diff --git a/sdk/voice/speechmatics/voice/_client.py b/sdk/voice/speechmatics/voice/_client.py
index b3cc134..2e1ee43 100644
--- a/sdk/voice/speechmatics/voice/_client.py
+++ b/sdk/voice/speechmatics/voice/_client.py
@@ -14,7 +14,10 @@
 from typing import Callable
 from typing import Optional
 from typing import Union
+from urllib.parse import parse_qs
 from urllib.parse import urlencode
+from urllib.parse import urlparse
+from urllib.parse import urlunparse
 
 from speechmatics.rt import AsyncClient
 from speechmatics.rt import AudioEncoding
@@ -1914,12 +1917,21 @@ def _get_endpoint_url(self, url: str, app: Optional[str] = None) -> str:
             app: The application name to use in the endpoint URL.
 
         Returns:
-        str: The formatted endpoint URL.
+            str: The formatted endpoint URL.
         """
 
-        query_params = {}
-        query_params["sm-app"] = app or f"voice-sdk/{__version__}"
-        query_params["sm-voice-sdk"] = f"{__version__}"
-        query = urlencode(query_params)
+        # Parse the URL to extract existing query parameters
+        parsed = urlparse(url)
 
-        return f"{url}?{query}"
+        # Extract existing params into a dict of lists, keeping params without values
+        params = parse_qs(parsed.query, keep_blank_values=True)
+
+        # Use the provided app name, or fallback to existing value, or use the default string
+        existing_app = params.get("sm-app", [None])[0]
+        app_name = app or existing_app or f"voice-sdk/{__version__}"
+        params["sm-app"] = [app_name]
+        params["sm-voice-sdk"] = [__version__]
+
+        # Re-encode the query string and reconstruct
+        updated_query = urlencode(params, doseq=True)
+        return urlunparse(parsed._replace(query=updated_query))
diff --git a/tests/voice/test_16_url.py b/tests/voice/test_16_url.py
new file mode 100644
index 0000000..c41dfc9
--- /dev/null
+++ b/tests/voice/test_16_url.py
@@ -0,0 +1,89 @@
+from dataclasses import dataclass
+from typing import Optional
+from urllib.parse import parse_qs
+from urllib.parse import urlparse
+
+import pytest
+from _utils import get_client
+
+from speechmatics.voice import __version__
+
+
+@dataclass
+class URLExample:
+    input_url: str
+    input_app: Optional[str] = None
+
+
+URLS: list[URLExample] = [
+    URLExample(
+        input_url="wss://dummy/ep",
+        input_app="dummy-0.1.2",
+    ),
+    URLExample(
+        input_url="wss://dummy:1234/ep?client=amz",
+        input_app="dummy-0.1.2",
+    ),
+    URLExample(
+        input_url="wss://dummy/ep?sm-app=dummy",
+    ),
+    URLExample(
+        input_url="ws://localhost:8080/ep?sm-app=dummy",
+        input_app="dummy-0.1.2",
+    ),
+    URLExample(
+        input_url="http://dummy/ep/v1/",
+        input_app="dummy-0.1.2",
+    ),
+    URLExample(
+        input_url="wss://dummy/ep",
+    ),
+    URLExample(
+        input_url="wss://dummy/ep",
+        input_app="client/a#b:c^d",
+    ),
+]
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize("test", URLS, ids=lambda s: s.input_url)
+async def test_url_endpoints(test: URLExample):
+    """Test URL endpoint construction."""
+
+    # Client
+    client = await get_client(
+        api_key="DUMMY",
+        connect=False,
+    )
+
+    # Parse the input parameters
+    input_parsed = urlparse(test.input_url)
+    input_params = parse_qs(input_parsed.query, keep_blank_values=True)
+
+    # URL test
+    generated_url = client._get_endpoint_url(test.input_url, test.input_app)
+
+    # Parse the URL
+    parsed_url = urlparse(generated_url)
+    parsed_params = parse_qs(parsed_url.query, keep_blank_values=True)
+
+    # Check the url scheme, netloc and path are preserved
+    assert parsed_url.scheme == input_parsed.scheme
+    assert parsed_url.netloc == input_parsed.netloc
+    assert parsed_url.path == input_parsed.path
+
+    # Validate `sm-app`
+    if test.input_app:
+        assert parsed_params["sm-app"] == [test.input_app]
+    elif "sm-app" in input_params:
+        assert parsed_params["sm-app"] == [input_params["sm-app"][0]]
+    else:
+        assert parsed_params["sm-app"] == [f"voice-sdk/{__version__}"]
+
+    # Validate `sm-voice-sdk`
+    assert parsed_params["sm-voice-sdk"] == [__version__]
+
+    # Check other original params are preserved
+    for key, value in input_params.items():
+        if key not in ["sm-app", "sm-voice-sdk"]:
+            assert parsed_params[key] == value