diff --git a/tests/unit/agents/conftest.py b/tests/unit/agents/conftest.py
new file mode 100644
index 00000000..2f8f1310
--- /dev/null
+++ b/tests/unit/agents/conftest.py
@@ -0,0 +1,68 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Shared fixtures for agent unit tests."""
+
+from typing import Any
+
+import pytest
+
+
+class StubChatModel:
+    """Stand-in for a resolved provider client.
+
+    Mirrors the only attribute ``VisualStepSummarizer.__init__`` reads back
+    (``model``), so the constructor's model-name resolution is still
+    exercised, and refuses to be invoked: a test that needs generation must
+    install its own fake on ``summarizer._llm``.
+    """
+
+    def __init__(self, model_name: str) -> None:
+        self.model = model_name
+
+    async def ainvoke(self, *_args: Any, **_kwargs: Any) -> Any:
+        raise AssertionError(
+            "The stubbed summarizer model was invoked. A unit test that needs "
+            "generation must assign its own fake to `summarizer._llm`."
+        )
+
+    def __repr__(self) -> str:  # pragma: no cover - debugging aid
+        return f"<StubChatModel model={self.model!r}>"
+
+
+@pytest.fixture
+def stub_summarizer_model_factories(monkeypatch):
+    """Resolve the summarizer's model without provider credentials.
+
+    ``VisualStepSummarizer.__init__`` asks ``get_llm`` for the utility model
+    and falls back to ``get_google_llm`` on *any* exception, so with no
+    ``GOOGLE_API_KEY`` present both paths end in a real
+    ``ChatGoogleGenerativeAI`` construction that raises. Replacing
+    ``summarizer._llm`` after the fact is too late -- the object never gets
+    built. Both factories are therefore stubbed at the summarizer module
+    boundary, which leaves the real summarizer, its retry loop, the
+    compressor, the ledger and the runner under test.
+    """
+
+    def _fake_get_google_llm(model_name: str | None = None, **_kwargs: Any) -> StubChatModel:
+        return StubChatModel(model_name or "gemini-2.5-flash-lite")
+
+    def _fake_get_llm(_ctx: Any, *, name: str = "", is_utils: bool = False, **_kwargs: Any):
+        return StubChatModel("gemini-2.5-flash-lite")
+
+    monkeypatch.setattr(
+        "artemis.agents.flash.summarizer.get_google_llm",
+        _fake_get_google_llm,
+    )
+    monkeypatch.setattr("artemis.agents.flash.summarizer.get_llm", _fake_get_llm)
diff --git a/tests/unit/agents/test_flash_runner.py b/tests/unit/agents/test_flash_runner.py
index f188b5e1..cc1eae73 100644
--- a/tests/unit/agents/test_flash_runner.py
+++ b/tests/unit/agents/test_flash_runner.py
@@ -29,6 +29,11 @@
 from artemis.context import ArtemisContext
 
 
+@pytest.fixture(autouse=True)
+def _isolate_summarizer_model_factories(stub_summarizer_model_factories):
+    """Keep this module runnable without provider credentials (see conftest)."""
+
+
 @pytest.fixture
 def mock_context():
     ctx = Mock(spec=ArtemisContext)
diff --git a/tests/unit/agents/test_flash_runner_ledger.py b/tests/unit/agents/test_flash_runner_ledger.py
index 57ff1ef8..78f257f6 100644
--- a/tests/unit/agents/test_flash_runner_ledger.py
+++ b/tests/unit/agents/test_flash_runner_ledger.py
@@ -56,6 +56,11 @@
 )
 
 
+@pytest.fixture(autouse=True)
+def _isolate_summarizer_model_factories(stub_summarizer_model_factories):
+    """Keep this module runnable without provider credentials (see conftest)."""
+
+
 @pytest.fixture
 def mock_context():
     ctx = Mock(spec=ArtemisContext)
diff --git a/tests/unit/agents/test_flash_scrub_edge.py b/tests/unit/agents/test_flash_scrub_edge.py
index 1524c3b9..9f4de4d0 100644
--- a/tests/unit/agents/test_flash_scrub_edge.py
+++ b/tests/unit/agents/test_flash_scrub_edge.py
@@ -30,6 +30,11 @@
 from artemis.context import ArtemisContext
 
 
+@pytest.fixture(autouse=True)
+def _isolate_summarizer_model_factories(stub_summarizer_model_factories):
+    """Keep this module runnable without provider credentials (see conftest)."""
+
+
 @pytest.fixture
 def mock_context():
     ctx = Mock(spec=ArtemisContext)
diff --git a/tests/unit/agents/test_flash_step_summarizer.py b/tests/unit/agents/test_flash_step_summarizer.py
index a26bc57b..d053d472 100644
--- a/tests/unit/agents/test_flash_step_summarizer.py
+++ b/tests/unit/agents/test_flash_step_summarizer.py
@@ -31,6 +31,11 @@
 from artemis.sdk.builders import Builders
 
 
+@pytest.fixture(autouse=True)
+def _isolate_summarizer_model_factories(stub_summarizer_model_factories):
+    """Keep this module runnable without provider credentials (see conftest)."""
+
+
 @pytest.fixture
 def mock_context():
     ctx = Mock(spec=ArtemisContext)
@@ -840,7 +845,7 @@ def test_flash_config_and_builder():
         step_summarizer=True,
         step_summarizer_model="gemini-2.5-flash-lite",
         prune_history_xml=True,
-    ).build()
+    ).build(validate_profiles=False)
 
     assert cfg.flash.max_turns == 25
     assert cfg.flash.explorer_mode == "flash"