perf(webview): send appended chat messages as deltas, not full history

Appending a new chat message used to call postTaskStateToWebview, which
re-serialized and re-sent the entire clineMessages array (and rebuilt the
full state object) over the webview postMessage bridge on every message.
For long or multiple concurrent conversations that array dominates the
per-update payload and hurts responsiveness.

Streaming token updates already use the single-message `messageUpdated`
delta; this closes the remaining full-payload path — message append.

- Add a `messageAdded` extension->webview message type carrying one
  ClineMessage.
- ClineProvider.postTaskMessageAddedToWebview: for the visible task, post the
  single new message then refresh the remaining task-scoped fields via
  postStateToWebviewWithoutClineMessages (omits clineMessages + taskHistory);
  for a background task, just schedule the active-conversations summary update.
- Task.addToClineMessages routes appends through this delta path, with a
  fallback to a full task-state push for provider shapes that lack it.
- Webview appends the delta in place (idempotent: replaces by ts if already
  present, e.g. if a later full-state push raced it).

taskHistory was already deltified previously; clineMessages was the remaining
large array re-sent on each update.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: allquixotic

packages/types/src/vscode-extension-host.ts

@@ -33,6 +33,7 @@ export interface ExtensionMessage {
 		| "workspaceUpdated"
 		| "invoke"
 		| "messageUpdated"
+		| "messageAdded"
 		| "mcpServers"
 		| "enhancedPrompt"
 		| "commitSearchResults"

src/core/task/Task.ts

@@ -719,6 +719,21 @@ export class Task extends EventEmitter<TaskEvents> implements TaskLike {
 		}
 	}
 
+	/**
+	 * Push a newly-appended message to the webview as a delta when the provider supports
+	 * it, falling back to a full task-state push for older provider shapes / tests.
+	 */
+	private async postTaskMessageAddedToWebview(message: ClineMessage): Promise<void> {
+		const provider = this.providerRef.deref()
+		if (typeof provider?.postTaskMessageAddedToWebview === "function") {
+			await provider.postTaskMessageAddedToWebview(this.taskId, message)
+			return
+		}
+		if (typeof provider?.postTaskStateToWebview === "function") {
+			await provider.postTaskStateToWebview(this.taskId)
+		}
+	}
+
 	/**
 	 * Wait for the task mode to be initialized before proceeding.
 	 * This method ensures that any operations depending on the task mode
@@ -1102,9 +1117,12 @@ export class Task extends EventEmitter<TaskEvents> implements TaskLike {
 
 	private async addToClineMessages(message: ClineMessage) {
 		this.clineMessages.push(message)
-		// Avoid resending large, mostly-static fields (notably taskHistory) on every chat message update.
-		// taskHistory is maintained in-memory in the webview and updated via taskHistoryItemUpdated.
-		await this.postTaskStateToWebview()
+		// Send only the newly-appended message as a delta instead of re-serializing and
+		// re-sending the entire clineMessages array (and taskHistory) on every chat message.
+		// The webview appends it in-place; the remaining task-scoped fields are refreshed
+		// without the large message/history arrays. Falls back to a full task-state push
+		// for provider shapes that don't support the delta channel.
+		await this.postTaskMessageAddedToWebview(message)
 		this.emit(RooCodeEventName.Message, { action: "created", message })
 		await this.saveClineMessages()
 
@@ -2912,9 +2930,13 @@ export class Task extends EventEmitter<TaskEvents> implements TaskLike {
 								if (signal.aborted) {
 									reject(new Error("Request cancelled by user"))
 								} else {
-									signal.addEventListener("abort", () => {
-										reject(new Error("Request cancelled by user"))
-									}, { once: true })
+									signal.addEventListener(
+										"abort",
+										() => {
+											reject(new Error("Request cancelled by user"))
+										},
+										{ once: true },
+									)
 								}
 							})
 							return await Promise.race([nextPromise, abortPromise])
@@ -4456,10 +4478,14 @@ export class Task extends EventEmitter<TaskEvents> implements TaskLike {
 		const iterator = stream[Symbol.asyncIterator]()
 
