fix: prevent empty history corruption during cancel/resume operations

- Make resume tolerant of empty API conversation history
- Standardize reads using helper functions instead of direct JSON.parse
- Add transaction helper for safe read-modify-write operations
- Guard against unintentional empty writes with allowEmpty flag

This fixes the issue where canceling during model's thinking/streaming
phase could leave the API history file as an empty array ([]), causing
the chat to lock when resuming.

Fixes #8153 and #5333

src/core/task-persistence/apiMessages.ts

@@ -81,3 +81,46 @@ export async function saveApiMessages({
 	const filePath = path.join(taskDir, GlobalFileNames.apiConversationHistory)
 	await safeWriteJson(filePath, messages)
 }
+
+/**
+ * Transaction helper for safe read-modify-write operations on API messages.
+ * Ensures atomic updates by reading, modifying, and writing under a conceptual lock.
+ *
+ * @param taskId - The task ID
+ * @param globalStoragePath - The global storage path
+ * @param updater - A pure function that takes the current messages and returns the updated messages
+ * @param options - Optional configuration
+ * @returns The updated messages
+ */
+export async function transactApiMessages({
+	taskId,
+	globalStoragePath,
+	updater,
+	options = {},
+}: {
+	taskId: string
+	globalStoragePath: string
+	updater: (messages: ApiMessage[]) => ApiMessage[]
+	options?: {
+		allowEmpty?: boolean
+	}
+}): Promise<ApiMessage[]> {
+	// Read current state
+	const currentMessages = await readApiMessages({ taskId, globalStoragePath })
+
+	// Apply the pure updater function
+	const updatedMessages = updater(currentMessages)
+
+	// Guard against unintentional empty writes
+	if (updatedMessages.length === 0 && currentMessages.length > 0 && !options.allowEmpty) {
+		console.warn(
+			`[transactApiMessages] Preventing empty write for taskId: ${taskId}. Current has ${currentMessages.length} messages. Use allowEmpty: true to force.`,
+		)
+		return currentMessages // Return unchanged
+	}
+
+	// Commit the changes
+	await saveApiMessages({ messages: updatedMessages, taskId, globalStoragePath })
+
+	return updatedMessages
+}

src/core/task/Task.ts

@@ -582,7 +582,16 @@ export class Task extends EventEmitter<TaskEvents> implements TaskLike {
 		await this.saveApiConversationHistory()
 	}
 
-	async overwriteApiConversationHistory(newHistory: ApiMessage[]) {
+	async overwriteApiConversationHistory(newHistory: ApiMessage[], allowEmpty: boolean = false) {
+		// Guard against unintentional empty writes
+		if (newHistory.length === 0 && this.apiConversationHistory.length > 0 && !allowEmpty) {
+			console.warn(
+				`[Task#overwriteApiConversationHistory] Preventing empty write for taskId: ${this.taskId}. ` +
+					`Current has ${this.apiConversationHistory.length} messages. Use allowEmpty: true to force.`,
+			)
+			return // Don't overwrite with empty array unless explicitly allowed
+		}
+
 		this.apiConversationHistory = newHistory
 		await this.saveApiConversationHistory()
 	}
@@ -1436,7 +1445,15 @@ export class Task extends EventEmitter<TaskEvents> implements TaskLike {
 				throw new Error("Unexpected: Last message is not a user or assistant message")
 			}
 		} else {
-			throw new Error("Unexpected: No existing API conversation history")
+			// Handle empty API conversation history gracefully instead of throwing
+			// This prevents the "chat locks until reopen" failure mode
+			this.say(
+				"text",
+				"[TASK RESUMPTION] Previous conversation history was empty. Starting with a fresh baseline.",
+			)
+			// Initialize with empty arrays to allow the task to continue
+			modifiedApiConversationHistory = []
+			modifiedOldUserContent = []
 		}
 
 		let newUserContent: Anthropic.Messages.ContentBlockParam[] = [...modifiedOldUserContent]

src/core/webview/ClineProvider.ts

@@ -918,8 +918,10 @@ export class ClineProvider
 						await task.overwriteClineMessages(task.clineMessages.slice(0, messageIndex))
 
 						if (apiConversationHistoryIndex !== -1) {
+							// Allow empty writes for edit operations after checkpoint restoration
 							await task.overwriteApiConversationHistory(
 								task.apiConversationHistory.slice(0, apiConversationHistoryIndex),
+								true, // allowEmpty: true for edit operations
 							)
 						}
 
