diff --git a/src/__tests__/history-resume-delegation.spec.ts b/src/__tests__/history-resume-delegation.spec.ts
index f3256bd1432..a78c41b7c06 100644
--- a/src/__tests__/history-resume-delegation.spec.ts
+++ b/src/__tests__/history-resume-delegation.spec.ts
@@ -387,6 +387,7 @@ describe("History resume delegation - parent metadata transitions", () => {
 
 	it("reopenParentFromDelegation emits events in correct order: TaskDelegationCompleted → TaskDelegationResumed", async () => {
 		const emitSpy = vi.fn()
+		const updateTaskHistory = vi.fn().mockResolvedValue([])
 
 		const provider = {
 			contextProxy: { globalStorageUri: { fsPath: "/tmp" } },
@@ -411,7 +412,7 @@ describe("History resume delegation - parent metadata transitions", () => {
 				overwriteClineMessages: vi.fn().mockResolvedValue(undefined),
 				overwriteApiConversationHistory: vi.fn().mockResolvedValue(undefined),
 			}),
-			updateTaskHistory: vi.fn().mockResolvedValue([]),
+			updateTaskHistory,
 		} as unknown as ClineProvider
 
 		vi.mocked(readTaskMessages).mockResolvedValue([])
@@ -433,6 +434,92 @@ describe("History resume delegation - parent metadata transitions", () => {
 		const resumedIdx = emitSpy.mock.calls.findIndex((c) => c[0] === RooCodeEventName.TaskDelegationResumed)
 		expect(completedIdx).toBeGreaterThanOrEqual(0)
 		expect(resumedIdx).toBeGreaterThan(completedIdx)
+
+		// RPD-05: verify parent metadata persistence happens before TaskDelegationCompleted emit
+		const parentUpdateCallIdx = updateTaskHistory.mock.calls.findIndex((call) => {
+			const item = call[0] as { id?: string; status?: string } | undefined
+			return item?.id === "p3" && item.status === "active"
+		})
+		expect(parentUpdateCallIdx).toBeGreaterThanOrEqual(0)
+
+		const parentUpdateCallOrder = updateTaskHistory.mock.invocationCallOrder[parentUpdateCallIdx]
+		const completedEmitCallOrder = emitSpy.mock.invocationCallOrder[completedIdx]
+		expect(parentUpdateCallOrder).toBeLessThan(completedEmitCallOrder)
+	})
+
+	it("reopenParentFromDelegation continues when overwrite operations fail and still resumes/emits (RPD-06)", async () => {
+		const emitSpy = vi.fn()
+		const parentInstance = {
+			resumeAfterDelegation: vi.fn().mockResolvedValue(undefined),
+			overwriteClineMessages: vi.fn().mockRejectedValue(new Error("ui overwrite failed")),
+			overwriteApiConversationHistory: vi.fn().mockRejectedValue(new Error("api overwrite failed")),
+		}
+
+		const provider = {
+			contextProxy: { globalStorageUri: { fsPath: "/tmp" } },
+			getTaskWithId: vi.fn().mockImplementation(async (id: string) => {
+				if (id === "parent-rpd06") {
+					return {
+						historyItem: {
+							id: "parent-rpd06",
+							status: "delegated",
+							awaitingChildId: "child-rpd06",
+							childIds: ["child-rpd06"],
+							ts: 800,
+							task: "Parent RPD-06",
+							tokensIn: 0,
+							tokensOut: 0,
+							totalCost: 0,
+						},
+					}
+				}
+
+				return {
+					historyItem: {
+						id: "child-rpd06",
+						status: "active",
+						ts: 801,
+						task: "Child RPD-06",
+						tokensIn: 0,
+						tokensOut: 0,
+						totalCost: 0,
+					},
+				}
+			}),
+			emit: emitSpy,
+			getCurrentTask: vi.fn(() => ({ taskId: "child-rpd06" })),
+			removeClineFromStack: vi.fn().mockResolvedValue(undefined),
+			createTaskWithHistoryItem: vi.fn().mockResolvedValue(parentInstance),
+			updateTaskHistory: vi.fn().mockResolvedValue([]),
+		} as unknown as ClineProvider
+
+		vi.mocked(readTaskMessages).mockResolvedValue([])
+		vi.mocked(readApiMessages).mockResolvedValue([])
+
+		await expect(
+			(ClineProvider.prototype as any).reopenParentFromDelegation.call(provider, {
+				parentTaskId: "parent-rpd06",
+				childTaskId: "child-rpd06",
+				completionResultSummary: "Subtask finished despite overwrite failures",
+			}),
+		).resolves.toBeUndefined()
+
+		expect(parentInstance.overwriteClineMessages).toHaveBeenCalledTimes(1)
+		expect(parentInstance.overwriteApiConversationHistory).toHaveBeenCalledTimes(1)
+		expect(parentInstance.resumeAfterDelegation).toHaveBeenCalledTimes(1)
+
+		expect(emitSpy).toHaveBeenCalledWith(
+			RooCodeEventName.TaskDelegationCompleted,
+			"parent-rpd06",
+			"child-rpd06",
+			"Subtask finished despite overwrite failures",
+		)
+		expect(emitSpy).toHaveBeenCalledWith(RooCodeEventName.TaskDelegationResumed, "parent-rpd06", "child-rpd06")
+
+		const completedIdx = emitSpy.mock.calls.findIndex((c) => c[0] === RooCodeEventName.TaskDelegationCompleted)
+		const resumedIdx = emitSpy.mock.calls.findIndex((c) => c[0] === RooCodeEventName.TaskDelegationResumed)
+		expect(completedIdx).toBeGreaterThanOrEqual(0)
+		expect(resumedIdx).toBeGreaterThan(completedIdx)
 	})
 
