studio/backend: forward top_k to Anthropic; filter OpenAI model list

Two paired changes so the frontend capability map has matching backend
behaviour:

1. ExternalProviderClient.stream_chat_completion now accepts top_k and
   forwards it to the Anthropic Messages body. OpenAI-compat providers
   (which all reject unknown sampling params) still receive only the
   fields they document. The proxy route in routes/inference.py passes
   payload.top_k through, so a UI request with top_k actually reaches
   Anthropic instead of being silently dropped at the boundary.

2. PROVIDER_REGISTRY['openai'] gains a model_id_allowlist regex that
   scopes the /models picker to current-gen ids (gpt-5.5 / gpt-5.4 /
   gpt-5.3 / gpt-4.5 / o3 families). The remote /v1/models listing
   otherwise returns dozens of historical snapshots, fine-tunes and
   non-chat models (embeddings, TTS, image, moderation) that we never
   want in the chat UI. default_models is refreshed to match.

studio/backend/core/inference/external_provider.py

@@ -67,6 +67,7 @@ class ExternalProviderClient:
         top_p: float = 0.95,
         max_tokens: Optional[int] = None,
         presence_penalty: float = 0.0,
+        top_k: Optional[int] = None,
         stream: bool = True,
     ) -> AsyncGenerator[str, None]:
         """
@@ -74,10 +75,15 @@ class ExternalProviderClient:
 
         For OpenAI-compatible providers, lines are forwarded verbatim.
         For Anthropic, the native Messages API SSE is translated to OpenAI format.
+
+        ``top_k`` and ``presence_penalty`` are forwarded only when the caller
+        supplies a value the provider accepts — the frontend's
+        provider-capability map already filters these per provider, so we
+        treat them as opt-in here.
         """
         if not self._is_openai_compatible():
             async for line in self._stream_anthropic(
-                messages, model, temperature, top_p, max_tokens
+                messages, model, temperature, top_p, max_tokens, top_k
             ):
                 yield line
             return
@@ -180,6 +186,7 @@ class ExternalProviderClient:
         temperature: float,
         top_p: float,
         max_tokens: Optional[int],
+        top_k: Optional[int] = None,
     ) -> AsyncGenerator[str, None]:
         """
         Call the Anthropic Messages API and translate its SSE to OpenAI format.
@@ -257,6 +264,8 @@ class ExternalProviderClient:
             # Anthropic rejects requests that set both temperature and top_p
             "stream": True,
         }
+        if top_k is not None and top_k > 0:
+            body["top_k"] = top_k
         if system:
             body["system"] = system
 

studio/backend/core/inference/providers.py

@@ -8,6 +8,7 @@ All providers expose OpenAI-compatible /v1/chat/completions endpoints
 with Bearer token authentication and SSE streaming support.
 """
 
+import re
 from typing import Any
 
 PROVIDER_REGISTRY: dict[str, dict[str, Any]] = {
@@ -15,18 +16,23 @@ PROVIDER_REGISTRY: dict[str, dict[str, Any]] = {
         "display_name": "OpenAI",
         "base_url": "https://api.openai.com/v1",
         "default_models": [
-            "gpt-4o",
-            "gpt-4o-mini",
-            "gpt-4.1",
-            "gpt-4.1-mini",
-            "gpt-4.1-nano",
-            "o3-mini",
+            "gpt-5.5",
+            "gpt-5.4",
+            "gpt-5.4-mini",
+            "gpt-5.3",
+            "o3",
         ],
         "supports_streaming": True,
         "supports_vision": True,
         "supports_tool_calling": True,
         "auth_header": "Authorization",
         "auth_prefix": "Bearer ",
+        # Keep the model picker scoped to the current generation. The remote
+        # /v1/models listing returns dozens of historical snapshots, fine-tunes
+        # and non-chat models (embeddings, TTS, image, moderation) that we
+        # never want to surface in the chat UI. Filtering here so backend
+        # is the single source of truth.
+        "model_id_allowlist": re.compile(r"^(gpt-5\.[345]|gpt-4\.5|o3)(?:[-.]|$)"),
     },
     "anthropic": {
         "display_name": "Anthropic",

studio/backend/routes/inference.py

@@ -1592,6 +1592,7 @@ async def _proxy_to_external_provider(
             top_p = payload.top_p,
             max_tokens = payload.max_tokens,
             presence_penalty = payload.presence_penalty,
+            top_k = payload.top_k,
             stream = payload.stream,
         )
         try:

studio/backend/routes/providers.py

@@ -281,6 +281,9 @@ async def list_provider_models(
 
     try:
         models = await client.list_models()
+        allowlist = info.get("model_id_allowlist")
+        if allowlist is not None:
+            models = [m for m in models if allowlist.match(m.get("id", ""))]
         return [
             ProviderModelInfo(
                 id = m.get("id", ""),