirects.py::test_cross_domain_redirect tests/client/test_redirects.py::test_same_domain_redirect tests/client/test_redirects.py::test_body_redirect tests/client/test_redirects.py::test_cannot_redirect_streaming_body
httpx/__init__.py httpx/exceptions.py httpx/models.py httpx/client.py
httpx/__init__.py httpx/exceptions.py httpx/models.py httpx/client.py