 	it("reopenParentFromDelegation does NOT emit TaskPaused or TaskUnpaused (new flow only)", async () => {
@@ -480,6 +567,162 @@ describe("History resume delegation - parent metadata transitions", () => {
 		expect(eventNames).not.toContain(RooCodeEventName.TaskSpawned)
 	})
 
+	it("reopenParentFromDelegation skips child close when current task differs and still reopens parent (RPD-02)", async () => {
+		const parentInstance = {
+			resumeAfterDelegation: vi.fn().mockResolvedValue(undefined),
+			overwriteClineMessages: vi.fn().mockResolvedValue(undefined),
+			overwriteApiConversationHistory: vi.fn().mockResolvedValue(undefined),
+		}
+
+		const updateTaskHistory = vi.fn().mockResolvedValue([])
+		const removeClineFromStack = vi.fn().mockResolvedValue(undefined)
+		const createTaskWithHistoryItem = vi.fn().mockResolvedValue(parentInstance)
+
+		const provider = {
+			contextProxy: { globalStorageUri: { fsPath: "/tmp" } },
+			getTaskWithId: vi.fn().mockImplementation(async (id: string) => {
+				if (id === "parent-rpd02") {
+					return {
+						historyItem: {
+							id: "parent-rpd02",
+							status: "delegated",
+							awaitingChildId: "child-rpd02",
+							childIds: ["child-rpd02"],
+							ts: 600,
+							task: "Parent RPD-02",
+							tokensIn: 0,
+							tokensOut: 0,
+							totalCost: 0,
+						},
+					}
+				}
+				return {
+					historyItem: {
+						id: "child-rpd02",
+						status: "active",
+						ts: 601,
+						task: "Child RPD-02",
+						tokensIn: 0,
+						tokensOut: 0,
+						totalCost: 0,
+					},
+				}
+			}),
+			emit: vi.fn(),
+			getCurrentTask: vi.fn(() => ({ taskId: "different-open-task" })),
+			removeClineFromStack,
+			createTaskWithHistoryItem,
+			updateTaskHistory,
+		} as unknown as ClineProvider
+
+		vi.mocked(readTaskMessages).mockResolvedValue([])
+		vi.mocked(readApiMessages).mockResolvedValue([])
+
+		await (ClineProvider.prototype as any).reopenParentFromDelegation.call(provider, {
+			parentTaskId: "parent-rpd02",
+			childTaskId: "child-rpd02",
+			completionResultSummary: "Child done without being current",
+		})
+
+		expect(removeClineFromStack).not.toHaveBeenCalled()
+		expect(updateTaskHistory).toHaveBeenCalledWith(
+			expect.objectContaining({
+				id: "child-rpd02",
+				status: "completed",
+			}),
+		)
+		expect(createTaskWithHistoryItem).toHaveBeenCalledWith(
+			expect.objectContaining({
+				id: "parent-rpd02",
+				status: "active",
+				completedByChildId: "child-rpd02",
+			}),
+			{ startTask: false },
+		)
+		expect(parentInstance.resumeAfterDelegation).toHaveBeenCalledTimes(1)
+	})
+
+	it("reopenParentFromDelegation logs child status persistence failure and continues reopen flow (RPD-04)", async () => {
+		const logSpy = vi.fn()
+		const emitSpy = vi.fn()
+		const parentInstance = {
