diff --git a/tests/agent/test_auxiliary_config_bridge.py b/tests/agent/test_auxiliary_config_bridge.py
index 66350519b0b..11fe9f71c23 100644
--- a/tests/agent/test_auxiliary_config_bridge.py
+++ b/tests/agent/test_auxiliary_config_bridge.py
@@ -200,7 +200,11 @@ class TestGatewayBridgeCodeParity:
     def test_gateway_has_auxiliary_bridge(self):
         """The gateway config bridge must include auxiliary.* bridging."""
         gateway_path = Path(__file__).parent.parent.parent / "gateway" / "run.py"
-        content = gateway_path.read_text()
+        # Pin encoding to UTF-8: source files in this repo are UTF-8, but
+        # Path.read_text() defaults to the system locale — which is cp1252
+        # on most Western Windows installs and crashes as soon as the file
+        # contains any non-ASCII byte (e.g. an em-dash in a comment).
+        content = gateway_path.read_text(encoding="utf-8")
         # Check for key patterns that indicate the bridge is present
         assert "AUXILIARY_VISION_PROVIDER" in content
         assert "AUXILIARY_VISION_MODEL" in content
@@ -214,7 +218,9 @@ class TestGatewayBridgeCodeParity:
     def test_gateway_no_compression_env_bridge(self):
         """Gateway should NOT bridge compression config to env vars (config-only)."""
         gateway_path = Path(__file__).parent.parent.parent / "gateway" / "run.py"
-        content = gateway_path.read_text()
+        # See note in test_gateway_has_auxiliary_bridge — pin UTF-8 so the
+        # test runs on Windows where the default locale is cp1252.
+        content = gateway_path.read_text(encoding="utf-8")
         assert "CONTEXT_COMPRESSION_PROVIDER" not in content
         assert "CONTEXT_COMPRESSION_MODEL" not in content
 
@@ -289,7 +295,9 @@ class TestCLIDefaultsHaveAuxiliaryKeys:
         # So auxiliary config from config.yaml gets merged even though
         # cli.py's defaults dict doesn't define it.
         import cli as _cli_mod
-        source = Path(_cli_mod.__file__).read_text()
+        # See note in test_gateway_has_auxiliary_bridge — pin UTF-8 so the
+        # test runs on Windows where the default locale is cp1252.
+        source = Path(_cli_mod.__file__).read_text(encoding="utf-8")
         assert "auxiliary_config = defaults.get(\"auxiliary\"" in source
         assert "AUXILIARY_VISION_PROVIDER" in source
         assert "AUXILIARY_VISION_MODEL" in source