Portability is an interface decision
Calling two model APIs does not create sovereignty. Provider-specific prompts, tool schemas, safety filters, embeddings and evaluation assumptions can still make migration expensive. Separate the task contract from the provider adapter and keep business rules in the control layer.
Route against an evaluation contract
Define quality, latency, cost, data location, retention and failure thresholds per task. A router chooses among approved deployments only when each candidate has passed the task’s evaluation set. Record the chosen route and reason for material requests.
Plan degradation
Fallback is not always another model. A safe path may reduce context, switch to retrieval-only output, require human review or stop the workflow. Define these modes before an outage and test them with unavailable providers and quota limits.
Exercise the exit path
At least periodically, replay representative traffic against the alternative provider or private deployment. Measure regressions and list provider-specific features that remain. An untested exit plan is only a procurement statement.