fix(plugin): guard assistant prefill message tails

src/plugin/messages-transform.ts

@@ -3,12 +3,75 @@ import type { Message, Part } from "@opencode-ai/sdk"
 import { log } from "../shared/logger"
 import type { CreatedHooks } from "../create-hooks"
 
+const ASSISTANT_PREFILL_RECOVERY_TEXT = "[internal] Continue from the previous assistant state."
+
 type MessageWithParts = {
   info: Message
   parts: Part[]
 }
 
 type MessagesTransformOutput = { messages: MessageWithParts[] }
+type UserMessageInfo = Extract<Message, { role: "user" }>
+
+function getSessionID(message: MessageWithParts): string | undefined {
+  return message.info.sessionID
+}
+
+function findLastUserMessage(messages: MessageWithParts[]): UserMessageInfo | undefined {
+  for (let index = messages.length - 1; index >= 0; index -= 1) {
+    const message = messages[index]
+    if (message?.info.role === "user") {
+      return message.info
+    }
+  }
+
+  return undefined
+}
+
+function createAssistantPrefillRecoveryMessage(
+  lastAssistantMessage: MessageWithParts,
+  messages: MessageWithParts[],
+): MessageWithParts {
+  const lastUserMessage = findLastUserMessage(messages)
+  const sessionID = getSessionID(lastAssistantMessage) ?? lastUserMessage?.sessionID ?? ""
+  const messageID = `${lastAssistantMessage.info.id}_prefill_recovery`
+  const model = lastUserMessage?.model ?? {
+    providerID: "internal",
+    modelID: "assistant-prefill-guard",
+  }
+
+  return {
+    info: {
+      id: messageID,
+      sessionID,
+      role: "user",
+      time: { created: Date.now() },
+      agent: lastUserMessage?.agent ?? "internal",
+      model,
+      ...(lastUserMessage?.system ? { system: lastUserMessage.system } : {}),
+      ...(lastUserMessage?.tools ? { tools: lastUserMessage.tools } : {}),
+    },
+    parts: [
+      {
+        id: `${messageID}_text`,
+        sessionID,
+        messageID,
+        type: "text",
+        text: ASSISTANT_PREFILL_RECOVERY_TEXT,
+        synthetic: true,
+      },
+    ],
+  }
+}
+
+function ensureUserTurnAfterAssistantTail(output: MessagesTransformOutput): void {
+  const lastMessage = output.messages.at(-1)
+  if (!lastMessage || lastMessage.info.role !== "assistant") {
+    return
+  }
+
+  output.messages.push(createAssistantPrefillRecoveryMessage(lastMessage, output.messages))
+}
 
 async function runMessagesTransformHookSafely<I, O>(
   hookName: string,
@@ -79,5 +142,7 @@ export function createMessagesTransformHandler(args: {
       input,
       output,
     )
+
+    ensureUserTurnAfterAssistantTail(output)
   }
 }