diff --git a/api/routes.py b/api/routes.py
index 455d5bed..c7f287ac 100644
--- a/api/routes.py
+++ b/api/routes.py
@@ -1512,7 +1512,12 @@ def _resolve_compatible_session_model_state(
         # qualifier — qualified strings require the catalog to decide whether
         # the qualifier matches the active provider (see slow path below).
         bare_model, explicit_provider = _split_provider_qualified_model(model)
-        if not explicit_provider:
+        model_prefix = model.split("/", 1)[0].strip().lower() if "/" in model else ""
+        stale_codex_openai_slash_id = (
+            requested_provider == "openai-codex"
+            and model_prefix == "openai"
+        )
+        if not explicit_provider and not stale_codex_openai_slash_id:
             return model, requested_provider, False
 
     catalog = get_available_models()
@@ -1533,7 +1538,14 @@ def _resolve_compatible_session_model_state(
 
     bare_for_context, explicit_provider = _split_provider_qualified_model(model)
     if requested_provider and not explicit_provider:
-        return model, requested_provider, False
+        model_prefix = model.split("/", 1)[0].strip().lower() if "/" in model else ""
+        stale_codex_openai_slash_id = (
+            raw_active_provider == "openai-codex"
+            and requested_provider == "openai-codex"
+            and model_prefix == "openai"
+        )
+        if not stale_codex_openai_slash_id:
+            return model, requested_provider, False
 
     if model.startswith("@") and ":" in model:
         provider_raw = explicit_provider or ""
@@ -1643,7 +1655,7 @@ def _resolve_compatible_session_model_state(
     if (
         raw_active_provider == "openai-codex"
         and model_provider == "openai"
-        and requested_provider is None
+        and requested_provider in {None, "openai-codex"}
         and default_model
     ):
         # Persist provider_context = "openai-codex" unconditionally on this
diff --git a/tests/test_issue1855_resolve_model_provider_fast_path.py b/tests/test_issue1855_resolve_model_provider_fast_path.py
index a7aa86f1..0cbb321d 100644
--- a/tests/test_issue1855_resolve_model_provider_fast_path.py
+++ b/tests/test_issue1855_resolve_model_provider_fast_path.py
@@ -61,13 +61,12 @@ class TestFastPathInvocation:
         )
         assert result == ("gpt-5.5", "openai-codex", False)
 
-    def test_fast_path_with_slash_qualified_model_skips_catalog(self):
-        """Slash-qualified IDs (openrouter/...) still hit the fast path.
+    def test_fast_path_with_openrouter_slash_qualified_model_skips_catalog(self):
+        """OpenRouter slash-qualified IDs still hit the fast path.
 
-        The fast path only excludes @provider:model strings, not slash-
-        qualified ones — those are valid model IDs that the picker emits
-        for OpenRouter and custom-provider routing, and a stored
-        model_provider is the authoritative routing decision.
+        Slash-qualified IDs are valid picker output for OpenRouter and a stored
+        model_provider is the authoritative routing decision. This remains fast
+        for explicit OpenRouter selections.
         """
         from api.routes import _resolve_compatible_session_model_state
 
@@ -80,6 +79,33 @@ class TestFastPathInvocation:
         assert mock_catalog.call_count == 0
         assert result == ("anthropic/claude-opus-4.7", "openrouter", False)
 
+    def test_codex_with_stale_openai_slash_id_uses_catalog_repair(self):
+        """Codex must repair stale OpenRouter-shaped OpenAI IDs.
+
+        Browser/localStorage state can submit ``openai/gpt-...`` while the
+        session/provider is ``openai-codex``. If the fast path preserves that
+        pair, runtime resolution routes through OpenRouter instead of Codex.
+        The Codex + ``openai/...`` shape must therefore use the slow-path repair
+        and normalize back to the active Codex default.
+        """
+        from api.routes import _resolve_compatible_session_model_state
+
+        with patch("api.routes.get_available_models") as mock_catalog:
+            mock_catalog.return_value = {
+                "active_provider": "openai-codex",
+                "default_model": "gpt-5.5",
+                "groups": [
+                    {"provider_id": "openai-codex", "models": [{"id": "gpt-5.5"}]}
+                ],
+            }
+            result = _resolve_compatible_session_model_state(
+                "openai/gpt-5.4-mini",
+                "openai-codex",
+            )
+
+        assert mock_catalog.call_count == 1
+        assert result == ("gpt-5.5", "openai-codex", True)
+
     def test_fast_path_normalizes_provider_default_alias(self):
         """`'default'` is treated as None by _clean_session_model_provider.