diff --git a/tests/test_tui_gateway_server.py b/tests/test_tui_gateway_server.py
index 1407c9586fb6a..8799be8afefc0 100644
--- a/tests/test_tui_gateway_server.py
+++ b/tests/test_tui_gateway_server.py
@@ -3125,10 +3125,11 @@ def update_session_cwd(self, *_args):
 
 
 def test_stored_session_runtime_overrides_skips_bare_billing_provider():
-    """A bare billing bucket ("custom"/"auto"/"openrouter") must not be restored as the
-    provider identity on resume. A custom endpoint that never used `/model` persists only
+    """A bare billing bucket ("custom"/"auto") must not be restored as the provider
+    identity on resume. A custom endpoint that never used `/model` persists only
     `billing_provider="custom"`; restoring that broke `session.resume` with "No LLM provider
-    configured" (agent_init treats it as non-routable). A real provider, or an explicit
+    configured" (agent_init treats it as non-routable). ``"openrouter"`` is NOT a bare bucket
+    — it is a fully routable provider; see #57588. A real provider, or an explicit
     `model_config.provider`, is still restored.
     """
     # Bare "custom" bucket, no explicit model_config.provider: no provider override restored.
@@ -3136,7 +3137,7 @@ def test_stored_session_runtime_overrides_skips_bare_billing_provider():
     assert "provider_override" not in ov
     assert ov["model_override"]["provider"] is None
 
-    for bare in ("auto", "openrouter", "custom"):
+    for bare in ("auto", "custom"):
         ov = server._stored_session_runtime_overrides({"model": "m", "billing_provider": bare})
         assert "provider_override" not in ov
 
@@ -3165,6 +3166,33 @@ def test_stored_session_runtime_overrides_restores_explicit_normal_tier():
     assert overrides["service_tier_override"] == ""
 
 
+def test_openrouter_session_resume_restores_provider():
+    """OpenRouter is a fully routable provider — sessions that used OpenRouter must
+    restore the "openrouter" provider override on resume, not fall through to whatever
+    the current global model is.  (#57588)
+    """
+    # OpenRouter session with no explicit model_config.provider (the common case
+    # for sessions that never used /model): billing_provider="openrouter" should
+    # be restored as the provider override.
+    ov = server._stored_session_runtime_overrides(
+        {"model": "anthropic/claude-opus-4.8", "billing_provider": "openrouter"}
+    )
+    assert ov["provider_override"] == "openrouter"
+    assert ov["model_override"]["provider"] == "openrouter"
+    assert ov["model_override"]["model"] == "anthropic/claude-opus-4.8"
+
+    # When an explicit model_config.provider exists, it takes precedence over
+    # billing_provider (this path was already correct).
+    ov = server._stored_session_runtime_overrides(
+        {
+            "model": "anthropic/claude-opus-4.8",
+            "billing_provider": "openrouter",
+            "model_config": {"provider": "openrouter", "base_url": "https://openrouter.ai/api/v1"},
+        }
+    )
+    assert ov["provider_override"] == "openrouter"
+
+
 def test_persist_live_session_runtime_preserves_resume_metadata(monkeypatch):
     updates = {}
 
diff --git a/tui_gateway/server.py b/tui_gateway/server.py
index 9c00bc2062eca..eb7e388cbde5c 100644
--- a/tui_gateway/server.py
+++ b/tui_gateway/server.py
@@ -3766,9 +3766,16 @@ def _resolve_startup_runtime() -> tuple[str, str | None]:
     return model, None
 
 
-# Bare billing buckets are not routable provider identities (kept in parity with the
-# provider gate in agent_init). Restoring one as a session provider override breaks resume.
-_BARE_BILLING_PROVIDERS = {"auto", "openrouter", "custom"}
+# Bare billing buckets are not routable provider identities; restoring one as a
+# session provider override breaks resume. (agent_init's fail-fast gate is a
+# DIFFERENT set that also skips "openrouter" — there it means "default route,
+# don't fail fast", not "unroutable".)
+# ``openrouter`` is deliberately excluded here — it is a fully routable provider
+# with its own API key and base_url. Sessions that used OpenRouter store
+# ``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"}
 
 
 def _stored_session_runtime_overrides(row: dict | None) -> dict: