API Error: 400 - messages.2.content.0.tool_result: content cannot be empty if `is_error` is true

[moz-mdauer]

I'm working on a multi-agent system using the Claude Agent SDK and after some time, I consistently encounter the following error, which is breaking the entire system:

UserMessage(content=[ToolResultBlock(tool_use_id='', content='', is_error=True)], parent_tool_use_id=None)
AssistantMessage(content=[TextBlock(text='API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages.2.content.0.tool_result: content cannot be empty if `is_error` is true"},"request_id":""}')], model='<synthetic>', parent_tool_use_id=None)
ResultMessage(subtype='success', duration_ms=28092, duration_api_ms=29761, is_error=True, num_turns=4, session_id='', total_cost_usd=0.07331955, usage={'input_tokens': 3, 'cache_creation_input_tokens': 11627, 'cache_read_input_tokens': 11933, 'output_tokens': 1133, 'server_tool_use': {'web_search_requests': 0}, 'service_tier': 'standard', 'cache_creation': {'ephemeral_1h_input_tokens': 0, 'ephemeral_5m_input_tokens': 11627}}, result='API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages.2.content.0.tool_result: content cannot be empty if `is_error` is true"},"request_id":""}')

It's unclear whether this issue originates from the SDK or the API itself, but opening the issue here for visibilty.