diff --git a/src/google/adk/tools/mcp_tool/mcp_session_manager.py b/src/google/adk/tools/mcp_tool/mcp_session_manager.py
index dc33f92609f..b11c7becf8b 100644
--- a/src/google/adk/tools/mcp_tool/mcp_session_manager.py
+++ b/src/google/adk/tools/mcp_tool/mcp_session_manager.py
@@ -193,6 +193,11 @@ class StdioConnectionParams(BaseModel):
   timeout: float = 5.0
 
 
+@runtime_checkable
+class CheckableMcpHttpClientFactory(McpHttpClientFactory, Protocol):
+  pass
+
+
 class SseConnectionParams(BaseModel):
   """Parameters for the MCP SSE connection.
 
@@ -219,11 +224,6 @@ class SseConnectionParams(BaseModel):
   httpx_client_factory: CheckableMcpHttpClientFactory = create_mcp_http_client
 
 
-@runtime_checkable
-class CheckableMcpHttpClientFactory(McpHttpClientFactory, Protocol):
-  pass
-
-
 class _DebugHttpxClientFactory:
   """A factory wrapper that hooks into the httpx.AsyncClient responses to capture debug info."""
 
diff --git a/tests/unittests/tools/mcp_tool/test_mcp_session_manager_model_completeness.py b/tests/unittests/tools/mcp_tool/test_mcp_session_manager_model_completeness.py
new file mode 100644
index 00000000000..365af736f3a
--- /dev/null
+++ b/tests/unittests/tools/mcp_tool/test_mcp_session_manager_model_completeness.py
@@ -0,0 +1,76 @@
+# 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.
+
+"""Fresh-process checks that the MCP connection params models are complete.
+
+Pydantic rebuilds an incomplete model lazily on first use, so an in-process
+assertion passes as soon as an earlier test builds one of these models. Every
+check here therefore runs in an interpreter that has just imported the module.
+"""
+
+from __future__ import annotations
+
+from google.adk.tools.mcp_tool.mcp_session_manager import SseConnectionParams
+import pydantic
+import pytest
+
+from ... import isolated_import_utils
+from ...isolated_import_utils import run_isolated
+
+pytestmark = pytest.mark.skipif(
+    not isolated_import_utils.SOURCE_ROOT.is_dir(),
+    reason='Import-time model checks need the source checkout layout.',
+)
+
+_MODELS = (
+    'StdioConnectionParams',
+    'SseConnectionParams',
+    'StreamableHTTPConnectionParams',
+)
+
+
+@pytest.mark.parametrize('model_name', _MODELS)
+def test_connection_params_complete_right_after_import(model_name):
+  result = run_isolated(f"""
+from google.adk.tools.mcp_tool import mcp_session_manager
+
+model = mcp_session_manager.{model_name}
+assert model.__pydantic_complete__ is True, (
+    '{model_name} is not fully defined right after import'
+)
+""")
+  assert result.returncode == 0, result.stderr
+
+
+def test_sse_connection_params_is_subclassable_from_another_module():
+  result = run_isolated("""
+from google.adk.tools.mcp_tool.mcp_session_manager import SseConnectionParams
+
+
+class CustomSseConnectionParams(SseConnectionParams):
+  retries: int = 3
+
+
+params = CustomSseConnectionParams(url='http://localhost:8080/sse')
+assert params.retries == 3
+""")
+  assert result.returncode == 0, result.stderr
+
+
+def test_sse_connection_params_rejects_a_non_factory_client_factory():
+  """The resolved annotation must still be the is-instance Protocol check."""
+  with pytest.raises(pydantic.ValidationError, match='httpx_client_factory'):
+    SseConnectionParams(
+        url='http://localhost:8080/sse', httpx_client_factory=123
+    )