 		// Set up abort handling - when the signal is aborted, clean up the controller reference
-		abortSignal.addEventListener("abort", () => {
-			console.log(`[Task#${this.taskId}.${this.instanceId}] AbortSignal triggered for current request`)
-			this.currentRequestAbortController = undefined
-		}, { once: true })
+		abortSignal.addEventListener(
+			"abort",
+			() => {
+				console.log(`[Task#${this.taskId}.${this.instanceId}] AbortSignal triggered for current request`)
+				this.currentRequestAbortController = undefined
+			},
+			{ once: true },
+		)
 
 		try {
 			// Awaiting first chunk to see if it will throw an error.
@@ -4471,9 +4497,13 @@ export class Task extends EventEmitter<TaskEvents> implements TaskLike {
 				if (abortSignal.aborted) {
 					reject(new Error("Request cancelled by user"))
 				} else {
-					abortSignal.addEventListener("abort", () => {
-						reject(new Error("Request cancelled by user"))
-					}, { once: true })
+					abortSignal.addEventListener(
+						"abort",
+						() => {
+							reject(new Error("Request cancelled by user"))
+						},
+						{ once: true },
+					)
 				}
 			})
 

src/core/webview/ClineProvider.ts

@@ -582,6 +582,29 @@ export class ClineProvider
 		this.scheduleActiveConversationsStateToWebview()
 	}
 
+	/**
+	 * Push a newly-appended chat message as a delta instead of re-sending the entire
+	 * clineMessages array.
+	 *
+	 * Appending a single message used to trigger a full task-state push, which
+	 * re-serialized and re-sent every message in the conversation over the webview
+	 * postMessage bridge. For long or multiple concurrent conversations that is the
+	 * dominant responsiveness cost. Here we send only the new message (`messageAdded`)
+	 * and then refresh the remaining task-scoped fields (token totals, todos, queue,
+	 * active conversations) WITHOUT the large clineMessages/taskHistory arrays.
+	 *
+	 * Streaming token updates already use the single-message `messageUpdated` delta;
+	 * this closes the remaining full-payload path (message append).
+	 */
+	public async postTaskMessageAddedToWebview(taskId: string, message: ClineMessage): Promise<void> {
+		if (this.isTaskVisible(taskId)) {
+			await this.postMessageToWebview({ type: "messageAdded", clineMessage: message })
+			await this.postStateToWebviewWithoutClineMessages()
+			return
+		}
+		this.scheduleActiveConversationsStateToWebview()
+	}
+
 	/**
 	 * Initialize the TaskHistoryStore and migrate from globalState if needed.
 	 */

webview-ui/src/context/ExtensionStateContext.tsx

@@ -412,6 +412,26 @@ export const ExtensionStateContextProvider: React.FC<{ children: React.ReactNode
 					})
 					break
 				}
+				case "messageAdded": {
+					// Delta append: a single newly-created message instead of a full
+					// clineMessages re-send. Idempotent — if a message with this ts is
+					// already present (e.g. a later full-state push raced us), replace it
+					// in place rather than duplicating.
+					const clineMessage = message.clineMessage!
+					setState((prevState) => {
+						const existingIndex = findLastIndex(
+							prevState.clineMessages,
+							(msg) => msg.ts === clineMessage.ts,
+						)
+						if (existingIndex !== -1) {
+							const newClineMessages = [...prevState.clineMessages]
+							newClineMessages[existingIndex] = clineMessage
+							return { ...prevState, clineMessages: newClineMessages }
+						}
+						return { ...prevState, clineMessages: [...prevState.clineMessages, clineMessage] }
+					})
+					break
+				}
 				case "skills": {
 					if (message.skills) {
 						setSkills(message.skills)