From 541b6c849657d13fed3580407a00a996e891ad9e Mon Sep 17 00:00:00 2001
From: hugocasa <hugo@casademont.ch>
Date: Mon, 24 Aug 2026 22:30:45 +0200
Subject: [PATCH] fix: keep ai chat messages when leaving the page
 mid-generation (#10809)

* fix: persist ai chat turns mid-generation so leaving the page keeps them

* fix: stop chat checkpoints once the turn commits, keep streamed text visible

* fix: checkpoint streamed answers as they grow and keep half-run tool batches

* fix: checkpoint text as received so a backgrounded tab keeps capturing

* fix: keep buffered tool screenshots in mid-batch chat checkpoints

* fix: decide committed-text at the flush site, condense checkpoint comments

* fix: checkpoint only live streamed text, never text the parser owns

* fix: don't swap the chat transcript out from under a running turn

* fix: close the pre-loading window in the conversation-switch guard
---
 .../copilot/chat/AIChatDisplay.svelte         |   6 +-
 .../copilot/chat/AIChatManager.svelte.ts      | 181 ++++++++-
 .../copilot/chat/AIChatManager.test.ts        | 362 ++++++++++++++++++
 .../components/copilot/chat/chatLoop.test.ts  |  26 +-
 .../lib/components/copilot/chat/chatLoop.ts   |  32 ++
 .../src/lib/components/copilot/chat/shared.ts |  25 +-
 .../copilot/chat/typewriterReveal.test.ts     |  20 +
 .../copilot/chat/typewriterReveal.ts          |   9 +
 8 files changed, 635 insertions(+), 26 deletions(-)

diff --git a/frontend/src/lib/components/copilot/chat/AIChatDisplay.svelte b/frontend/src/lib/components/copilot/chat/AIChatDisplay.svelte
index 2dd8f7bef7..9b55bd9217 100644
--- a/frontend/src/lib/components/copilot/chat/AIChatDisplay.svelte
+++ b/frontend/src/lib/components/copilot/chat/AIChatDisplay.svelte
@@ -611,7 +611,11 @@ the panel, or the Escape-to-stop focus check would wrongly reject them. -->
 								<div class="flex flex-col">
 									{#each pastChats as chat (chat.id)}
 										<button
-											class="text-left flex flex-row items-center gap-2 justify-between hover:bg-gray-100 dark:hover:bg-gray-700 rounded-md p-1"
+											class="text-left flex flex-row items-center gap-2 justify-between hover:bg-gray-100 dark:hover:bg-gray-700 rounded-md p-1 disabled:opacity-50 disabled:cursor-not-allowed disabled:hover:bg-transparent dark:disabled:hover:bg-transparent"
+											disabled={aiChatManager.loading || aiChatManager.sendInFlight}
+											title={aiChatManager.loading || aiChatManager.sendInFlight
+												? 'Stop the current answer to switch conversation'
+												: undefined}
 											onclick={() => {
 												loadPastChat(chat.id)
 												close()
diff --git a/frontend/src/lib/components/copilot/chat/AIChatManager.svelte.ts b/frontend/src/lib/components/copilot/chat/AIChatManager.svelte.ts
index f68501b293..fdbea71c4b 100644
--- a/frontend/src/lib/components/copilot/chat/AIChatManager.svelte.ts
+++ b/frontend/src/lib/components/copilot/chat/AIChatManager.svelte.ts
@@ -29,6 +29,7 @@ import {
 	completedJobToolStatus,
 	backgroundJobCompletionNote,
 	deriveChatJobStatus,
+	pendingToolImagesMessage,
 	trimJob
 } from './shared'
 import type {
@@ -99,7 +100,7 @@ import {
 import type { Selection } from 'monaco-editor'
 import type AIChatInput from './AIChatInput.svelte'
 import { prepareApiSystemMessage, prepareApiUserMessage } from './api/core'
-import { runChatLoop, truncateToToolPairedPrefix } from './chatLoop'
+import { closeInterruptedToolBatch, runChatLoop, truncateToToolPairedPrefix } from './chatLoop'
 import { sanitizeToolCallArguments } from './toolCallArguments'
 import {
 	billedTokens,
@@ -164,6 +165,15 @@ function prefersInstantReveal(): boolean {
 // schema changes from mode switches, and the estimate's chars/4 error.
 const COMPACTION_TRIGGER_RATIO = 0.8
 const COMPACTION_TARGET_RATIO = 0.7
+// How often a running turn is offered to the mid-turn checkpoint (see
+// sendRequest). The whole transcript is rewritten on each accepted checkpoint,
+// so this bounds the write rate; it also bounds how much of a turn a tab that
+// dies without warning can lose.
+const CHECKPOINT_INTERVAL_MS = 2000
+// Stands in for the result of a tool call that had not finished when the
+// transcript was checkpointed — still running, or still waiting to be confirmed.
+// The model reads the step as unfinished, which is what the card tells the reader.
+const INTERRUPTED_TOOL_RESULT = 'Interrupted: the chat was closed before this tool finished'
 // Flat per-image token estimate for a downscaled (≤1568px) vision image. Used instead
 // of chars/4 on the base64 data URL, which would overcount by ~50x.
 const IMAGE_TOKEN_ESTIMATE = 1200
@@ -2296,27 +2306,63 @@ export class AIChatManager {
 		}
 	}
 
