diff --git a/studio/backend/core/inference/external_provider.py b/studio/backend/core/inference/external_provider.py
index f18075a6a3..04f9b15c74 100644
--- a/studio/backend/core/inference/external_provider.py
+++ b/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
 
diff --git a/studio/frontend/src/features/chat/provider-capabilities.ts b/studio/frontend/src/features/chat/provider-capabilities.ts
index fd96aa3d6b..b02ec4eaaa 100644
--- a/studio/frontend/src/features/chat/provider-capabilities.ts
+++ b/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,