Fix format_url to preserve trailing slash when url_template is query-string only

Co-authored-by: l0lawrence <100643745+l0lawrence@users.noreply.github.com>

Author: Copilot

sdk/core/azure-core/CHANGELOG.md

@@ -8,6 +8,8 @@
 
 ### Bugs Fixed
 
+- Fixed `PipelineClient.format_url` to preserve trailing slash in the base URL when the URL template is a query-string only (e.g. `?key=value`). #40426
+
 ### Other Changes
 
 ## 1.38.2 (2026-02-18)

sdk/core/azure-core/azure/core/pipeline/transport/_base.py

@@ -663,7 +663,7 @@ def format_url(self, url_template: str, **kwargs: Any) -> str:
             parsed = urlparse(url)
             if not parsed.scheme or not parsed.netloc:
                 try:
-                    base = self._base_url.format(**kwargs).rstrip("/")
+                    base = self._base_url.format(**kwargs)
                 except KeyError as key:
                     err_msg = "The value provided for the url part {} was incorrect, and resulted in an invalid url"
                     raise ValueError(err_msg.format(key.args[0])) from key

sdk/core/azure-core/tests/test_pipeline.py

@@ -225,6 +225,14 @@ def test_format_url_query_strings():
     assert formatted == "https://foo.core.windows.net/Tables?a=X&c=Y"
 
 
+def test_format_url_trailing_slash_preserved_with_query_only():
+    # Test that trailing slash in base URL is preserved when url_template is query-string only
+    # https://github.com/Azure/azure-sdk-for-python/issues/XXXXX
+    client = PipelineClientBase("{url}")
+    formatted = client.format_url("?versionid=2026-02-25", url="https://storage.blob.core.windows.net/sample//a/a/")
+    assert formatted == "https://storage.blob.core.windows.net/sample//a/a/?versionid=2026-02-25"
+
+
 def test_format_url_from_http_request():
     client = PipelineClientBase("https://foo.core.windows.net")