@@ -1453,6 +1455,7 @@ export class ClineProvider
 
 		if (historyItem) {
 			const { getTaskDirectoryPath } = await import("../../utils/storage")
+			const { readApiMessages } = await import("../task-persistence/apiMessages")
 			const globalStoragePath = this.contextProxy.globalStorageUri.fsPath
 			const taskDirPath = await getTaskDirectoryPath(globalStoragePath, id)
 			const apiConversationHistoryFilePath = path.join(taskDirPath, GlobalFileNames.apiConversationHistory)
@@ -1460,7 +1463,8 @@ export class ClineProvider
 			const fileExists = await fileExistsAtPath(apiConversationHistoryFilePath)
 
 			if (fileExists) {
-				const apiConversationHistory = JSON.parse(await fs.readFile(apiConversationHistoryFilePath, "utf8"))
+				// Use the helper reader for unified behavior/logging instead of direct JSON.parse
+				const apiConversationHistory = await readApiMessages({ taskId: id, globalStoragePath })
 
 				return {
 					historyItem,

src/core/webview/__tests__/webviewMessageHandler.edit.spec.ts

@@ -135,7 +135,7 @@ describe("webviewMessageHandler - Edit Message with Timestamp Fallback", () => {
 		)
 
 		// API history should be truncated from first message at/after edited timestamp (fallback)
-		expect(mockCurrentTask.overwriteApiConversationHistory).toHaveBeenCalledWith([])
+		expect(mockCurrentTask.overwriteApiConversationHistory).toHaveBeenCalledWith([], true)
 	})
 
 	it("should preserve messages before the edited message when message not in API history", async () => {
@@ -197,13 +197,16 @@ describe("webviewMessageHandler - Edit Message with Timestamp Fallback", () => {
 		])
 
 		// API history should be truncated from the first API message at/after the edited timestamp (fallback)
-		expect(mockCurrentTask.overwriteApiConversationHistory).toHaveBeenCalledWith([
-			{
-				ts: earlierMessageTs,
-				role: "user",
-				content: [{ type: "text", text: "Earlier message" }],
-			},
-		])
+		expect(mockCurrentTask.overwriteApiConversationHistory).toHaveBeenCalledWith(
+			[
+				{
+					ts: earlierMessageTs,
+					role: "user",
+					content: [{ type: "text", text: "Earlier message" }],
+				},
+			],
+			true,
+		)
 	})
 
 	it("should not use fallback when exact apiConversationHistoryIndex is found", async () => {
@@ -248,7 +251,7 @@ describe("webviewMessageHandler - Edit Message with Timestamp Fallback", () => {
 
 		// Both should be truncated at index 0
 		expect(mockCurrentTask.overwriteClineMessages).toHaveBeenCalledWith([])
-		expect(mockCurrentTask.overwriteApiConversationHistory).toHaveBeenCalledWith([])
+		expect(mockCurrentTask.overwriteApiConversationHistory).toHaveBeenCalledWith([], true)
 	})
 
 	it("should handle case where no API messages match timestamp criteria", async () => {
@@ -385,6 +388,6 @@ describe("webviewMessageHandler - Edit Message with Timestamp Fallback", () => {
 		expect(mockCurrentTask.overwriteClineMessages).toHaveBeenCalledWith([])
 
 		// API history should be truncated from first message at/after edited timestamp (fallback)
-		expect(mockCurrentTask.overwriteApiConversationHistory).toHaveBeenCalledWith([])
+		expect(mockCurrentTask.overwriteApiConversationHistory).toHaveBeenCalledWith([], true)
 	})
 })

src/core/webview/webviewMessageHandler.ts

@@ -108,8 +108,10 @@ export const webviewMessageHandler = async (
 		await currentCline.overwriteClineMessages(currentCline.clineMessages.slice(0, messageIndex))
 
 		if (apiConversationHistoryIndex !== -1) {
+			// Allow empty writes for edit/delete operations
 			await currentCline.overwriteApiConversationHistory(
 				currentCline.apiConversationHistory.slice(0, apiConversationHistoryIndex),
+				true, // allowEmpty: true for edit/delete operations
 			)
 		}
 	}