fix: use user message instead of assistant prefill for continuation retry (#3027)

e2e-tests/snapshots/partial_response.spec.ts_partial-message-is-resumed-1.txt

@@ -132,3 +132,8 @@ role: assistant
 message: START OF MESSAGE
 <dyad-write path="src/new-file.ts" description="this file will be partially written">
 const a = "[[STRING_TO_BE_FINISHED]]
+
+
+===
+role: user
+message: Your previous response did not finish completely. Continue exactly where you left off without any preamble.
\ No newline at end of file

src/ipc/handlers/chat_stream_handlers.ts

@@ -1453,10 +1453,18 @@ ${formattedSearchReplaceIssues}`,
               continuationAttempts++;
 
               const { fullStream: contStream } = await simpleStreamText({
-                // Build messages: replay history then pre-fill assistant with current partial.
+                // Build messages: replay history, then ask the model to continue from the partial response.
                 chatMessages: [
                   ...chatMessages,
-                  { role: "assistant", content: fullResponse },
+                  {
+                    role: "assistant",
+                    content: fullResponse,
+                  },
+                  {
+                    role: "user",
+                    content:
+                      "Your previous response did not finish completely. Continue exactly where you left off without any preamble.",
+                  },
                 ],
                 modelClient,
                 files: files,

testing/fake-llm-server/chatCompletionHandler.ts

@@ -59,7 +59,8 @@ export const createChatCompletionHandler =
     if (
       !localAgentFixture &&
       (userTextContent.includes("incomplete todo(s)") ||
-        userTextContent.includes("previous response stream was interrupted"))
+        userTextContent.includes("previous response stream was interrupted") ||
+        userTextContent.includes("did not finish completely"))
     ) {
       for (const msg of userMessages) {
         const textContent = getTextContent(msg);
@@ -334,11 +335,13 @@ export default Index;
       }
     }
 
+    // Continuation requests: the partial assistant output is in a preceding assistant
+    // message, then a user message asks to continue ("did not finish completely").
+    // Check any message for the marker. See chat_stream_handlers continuation prompt.
     if (
-      lastMessage &&
-      lastMessage.content &&
-      typeof lastMessage.content === "string" &&
-      lastMessage.content.trim().endsWith("[[STRING_TO_BE_FINISHED]]")
+      messages.some((m: any) =>
+        getTextContent(m).includes("[[STRING_TO_BE_FINISHED]]"),
+      )
     ) {
       messageContent = `[[STRING_IS_FINISHED]]";</dyad-write>\nFinished writing file.`;
       messageContent += "\n\n" + generateDump(req);