+			resumeAfterDelegation: vi.fn().mockResolvedValue(undefined),
+			overwriteClineMessages: vi.fn().mockResolvedValue(undefined),
+			overwriteApiConversationHistory: vi.fn().mockResolvedValue(undefined),
+		}
+
+		const updateTaskHistory = vi.fn().mockImplementation(async (historyItem: { id?: string }) => {
+			if (historyItem.id === "child-rpd04") {
+				throw new Error("child status persist failed")
+			}
+			return []
+		})
+
+		const provider = {
+			contextProxy: { globalStorageUri: { fsPath: "/tmp" } },
+			getTaskWithId: vi.fn().mockImplementation(async (id: string) => {
+				if (id === "parent-rpd04") {
+					return {
+						historyItem: {
+							id: "parent-rpd04",
+							status: "delegated",
+							awaitingChildId: "child-rpd04",
+							childIds: ["child-rpd04"],
+							ts: 700,
+							task: "Parent RPD-04",
+							tokensIn: 0,
+							tokensOut: 0,
+							totalCost: 0,
+						},
+					}
+				}
+				return {
+					historyItem: {
+						id: "child-rpd04",
+						status: "active",
+						ts: 701,
+						task: "Child RPD-04",
+						tokensIn: 0,
+						tokensOut: 0,
+						totalCost: 0,
+					},
+				}
+			}),
+			emit: emitSpy,
+			log: logSpy,
+			getCurrentTask: vi.fn(() => ({ taskId: "child-rpd04" })),
+			removeClineFromStack: vi.fn().mockResolvedValue(undefined),
+			createTaskWithHistoryItem: vi.fn().mockResolvedValue(parentInstance),
+			updateTaskHistory,
+		} as unknown as ClineProvider
+
+		vi.mocked(readTaskMessages).mockResolvedValue([])
+		vi.mocked(readApiMessages).mockResolvedValue([])
+
+		await expect(
+			(ClineProvider.prototype as any).reopenParentFromDelegation.call(provider, {
+				parentTaskId: "parent-rpd04",
+				childTaskId: "child-rpd04",
+				completionResultSummary: "Child completion with persistence failure",
+			}),
+		).resolves.toBeUndefined()
+
+		expect(logSpy).toHaveBeenCalledWith(
+			expect.stringContaining(
+				"[reopenParentFromDelegation] Failed to persist child completed status for child-rpd04:",
+			),
+		)
+		expect(updateTaskHistory).toHaveBeenCalledWith(
+			expect.objectContaining({
+				id: "parent-rpd04",
+				status: "active",
+				completedByChildId: "child-rpd04",
+			}),
+		)
+		expect(parentInstance.resumeAfterDelegation).toHaveBeenCalledTimes(1)
+		expect(emitSpy).toHaveBeenCalledWith(RooCodeEventName.TaskDelegationResumed, "parent-rpd04", "child-rpd04")
+	})
+
 	it("handles empty history gracefully when injecting synthetic messages", async () => {
 		const provider = {
 			contextProxy: { globalStorageUri: { fsPath: "/tmp" } },
diff --git a/src/core/webview/ClineProvider.ts b/src/core/webview/ClineProvider.ts
index fc15a8dd5c3..62848a9b0c6 100644
--- a/src/core/webview/ClineProvider.ts
+++ b/src/core/webview/ClineProvider.ts
@@ -149,6 +149,7 @@ export class ClineProvider
 	private currentWorkspacePath: string | undefined
 
