Known limitations

serve() is single-threaded and blocking — one request at a time, no concurrency
ton.int and ton.float are the same internal number type (no integer-only enforcement)
Dict keys are always strings (no numeric or nested-structural keys)
Classes have no static fields, no private members and no operator overloading (== compares objects by identity)
ton.requests only speaks plain http:// — no TLS/https://, no redirects, no chunked transfer-encoding
The browser playground (ton618-lite.js) doesn't implement ton.encoding/ton.regex/ton.path/ton.tensor, and can print floats with more decimal digits than the real interpreter (a JavaScript-vs-C++ number-formatting difference, not a value difference)