fix: ACP 模式下 extended thinking + tool_use 触发连续 user 消息导致 400 (CC-1215)

src/utils/messages.ts

@@ -5829,11 +5829,15 @@ export function ensureToolResultPairing(
         )
       } else {
         // Content is empty after stripping orphaned tool_results. We still
-        // need a user message here to maintain role alternation — otherwise
-        // the assistant placeholder we just pushed would be immediately
-        // followed by the NEXT assistant message, which the API rejects with
-        // a role-alternation 400 (not the duplicate-id 400 we handle).
+        // need a user message here to maintain role alternation — unless the
+        // previous result entry is already a user message, in which case
+        // inserting another user placeholder creates consecutive-user messages
+        // that Anthropic rejects with a misleading "tool_use without
+        // tool_result" 400 (CC-1215).
         i++
+        if (result.at(-1)?.type === 'user') {
+          continue
+        }
         result.push(
           createUserMessage({
             content: NO_CONTENT_MESSAGE,