fix(headers): merge tracking headers regardless of casing

src/google/adk/tools/mcp_tool/mcp_session_manager.py

@@ -108,12 +108,6 @@ class AsyncAuthorizedSession:  # pylint: disable=g-bad-classes
 # the manager, so credentials granted while the process runs are picked up.
 _MTLS_PROBE_RETRY_INTERVAL_SECONDS = 300.0
 
-# The headers `merge_tracking_headers` writes, spelled the way it spells them.
-# HTTP header names are case-insensitive and a caller may have used any casing,
-# but a dict is not: leaving their spelling alongside ours would put the header
-# on the wire twice, so theirs is folded onto ours before merging.
-_TRACKING_HEADER_NAMES = frozenset(('user-agent', 'x-goog-api-client'))
-
 
 def create_mcp_http_client(
     headers: dict[str, str] | None = None,
@@ -998,10 +992,7 @@ def _merge_headers(
     if additional_headers:
       base_headers.update(additional_headers)
 
-    return merge_tracking_headers({
-        key.lower() if key.lower() in _TRACKING_HEADER_NAMES else key: value
-        for key, value in base_headers.items()
-    })
+    return merge_tracking_headers(base_headers)
 
   def _is_session_disconnected(self, session: ClientSession) -> bool:
     """Checks if a session is disconnected or closed.

src/google/adk/utils/_google_client_headers.py

@@ -62,19 +62,20 @@ def merge_tracking_headers(
   Returns:
     A dictionary of HTTP headers with tracking headers merged.
   """
-  new_headers = (headers or {}).copy()
-  for key, tracking_header_value in get_tracking_headers(
-      framework_label=framework_label
-  ).items():
-    custom_value = new_headers.get(key, None)
-    if not custom_value:
-      new_headers[key] = tracking_header_value
-      continue
-
+  tracking_headers = get_tracking_headers(framework_label=framework_label)
+  new_headers = {
+      key: value
+      for key, value in (headers or {}).items()
+      if key.lower() not in tracking_headers
+  }
+  for key, tracking_header_value in tracking_headers.items():
     # Merge tracking headers with existing headers and avoid duplicates.
     value_parts = tracking_header_value.split(" ")
-    for custom_value_part in custom_value.split(" "):
-      if custom_value_part not in value_parts:
-        value_parts.append(custom_value_part)
+    for header_key, custom_value in (headers or {}).items():
+      if header_key.lower() != key or not custom_value:
+        continue
+      for custom_value_part in custom_value.split(" "):
+        if custom_value_part not in value_parts:
+          value_parts.append(custom_value_part)
     new_headers[key] = " ".join(value_parts)
   return new_headers

tests/unittests/tools/mcp_tool/test_mcp_session_manager.py

@@ -446,6 +446,36 @@ def test_merge_headers_keeps_custom_user_agent(self):
     assert merged["user-agent"].startswith("google-adk/")
     assert merged["user-agent"].endswith(" my-app/1.0")
 
+  def test_merge_headers_preserves_tracking_case_aliases(self):
+    """MCP keeps base and additional custom tokens when their casing differs."""
+    base_headers = {
+        "User-Agent": "base-client/1 shared/1",
+        "X-Goog-Api-Client": "base-sdk/1 shared/1",
+        "X-Custom": "base",
+    }
+    additional = {
+        "user-agent": "extra-client/1 shared/1",
+        "x-goog-api-client": "extra-sdk/1 shared/1",
+        "X-Custom": "extra",
+    }
+    manager = MCPSessionManager(
+        SseConnectionParams(url="https://example.com/mcp", headers=base_headers)
+    )
+
+    merged = manager._merge_headers(additional)
+    request = httpx.Request("GET", "https://example.com/mcp", headers=merged)
+
+    for key, expected_custom in (
+        ("user-agent", "base-client/1 shared/1 extra-client/1"),
+        ("x-goog-api-client", "base-sdk/1 shared/1 extra-sdk/1"),
+    ):
+      values = request.headers.get_list(key)
+      assert len(values) == 1
+      assert values[0].endswith(" " + expected_custom)
+    assert merged["X-Custom"] == "extra"
+    assert base_headers["User-Agent"] == "base-client/1 shared/1"
+    assert additional["user-agent"] == "extra-client/1 shared/1"
+
   def test_is_session_disconnected(self):
     """Test session disconnection detection."""
     manager = MCPSessionManager(self.mock_stdio_connection_params)

tests/unittests/utils/test_google_client_headers.py

@@ -16,6 +16,7 @@
 
 from google.adk import version
 from google.adk.utils import _google_client_headers
+import httpx
 import pytest
 
 _EXPECTED_BASE_HEADER = (
@@ -79,6 +80,64 @@ def test_merge_tracking_headers(input_headers, expected_headers):
   assert headers == expected_headers
 
 
+@pytest.mark.parametrize(
+    "user_agent_key, api_client_key",
+    [
+        ("User-Agent", "X-Goog-Api-Client"),
+        ("USER-AGENT", "X-GOOG-API-CLIENT"),
+    ],
+)
+def test_merge_tracking_headers_merges_case_insensitively(
+    user_agent_key, api_client_key
+):
+  """Each tracking header reaches the transport once with custom tokens kept."""
+  input_headers = {
+      user_agent_key: "custom-client/1",
+      api_client_key: "custom-sdk/1",
+      "X-Custom": "value",
+  }
+
+  headers = _google_client_headers.merge_tracking_headers(input_headers)
+  request = httpx.Request("GET", "https://example.test", headers=headers)
+
+  assert request.headers.get_list("user-agent") == [
+      f"{_EXPECTED_BASE_HEADER} custom-client/1"
+  ]
+  assert request.headers.get_list("x-goog-api-client") == [
+      f"{_EXPECTED_BASE_HEADER} custom-sdk/1"
+  ]
+  assert headers["X-Custom"] == "value"
+  assert input_headers == {
+      user_agent_key: "custom-client/1",
+      api_client_key: "custom-sdk/1",
+      "X-Custom": "value",
+  }
+
+
+def test_merge_tracking_headers_preserves_all_case_aliases():
+  """Case aliases merge into one transport field without losing custom tokens."""
+  input_headers = {
+      "User-Agent": f"first-client/1 shared/1 {_EXPECTED_BASE_HEADER}",
+      "user-agent": "second-client/1 shared/1",
+      "X-Goog-Api-Client": "first-sdk/1 shared/1",
+      "x-goog-api-client": "second-sdk/1 shared/1",
+      "X-Custom": "value",
+  }
+  original_headers = input_headers.copy()
+
+  headers = _google_client_headers.merge_tracking_headers(input_headers)
+  request = httpx.Request("GET", "https://example.test", headers=headers)
+
+  assert request.headers.get_list("user-agent") == [
+      f"{_EXPECTED_BASE_HEADER} first-client/1 shared/1 second-client/1"
+  ]
+  assert request.headers.get_list("x-goog-api-client") == [
+      f"{_EXPECTED_BASE_HEADER} first-sdk/1 shared/1 second-sdk/1"
+  ]
+  assert headers["X-Custom"] == "value"
+  assert input_headers == original_headers
+
+
 def test_get_tracking_http_options():
   """get_tracking_http_options returns HttpOptions carrying tracking headers."""
   http_options = _google_client_headers.get_tracking_http_options()