 	private recentTasksCache?: string[]
+	private taskHistoryWriteLock: Promise<void> = Promise.resolve()
 	private pendingOperations: Map<string, PendingEditOperation> = new Map()
 	private static readonly PENDING_OPERATION_TIMEOUT_MS = 30000 // 30 seconds
 
@@ -1803,10 +1804,12 @@ export class ClineProvider
 			}
 
 			// Delete all tasks from state in one batch
-			const taskHistory = this.getGlobalState("taskHistory") ?? []
-			const updatedTaskHistory = taskHistory.filter((task) => !allIdsToDelete.includes(task.id))
-			await this.updateGlobalState("taskHistory", updatedTaskHistory)
-			this.recentTasksCache = undefined
+			await this.withTaskHistoryLock(async () => {
+				const taskHistory = this.getGlobalState("taskHistory") ?? []
+				const updatedTaskHistory = taskHistory.filter((task) => !allIdsToDelete.includes(task.id))
+				await this.updateGlobalState("taskHistory", updatedTaskHistory)
+				this.recentTasksCache = undefined
+			})
 
 			// Delete associated shadow repositories or branches and task directories
 			const globalStorageDir = this.contextProxy.globalStorageUri.fsPath
@@ -1847,10 +1850,12 @@ export class ClineProvider
 	}
 
 	async deleteTaskFromState(id: string) {
-		const taskHistory = this.getGlobalState("taskHistory") ?? []
-		const updatedTaskHistory = taskHistory.filter((task) => task.id !== id)
-		await this.updateGlobalState("taskHistory", updatedTaskHistory)
-		this.recentTasksCache = undefined
+		await this.withTaskHistoryLock(async () => {
+			const taskHistory = this.getGlobalState("taskHistory") ?? []
+			const updatedTaskHistory = taskHistory.filter((task) => task.id !== id)
+			await this.updateGlobalState("taskHistory", updatedTaskHistory)
+			this.recentTasksCache = undefined
+		})
 		await this.postStateToWebview()
 	}
 
@@ -2511,6 +2516,19 @@ export class ClineProvider
 		}
 	}
 
+	/**
+	 * Serializes all read-modify-write operations on taskHistory to prevent
+	 * concurrent interleaving that can cause entries to vanish.
+	 */
+	private withTaskHistoryLock<T>(fn: () => Promise<T>): Promise<T> {
+		const result = this.taskHistoryWriteLock.then(fn, fn) // run even if previous write errored
+		this.taskHistoryWriteLock = result.then(
+			() => {},
+			() => {},
+		) // swallow for chain continuity
+		return result
+	}
+
 	/**
 	 * Updates a task in the task history and optionally broadcasts the updated history to the webview.
 	 * @param item The history item to update or add
@@ -2518,34 +2536,36 @@ export class ClineProvider
 	 * @returns The updated task history array
 	 */
 	async updateTaskHistory(item: HistoryItem, options: { broadcast?: boolean } = {}): Promise<HistoryItem[]> {
-		const { broadcast = true } = options
-		const history = (this.getGlobalState("taskHistory") as HistoryItem[] | undefined) || []
-		const existingItemIndex = history.findIndex((h) => h.id === item.id)
-		const wasExisting = existingItemIndex !== -1
-
-		if (wasExisting) {
-			// Preserve existing metadata (e.g., delegation fields) unless explicitly overwritten.
-			// This prevents loss of status/awaitingChildId/delegatedToId when tasks are reopened,
-			// terminated, or when routine message persistence occurs.
-			history[existingItemIndex] = {
-				...history[existingItemIndex],
-				...item,
+		return this.withTaskHistoryLock(async () => {
+			const { broadcast = true } = options
+			const history = (this.getGlobalState("taskHistory") as HistoryItem[] | undefined) || []
+			const existingItemIndex = history.findIndex((h) => h.id === item.id)
+			const wasExisting = existingItemIndex !== -1
+
+			if (wasExisting) {
+				// Preserve existing metadata (e.g., delegation fields) unless explicitly overwritten.
+				// This prevents loss of status/awaitingChildId/delegatedToId when tasks are reopened,
+				// terminated, or when routine message persistence occurs.
+				history[existingItemIndex] = {
+					...history[existingItemIndex],
+					...item,
+				}
+			} else {
+				history.push(item)
 			}
-		} else {
-			history.push(item)
-		}
 
