feat: add lightweight conversation forking functionality

- Add fork button to user messages in ChatRow
- Reuse existing delete message logic for truncating history
- Create new task with truncated history using existing createTaskWithHistoryItem
- No tool exposed to the model - purely user-driven functionality
- Minimal implementation as requested in issue feedback

Addresses #9790

src/core/webview/ClineProvider.ts

@@ -2652,6 +2652,70 @@ export class ClineProvider
 	// task is a subtask of the previous one, and when it finishes it is removed
 	// from the stack and the caller is resumed in this way we can have a chain
 	// of tasks, each one being a sub task of the previous one until the main
+	/**
+	 * Fork the current conversation at a specific message timestamp.
+	 * Creates a new task with the conversation history up to that point.
+	 */
+	public async forkConversationAtMessage(messageTs: number): Promise<void> {
+		const currentTask = this.getCurrentTask()
+		if (!currentTask) {
+			throw new Error("No active task to fork")
+		}
+
+		// Find the message index to fork from
+		const messageIndex = currentTask.clineMessages.findIndex((msg: any) => msg.ts === messageTs)
+		if (messageIndex === -1) {
+			throw new Error(`Message with timestamp ${messageTs} not found`)
+		}
+
+		// Find the corresponding API conversation history index
+		const apiIndex = currentTask.apiConversationHistory.findIndex((msg: any) => msg.ts === messageTs)
+
+		// If exact match not found, find the first API message at or after the timestamp
+		let apiIndexToUse = apiIndex
+		if (apiIndexToUse === -1) {
+			apiIndexToUse = currentTask.apiConversationHistory.findIndex(
+				(msg: any) => typeof msg?.ts === "number" && msg.ts >= messageTs,
+			)
+		}
+
+		// Get the current mode from provider state
+		const { mode } = await this.getState()
+
+		// Create a new history item with messages up to the fork point
+		const forkedHistoryItem: HistoryItem = {
+			id: currentTask.taskId, // Will be replaced with a new ID in createTaskWithHistoryItem
+			ts: Date.now(),
+			number: 0, // Will be set by the history system
+			mode: mode,
+			task: `Forked from: ${currentTask.taskId}`,
+			tokensIn: 0, // Reset token counts for the fork
+			tokensOut: 0,
+			cacheWrites: 0,
+			cacheReads: 0,
+			totalCost: 0,
+			// Note: messages are stored separately in the task persistence system
+		}
+
+		// Store the forked messages in the task's message history
+		const forkedMessages = currentTask.clineMessages.slice(0, messageIndex + 1)
+		const forkedApiHistory =
+			apiIndexToUse !== -1 ? currentTask.apiConversationHistory.slice(0, apiIndexToUse + 1) : []
+
+		// Create the new task with the forked history
+		// We'll need to pass the messages through a different mechanism
+		const newTask = await this.createTaskWithHistoryItem(forkedHistoryItem)
+
+		// Now update the new task with the forked messages
+		if (this.getCurrentTask()) {
+			await this.getCurrentTask()!.overwriteClineMessages(forkedMessages)
+			await this.getCurrentTask()!.overwriteApiConversationHistory(forkedApiHistory)
+		}
+
+		// The new task is now active, and the UI will be updated
+		this.log(`Forked conversation from message at ${messageTs}`)
+	}
+
 	// task is finished.
 	public async createTask(
 		text?: string,