-	// Commit an interrupted turn's usable output as context for a follow-up:
-	// the tool-paired prefix of completed steps (a dangling tool call would
-	// make providers reject the next request) plus the partial answer text.
-	// A reasoning-only interrupt instead drops its stuck-open bubble.
-	private commitInterruptedTurn = (
+	// The transcript an interrupted turn leaves behind: the stored history, the
+	// tool-paired prefix of the turn's completed steps (a dangling tool call
+	// would make providers reject the next request), and the partial answer text
+	// when it isn't already inside that prefix. Pure — the caller decides whether
+	// this becomes the live transcript or only a persisted checkpoint.
+	private interruptedTurnMessages = (
 		collectedMessages: ChatCompletionMessageParam[],
-		partialReply: string
-	) => {
-		const prefix = truncateToToolPairedPrefix(collectedMessages)
-		this.messages = [...this.messages, ...prefix]
+		partialReply: string,
+		// Passed only by the mid-turn checkpoint, whose result must outlive its turn.
+		// A turn committed for a follow-up is still live, so it would rather truncate
+		// a half-run batch and rerun it than read results nothing produced.
+		snapshot?: {
+			/** Result to synthesize for the calls of a batch caught mid-execution. */
+			interruptedToolContent: string
+			/** Images a tool has produced that the turn has not yet turned into a
+			 *  message (see appendPendingToolImages). */
+			bufferedImages?: ChatCompletionMessageParam
+		}
+	): { messages: ChatCompletionMessageParam[]; keptPartialReply: boolean } => {
+		const prefix = snapshot
+			? closeInterruptedToolBatch(collectedMessages, snapshot.interruptedToolContent)
+			: truncateToToolPairedPrefix(collectedMessages)
 		// partialReply can be stale — equal to text already committed inside the
-		// prefix (see its capture in onMessageEnd) — so only append when new.
+		// prefix — so only append when new. A snapshot is exempt: it passes only
+		// live streaming text, which is never in the prefix, and identical text can
+		// legitimately recur across iterations where content alone cannot judge it.
 		const lastCommittedText = [...prefix]
 			.reverse()
 			.find(
 				(m): m is ChatCompletionMessageParam & { content: string } =>
 					m.role === 'assistant' && typeof m.content === 'string' && !!m.content.trim()
 			)?.content
-		if (partialReply.trim() && partialReply !== lastCommittedText) {
-			this.messages = [...this.messages, { role: 'assistant', content: partialReply }]
-		} else {
+		const keptPartialReply =
+			!!partialReply.trim() && (!!snapshot || partialReply !== lastCommittedText)
+		// Images sit between the batch that produced them and whatever the model
+		// said next, matching where appendPendingToolImages puts them live.
+		const tail = snapshot?.bufferedImages ? [...prefix, snapshot.bufferedImages] : prefix
+		return {
+			messages: keptPartialReply
+				? [...this.messages, ...tail, { role: 'assistant', content: partialReply }]
+				: [...this.messages, ...tail],
+			keptPartialReply
+		}
+	}
+
+	// Commit an interrupted turn's usable output as context for a follow-up.
+	// A reasoning-only interrupt instead drops its stuck-open bubble.
+	private commitInterruptedTurn = (
+		collectedMessages: ChatCompletionMessageParam[],
+		partialReply: string
+	) => {
+		const { messages, keptPartialReply } = this.interruptedTurnMessages(
+			collectedMessages,
+			partialReply
+		)
+		this.messages = messages
+		if (!keptPartialReply) {
 			const last = this.displayMessages[this.displayMessages.length - 1]
 			if (last?.role === 'assistant' && !last.content.trim() && !!last.reasoning) {
 				this.displayMessages = this.displayMessages.slice(0, -1)
@@ -3036,6 +3082,83 @@ export class AIChatManager {
 		// auto-sends the next queued message. Cancel, error, and empty-response
 		// rollbacks leave it false so queued text is restored to the input.
 		let turnCommittedCleanly = false
+		// A turn's output only reaches history when the turn ends, so a tab closed
+		// mid-turn loses every step it had taken. Persist progress WITHOUT
+		// committing it: the outcome branches still need `this.messages`
+		// unmodified to roll the turn back, and their save overwrites this.
+		let checkpointedShape = ''
+		const checkpointTurn = async (force = false) => {
+			// Text as received, not as painted: a hidden tab pauses the reveal loop
+			// while text keeps arriving, so fingerprinting painted text would stall
+			// the poll exactly when nobody is watching. `pending` reads it without
+			// disturbing the animation.
+			const streaming = this.currentReply + this.replyReveal.pending
+			// Write only when the turn advanced, so a parked confirmation costs
+			// nothing and the rate follows steps taken rather than time.
