fix(plugin): enforce user tail after message transforms

Promote assistant-tail prefill repair from model-specific targeting to a transform-level invariant. The plugin now avoids sending assistant-ending histories after message transforms regardless of provider metadata.

src/plugin/messages-transform.test.ts

@@ -181,7 +181,7 @@ describe("createMessagesTransformHandler", () => {
     await runHandler(hooks, [])
   })
 
-  it("#given a completed assistant response tail #when messages transform runs again #then it does not synthesize a continuation user turn", async () => {
+  it("#given a completed assistant response tail #when messages transform runs again #then it appends a synthetic user recovery turn", async () => {
     //#given
     const messages: TestMessage[] = [
       { info: { role: "user" }, parts: [{ type: "text", text: "work on this" }] },
@@ -192,8 +192,13 @@ describe("createMessagesTransformHandler", () => {
     await runHandler(makeHooks({}), messages)
 
     //#then
-    expect(messages).toHaveLength(2)
-    expect(messages.at(-1)?.info.role).toBe("assistant")
+    expect(messages).toHaveLength(3)
+    expect(messages.at(-1)?.info.role).toBe("user")
+    expect(messages.at(-1)?.parts[0]).toMatchObject({
+      type: "text",
+      text: "[internal] Continue from the previous assistant state.",
+      synthetic: true,
+    })
   })
 
   it("#given an Anthropic Opus 4.7 history ends with an ordinary assistant tail #when messages transform runs #then it appends a synthetic user recovery turn", async () => {
@@ -415,7 +420,7 @@ describe("createMessagesTransformHandler", () => {
     })
   })
 
-  it("#given models that still allow assistant prefill or missing model metadata #when messages transform runs #then it keeps the assistant tail unchanged", async () => {
+  it("#given any provider or missing model metadata ends with an assistant tail #when messages transform runs #then it appends a synthetic user recovery turn", async () => {
     //#given
     const scenarios: Array<{ name: string; userInfo: TestMessage["info"] }> = [
       {
@@ -455,8 +460,13 @@ describe("createMessagesTransformHandler", () => {
       await runHandler(makeHooks({}), messages)
 
       //#then
-      expect(messages, scenario.name).toHaveLength(2)
-      expect(messages.at(-1)?.info.role, scenario.name).toBe("assistant")
+      expect(messages, scenario.name).toHaveLength(3)
+      expect(messages.at(-1)?.info.role, scenario.name).toBe("user")
+      expect(messages.at(-1)?.parts[0], scenario.name).toMatchObject({
+        type: "text",
+        text: "[internal] Continue from the previous assistant state.",
+        synthetic: true,
+      })
     }
   })
 

src/plugin/messages-transform.ts

@@ -1,20 +1,9 @@
 import type { Message, Part } from "@opencode-ai/sdk"
 
 import { log } from "../shared/logger"
-import { normalizeModelID } from "../shared/model-normalization"
 import type { CreatedHooks } from "../create-hooks"
 
 const ASSISTANT_PREFILL_RECOVERY_TEXT = "[internal] Continue from the previous assistant state."
-const ASSISTANT_PREFILL_UNSUPPORTED_PROVIDERS = new Set([
-  "anthropic",
-  "google-vertex-anthropic",
-])
-const ASSISTANT_PREFILL_UNSUPPORTED_MODEL_PREFIXES = [
-  "claude-opus-4-7",
-  "claude-opus-4-6",
-  "claude-sonnet-4-6",
-  "claude-mythos",
-]
 
 type MessageWithParts = {
   info: Message
@@ -43,17 +32,6 @@ function findLastUserMessage(messages: MessageWithParts[]): UserMessageInfo | un
   return undefined
 }
 
-function findLastUserTurn(messages: MessageWithParts[]): MessageWithParts | undefined {
-  for (let index = messages.length - 1; index >= 0; index -= 1) {
-    const message = messages[index]
-    if (message?.info.role === "user") {
-      return message
-    }
-  }
-
-  return undefined
-}
-
 function isRecord(value: unknown): value is Record<string, unknown> {
   return typeof value === "object" && value !== null
 }
@@ -80,44 +58,6 @@ function readModelIdentifier(info: unknown): ModelIdentifier | undefined {
   return providerID && modelID ? { providerID, modelID } : undefined
 }
 
-function findLastUserModel(messages: MessageWithParts[]): ModelIdentifier | undefined {
-  for (let index = messages.length - 1; index >= 0; index -= 1) {
-    const message = messages[index]
-    if (message?.info.role === "user") {
-      return readModelIdentifier(message.info)
-    }
-  }
-
-  return undefined
-}
-
-function shouldRepairAssistantPrefillForModel(model: ModelIdentifier | undefined): boolean {
-  if (!model) {
-    return false
-  }
-
-  const providerID = model.providerID.toLowerCase()
-  if (!ASSISTANT_PREFILL_UNSUPPORTED_PROVIDERS.has(providerID)) {
-    return false
-  }
-
-  const modelID = normalizeModelID(model.modelID.toLowerCase())
-  return ASSISTANT_PREFILL_UNSUPPORTED_MODEL_PREFIXES.some((prefix) => modelID.startsWith(prefix))
-}
-
-function isCompactionContinuationPart(part: unknown): boolean {
-  if (!isRecord(part)) {
-    return false
-  }
-
-  const metadata = part["metadata"]
-  return isRecord(metadata) && metadata["compaction_continue"] === true
-}
-
-function hasInternalContinuationTrigger(messages: MessageWithParts[]): boolean {
-  return findLastUserTurn(messages)?.parts.some(isCompactionContinuationPart) === true
-}
-
 function createAssistantPrefillRecoveryMessage(
   lastAssistantMessage: MessageWithParts,
   messages: MessageWithParts[],
@@ -160,13 +100,6 @@ function ensureUserTurnAfterAssistantTail(output: MessagesTransformOutput): void
     return
   }
 
-  const shouldRepairAssistantTail = hasInternalContinuationTrigger(output.messages) ||
-    shouldRepairAssistantPrefillForModel(findLastUserModel(output.messages)) ||
-    shouldRepairAssistantPrefillForModel(readModelIdentifier(lastMessage.info))
-  if (!shouldRepairAssistantTail) {
-    return
-  }
-
   output.messages.push(createAssistantPrefillRecoveryMessage(lastMessage, output.messages))
 }