-		await this.updateGlobalState("taskHistory", history)
-		this.recentTasksCache = undefined
+			await this.updateGlobalState("taskHistory", history)
+			this.recentTasksCache = undefined
 
-		// Broadcast the updated history to the webview if requested.
-		// Prefer per-item updates to avoid repeatedly cloning/sending the full history.
-		if (broadcast && this.isViewLaunched) {
-			const updatedItem = wasExisting ? history[existingItemIndex] : item
-			await this.postMessageToWebview({ type: "taskHistoryItemUpdated", taskHistoryItem: updatedItem })
-		}
+			// Broadcast the updated history to the webview if requested.
+			// Prefer per-item updates to avoid repeatedly cloning/sending the full history.
+			if (broadcast && this.isViewLaunched) {
+				const updatedItem = wasExisting ? history[existingItemIndex] : item
+				await this.postMessageToWebview({ type: "taskHistoryItemUpdated", taskHistoryItem: updatedItem })
+			}
 
-		return history
+			return history
+		})
 	}
 
 	/**
@@ -3416,7 +3436,19 @@ export class ClineProvider
 
 		await saveApiMessages({ messages: parentApiMessages as any, taskId: parentTaskId, globalStoragePath })
 
-		// 3) Update child metadata to "completed" status
+		// 3) Close child instance if still open (single-open-task invariant).
+		//    This MUST happen BEFORE updating the child's status to "completed" because
+		//    removeClineFromStack() → abortTask(true) → saveClineMessages() writes
+		//    the historyItem with initialStatus (typically "active"), which would
+		//    overwrite a "completed" status set earlier.
+		const current = this.getCurrentTask()
+		if (current?.taskId === childTaskId) {
+			await this.removeClineFromStack()
+		}
+
+		// 4) Update child metadata to "completed" status.
+		//    This runs after the abort so it overwrites the stale "active" status
+		//    that saveClineMessages() may have written during step 3.
 		try {
 			const { historyItem: childHistory } = await this.getTaskWithId(childTaskId)
 			await this.updateTaskHistory({
@@ -3431,7 +3463,7 @@ export class ClineProvider
 			)
 		}
 
-		// 4) Update parent metadata and persist BEFORE emitting completion event
+		// 5) Update parent metadata and persist BEFORE emitting completion event
 		const childIds = Array.from(new Set([...(historyItem.childIds ?? []), childTaskId]))
 		const updatedHistory: typeof historyItem = {
 			...historyItem,
@@ -3443,19 +3475,13 @@ export class ClineProvider
 		}
 		await this.updateTaskHistory(updatedHistory)
 
-		// 5) Emit TaskDelegationCompleted (provider-level)
+		// 6) Emit TaskDelegationCompleted (provider-level)
 		try {
 			this.emit(RooCodeEventName.TaskDelegationCompleted, parentTaskId, childTaskId, completionResultSummary)
 		} catch {
 			// non-fatal
 		}
 