+			const shape = `${collectedMessages.length}:${this.displayMessages.length}:${streaming.length}`
+			if (!force && shape === checkpointedShape) return
+			// Live text only. Text the parsers have flushed is theirs to push, and
+			// they do so before the tool execution a checkpoint is likely to land in
+			// — so reading it here would mean re-appending what the transcript
+			// already holds. The abort path still recovers it via partialReply.
+			const { messages, keptPartialReply } = this.interruptedTurnMessages(
+				collectedMessages,
+				streaming,
+				{
+					interruptedToolContent: INTERRUPTED_TOOL_RESULT,
+					// A screenshot's image becomes a message only once its whole batch
+					// does, so a batch closed mid-flight would restore a result
+					// announcing a screenshot the model cannot see. Read without
+					// draining: the live turn still owns the buffer.
+					bufferedImages: pendingToolImagesMessage([...this.pendingToolImages.values()].flat())
+				}
+			)
+			if (messages.length === this.messages.length) return
+			checkpointedShape = shape
+			const display = this.settledToolDisplay(this.displayMessages, 'Interrupted')
+			// onMessageEnd is what gives streamed text its bubble, and it clears
+			// currentReply doing so — text still there has none, and without one the
+			// reply returns as context the reader cannot see.
+			const withStreamed =
+				streaming && keptPartialReply
+					? [...display, { role: 'assistant' as const, content: streaming }]
+					: display
+			// Best-effort: the turn-end save is the authoritative one, so a failed
+			// checkpoint must never break the turn it is only shadowing.
+			try {
+				await this.historyManager.saveChat(
+					withStreamed,
+					messages,
+					// No report describes this transcript: `contextUsage` still measures
+					// the pre-turn history while these messages already carry part of the
+					// turn. Storing it would under-report a restored chat by the whole
+					// partial turn — enough to skip the compaction its next send needs.
+					// Omitting drops the field, which is the "readers estimate" fallback.
+					undefined,
+					this.modifiedItems ? [...this.modifiedItems] : undefined
+				)
+			} catch (e) {
+				console.error('Failed to checkpoint chat mid-turn', e)
+			}
+		}
+		const checkpointTimer = setInterval(() => void checkpointTurn(), CHECKPOINT_INTERVAL_MS)
+		// `hidden` precedes pagehide on close, reload and navigation and still runs a
+		// live document, so it is the last point a write can land. A navigation can
+		// outrun it — the poll, not this, carries the guarantee. `document` is absent
+		// under SSR and the node test env.
+		const hideTarget = typeof document !== 'undefined' ? document : undefined
+		const checkpointOnHide = () => {
+			if (hideTarget?.visibilityState === 'hidden') void checkpointTurn(true)
+		}
+		hideTarget?.addEventListener('visibilitychange', checkpointOnHide)
+		// Must stop when the loop hands back, not in `finally`: the outcome branches
+		// merge the turn into `this.messages` before awaiting their save, so a
+		// checkpoint there would re-append the same messages and, queued behind that
+		// save, persist the duplicate. Idempotent — every exit path calls it.
+		const stopCheckpoints = () => {
+			clearInterval(checkpointTimer)
+			hideTarget?.removeEventListener('visibilitychange', checkpointOnHide)
+		}
 		try {
 			// A queued message carries its own context snapshot (contextOverride); use
 			// it verbatim and leave the live selection alone (it belongs to whatever the
@@ -3407,6 +3530,7 @@ export class AIChatManager {
 					webSearchUnavailable = true
 				}
 			})
+			stopCheckpoints()
 			const wasAborted = this.abortController?.signal.aborted ?? false
 			// Pure reasoning doesn't count as usable: it's not replayed as context,
 			// so a reasoning-only turn is as unsent as a literally empty one.
@@ -3518,6 +3642,7 @@ export class AIChatManager {
 				}
 			}
 		} catch (err) {
+			stopCheckpoints()
 			console.error(err)
 			// Request failure: keep the usable output as context for a follow-up.
 			// Skipped when the throw came from post-outcome code (e.g. saveChat) —
@@ -3576,6 +3701,9 @@ export class AIChatManager {
 			sendUserToast(getSendRequestErrorMessage(err, webSearchUnavailable), true)
 		} finally {
 			this.loading = false
+			// Backstop for the paths that leave the try without reaching either call
+			// above (a pre-flight throw, an aborted compaction).
+			stopCheckpoints()
 			// Turn teardown: cancel any in-flight reveal frame and drop leftover
 			// backlog. onMessageEnd already flushed on every outcome, so this only
 			// releases the loop; it never discards uncommitted text.
@@ -3802,6 +3930,11 @@ export class AIChatManager {
 	}
 
