diff --git a/gabbi/driver.py b/gabbi/driver.py
index 9cb88fe..22a48c4 100644
--- a/gabbi/driver.py
+++ b/gabbi/driver.py
@@ -29,8 +29,10 @@ import os
 import unittest
 from unittest import suite
 import uuid
+import warnings
 
 from gabbi import case
+from gabbi import exception
 from gabbi import handlers
 from gabbi import reporter
 from gabbi import suitemaker
@@ -83,6 +85,10 @@ def build_tests(path, loader, host=None, port=8001, intercept=None,
 
     top_suite = suite.TestSuite()
     for test_file in glob.iglob('%s/*.yaml' % path):
+        if '_' in os.path.basename(test_file):
+            warnings.warn(exception.GabbiSyntaxWarning(
+                "'_' in test filename %s. This can break suite grouping."
+                % test_file))
         if intercept:
             host = str(uuid.uuid4())
         suite_dict = utils.load_yaml(yaml_file=test_file)
@@ -134,7 +140,6 @@ def py_test_generator(test_dir, host=None, port=8001, intercept=None,
 def test_suite_from_yaml(loader, test_base_name, test_yaml, test_directory,
                          host, port, fixture_module, intercept, prefix=''):
     """Legacy wrapper retained for backwards compatibility."""
-    import warnings
 
     with warnings.catch_warnings():  # ensures warnings filter is restored
         warnings.simplefilter('default', DeprecationWarning)
diff --git a/gabbi/exception.py b/gabbi/exception.py
index 3d4ef45..2bc93e4 100644
--- a/gabbi/exception.py
+++ b/gabbi/exception.py
@@ -16,3 +16,8 @@
 class GabbiFormatError(ValueError):
     """An exception to encapsulate poorly formed test data."""
     pass
+
+
+class GabbiSyntaxWarning(SyntaxWarning):
+    """A warning about syntax that is not desirable."""
+    pass

Q: What characters are legal / recommended for test name in YAML?
I suspect plain alphanum + spaces is recommended, but I might sometimes want to do hyphen or parens.  So, just being thorough.

Thanks!

diff --git a/gabbi/tests/gabbits_intercept/json_extensions.yaml b/gabbi/tests/gabbits_intercept/json-extensions.yaml
similarity index 100%
rename from gabbi/tests/gabbits_intercept/json_extensions.yaml
rename to gabbi/tests/gabbits_intercept/json-extensions.yaml
diff --git a/gabbi/tests/gabbits_intercept/last_url.yaml b/gabbi/tests/gabbits_intercept/last-url.yaml
similarity index 100%
rename from gabbi/tests/gabbits_intercept/last_url.yaml
rename to gabbi/tests/gabbits_intercept/last-url.yaml
diff --git a/gabbi/tests/gabbits_intercept/method_shortcut.yaml b/gabbi/tests/gabbits_intercept/method-shortcut.yaml
similarity index 100%
rename from gabbi/tests/gabbits_intercept/method_shortcut.yaml
rename to gabbi/tests/gabbits_intercept/method-shortcut.yaml
diff --git a/gabbi/tests/test_syntax_warning.py b/gabbi/tests/test_syntax_warning.py
new file mode 100644
index 0000000..529dbf6
--- /dev/null
+++ b/gabbi/tests/test_syntax_warning.py
@@ -0,0 +1,41 @@
+#
+# 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.
+"""Test that the driver warns on bad yaml name."""
+
+import os
+import unittest
+import warnings
+
+from gabbi import driver
+from gabbi import exception
+
+
+TESTS_DIR = 'warning_gabbits'
+
+
+class DriverTest(unittest.TestCase):
+
+    def setUp(self):
+        super(DriverTest, self).setUp()
+        self.loader = unittest.defaultTestLoader
+        self.test_dir = os.path.join(os.path.dirname(__file__), TESTS_DIR)
+
+    def test_driver_warngs_on_files(self):
+        with warnings.catch_warnings(record=True) as the_warnings:
+            driver.build_tests(
+                self.test_dir, self.loader, host='localhost', port=8001)
+            self.assertEqual(1, len(the_warnings))
+            the_warning = the_warnings[-1]
+            self.assertEqual(
+                the_warning.category, exception.GabbiSyntaxWarning)
+            self.assertIn("'_' in test filename", str(the_warning.message))
diff --git a/gabbi/tests/warning_gabbits/underscore_sample.yaml b/gabbi/tests/warning_gabbits/underscore_sample.yaml
new file mode 100644
index 0000000..185e378
--- /dev/null
+++ b/gabbi/tests/warning_gabbits/underscore_sample.yaml
@@ -0,0 +1,6 @@
+
+tests:
+    - name: one
+      url: /
+    - name: two
+      url: http://example.com/moo

