diff --git a/packages/data-designer/src/data_designer/interface/data_designer.py b/packages/data-designer/src/data_designer/interface/data_designer.py
index e296d756c..b1c6dcfd3 100644
--- a/packages/data-designer/src/data_designer/interface/data_designer.py
+++ b/packages/data-designer/src/data_designer/interface/data_designer.py
@@ -166,13 +166,31 @@ def __init__(
             self._model_providers = self._resolve_model_providers(model_providers)
             default_provider_name = None
         self._mcp_providers = mcp_providers or []
-        # When the YAML carries a default, ``get_default_provider_name`` already
-        # nudged the user with a ``DeprecationWarning``. Building the registry
-        # below would re-fire ``ModelProviderRegistry._warn_on_explicit_default``
-        # for the same root cause, so suppress that second warning. See PR #594
-        # review.
+        # Suppress ``ModelProviderRegistry._warn_on_explicit_default`` whenever
+        # *we* are filling ``default=`` on the user's behalf rather than the
+        # user actively opting into the deprecated registry-level default. Two
+        # such cases:
+        #   1. ``model_providers is None`` — the caller passed nothing, so we
+        #      load the YAML's ``providers:`` list and (in the multi-provider
+        #      case) ``resolve_model_provider_registry`` synthesises
+        #      ``default=providers[0].name`` to satisfy ``check_implicit_default``.
+        #      The fresh-install YAML ships three providers and no ``default:``
+        #      key, so this fires for every default ``DataDesigner()``
+        #      construction. The user has no actionable lever here, and the
+        #      warning's "Specify provider= on each ModelConfig" remediation
+        #      doesn't apply when they haven't built a ``ModelConfig`` at all.
+        #   2. ``default_provider_name is not None`` — the YAML carried a
+        #      ``default:`` key and ``get_default_provider_name`` already
+        #      emitted the YAML-level ``DeprecationWarning``. The registry
+        #      warning would fire for the same root cause, so suppress it to
+        #      avoid double-warning. See PR #594 review.
+        # Users who hand-construct a multi-provider list in Python still see
+        # the warning (they wrote the multi-provider intent themselves), and
+        # users who hand-construct ``ModelProviderRegistry(default=...)``
+        # directly always see it — those are the entry points #589 targets.
+        library_synthesised_default = model_providers is None or default_provider_name is not None
         with warnings.catch_warnings():
-            if default_provider_name is not None:
+            if library_synthesised_default:
                 warnings.filterwarnings(
                     "ignore",
                     message="ModelProviderRegistry.default is deprecated",
diff --git a/packages/data-designer/tests/interface/test_data_designer.py b/packages/data-designer/tests/interface/test_data_designer.py
index c98987103..5e7de7dc6 100644
--- a/packages/data-designer/tests/interface/test_data_designer.py
+++ b/packages/data-designer/tests/interface/test_data_designer.py
@@ -654,6 +654,60 @@ def test_init_yaml_default_emits_single_deprecation_warning(
     )
 
 
+def test_init_no_user_providers_no_yaml_default_stays_quiet(
+    stub_artifact_path: Path,
+    stub_managed_assets_path: Path,
+) -> None:
+    """Pin the bare-``DataDesigner()`` happy path: when the caller passes
+    nothing and the YAML carries multiple ``providers:`` but no ``default:``
+    key, ``resolve_model_provider_registry`` synthesises
+    ``default=providers[0].name`` to satisfy ``check_implicit_default``. The
+    user did not opt into the deprecated registry-level default — the library
+    filled it in on their behalf — so ``_warn_on_explicit_default`` must stay
+    quiet. The fresh-install YAML ships exactly this shape (3 providers, no
+    ``default:``), so a regression here is what every user sees on their first
+    ``DataDesigner()`` call.
+
+    Counterpart to ``test_init_user_supplied_providers_preserve_first_wins_over_yaml_default``,
+    which pins that the warning *does* fire when the caller hand-builds a
+    multi-provider list themselves (they wrote the multi-provider intent, so
+    the deprecation nudge applies).
+    """
+    yaml_providers = [
+        ModelProvider(
+            name="yaml-first",
+            endpoint="https://yaml-first.example.com/v1",
+            api_key="yaml-first-key",
+        ),
+        ModelProvider(
+            name="yaml-second",
+            endpoint="https://yaml-second.example.com/v1",
+            api_key="yaml-second-key",
+        ),
+    ]
+
+    with warnings.catch_warnings(record=True) as caught:
+        warnings.simplefilter("always", DeprecationWarning)
+        with (
+            patch.object(dd_mod, "get_default_providers", return_value=yaml_providers),
+            patch.object(dd_mod, "get_default_provider_name", return_value=None),
+        ):
+            data_designer = DataDesigner(
+                artifact_path=stub_artifact_path,
+                secret_resolver=PlaintextResolver(),
+                managed_assets_path=stub_managed_assets_path,
+            )
+
+    deprecation_messages = [str(w.message) for w in caught if issubclass(w.category, DeprecationWarning)]
+    registry_default_warnings = [m for m in deprecation_messages if "ModelProviderRegistry.default is deprecated" in m]
+    assert registry_default_warnings == [], (
+        "Library-synthesised default must not emit the registry-level deprecation; "
+        f"the user did not opt into it. Saw: {deprecation_messages}"
+    )
+    # Behavioral pin: first-wins still resolves correctly.
+    assert data_designer.model_provider_registry.get_default_provider_name() == "yaml-first"
+
+
 def test_run_config_setting_persists(stub_artifact_path, stub_model_providers):
     """Test that run config setting persists across multiple calls."""
     data_designer = DataDesigner(artifact_path=stub_artifact_path, model_providers=stub_model_providers)