Portal will serve anthropic/* from more than one upstream (OpenRouter
passthrough today; GMI/Vertex once it is back online). The native Messages
wire is the better transport but is only safe where the upstream keeps
prompt-cache routing sticky: measured false on the OpenRouter path (14-20% of
consecutive calls re-write the previous turn; #104284 moved the default to
chat), untested on GMI. Hermes cannot see the upstream in the request, only
in the response: OpenRouter stamps `provider` (chat wire) and mints
`gen-<unix>-<rand>` ids; GMI/Vertex returns Anthropic-native `msg_...` ids
and no provider.
`auto` therefore starts every session on chat (correct on both upstreams),
classifies the first response, and switches that session to native only
when the upstream is GMI AND `agent/nous_wire.py::GMI_NATIVE_WIRE_CLEARED`
is True. The switch is scheduled at response time and applied at the start
of the next iteration (turn_iteration_prep), so nothing is rebuilt while a
response is being consumed; it goes through switch_model so the client,
cache policy and _primary_runtime stay consistent. One decision per session,
call 1 only; unknown upstream never switches; a failed switch logs and stays.
GMI_NATIVE_WIRE_CLEARED is False: until the 20x6 concurrency probe
(evals/postmortem/live_ab) is clean on a GMI-served anthropic/* id on the
native wire, `auto` behaves exactly like `chat`. Flipping it is the whole
rollout once GMI is measured. Default stays `chat`.
Tests (17): classifier on real Portal response shapes from both wires and
both upstreams; chat for openrouter/unknown, GMI gated on the flag; one
decision per session, call 1 only, explicit chat/native never auto-switch,
other providers/models untouched, switch failure swallowed and final;
record_response_usage on a real AIAgent invokes the hook once.
Live (auto, real Portal, Fable 5.1): arm A, real classification
(OpenRouter today) - stays on chat through a tool loop and a second turn,
cache 97-99%. Arm B, classifier forced to gmi with the flag on - call 1 on
chat, switch applied before call 2, calls 2-3 on the native wire in the
same session, tool result and both turns correct, cache 97-99%. An earlier
shape that switched inside the response path broke call 1 (SimpleNamespace
has no .content); the scheduled apply is why.
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