I haven't looked at the OpenAI tool calling spec, but the lack of return types in MCP, as reported by Erik Meijers, makes composition hard.

Additionally, the lack of typed encodings makes I/O unavoidable because the model has to interpret the schema of returned text values first to make sense of it before passing it as input to other tools. Makes it impossible to pre-compile transformations while you wait on tool results.

IMO endgame for MCP is to delete MCP and give AI access to a REPL with eval authorized at function-level.

This is why, in the age of AI, I am long dynamic languages like Clojure.