diff --git a/api/config.py b/api/config.py
index 0839634..55df1af 100644
--- a/api/config.py
+++ b/api/config.py
@@ -637,7 +637,10 @@ def resolve_model_provider(model_id: str) -> tuple:
         # just because the model name contains a slash (e.g. google/gemma-4-26b-a4b).
         # The user has explicitly pointed at a base_url, so trust their routing config.
         if config_base_url:
-            return model_id, config_provider, config_base_url
+            # Strip provider prefix (e.g. 'openai/gpt-5.4' -> 'gpt-5.4') so prefixed
+            # model IDs from previous sessions don't break custom endpoint routing.
+            bare_model = model_id.split('/', 1)[-1]
+            return bare_model, config_provider, config_base_url
         # If prefix does NOT match config provider, the user picked a cross-provider model
         # from the OpenRouter dropdown (e.g. config=anthropic but picked openai/gpt-5.4-mini).
         # In this case always route through openrouter with the full provider/model string.
diff --git a/tests/test_model_resolver.py b/tests/test_model_resolver.py
index 6176e84..c0a2b4e 100644
--- a/tests/test_model_resolver.py
+++ b/tests/test_model_resolver.py
@@ -403,8 +403,10 @@ def test_custom_endpoint_slash_model_routes_to_custom_not_openrouter():
     assert base_url == 'http://127.0.0.1:1234/v1', (
         "Expected base_url 'http://127.0.0.1:1234/v1', got '{}'.".format(base_url)
     )
-    assert model == 'google/gemma-4-26b-a4b', (
-        "Model name should be preserved as-is, got '{}'.".format(model)
+    # Fix #433: provider prefix is now stripped for custom endpoints so stale
+    # prefixed model IDs from previous sessions do not break custom endpoint routing.
+    assert model == 'gemma-4-26b-a4b', (
+        "Model name prefix should be stripped for custom base_url endpoint, got '{}'.".format(model)
     )
 
     # --- openrouter with slash model name MUST still route to openrouter -----