studio: gate Anthropic top_k drop to Claude 4.7 only

Previous commit (b5aa6ffd) dropped top_k for every Anthropic call,
but only Claude 4.7 (Opus/Sonnet/Haiku) actually rejects it. 4.6, 4.5,
and the 3.x line still accept top_k and use it as documented.

Backend: _stream_anthropic matches the model id against
^claude-(opus|sonnet|haiku)-4-7(-|.|$) and only strips top_k when it
hits. Every other Claude generation continues to receive the value
from the chat settings panel.

Frontend: anthropic.topK is restored to true so the Top K slider is
visible again — the backend handles the per-model drop, and the
4.7 case is silent (request still succeeds without top_k).

studio/backend/core/inference/external_provider.py

@@ -9,12 +9,19 @@ Anthropic uses native Messages API with translation in this client.
 """
 
 import logging
+import re
 from typing import Any, AsyncGenerator, Optional
 
 import httpx
 
 logger = logging.getLogger(__name__)
 
+# Claude 4.7 (Opus/Sonnet/Haiku) deprecated top_k and returns 400
+# "top_k is deprecated for this model" when it is set. 3.x and 4.5/4.6
+# still accept it. Match the 4-7 line specifically so we keep the knob
+# live on every other Claude generation.
+_ANTHROPIC_TOP_K_DEPRECATED = re.compile(r"^claude-(?:opus|sonnet|haiku)-4-7(?:[-.]|$)")
+
 # Shared client reused across all requests for HTTP connection pooling.
 # Auth headers and timeouts are passed per-request, so a single client
 # handles every provider without storing credentials.
@@ -276,13 +283,15 @@ class ExternalProviderClient:
             # Anthropic rejects requests that set both temperature and top_p
             "stream": True,
         }
-        # 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
+        # top_k is deprecated on Claude 4.7 (Opus/Sonnet/Haiku) — the API
+        # returns 400 "top_k is deprecated for this model" when it is set.
+        # 3.x and 4.5/4.6 still accept it, so gate strictly on the 4.7 ids.
+        if (
+            top_k is not None
+            and top_k > 0
+            and not _ANTHROPIC_TOP_K_DEPRECATED.match(model)
+        ):
+            body["top_k"] = top_k
         if system:
             body["system"] = system
 

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

@@ -75,14 +75,17 @@ const PROVIDER_CAPABILITIES: Record<string, ProviderCapabilities> = {
     repetitionPenalty: false,
     presencePenalty: false,
   },
-  // 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's Messages API accepts top_k on 3.x and 4.5/4.6, but Claude
+  // 4.7 (Opus/Sonnet/Haiku) deprecated it and returns 400 if it is set.
+  // We surface top_k in the panel for all Anthropic providers and let the
+  // backend strip it per-model — see _stream_anthropic in
+  // studio/backend/core/inference/external_provider.py.
+  // Presence/frequency penalty is not part of the Messages API on any
+  // Claude generation.
   anthropic: {
     temperature: true,
     topP: true,
-    topK: false,
+    topK: true,
     minP: false,
     repetitionPenalty: false,
     presencePenalty: false,