diff --git a/gateway/slash_commands.py b/gateway/slash_commands.py
index eb19f61bd0c52..52d8fc78b3e1d 100644
--- a/gateway/slash_commands.py
+++ b/gateway/slash_commands.py
@@ -1937,7 +1937,8 @@ async def _on_model_selected_scoped(
                                     event.source
                                 )
                                 await _sess_db.update_session_model(
-                                    _sess_entry.session_id, result.new_model
+                                    _sess_entry.session_id, result.new_model,
+                                    provider=result.target_provider,
                                 )
                             except Exception as exc:
                                 logger.debug(
@@ -2247,7 +2248,8 @@ async def _finish_switch() -> str:
                     if getattr(_sess_entry, "was_auto_reset", False):
                         _sess_entry.was_auto_reset = False
                     await _sess_db.update_session_model(
-                        _sess_entry.session_id, result.new_model
+                        _sess_entry.session_id, result.new_model,
+                        provider=result.target_provider,
                     )
                 except Exception as exc:
                     logger.debug(
diff --git a/hermes_state.py b/hermes_state.py
index 0188e5d59bd12..1f44cfc184f5d 100644
--- a/hermes_state.py
+++ b/hermes_state.py
@@ -255,6 +255,15 @@ def _delegate_from_json(col: str = "model_config") -> str:
 # None result ("merged config is empty → store NULL").
 _MODEL_CONFIG_ROW_MISSING = object()
 
+# Billing-bucket classes that aren't a routable provider identity on their
+# own — used by session_gateway_runtime's billing_provider fallback and by
+# tui_gateway.server._stored_session_runtime_overrides. A session that
+# persisted only one of these (never ran /model) must fall back to the
+# ambient config default rather than restore a bare bucket. Shared here so
+# both consumers stay in sync (previously duplicated as a set in
+# tui_gateway/server.py).
+_BARE_BILLING_PROVIDERS = frozenset({"auto", "custom"})
+
 
 def _cwd_prefix_clause(cwd_prefix: str) -> Tuple[str, List[str]]:
     prefix = cwd_prefix.rstrip("/\\") or cwd_prefix
@@ -5893,7 +5902,9 @@ def _do(conn):
             self._delete_unreferenced_system_prompts(conn)
         self._execute_write(_do)
 
-    def update_session_model(self, session_id: str, model: str) -> None:
+    def update_session_model(
+        self, session_id: str, model: str, provider: Optional[str] = None
+    ) -> None:
         """Update the model for a session after a mid-session switch.
 
         Unlike ``update_token_counts`` which uses ``COALESCE(model, ?)``
@@ -5903,6 +5914,13 @@ def update_session_model(self, session_id: str, model: str) -> None:
         footer metadata is rebuilt on the next turn. A successful /model
         switch explicitly replaces any confirmed Browser runtime lock while
         preserving unrelated lineage markers in ``model_config``.
+
+        When *provider* is given, it is merged into ``model_config``
+        alongside the model (``$.model`` / ``$.provider``) so a later
+        resume recombines the persisted model with the provider that
+        actually serves it instead of the config.yaml primary provider
+        (#79536). Callers without provider knowledge leave any stored
+        provider untouched.
         """
         # This write bypasses the token queue, so deltas enqueued before the
         # switch must land first: a still-queued first delta carries the
@@ -5913,19 +5931,24 @@ def update_session_model(self, session_id: str, model: str) -> None:
         self.flush_token_counts()
 