pip install -e .
parse_log_pytest
requirements.txt
mock testrepository coverage hacking sphinx pytest
3.5
requirements.txt
pytest --no-header -rA --tb=line --color=no -p no:cacheprovider -W ignore::DeprecationWarning
alabaster==0.7.13
attrs==22.2.0
Babel==2.11.0
certifi==2021.5.30
charset-normalizer==2.0.12
colorama==0.4.5
coverage==6.2
decorator==5.1.1
docutils==0.18.1
extras==1.0.0
fixtures==4.0.1
flake8==3.8.4
-e git+https://github.com/cdent/gabbi.git@1b9a0be830dac86865bee85c33886d3b2fb4d37b#egg=gabbi
hacking==4.1.0
idna==3.10
imagesize==1.4.1
importlib-metadata==4.8.3
iniconfig==1.1.1
iso8601==1.1.0
Jinja2==3.0.3
jsonpath-rw==1.4.0
jsonpath-rw-ext==1.2.2
MarkupSafe==2.0.1
mccabe==0.6.1
mock==5.2.0
packaging==21.3
pbr==6.1.1
pluggy==1.0.0
ply==3.11
py==1.11.0
pycodestyle==2.6.0
pyflakes==2.2.0
Pygments==2.14.0
pyparsing==3.1.4
pytest==7.0.1
python-subunit==1.4.2
pytz==2025.2
PyYAML==6.0.1
requests==2.27.1
six==1.17.0
snowballstemmer==2.2.0
Sphinx==5.3.0
sphinxcontrib-applehelp==1.0.2
sphinxcontrib-devhelp==1.0.2
sphinxcontrib-htmlhelp==2.0.0
sphinxcontrib-jsmath==1.0.1
sphinxcontrib-qthelp==1.0.3
sphinxcontrib-serializinghtml==1.1.5
testrepository==0.0.21
testtools==2.6.0
tomli==1.2.3
typing_extensions==4.1.1
urllib3==1.26.20
wsgi_intercept==1.13.1
zipp==3.6.0

name: gabbi
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
  - certifi=2021.5.30=py36h06a4308_0
  - ld_impl_linux-64=2.40=h12ee557_0
  - libffi=3.3=he6710b0_2
  - 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=1.1.1w=h7f8727e_0
  - pip=21.2.2=py36h06a4308_0
  - python=3.6.13=h12debd9_1
  - readline=8.2=h5eee18b_0
  - setuptools=58.0.4=py36h06a4308_0
  - sqlite=3.45.3=h5eee18b_0
  - tk=8.6.14=h39e8969_0
  - wheel=0.37.1=pyhd3eb1b0_0
  - xz=5.6.4=h5eee18b_1
  - zlib=1.2.13=h5eee18b_1
  - pip:
      - alabaster==0.7.13
      - attrs==22.2.0
      - babel==2.11.0
      - charset-normalizer==2.0.12
      - colorama==0.4.5
      - coverage==6.2
      - decorator==5.1.1
      - docutils==0.18.1
      - extras==1.0.0
      - fixtures==4.0.1
      - flake8==3.8.4
      - hacking==4.1.0
      - idna==3.10
      - imagesize==1.4.1
      - importlib-metadata==4.8.3
      - iniconfig==1.1.1
      - iso8601==1.1.0
      - jinja2==3.0.3
      - jsonpath-rw==1.4.0
      - jsonpath-rw-ext==1.2.2
      - markupsafe==2.0.1
      - mccabe==0.6.1
      - mock==5.2.0
      - packaging==21.3
      - pbr==6.1.1
      - pluggy==1.0.0
      - ply==3.11
      - py==1.11.0
      - pycodestyle==2.6.0
      - pyflakes==2.2.0
      - pygments==2.14.0
      - pyparsing==3.1.4
      - pytest==7.0.1
      - python-subunit==1.4.2
      - pytz==2025.2
      - pyyaml==6.0.1
      - requests==2.27.1
      - six==1.17.0
      - snowballstemmer==2.2.0
      - sphinx==5.3.0
      - sphinxcontrib-applehelp==1.0.2
      - sphinxcontrib-devhelp==1.0.2
      - sphinxcontrib-htmlhelp==2.0.0
      - sphinxcontrib-jsmath==1.0.1
      - sphinxcontrib-qthelp==1.0.3
      - sphinxcontrib-serializinghtml==1.1.5
      - testrepository==0.0.21
      - testtools==2.6.0
      - tomli==1.2.3
      - typing-extensions==4.1.1
      - urllib3==1.26.20
      - wsgi-intercept==1.13.1
      - zipp==3.6.0
prefix: /opt/conda/envs/gabbi

gabbi/tests/test_syntax_warning.py::DriverTest::test_driver_warngs_on_files
gabbi/exception.py gabbi/driver.py
gabbi/exception.py gabbi/driver.py