fix: 改进流式 write_file 沙箱预览与 Mermaid 旧浏览器兼容

同轮次非末尾 write_file 也能自动打开沙箱并跳过 JSON 大文件预览；Mermaid 改用 SVG 标签；agentfx 强制先写 md 再写 json。

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend-web/src/components/workspace/chats/chat-box.tsx

@@ -18,6 +18,7 @@ import {
   listThreadArtifacts,
   mergeArtifactPaths,
 } from "@/core/artifacts/listing";
+import { latestInFlightWriteFilePreview } from "@/core/artifacts/write-file-in-flight";
 import { getBackendBaseURL } from "@/core/config";
 import { isThreadRagModeLocked, useLocalSettings } from "@/core/settings";
 import { isSingleRoundRagMessage } from "@/core/threads/use-single-round-rag";
@@ -174,6 +175,7 @@ const ChatBox: React.FC<{
     deselect,
     selectedArtifact,
     selectedArtifactKey,
+    openStreamedArtifact,
   } = useArtifacts();
 
   const [autoSelectFirstArtifact, setAutoSelectFirstArtifact] = useState(true);
@@ -424,6 +426,26 @@ const ChatBox: React.FC<{
     disableArtifacts,
   ]);
 
+  // Live sandbox: open as soon as any in-flight write_file has a path or
+  // the first content bytes — not only when that tool is the last CoT step.
+  // Agentfx (and other agents) often emit write_file + present_files/bash in
+  // the same turn; the collapsed earlier write would otherwise never auto-open.
+  const streamingWritePreview = useMemo(
+    () =>
+      disableArtifacts || !thread.isLoading
+        ? null
+        : latestInFlightWriteFilePreview(thread.messages ?? []),
+    [disableArtifacts, thread.isLoading, thread.messages],
+  );
+  useEffect(() => {
+    if (!streamingWritePreview) return;
+    openStreamedArtifact(
+      streamingWritePreview.artifactPath,
+      streamingWritePreview.previewKey,
+    );
+    setSideTab("document");
+  }, [openStreamedArtifact, streamingWritePreview]);
+
   const directoryModeWasEnabledRef = useRef(artifactDirectoryModeEnabled);
   useEffect(() => {
     const wasEnabled = directoryModeWasEnabledRef.current;

frontend-web/src/components/workspace/messages/message-group.tsx

@@ -42,6 +42,7 @@ import {
 import {
   buildWriteFileVirtualUrl,
   hasCompleteArtifactExtension,
+  languageOfStreamingArtifact,
   normalizeStreamingArtifactPath,
   provisionalWriteFilePath,
 } from "@/core/artifacts/streaming-write-preview";
@@ -754,7 +755,6 @@ function ArtifactWriteToolCall({
   args,
   result,
   description,
-  isLast,
   isLoading,
 }: {
   id?: string;
@@ -793,19 +793,20 @@ function ArtifactWriteToolCall({
       !artifactPath ||
       !previewKey ||
       !isLoading ||
-      !isLast ||
-      result !== undefined
+      result !== undefined ||
+      !sourcePath ||
+      !languageOfStreamingArtifact(sourcePath)
     ) {
       return;
     }
     openStreamedArtifact(artifactPath, previewKey);
   }, [
     artifactPath,
-    isLast,
     isLoading,
     openStreamedArtifact,
     previewKey,
     result,
+    sourcePath,
   ]);
 
   return (