diff --git a/src/kimi_cli/auth/oauth.py b/src/kimi_cli/auth/oauth.py
index 96d025d96..2976c0565 100644
--- a/src/kimi_cli/auth/oauth.py
+++ b/src/kimi_cli/auth/oauth.py
@@ -195,10 +195,10 @@ def get_device_id() -> str:
 def _ascii_header_value(value: str, *, fallback: str = "unknown") -> str:
     try:
         value.encode("ascii")
-        return value
+        sanitized = value.strip()
     except UnicodeEncodeError:
         sanitized = value.encode("ascii", errors="ignore").decode("ascii").strip()
-        return sanitized or fallback
+    return sanitized or fallback
 
 
 def _common_headers() -> dict[str, str]:
diff --git a/tests/utils/test_ascii_header_value.py b/tests/utils/test_ascii_header_value.py
new file mode 100644
index 000000000..5b2f04b06
--- /dev/null
+++ b/tests/utils/test_ascii_header_value.py
@@ -0,0 +1,40 @@
+from __future__ import annotations
+
+from kimi_cli.auth.oauth import _ascii_header_value
+
+
+def test_strips_trailing_whitespace() -> None:
+    """platform.version() on some Linux systems returns a trailing space."""
+    value = "#100~22.04.1-Ubuntu SMP PREEMPT_DYNAMIC Mon Jan 19 17:10:19 UTC "
+    assert _ascii_header_value(value) == value.strip()
+
+
+def test_strips_leading_whitespace() -> None:
+    value = " some-value"
+    assert _ascii_header_value(value) == "some-value"
+
+
+def test_clean_ascii_value_unchanged() -> None:
+    value = "KimiCLI/1.13.0"
+    assert _ascii_header_value(value) == value
+
+
+def test_non_ascii_stripped_and_trimmed() -> None:
+    value = "héllo wörld "
+    result = _ascii_header_value(value)
+    assert result == "hllo wrld"
+
+
+def test_empty_after_sanitize_returns_fallback() -> None:
+    value = "日本語"
+    assert _ascii_header_value(value) == "unknown"
+
+
+def test_custom_fallback() -> None:
+    value = "日本語"
+    assert _ascii_header_value(value, fallback="n/a") == "n/a"
+
+
+def test_whitespace_only_returns_fallback() -> None:
+    value = "   "
+    assert _ascii_header_value(value) == "unknown"