 	loadPastChat = async (id: string) => {
+		// A turn commits into whatever transcript it finds when it ends, so swapping
+		// one in underneath it misfiles the turn — or duplicates it, when the loaded
+		// chat already carries the turn's own checkpoint. Gated on `sendInFlight`
+		// too, for the pre-`loading` window `sendOrQueue` documents.
+		if (this.loading || this.sendInFlight) return
 		const chat = await this.historyManager.loadPastChat(id)
 		if (chat) {
 			// Drop any message queued in the current conversation so it doesn't
@@ -4155,13 +4288,26 @@ export class AIChatManager {
 		}
 	}
 
-	cancelLoadingTools = (messageText: 'Canceled' | 'Error' = 'Canceled') => {
-		this.displayMessages = this.displayMessages.map((message) => {
+	// In-flight and queued tool cards settled into a terminal state. Persisting
+	// one as-is restores a card that spins forever, and an unanswered question
+	// keeps the composer disabled (see isActiveUserQuestion) with nothing left
+	// running to answer it — so every transcript that outlives its turn goes
+	// through here first.
+	private settledToolDisplay = (
+		messages: DisplayMessage[],
+		messageText: string
+	): DisplayMessage[] =>
+		messages.map((message) => {
 			if (message.role === 'tool' && (message.isLoading || message.isQueued)) {
 				return {
 					...message,
 					isLoading: false,
 					isQueued: false,
+					// Both render live affordances on their own, without consulting
+					// isLoading: a Run/Reject footer for a call nothing is waiting on,
+					// and a card that hides its result as still-streaming.
+					needsConfirmation: false,
+					isStreamingArguments: false,
 					// A question's card disappears once canceled, so keep the question
 					// itself readable in the collapsed header.
 					content: message.userQuestion
@@ -4175,6 +4321,9 @@ export class AIChatManager {
 			}
 			return message
 		})
+
+	cancelLoadingTools = (messageText: 'Canceled' | 'Error' = 'Canceled') => {
+		this.displayMessages = this.settledToolDisplay(this.displayMessages, messageText)
 	}
 }
 
diff --git a/frontend/src/lib/components/copilot/chat/AIChatManager.test.ts b/frontend/src/lib/components/copilot/chat/AIChatManager.test.ts
index 0a0330d939..68eb4cbab1 100644
--- a/frontend/src/lib/components/copilot/chat/AIChatManager.test.ts
+++ b/frontend/src/lib/components/copilot/chat/AIChatManager.test.ts
@@ -4,6 +4,7 @@ import type { PipelineAIChatHelpers } from './pipeline/core'
 import type { CurrentEditor } from '$lib/components/flows/types'
 import type { ReviewChangesOpts } from './monaco-adapter'
 import type { ChatCompletionMessageParam } from 'openai/resources/chat/completions.mjs'
+import type { DisplayMessage } from './shared'
 import type { AttachedImage } from './imageUtils'
 import { AIChatManager, AIMode, AIAutonomyMode } from './AIChatManager.svelte'
 import { chatState } from './sharedChatState.svelte'
@@ -1800,6 +1801,312 @@ describe('AIChatManager queued messages', () => {
 		expect(input.restoreInstructions).not.toHaveBeenCalled()
 	})
 