-		// 6) Close child instance if still open (single-open-task invariant)
-		const current = this.getCurrentTask()
-		if (current?.taskId === childTaskId) {
-			await this.removeClineFromStack()
-		}
-
 		// 7) Reopen the parent from history as the sole active task (restores saved mode)
 		//    IMPORTANT: startTask=false to suppress resume-from-history ask scheduling
 		const parentInstance = await this.createTaskWithHistoryItem(updatedHistory, { startTask: false })
diff --git a/src/core/webview/__tests__/ClineProvider.taskHistory.spec.ts b/src/core/webview/__tests__/ClineProvider.taskHistory.spec.ts
index e0f1d2dc29a..aefed797443 100644
--- a/src/core/webview/__tests__/ClineProvider.taskHistory.spec.ts
+++ b/src/core/webview/__tests__/ClineProvider.taskHistory.spec.ts
@@ -420,6 +420,74 @@ describe("ClineProvider Task History Synchronization", () => {
 			expect(taskHistoryItemUpdatedCalls.length).toBe(0)
 		})
 
+		it("preserves delegated metadata on partial update unless explicitly overwritten (UTH-02)", async () => {
+			await provider.resolveWebviewView(mockWebviewView)
+			provider.isViewLaunched = true
+
+			const initial = createHistoryItem({
+				id: "task-delegated-metadata",
+				task: "Delegated task",
+				status: "delegated",
+				delegatedToId: "child-1",
+				awaitingChildId: "child-1",
+				childIds: ["child-1"],
+			})
+
+			await provider.updateTaskHistory(initial, { broadcast: false })
+
+			// Partial update intentionally omits delegated metadata fields.
+			const partialUpdate: HistoryItem = {
+				...createHistoryItem({ id: "task-delegated-metadata", task: "Delegated task (updated)" }),
+				status: "active",
+			}
+
+			const updatedHistory = await provider.updateTaskHistory(partialUpdate, { broadcast: false })
+			const updatedItem = updatedHistory.find((item) => item.id === "task-delegated-metadata")
+
+			expect(updatedItem).toBeDefined()
+			expect(updatedItem?.status).toBe("active")
+			expect(updatedItem?.delegatedToId).toBe("child-1")
+			expect(updatedItem?.awaitingChildId).toBe("child-1")
+			expect(updatedItem?.childIds).toEqual(["child-1"])
+		})
+
+		it("invalidates recentTasksCache on updateTaskHistory (UTH-04)", async () => {
+			const workspace = provider.cwd
+			const tsBase = Date.now()
+
+			await provider.updateTaskHistory(
+				createHistoryItem({
+					id: "cache-seed",
+					task: "Cache seed",
+					workspace,
+					ts: tsBase,
+				}),
+				{ broadcast: false },
+			)
+
+			const initialRecent = provider.getRecentTasks()
+			expect(initialRecent).toContain("cache-seed")
+
+			// Prime cache and verify internal cache is set.
+			expect((provider as unknown as { recentTasksCache?: string[] }).recentTasksCache).toEqual(initialRecent)
+
+			await provider.updateTaskHistory(
+				createHistoryItem({
+					id: "cache-new",
+					task: "Cache new",
+					workspace,
+					ts: tsBase + 1,
+				}),
+				{ broadcast: false },
+			)
+
+			// Direct assertion for invalidation side-effect.
+			expect((provider as unknown as { recentTasksCache?: string[] }).recentTasksCache).toBeUndefined()
+
+			const recomputedRecent = provider.getRecentTasks()
+			expect(recomputedRecent).toContain("cache-new")
+		})
+
 		it("updates existing task in history", async () => {
 			await provider.resolveWebviewView(mockWebviewView)
 			provider.isViewLaunched = true
@@ -597,4 +665,97 @@ describe("ClineProvider Task History Synchronization", () => {
 			expect(state.taskHistory.some((item: HistoryItem) => item.workspace === "/different/workspace")).toBe(true)
 		})
 	})
+
+	describe("taskHistory write lock (mutex)", () => {
+		it("serializes concurrent updateTaskHistory calls so no entries are lost", async () => {
