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Salvage of #79604 (webtecnica) + #85721 (pierrenode), combined and rebased onto current main with simplify-code findings folded in. #79604: update_session_model() wrote the model name to sessions.model but never persisted the provider into model_config. On resume, the runtime recombined the persisted model with the config.yaml primary provider (which may not serve that model), producing auth errors. Fix: add optional provider parameter to update_session_model, merged into model_config via the shared _merge_model_config_json helper (not hand-rolled SQL). Wire both gateway /model call sites to pass result.target_provider. #85721: session_gateway_runtime() had no billing_provider fallback. A CLI session that never ran /model has no gateway_runtime or top-level provider in model_config — billing_provider (written on every session's first accounted API call) is the only durable record. Fix: add billing_provider as the last-resort fallback in session_gateway_runtime(), filtering bare billing buckets (auto/custom) that are not routable identities. Simplify-code findings addressed: - Use _merge_model_config_json instead of 40 lines of branched SQL - Share _BARE_BILLING_PROVIDERS from hermes_state.py (was duplicated as a set in tui_gateway/server.py) - Merge None-filtering from #85920 with the billing_provider fallback into one coherent return path Co-authored-by: pierrenode <298902573+pierrenode@users.noreply.github.com>