feat: Introduce state_header_strict config for strict state header type validation and add a test.

Author: timof1308

src/google/adk/tools/mcp_tool/mcp_toolset.py

@@ -367,6 +367,7 @@ def from_config(
               header_name=header_name,
               header_format=state_format.get(header_name, "{value}"),
               default_value=None,
+              strict=mcp_toolset_config.state_header_strict,
           )
           for state_key, header_name in state_mapping.items()
       ]
@@ -450,6 +451,21 @@ class McpToolsetConfig(BaseToolConfig):
   used as-is.
   """
 
+  state_header_strict: bool = False
+  """Enable strict type validation for state header values.
+
+  When True, raises ValueError if state values are non-primitive types
+  (not str, int, float, or bool). This helps catch configuration errors
+  early by preventing accidental serialization of complex objects into headers.
+
+  When False (default), non-primitive types trigger a warning but are still
+  formatted into headers.
+
+  Example::
+
+      state_header_strict: true  # Raises error on non-primitive types
+  """
+
   @model_validator(mode="after")
   def _check_only_one_params_field(self):
     param_fields = [

tests/unittests/tools/mcp_tool/test_jwt_token_propagation.py

@@ -318,3 +318,27 @@ def test_from_config_no_state_mapping_no_provider(self):
 
     # No header provider should be created
     assert toolset._header_provider is None
+
+  def test_from_config_with_strict_mode(self):
+    """Test that from_config respects state_header_strict setting."""
+    from google.adk.tools.mcp_tool.mcp_toolset import McpToolset
+    from google.adk.tools.tool_configs import ToolArgsConfig
+
+    config = ToolArgsConfig(
+        stdio_server_params={"command": "test_command", "args": []},
+        state_header_mapping={"data": "X-Data"},
+        state_header_strict=True,  # Enable strict mode
+    )
+
+    toolset = McpToolset.from_config(config, "/fake/path")
+
+    # Test with non-primitive type - should raise ValueError
+    mock_context = Mock(spec=ReadonlyContext)
+    mock_context.state = {"data": {"nested": "object"}}
+
+    with pytest.raises(ValueError) as exc_info:
+      toolset._header_provider(mock_context)
+
+    assert "data" in str(exc_info.value)
+    assert "dict" in str(exc_info.value)
+