diff --git a/src/adcp/types/__init__.py b/src/adcp/types/__init__.py
index e6dd78d6..9947d9e2 100644
--- a/src/adcp/types/__init__.py
+++ b/src/adcp/types/__init__.py
@@ -184,7 +184,6 @@
     GetProductsResponse,
     GetPropertyListRequest,
     GetPropertyListResponse,
-    GetSignalsRequest,
     GetSignalsResponse,
     Gtin,
     HtmlAsset,
@@ -399,6 +398,7 @@
     GetProductsWholesaleRequest,
     GetSignalsDiscoveryRequest,
     GetSignalsLookupRequest,
+    GetSignalsRequest,
     HtmlPreviewRender,
     InlineDaastAsset,
     InlineVastAsset,
diff --git a/src/adcp/types/aliases.py b/src/adcp/types/aliases.py
index 20620347..af0aeee3 100644
--- a/src/adcp/types/aliases.py
+++ b/src/adcp/types/aliases.py
@@ -506,6 +506,9 @@ def process_result(result: SyncCatalogResult) -> None:
 """Get products in wholesale mode - buying_mode='wholesale', raw inventory."""
 
 # Get Signals Request Variants
+GetSignalsRequest = GetSignalsRequest1 | GetSignalsRequest2
+"""Union of GetSignalsRequest variants. Use instead of the RootModel wrapper to allow subclassing."""
+
 GetSignalsDiscoveryRequest = GetSignalsRequest1
 """Discover signals by natural language spec - signal_spec required."""
 
diff --git a/tests/test_type_aliases.py b/tests/test_type_aliases.py
index 5d9a36d8..cec39528 100644
--- a/tests/test_type_aliases.py
+++ b/tests/test_type_aliases.py
@@ -808,3 +808,48 @@ def test_destination_union_contains_all_variants():
     }
 
     assert union_args == expected_variants
+
+
+def test_get_signals_request_is_union_not_root_model():
+    """GetSignalsRequest is a plain union alias, not a RootModel.
+
+    This allows consumers to subclass GetSignalsRequest1 or GetSignalsRequest2
+    with custom model_config (e.g. extra='forbid' in CI, extra='ignore' in prod).
+    See: https://github.com/adcontextprotocol/adcp-client-python/issues/138
+    """
+    import types
+
+    from pydantic import RootModel
+
+    from adcp import GetSignalsRequest
+
+    # Must be a union, not a RootModel subclass
+    assert isinstance(GetSignalsRequest, types.UnionType)
+    assert not (isinstance(GetSignalsRequest, type) and issubclass(GetSignalsRequest, RootModel))
+
+
+def test_get_signals_request_union_contains_variants():
+    """GetSignalsRequest union contains the two concrete variants."""
+    from typing import get_args
+
+    from adcp import GetSignalsDiscoveryRequest, GetSignalsLookupRequest, GetSignalsRequest
+
+    union_args = set(get_args(GetSignalsRequest))
+    assert union_args == {GetSignalsDiscoveryRequest, GetSignalsLookupRequest}
+
+
+def test_get_signals_request_variants_are_subclassable():
+    """Consumers can subclass GetSignalsRequest variants with custom model_config."""
+    from pydantic import ConfigDict
+
+    from adcp import GetSignalsDiscoveryRequest, GetSignalsLookupRequest
+
+    class MyDiscoveryRequest(GetSignalsDiscoveryRequest):
+        model_config = ConfigDict(extra="forbid")
+
+    class MyLookupRequest(GetSignalsLookupRequest):
+        model_config = ConfigDict(extra="forbid")
+
+    # Should construct without error
+    req = MyDiscoveryRequest(signal_spec="targeting signals for automotive in-market buyers")
+    assert req.signal_spec == "targeting signals for automotive in-market buyers"