From 41629b526284575b28b142696b17d8015f96394e Mon Sep 17 00:00:00 2001
From: YeonGyu-Kim <code.yeon.gyu@gmail.com>
Date: Sun, 10 May 2026 15:54:29 +0900
Subject: [PATCH] fix(plugin): guard assistant prefill message tails

---
 src/plugin/messages-transform.test.ts | 24 +++++++++-
 src/plugin/messages-transform.ts      | 65 +++++++++++++++++++++++++++
 2 files changed, 88 insertions(+), 1 deletion(-)

diff --git a/src/plugin/messages-transform.test.ts b/src/plugin/messages-transform.test.ts
index d3c0d6315..d4cb0637d 100644
--- a/src/plugin/messages-transform.test.ts
+++ b/src/plugin/messages-transform.test.ts
@@ -7,10 +7,13 @@ import type { CreatedHooks } from "../create-hooks"
 type TestPart = {
   type: string
   id?: string
+  sessionID?: string
+  messageID?: string
   callID?: string
   tool_use_id?: string
   content?: string
   text?: string
+  synthetic?: boolean
 }
 
 type TestMessage = {
@@ -38,7 +41,7 @@ function makeHooks(overrides: {
     contextInjectorMessagesTransform: overrides.contextInjector ? makeHook(overrides.contextInjector) : undefined,
     thinkingBlockValidator: overrides.thinkingBlock ? makeHook(overrides.thinkingBlock) : undefined,
     toolPairValidator: overrides.toolPair ? makeHook(overrides.toolPair) : undefined,
-  } as unknown as CreatedHooks
+  } as CreatedHooks
 }
 
 async function runHandler(
@@ -157,6 +160,25 @@ describe("createMessagesTransformHandler", () => {
     //#when / #then
     await runHandler(hooks, [])
   })
+
+  it("appends a synthetic user turn when transformed messages end with assistant prefill", async () => {
+    //#given
+    const messages: TestMessage[] = [
+      { info: { role: "user" }, parts: [{ type: "text", text: "work on this" }] },
+      { info: { role: "assistant" }, parts: [{ type: "text", text: "partial assistant tail" }] },
+    ]
+
+    //#when
+    await runHandler(makeHooks({}), messages)
+
+    //#then
+    expect(messages.at(-1)?.info).toMatchObject({ role: "user" })
+    expect(messages.at(-1)?.parts[0]).toMatchObject({
+      type: "text",
+      text: "[internal] Continue from the previous assistant state.",
+      synthetic: true,
+    })
+  })
 })
 
 function createRealToolPairValidator(): TransformHook {
diff --git a/src/plugin/messages-transform.ts b/src/plugin/messages-transform.ts
index 1eaef3eba..ab3999456 100644
--- a/src/plugin/messages-transform.ts
+++ b/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)
   }
 }