diff --git a/agentflow/core/llm/client_factory.py b/agentflow/core/llm/client_factory.py
index 35cdfb3..5fb8329 100644
--- a/agentflow/core/llm/client_factory.py
+++ b/agentflow/core/llm/client_factory.py
@@ -72,9 +72,7 @@ def detect_provider(model: str, use_vertex_ai: bool = False) -> str:
     return "openai"
 
 
-def resolve_provider_and_model(
-    model: str, use_vertex_ai: bool = False
-) -> tuple[str, str]:
+def resolve_provider_and_model(model: str, use_vertex_ai: bool = False) -> tuple[str, str]:
     """Resolve a model string into a concrete ``(provider, model)`` pair.
 
     Unlike :func:`detect_provider`, this also returns the model name that should
diff --git a/tests/conftest.py b/tests/conftest.py
index 1aa85f8..54c8b8f 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -34,6 +34,13 @@ def setup_test_environment():
     # Set dummy Google API key for tests
     os.environ.setdefault("GEMINI_API_KEY", "dummy-gemini-key-for-testing-only")
 
+    # Vertex AI selection must NOT be inherited from a developer's .env / shell.
+    # Agent() reads GOOGLE_GENAI_USE_VERTEXAI as the default for use_vertex_ai, so
+    # an ambient "true" would make provider auto-detection resolve to "google" for
+    # every model and break deterministic unit tests. Force it off for the suite;
+    # tests that exercise Vertex pass use_vertex_ai=True explicitly.
+    os.environ.pop("GOOGLE_GENAI_USE_VERTEXAI", None)
+
     # Keep tests compatible while core graph transitions from
     # Node(name, func, publisher) to Node(name, func).
     Node.__init__ = _compat_node_init
diff --git a/tests/normal_tests/test_custom_state_comprehensive.py b/tests/normal_tests/test_custom_state_comprehensive.py
index 8d523c4..bb8c01f 100644
--- a/tests/normal_tests/test_custom_state_comprehensive.py
+++ b/tests/normal_tests/test_custom_state_comprehensive.py
@@ -6,8 +6,6 @@
 from dataclasses import dataclass, field
 from typing import Any
 
-from dotenv import load_dotenv
-
 from agentflow.storage.checkpointer import InMemoryCheckpointer
 from agentflow.core.graph import StateGraph
 from agentflow.core.state.agent_state import AgentState
@@ -15,9 +13,6 @@
 from agentflow.utils.converter import convert_messages
 
 
-load_dotenv()
-
-
 @dataclass
 class TestCustomState(AgentState):
     """Test state with various field types."""