         def _do(conn):
+            # Use the shared merge discipline so lineage markers like
+            # _branched_from / _delegate_from survive. browser_model_lock
+            # is deleted via a None patch value (same semantics as the
+            # old json_remove).
+            patch: Dict[str, Any] = {"browser_model_lock": None}
+            if model:
+                patch["model"] = model
+            if provider:
+                patch["provider"] = provider
+            merged = self._merge_model_config_json(conn, session_id, patch)
+            if merged is _MODEL_CONFIG_ROW_MISSING:
+                return
             conn.execute(
-                """UPDATE sessions SET
-                   model = ?,
-                   model_config = CASE
-                       WHEN model_config IS NULL THEN NULL
-                       WHEN json_valid(model_config)
-                           THEN json_remove(model_config, '$.browser_model_lock')
-                       ELSE model_config
-                   END,
-                   system_prompt = NULL,
-                   system_prompt_hash = NULL
-                   WHERE id = ?""",
-                (model, session_id),
+                "UPDATE sessions SET "
+                "model = ?, model_config = ?, "
+                "system_prompt = NULL, system_prompt_hash = NULL "
+                "WHERE id = ?",
+                (model, merged, session_id),
             )
             self._delete_unreferenced_system_prompts(conn)
         self._execute_write(_do)
@@ -6118,18 +6141,21 @@ def session_gateway_runtime(session_meta: Optional[Dict[str, Any]]) -> Dict[str,
         ``gateway_runtime`` key (written by the gateway's
         ``_sync_session_model_from_agent`` and the CLI ``/model`` persist),
         falling back to the top-level ``provider``/``base_url``/``api_mode``
-        keys the TUI gateway's ``_runtime_model_config`` writes. Returns an
-        empty dict on any parse failure — resume falls back to ambient
-        config resolution.
+        keys the TUI gateway's ``_runtime_model_config`` writes. As a last
+        resort, falls back to the ``billing_provider`` column (written on
+        every session's first accounted API call) so sessions that never ran
+        ``/model`` still restore the provider that actually served them.
+        Returns an empty dict on any parse failure — resume falls back to
+        ambient config resolution.
         """
         raw = (session_meta or {}).get("model_config")
         if isinstance(raw, str):
             try:
                 raw = json.loads(raw)
             except Exception:
-                return {}
+                raw = {}
         if not isinstance(raw, dict):
-            return {}
+            raw = {}
         runtime = raw.get("gateway_runtime")
         if isinstance(runtime, dict) and runtime.get("provider"):
             # Filter None values: the persist path writes or-None to trigger
@@ -6143,7 +6169,21 @@ def session_gateway_runtime(session_meta: Optional[Dict[str, Any]]) -> Dict[str,
         }
         if top_level:
             return top_level
-        return dict(runtime) if isinstance(runtime, dict) else {}
+        # Last resort: billing_provider column. Written via COALESCE on every
+        # session's first accounted API call — the only durable record for
+        # sessions that never ran /model. Mirrors the TUI gateway's
+        # _stored_session_runtime_overrides fallback. Bare billing buckets
+        # ("auto"/"custom") are not routable identities — filter them out so
+        # resume falls back to the ambient config default instead.
+        billing_provider = str(
+            (session_meta or {}).get("billing_provider") or ""
+        ).strip()
+        if (
+            billing_provider
+            and billing_provider.lower() not in _BARE_BILLING_PROVIDERS
+        ):
+            return {"provider": billing_provider}
+        return {k: v for k, v in (runtime or {}).items() if v is not None} if isinstance(runtime, dict) else {}
 
