PreProcessResult(
    url="https://api.example.com/chat",  # required
    method="POST",                       # optional — HTTP method (default: "POST")
    headers={},                          # optional — request headers
    json_body=None,                      # optional — JSON body (dict or list)
    data=None,                           # optional — form or raw body
    params=None,                         # optional — query-string parameters
)

PostProcessResult(
    output="the target's reply",  # required — the reply text
    session_state=None,           # optional — set only if session state changed this turn;
                                  # None keeps the existing session
)

CallTargetResult(
    output="the target's reply",  # required — the reply text
    session_state=None,           # optional — set only if session state changed this turn;
                                  # None keeps the existing session
)

AuthResult(
    ttl=3600,               # required — seconds to cache before re-authenticating
    data={"token": "..."},  # required — any dict; available as context.auth in other functions
)

SessionPreProcessResult(
    session_state={"session_id": "..."},  # required — any dict; available as context.session
                                          # in pre_process, post_process, and call_target
)