diff --git a/python/packages/core/agent_framework/_telemetry.py b/python/packages/core/agent_framework/_telemetry.py
index af6309e5d3..35500240bf 100644
--- a/python/packages/core/agent_framework/_telemetry.py
+++ b/python/packages/core/agent_framework/_telemetry.py
@@ -212,6 +212,16 @@ def prepend_agent_framework_to_user_agent(headers: dict[str, Any] | None = None)
     user_agent = get_user_agent()
     if not headers:
         return {USER_AGENT_KEY: user_agent}
-    headers[USER_AGENT_KEY] = f"{user_agent} {headers[USER_AGENT_KEY]}" if USER_AGENT_KEY in headers else user_agent
+    # HTTP header names are case-insensitive; detect an existing User-Agent entry
+    # regardless of casing so the framework value is prepended to it instead of
+    # creating a second, duplicate User-Agent header.
+    existing_key = next(
+        (key for key in headers if isinstance(key, str) and key.lower() == USER_AGENT_KEY.lower()),
+        None,
+    )
+    if existing_key is None:
+        headers[USER_AGENT_KEY] = user_agent
+    else:
+        headers[existing_key] = f"{user_agent} {headers[existing_key]}"
 
     return headers
diff --git a/python/packages/core/tests/core/test_telemetry.py b/python/packages/core/tests/core/test_telemetry.py
index 7d7fc77256..e9b4e2f9cf 100644
--- a/python/packages/core/tests/core/test_telemetry.py
+++ b/python/packages/core/tests/core/test_telemetry.py
@@ -343,6 +343,47 @@ def test_modifies_original_dict():
     assert "User-Agent" in headers
 
 
+def test_prepend_to_lowercase_user_agent():
+    """Test prepending to an existing lowercase user-agent entry.
+
+    HTTP header names are case-insensitive, so callers may pass
+    ``user-agent`` instead of ``User-Agent``. The framework value must be
+    prepended to the existing entry rather than adding a second
+    User-Agent header alongside it.
+    """
+    headers = {"user-agent": "existing-agent/1.0"}
+    result = prepend_agent_framework_to_user_agent(headers)
+
+    user_agent_values = [value for key, value in result.items() if key.lower() == "user-agent"]
+    assert len(user_agent_values) == 1
+    assert user_agent_values[0].startswith("agent-framework-python/")
+    assert "existing-agent/1.0" in user_agent_values[0]
+
+
+def test_prepend_to_uppercase_user_agent():
+    """Test prepending to an all-caps USER-AGENT entry."""
+    headers = {"USER-AGENT": "existing-agent/2.0"}
+    result = prepend_agent_framework_to_user_agent(headers)
+
+    user_agent_values = [value for key, value in result.items() if key.lower() == "user-agent"]
+    assert len(user_agent_values) == 1
+    assert user_agent_values[0].startswith("agent-framework-python/")
+    assert "existing-agent/2.0" in user_agent_values[0]
+
+
+def test_no_duplicate_user_agent_after_prepend(monkeypatch):
+    """Test httpx sees a single User-Agent header after the prepend."""
+    import httpx
+
+    headers = {"user-agent": "existing-agent/1.0"}
+    result = prepend_agent_framework_to_user_agent(headers)
+    request = httpx.Request("GET", "https://example.com", headers=result)
+    raw_values = [value for name, value in request.headers.raw if name.lower() == b"user-agent"]
+
+    assert len(raw_values) == 1
+    assert raw_values[0].startswith(b"agent-framework-python/")
+
+
 # region Test _add_user_agent_prefix