fix(plugin): repair anthropic assistant prefill tails

src/plugin/messages-transform.test.ts

@@ -419,7 +419,48 @@ 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 Anthropic Claude history ends with an assistant tail #when messages transform runs #then it appends a synthetic user recovery turn", async () => {
+    //#given
+    const messages: TestMessage[] = [
+      {
+        info: {
+          id: "msg_user_any_anthropic",
+          role: "user",
+          sessionID: "ses_any_anthropic_prefill",
+          agent: "sisyphus",
+          model: { providerID: "anthropic", modelID: "claude-sonnet-4-5" },
+        },
+        parts: [{ type: "text", text: "continue" }],
+      },
+      {
+        info: {
+          id: "msg_assistant_any_anthropic",
+          role: "assistant",
+          sessionID: "ses_any_anthropic_prefill",
+        },
+        parts: [{ type: "text", text: "done" }],
+      },
+    ]
+
+    //#when
+    await runHandler(makeHooks({}), messages)
+
+    //#then
+    expect(messages).toHaveLength(3)
+    expect(messages.at(-1)?.info).toMatchObject({
+      role: "user",
+      sessionID: "ses_any_anthropic_prefill",
+      agent: "sisyphus",
+      model: { providerID: "anthropic", modelID: "claude-sonnet-4-5" },
+    })
+    expect(messages.at(-1)?.parts[0]).toMatchObject({
+      type: "text",
+      text: "[internal] Continue from the previous assistant state.",
+      synthetic: true,
+    })
+  })
+
+  it("#given non-Anthropic models or missing model metadata #when messages transform runs #then it keeps the assistant tail unchanged", async () => {
     //#given
     const scenarios: Array<{ name: string; userInfo: TestMessage["info"] }> = [
       {
@@ -429,13 +470,6 @@ describe("createMessagesTransformHandler", () => {
           model: { providerID: "openai", modelID: "gpt-5.4" },
         },
       },
-      {
-        name: "anthropic allowed",
-        userInfo: {
-          role: "user",
-          model: { providerID: "anthropic", modelID: "claude-sonnet-4-5" },
-        },
-      },
       {
         name: "missing model",
         userInfo: { role: "user" },

src/plugin/messages-transform.ts

@@ -9,12 +9,6 @@ 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
@@ -101,8 +95,7 @@ function shouldRepairAssistantPrefillForModel(model: ModelIdentifier | undefined
     return false
   }
 
-  const modelID = normalizeModelID(model.modelID.toLowerCase())
-  return ASSISTANT_PREFILL_UNSUPPORTED_MODEL_PREFIXES.some((prefix) => modelID.startsWith(prefix))
+  return normalizeModelID(model.modelID.toLowerCase()).startsWith("claude-")
 }
 
 function isCompactionContinuationPart(part: unknown): boolean {