studio: stop forwarding top_k to Anthropic

Claude 4.x (Opus / Sonnet / Haiku 4.x) returns 400 'top_k is
deprecated for this model' on any request that includes top_k. It
was always optional on the older 3.x line, so dropping it
unconditionally for every Anthropic call is the simplest path —
no per-model gate to maintain.

- external_provider._stream_anthropic no longer adds top_k to the
  Messages body (kept on the method signature for API symmetry).
- provider-capabilities sets anthropic.topK = false so the chat
  settings panel hides the Top K slider for Anthropic providers
  and chat-adapter does not send top_k in the external request.

studio/backend/core/inference/external_provider.py

@@ -276,8 +276,13 @@ 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
+        # top_k is deprecated on Claude 4.x (Opus / Sonnet / Haiku 4.x return
+        # 400 with `top_k is deprecated for this model`). It was always
+        # optional on the older 3.x line too, so we just stop forwarding it
+        # for every Anthropic call rather than maintaining a per-model gate.
+        # ``top_k`` is still accepted on the method signature for API
+        # symmetry with the other stream methods.
+        del top_k
         if system:
             body["system"] = system
 

studio/frontend/src/features/chat/provider-capabilities.ts

@@ -75,11 +75,14 @@ const PROVIDER_CAPABILITIES: Record<string, ProviderCapabilities> = {
     repetitionPenalty: false,
     presencePenalty: false,
   },
-  // Anthropic's Messages API accepts top_k but not presence/frequency penalty.
+  // Anthropic's Messages API rejects presence/frequency penalty, and top_k
+  // is now deprecated across the Claude 4.x line (Opus / Sonnet / Haiku 4.x
+  // 400 with "top_k is deprecated for this model"). It was always optional
+  // on the older 3.x line, so we just drop it for every Anthropic call.
   anthropic: {
     temperature: true,
     topP: true,
-    topK: true,
+    topK: false,
     minP: false,
     repetitionPenalty: false,
     presencePenalty: false,