diff --git a/nominal/core/client.py b/nominal/core/client.py
index fc494632..0303a832 100644
--- a/nominal/core/client.py
+++ b/nominal/core/client.py
@@ -3,6 +3,7 @@
 import enum
 import logging
 import uuid
+import warnings
 from dataclasses import dataclass, field
 from datetime import datetime, timedelta
 from io import TextIOBase
@@ -171,6 +172,7 @@ def from_token(
         connect_timeout: timedelta | float = DEFAULT_CONNECT_TIMEOUT,
         extra_headers: HeaderProvider | Mapping[str, str] | None = None,
         _profile: str | None = None,
+        _workspace_warning_emitted: bool = False,
     ) -> Self:
         """Create a connection to the Nominal platform from a token.
 
@@ -184,6 +186,13 @@ def from_token(
             connect_timeout: Request connection timeout.
             extra_headers: Extra request headers, either as a mapping or HeaderProvider.
         """
+        if workspace_rid is None and not _workspace_warning_emitted:
+            warnings.warn(
+                "NominalClient will soon require a workspace RID. "
+                "Any client which doesn't have a workspace RID specified will fail.",
+                UserWarning,
+                stacklevel=2,
+            )
         trust_store_path = certifi.where() if trust_store_path is None else trust_store_path
         timeout_seconds = connect_timeout.total_seconds() if isinstance(connect_timeout, timedelta) else connect_timeout
         cfg = ServiceConfiguration(
diff --git a/nominal/experimental/impersonation/__init__.py b/nominal/experimental/impersonation/__init__.py
index dd2c41a2..a0b66fe6 100644
--- a/nominal/experimental/impersonation/__init__.py
+++ b/nominal/experimental/impersonation/__init__.py
@@ -18,4 +18,5 @@ def as_user(client: NominalClient, user_rid: str) -> NominalClient:
         connect_timeout=client._clients._service_config.connect_timeout,
         extra_headers={ON_BEHALF_OF_USER_RID_HEADER: user_rid},
         _profile=client._profile,
+        _workspace_warning_emitted=True,
     )
diff --git a/tests/core/test_clientsbunch.py b/tests/core/test_clientsbunch.py
index bf24dc98..577811f9 100644
--- a/tests/core/test_clientsbunch.py
+++ b/tests/core/test_clientsbunch.py
@@ -222,7 +222,7 @@ def test_experimental_as_user_returns_derived_nominal_client(monkeypatch):
             "https://api.nominal.test",
             "test-agent",
             "token",
-            None,
+            "ri.workspace.main.workspace.test",
         )
     )
 
diff --git a/tests/e2e/test_workspace_resolution.py b/tests/e2e/test_workspace_resolution.py
index 377cfe89..2a418100 100644
--- a/tests/e2e/test_workspace_resolution.py
+++ b/tests/e2e/test_workspace_resolution.py
@@ -45,7 +45,8 @@ def test_workspace_rid_for_search_returns_none_for_all(client: NominalClient) ->
 
 def test_unconfigured_client_uses_workspace_service_default(client: NominalClient, pytestconfig) -> None:
     """Without a pinned workspace, DEFAULT should resolve to the tenant's service-side default."""
-    unconfigured_client = _client_with_workspace_override(client, pytestconfig, workspace_rid=None)
+    with pytest.warns(UserWarning, match="NominalClient will soon require a workspace RID"):
+        unconfigured_client = _client_with_workspace_override(client, pytestconfig, workspace_rid=None)
     expected_workspace_rid = _get_service_default_workspace_rid(unconfigured_client)
 
     assert unconfigured_client._clients.resolve_default_workspace_rid() == expected_workspace_rid
@@ -54,7 +55,8 @@ def test_unconfigured_client_uses_workspace_service_default(client: NominalClien
 
 def test_configured_workspace_rid_takes_precedence_over_service_default(client: NominalClient, pytestconfig) -> None:
     """A pinned workspace RID should win over the tenant default exposed by the workspace service."""
-    unconfigured_client = _client_with_workspace_override(client, pytestconfig, workspace_rid=None)
+    with pytest.warns(UserWarning, match="NominalClient will soon require a workspace RID"):
+        unconfigured_client = _client_with_workspace_override(client, pytestconfig, workspace_rid=None)
     service_default_workspace_rid = _get_service_default_workspace_rid(unconfigured_client)
     configured_workspace = next(
         (