src/routes/chat.ts

@@ -35,6 +35,7 @@ const payloadSchema = z.object({
   session_id: z.string().max(128).optional(),
   model: z.enum(supportedModels).optional(),
   approval_mode: z.enum(["request", "always"]).optional().default("request"),
+  regenerate_from_message_index: z.coerce.number().int().min(0).optional(),
 });
 
 const abortPayloadSchema = z.object({
@@ -955,6 +956,11 @@ export const buildChatRouter = (
       const initialSessionState = await sessionUiStateStore.read(
         toSessionUiStateContext(activeSessionRecord),
       );
+      const persistedMessages = initialSessionState?.messages ?? [];
+      const baseMessages =
+        parsed.data.regenerate_from_message_index !== undefined
+          ? persistedMessages.slice(0, parsed.data.regenerate_from_message_index)
+          : persistedMessages;
       if (activeRuns.get(activeSessionRecord.sessionId)?.status === "running") {
         res.status(409).json({
           message: "session is already streaming",
@@ -988,7 +994,7 @@ export const buildChatRouter = (
       const abortController = new AbortController();
       sessionBridge.registerAbortController(clientSessionId, abortController);
       const initialMessages = createInitialStreamingMessages(
-        initialSessionState?.messages ?? [],
+        baseMessages,
         parsed.data.message,
       );
       const branchGroups = initialSessionState?.branchGroups ?? [];
@@ -1122,13 +1128,17 @@ export const buildChatRouter = (
       };
 
       try {
+        if (parsed.data.regenerate_from_message_index !== undefined) {
+          await runtime.revertLastUserMessage(binding.sessionId);
+        }
+
         const preparedMessage = await buildPromptWithLearningContext(
           memoryStore,
           requestContext.actorKey,
           requestContext.projectKey,
           {
             recentTurns,
-            persistedMessages: initialSessionState?.messages,
+            persistedMessages: baseMessages,
             message: parsed.data.message,
             restoreConversation: !hadExistingRuntimeSession,
           },

src/runtime/opencode.ts

@@ -99,6 +99,28 @@ export class OpencodeRuntimeAdapter {
     return requireData(messages.data, "session.messages");
   }
 
+  async revertMessage(sessionId: string, messageId: string) {
+    const client = await this.ensureClient();
+    const response = await client.session.revert({
+      sessionID: sessionId,
+      messageID: messageId,
+    });
+    return response.data;
+  }
+
+  async revertLastUserMessage(sessionId: string) {
+    const messages = await this.messages(sessionId, 40);
+    const lastUserMessage = [...messages]
+      .reverse()
+      .find((message) => message.info.role === "user");
+
+    if (!lastUserMessage) {
+      throw new Error("no user message found to revert");
+    }
+
+    return this.revertMessage(sessionId, lastUserMessage.info.id);
+  }
+
   async abortSession(sessionId: string) {
     const client = await this.ensureClient();
     const response = await client.session.abort({