why does every agent framework assume tools return json

tested langchain, autogen, crewai, haystack - all of them assume mcp tools return json. but lots of real tools return plain text, binary data, or streaming responses. if you have a tool that returns a 4mb log file as text, the framework tries to parse it as json and crashes. no fallback, no content-type checking, just assumes everything is json. is there a spec reason for this or did everyone just copy langchain's assumptions