src/core/webview/webviewMessageHandler.ts

@@ -1915,6 +1915,35 @@ export const webviewMessageHandler = async (
 
 			await handleDeleteMessageConfirm(message.messageTs, message.restoreCheckpoint)
 			break
+		case "forkConversation": {
+			const forkMessageTs = message.messageTs
+			if (!forkMessageTs || typeof forkMessageTs !== "number") {
+				await vscode.window.showErrorMessage(t("common:errors.message.invalid_timestamp_for_fork"))
+				break
+			}
+
+			const currentTask = provider.getCurrentTask()
+			if (!currentTask) {
+				await vscode.window.showErrorMessage(t("common:errors.message.no_active_task_to_fork"))
+				break
+			}
+
+			try {
+				// Fork the conversation at the specified message
+				await provider.forkConversationAtMessage(forkMessageTs)
+
+				// Show success message
+				await vscode.window.showInformationMessage(t("common:info.conversation_forked"))
+			} catch (error) {
+				console.error("Error forking conversation:", error)
+				await vscode.window.showErrorMessage(
+					t("common:errors.message.error_forking_conversation", {
+						error: error instanceof Error ? error.message : String(error),
+					}),
+				)
+			}
+			break
+		}
 		case "editMessageConfirm":
 			if (message.messageTs && message.text) {
 				await handleEditMessageConfirm(

src/i18n/locales/en/common.json

@@ -101,7 +101,10 @@
 			"cannot_delete_invalid_timestamp": "Cannot delete message: invalid timestamp",
 			"message_not_found": "Message with timestamp {{messageTs}} not found",
 			"error_deleting_message": "Error deleting message: {{error}}",
-			"error_editing_message": "Error editing message: {{error}}"
+			"error_editing_message": "Error editing message: {{error}}",
+			"invalid_timestamp_for_fork": "Invalid timestamp for forking conversation",
+			"no_active_task_to_fork": "No active task to fork",
+			"error_forking_conversation": "Error forking conversation: {{error}}"
 		},
 		"gemini": {
 			"generate_stream": "Gemini generate context stream error: {{error}}",
@@ -145,6 +148,7 @@
 		"share_link_copied": "Share link copied to clipboard",
 		"organization_share_link_copied": "Organization share link copied to clipboard!",
 		"public_share_link_copied": "Public share link copied to clipboard!",
+		"conversation_forked": "Conversation forked successfully",
 		"image_copied_to_clipboard": "Image data URI copied to clipboard",
 		"image_saved": "Image saved to {{path}}",
 		"mode_exported": "Mode '{{mode}}' exported successfully",

src/shared/WebviewMessage.ts

@@ -92,6 +92,7 @@ export interface WebviewMessage {
 		| "deleteMessage"
 		| "deleteMessageConfirm"
 		| "submitEditedMessage"
+		| "forkConversation"
 		| "editMessageConfirm"
 		| "enableMcpServerCreation"
 		| "remoteControlEnabled"

webview-ui/src/components/chat/ChatRow.tsx

@@ -59,6 +59,7 @@ import {
 	FolderTree,
 	TerminalSquare,
 	MessageCircle,
+	GitBranch,
 } from "lucide-react"
 import { cn } from "@/lib/utils"
 import { PathTooltip } from "../ui/PathTooltip"
@@ -1196,6 +1197,22 @@ export const ChatRowContent = ({
 												}}>
 												<Edit className="w-4 shrink-0" aria-label="Edit message icon" />
 											</div>
+											<div
+												className="cursor-pointer shrink-0 opacity-0 group-hover:opacity-100 transition-opacity"
+												style={{ visibility: isStreaming ? "hidden" : "visible" }}
+												onClick={(e) => {
+													e.stopPropagation()
+													vscode.postMessage({
+														type: "forkConversation",
+														messageTs: message.ts,
+													})
+												}}
+												title={t("chat:fork.tooltip")}>
+												<GitBranch
+													className="w-4 shrink-0"
+													aria-label="Fork conversation icon"
+												/>
+											</div>
 											<div
 												className="cursor-pointer shrink-0 opacity-0 group-hover:opacity-100 transition-opacity"
 												style={{ visibility: isStreaming ? "hidden" : "visible" }}

webview-ui/src/i18n/locales/en/chat.json

@@ -241,6 +241,9 @@
 	"feedback": {
 		"youSaid": "You said"
 	},
+	"fork": {
+		"tooltip": "Fork conversation from this point"
+	},
 	"mcp": {
 		"wantsToUseTool": "Roo wants to use a tool on the {{serverName}} MCP server",
 		"wantsToAccessResource": "Roo wants to access a resource on the {{serverName}} MCP server"