     def update_session_billing_route(
         self,
diff --git a/tests/cli/test_resume_model_restore.py b/tests/cli/test_resume_model_restore.py
index 766490c54d88e..aa01462527e7d 100644
--- a/tests/cli/test_resume_model_restore.py
+++ b/tests/cli/test_resume_model_restore.py
@@ -278,3 +278,90 @@ def test_round_trip_persist_then_restore(tmp_path, monkeypatch):
     assert restored.model == "deepseek-v4-flash-free"
     assert restored.provider == "custom:opencode-zen"
     assert restored.base_url == "https://oz/v1"
+
+
+# ── update_session_model provider persistence (#79536) ──────────────
+
+
+def test_update_session_model_persists_provider(tmp_path, monkeypatch):
+    """update_session_model writes $.model + $.provider into model_config."""
+    monkeypatch.setenv("HERMES_HOME", str(tmp_path / ".hermes"))
+    db = SessionDB(db_path=tmp_path / "state.db")
+    db.create_session(session_id="s1", source="cli", model="m0")
+    db.update_session_model("s1", "claude-x", provider="custom:feather")
+    meta = db.get_session("s1")
+    assert meta["model"] == "claude-x"
+    config = json.loads(meta["model_config"])
+    assert config["model"] == "claude-x"
+    assert config["provider"] == "custom:feather"
+
+
+def test_update_session_model_without_provider_preserves_existing(tmp_path, monkeypatch):
+    """Without provider, existing $.provider is left untouched."""
+    monkeypatch.setenv("HERMES_HOME", str(tmp_path / ".hermes"))
+    db = SessionDB(db_path=tmp_path / "state.db")
+    db.create_session(session_id="s2", source="cli", model="m0")
+    db.update_session_model("s2", "claude-x", provider="custom:feather")
+    db.update_session_model("s2", "gpt-5.4")  # no provider
+    meta = db.get_session("s2")
+    config = json.loads(meta["model_config"])
+    assert config["model"] == "gpt-5.4"
+    assert config["provider"] == "custom:feather"  # preserved
+
+
+def test_update_session_model_null_model_config_with_provider(tmp_path, monkeypatch):
+    """Provider persistence works when model_config starts as NULL."""
+    monkeypatch.setenv("HERMES_HOME", str(tmp_path / ".hermes"))
+    db = SessionDB(db_path=tmp_path / "state.db")
+    db.create_session(session_id="s3", source="cli", model="m0")
+    # model_config is NULL at creation — update_session_model must create it
+    db.update_session_model("s3", "claude-x", provider="minimax")
+    meta = db.get_session("s3")
+    config = json.loads(meta["model_config"])
+    assert config["model"] == "claude-x"
+    assert config["provider"] == "minimax"
+
+
+# ── session_gateway_runtime billing_provider fallback (#85721) ─────
+
+
+def test_session_gateway_runtime_falls_back_to_billing_provider():
+    """Sessions that never ran /model have only billing_provider."""
+    meta = {
+        "model": "glm-4.7",
+        "model_config": None,
+        "billing_provider": "minimax",
+    }
+    runtime = SessionDB.session_gateway_runtime(meta)
+    assert runtime == {"provider": "minimax"}
+
+
+def test_session_gateway_runtime_billing_provider_bare_bucket_ignored():
+    """Bare billing buckets (auto/custom) are not routable — skip them."""
+    for bare in ("auto", "custom"):
+        meta = {
+            "model": "m",
+            "model_config": None,
+            "billing_provider": bare,
+        }
+        assert SessionDB.session_gateway_runtime(meta) == {}
+
+
+def test_session_gateway_runtime_explicit_provider_wins_over_billing():
+    """Explicit model_config provider takes precedence over billing_provider."""
+    meta = _row(model_config={"provider": "nous"})
+    meta["billing_provider"] = "minimax"
+    runtime = SessionDB.session_gateway_runtime(meta)
+    assert runtime == {"provider": "nous"}
+
+
+def test_restore_session_model_restores_billing_provider_fallback():
+    """End-to-end: _restore_session_model uses billing_provider fallback."""
+    stub = _make_stub()
+    stub._restore_session_model({
+        "model": "glm-4.7",
+        "model_config": None,
+        "billing_provider": "minimax",
+    })
+    assert stub.model == "glm-4.7"
+    assert stub.provider == "minimax"
diff --git a/tui_gateway/server.py b/tui_gateway/server.py
index 04b3c956e090f..d1b58dd185500 100644
--- a/tui_gateway/server.py
+++ b/tui_gateway/server.py
@@ -3775,7 +3775,7 @@ def _resolve_startup_runtime() -> tuple[str, str | None]:
 # ``billing_provider="openrouter"``; dropping it forces resume to the current
 # global model (e.g. a custom endpoint), which is the wrong provider for the
 # stored model. See #57588.
-_BARE_BILLING_PROVIDERS = {"auto", "custom"}
+from hermes_state import _BARE_BILLING_PROVIDERS
 
 
 def _stored_session_runtime_overrides(row: dict | None) -> dict: