From 8c7475aafee57124b419422ae935bac0c49bafab Mon Sep 17 00:00:00 2001
From: "rosetta-livekit-bot[bot]"
 <282703043+rosetta-livekit-bot[bot]@users.noreply.github.com>
Date: Fri, 26 Jun 2026 12:57:25 +0000
Subject: [PATCH] fix(anthropic): add non-whitespace trailing dummy

---
 plugins/anthropic/src/llm.ts | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/plugins/anthropic/src/llm.ts b/plugins/anthropic/src/llm.ts
index 4f963d462..33fdf6c57 100644
--- a/plugins/anthropic/src/llm.ts
+++ b/plugins/anthropic/src/llm.ts
@@ -101,6 +101,8 @@ export class LLM extends llm.LLM {
    *   strict alternating-turn requirement.
    * - A dummy `(empty)` user message is injected if the conversation doesn't
    *   start with a user turn.
+   * - A dummy user message is appended if the conversation ends with an
+   *   assistant turn, since Claude 4.6+ does not support prefilling.
    */
   protected _buildAnthropicContext(chatCtx: llm.ChatContext): {
     system: Anthropic.TextBlockParam[];
@@ -172,6 +174,11 @@ export class LLM extends llm.LLM {
       messages.unshift({ role: 'user', content: '(empty)' });
     }
 
+    // Claude 4.6+ does not support prefilling (trailing assistant messages).
+    if (messages.length > 0 && messages[messages.length - 1]!.role === 'assistant') {
+      messages.push({ role: 'user', content: [{ type: 'text', text: '.' }] });
+    }
+
     return { system, messages };
   }