Mbed TLS doesn't support the TLS 1.3 protocol yet, but a prototype is in development. Stable parts of this prototype that can be independently tested are being successively upstreamed under the guard of the following macro:
MBEDTLS_SSL_PROTO_TLS1_3_EXPERIMENTAL
This macro will likely be renamed to MBEDTLS_SSL_PROTO_TLS1_3 once a minimal viable implementation of the TLS 1.3 protocol is available.
See the documentation of MBEDTLS_SSL_PROTO_TLS1_3_EXPERIMENTAL for more information.
The following lists which parts of the TLS 1.3 prototype have already been upstreamed together with their level of testing:
TLS 1.3 record protection mechanisms
The record protection routines mbedtls_ssl_{encrypt|decrypt}_buf() have been extended to support the modified TLS 1.3 record protection mechanism, including modified computation of AAD, IV, and the introduction of a flexible padding.
Those record protection routines have unit tests in test_suite_ssl alongside the tests for the other record protection routines.
TODO: Add some test vectors from RFC 8448.
MBEDTLS_HKDF_C independently of the development of the TLS 1.3 prototype.