fix: avoid leading separator in user_agent when no caller-supplied agent is provided

kernels/src/kernels/utils.py

@@ -627,31 +627,41 @@ def _get_hf_api(user_agent: str | dict | None = None) -> HfApi:
 
     user_agent_str = ""
     if not constants.HF_HUB_DISABLE_TELEMETRY:
+        parts: list[str] = []
+
         # User-defined info
         if isinstance(user_agent, dict):
-            user_agent_str = "; ".join(f"{k}/{v}" for k, v in user_agent.items())
-        if isinstance(user_agent, str):
-            user_agent_str = user_agent
+            parts.extend(f"{k}/{v}" for k, v in user_agent.items())
+        elif isinstance(user_agent, str) and user_agent:
+            parts.append(user_agent)
 
         # System info
         python = ".".join(platform.python_version_tuple()[:2])
         backend = _select_backend(None).variant_str
-        user_agent_str += (
-            f"; kernels/{__version__}; python/{python}; backend/{backend}; platform/{_platform()}; file_type/kernel"
+        parts.extend(
+            [
+                f"kernels/{__version__}",
+                f"python/{python}",
+                f"backend/{backend}",
+                f"platform/{_platform()}",
+                "file_type/kernel",
+            ]
         )
 
         if has_torch:
             import torch
 
-            user_agent_str += f"; torch/{torch.__version__}"
+            parts.append(f"torch/{torch.__version__}")
         if has_tvm_ffi:
             import tvm_ffi
 
-            user_agent_str += f"; tvm-ffi/{tvm_ffi.__version__}"
+            parts.append(f"tvm-ffi/{tvm_ffi.__version__}")
 
         # Add glibc version if available
         glibc = glibc_version()
         if glibc is not None:
-            user_agent_str += f"; glibc/{glibc}"
+            parts.append(f"glibc/{glibc}")
+
+        user_agent_str = "; ".join(parts)
 
     return HfApi(library_name="kernels", library_version=__version__, user_agent=user_agent_str)

kernels/tests/test_user_agent.py

@@ -69,3 +69,16 @@ def test_platform_format():
     parts = plat.split("-")
     assert len(parts) == 2
     assert parts[1] in ("linux", "darwin", "windows")
+
+
+def test_user_agent_no_leading_or_empty_segment():
+    # Regression: when no caller-supplied user_agent is passed, the resulting
+    # string must not start with a separator and must not contain empty
+    # segments. Empty segments downstream produce malformed User-Agent headers
+    # (e.g. trailing "; ") which strict HTTP clients reject.
+    for ua_input in (None, "", {}):
+        api = _get_hf_api(user_agent=ua_input)
+        ua = api.user_agent
+        assert not ua.startswith(";"), f"user_agent must not start with ';': {ua!r}"
+        for segment in ua.split(";"):
+            assert segment.strip() != "", f"empty segment found in user_agent: {ua!r}"