feat: Add warning for non-primitive state values that may not serialize correctly into headers.

Author: timof1308

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

@@ -103,6 +103,14 @@ def provider(ctx: ReadonlyContext) -> Dict[str, str]:
     value = ctx.state.get(state_key, default_value)
     if value is None:
       return {}
+    if not isinstance(value, (str, int, float, bool)):
+      logger.warning(
+          'Value for state key "%s" is of type %s, which may not serialize'
+          ' correctly into a header. Consider pre-serializing complex values or'
+          ' using a different header_format.',
+          state_key,
+          type(value).__name__,
+      )
     formatted_value = header_format.format(value=value)
     return {header_name: formatted_value}
 
@@ -297,6 +305,14 @@ def config_based_header_provider(
         for state_key, header_name in state_mapping.items():
           value = ctx.state.get(state_key)
           if value is not None:
+            if not isinstance(value, (str, int, float, bool)):
+              logger.warning(
+                  'Value for state key "%s" is of type %s, which may not'
+                  ' serialize correctly into a header. Consider pre-serializing'
+                  ' complex values or using a different header_format.',
+                  state_key,
+                  type(value).__name__,
+              )
             # Apply formatting if specified for this header
             if header_name in state_format:
               formatted_value = state_format[header_name].format(value=value)