+			await provider.resolveWebviewView(mockWebviewView)
+
+			// Fire 5 concurrent updateTaskHistory calls
+			const items = Array.from({ length: 5 }, (_, i) =>
+				createHistoryItem({ id: `concurrent-${i}`, task: `Task ${i}` }),
+			)
+
+			await Promise.all(items.map((item) => provider.updateTaskHistory(item, { broadcast: false })))
+
+			// All 5 entries must survive
+			const history = (provider as any).contextProxy.getGlobalState("taskHistory") as HistoryItem[]
+			const ids = history.map((h: HistoryItem) => h.id)
+			for (const item of items) {
+				expect(ids).toContain(item.id)
+			}
+			expect(history.length).toBe(5)
+		})
+
+		it("serializes concurrent update and deleteTaskFromState so they don't corrupt each other", async () => {
+			await provider.resolveWebviewView(mockWebviewView)
+
+			// Seed with two items
+			const keep = createHistoryItem({ id: "keep-me", task: "Keep" })
+			const remove = createHistoryItem({ id: "remove-me", task: "Remove" })
+			await provider.updateTaskHistory(keep, { broadcast: false })
+			await provider.updateTaskHistory(remove, { broadcast: false })
+
+			// Concurrently: add a new item AND delete "remove-me"
+			const newItem = createHistoryItem({ id: "new-item", task: "New" })
+			await Promise.all([
+				provider.updateTaskHistory(newItem, { broadcast: false }),
+				provider.deleteTaskFromState("remove-me"),
+			])
+
+			const history = (provider as any).contextProxy.getGlobalState("taskHistory") as HistoryItem[]
+			const ids = history.map((h: HistoryItem) => h.id)
+			expect(ids).toContain("keep-me")
+			expect(ids).toContain("new-item")
+			expect(ids).not.toContain("remove-me")
+		})
+
+		it("does not block subsequent writes when a previous write errors", async () => {
+			await provider.resolveWebviewView(mockWebviewView)
+
+			// Temporarily make updateGlobalState throw
+			const origUpdateGlobalState = (provider as any).updateGlobalState.bind(provider)
+			let callCount = 0
+			;(provider as any).updateGlobalState = vi.fn().mockImplementation((...args: unknown[]) => {
+				callCount++
+				if (callCount === 1) {
+					return Promise.reject(new Error("simulated write failure"))
+				}
+				return origUpdateGlobalState(...args)
+			})
+
+			// First call should fail
+			const item1 = createHistoryItem({ id: "fail-item", task: "Fail" })
+			await expect(provider.updateTaskHistory(item1, { broadcast: false })).rejects.toThrow(
+				"simulated write failure",
+			)
+
+			// Second call should still succeed (lock not stuck)
+			const item2 = createHistoryItem({ id: "ok-item", task: "OK" })
+			const result = await provider.updateTaskHistory(item2, { broadcast: false })
+			expect(result.some((h) => h.id === "ok-item")).toBe(true)
+		})
+
+		it("serializes concurrent updates to the same item preserving the last write", async () => {
+			await provider.resolveWebviewView(mockWebviewView)
+
+			const base = createHistoryItem({ id: "race-item", task: "Original" })
+			await provider.updateTaskHistory(base, { broadcast: false })
+
+			// Fire two concurrent updates to the same item
+			await Promise.all([
+				provider.updateTaskHistory(createHistoryItem({ id: "race-item", task: "Original", tokensIn: 111 }), {
+					broadcast: false,
+				}),
+				provider.updateTaskHistory(createHistoryItem({ id: "race-item", task: "Original", tokensIn: 222 }), {
+					broadcast: false,
+				}),
+			])
+
+			const history = (provider as any).contextProxy.getGlobalState("taskHistory") as HistoryItem[]
+			const item = history.find((h: HistoryItem) => h.id === "race-item")
+			expect(item).toBeDefined()
+			// The second write (tokensIn: 222) should be the last one since writes are serialized
+			expect(item!.tokensIn).toBe(222)
+		})
+	})
 })