+	// This suite runs under the node env, so stand up the minimum document the
+	// manager needs to hear the page going away. Returns the registered listeners
+	// so a test can play the user leaving.
+	function stubHidingPage() {
+		const leavePage = new Set<() => void>()
+		vi.stubGlobal('document', {
+			visibilityState: 'hidden',
+			addEventListener: (_: string, fn: () => void) => leavePage.add(fn),
+			removeEventListener: (_: string, fn: () => void) => leavePage.delete(fn)
+		})
+		return leavePage
+	}
+
+	// Every checkpoint test installs a fake document; leaking one would make a
+	// single failure cascade through every later test in the file.
+	afterEach(() => {
+		vi.unstubAllGlobals()
+		vi.useRealTimers()
+	})
+
+	it('keeps re-checkpointing a streamed answer as it grows', async () => {
+		vi.useFakeTimers({ shouldAdvanceTime: true })
+		const manager = createManager()
+		const saveChat = vi.spyOn(manager.historyManager, 'saveChat').mockResolvedValue(undefined)
+		// One poll interval, per CHECKPOINT_INTERVAL_MS in AIChatManager.
+		const pastOnePoll = 2100
+
+		mocks.runChatLoop.mockImplementationOnce(async (config: any) => {
+			// A text-only answer: nothing lands in addedMessages and no card appears,
+			// so the growing reply is the only thing that can drive the poll.
+			for (const text of ['the first part', 'the first part and more', 'the whole answer']) {
+				manager.currentReply = text
+				await vi.advanceTimersByTimeAsync(pastOnePoll)
+			}
+			const message = { role: 'assistant' as const, content: manager.currentReply }
+			config.addedMessages.push(message)
+			return {
+				addedMessages: [message],
+				tokenUsage: { prompt: 0, completion: 0, total: 0 },
+				hitMaxIterations: false
+			}
+		})
+
+		await manager.sendRequest({ instructions: 'write me something long' })
+
+		// A fingerprint blind to the reply would freeze at whatever the first tick
+		// captured, so each tick must persist strictly more of the answer.
+		const persisted = saveChat.mock.calls
+			.map(([, messages]) => messages as ChatCompletionMessageParam[])
+			.map((messages) => messages[messages.length - 1])
+			.filter((m) => m?.role === 'assistant' && typeof m.content === 'string')
+			.map((m) => String(m.content))
+			.filter((c) => c.startsWith('the first part'))
+		expect(persisted).toEqual(['the first part', 'the first part and more'])
+	})
+
+	it('carries a completed screenshot into a checkpoint of the batch it came from', async () => {
+		const leavePage = stubHidingPage()
+		const manager = createManager()
+		const saveChat = vi.spyOn(manager.historyManager, 'saveChat').mockResolvedValue(undefined)
+
+		mocks.runChatLoop.mockImplementationOnce(async (config: any) => {
+			// take_screenshot finished and buffered its image, but the batch it belongs
+			// to has another call still pending — so the loop has not yet turned the
+			// buffer into a message.
+			config.addedMessages.push(
+				{
+					role: 'assistant' as const,
+					content: '',
+					tool_calls: [
+						{
+							id: 'shot',
+							type: 'function' as const,
+							function: { name: 'take_screenshot', arguments: '{}' }
+						},
+						{
+							id: 'next',
+							type: 'function' as const,
+							function: { name: 'do_thing', arguments: '{}' }
+						}
+					]
+				},
+				{ role: 'tool' as const, tool_call_id: 'shot', content: 'Screenshot attached below' }
+			)
+			config.callbacks.attachToolImage('shot', {
+				dataUrl: 'data:image/png;base64,iVBORw0KGgo=',
+				name: 'shot.png'
+			})
+			leavePage.forEach((fn) => fn())
+			return {
+				addedMessages: config.addedMessages,
+				tokenUsage: { prompt: 0, completion: 0, total: 0 },
+				hitMaxIterations: false
+			}
+		})
+
+		await manager.sendRequest({ instructions: 'look at the app' })
+
+		const [, actual] = saveChat.mock.calls.find(
+			([, messages]) => messages.length > 1
+		) as unknown as [DisplayMessage[], ChatCompletionMessageParam[]]
+		// Without the image the restored history announces a screenshot the model
+		// cannot see, so the next turn cannot answer anything about it.
+		const imageParts = actual.flatMap((m) =>
+			Array.isArray(m.content) ? m.content.filter((p: any) => p.type === 'image_url') : []
+		)
+		expect(imageParts).toHaveLength(1)
+		// And it sits after the batch that produced it, where the live path puts it.
+		const imageIdx = actual.findIndex((m) => Array.isArray(m.content))
+		const resultIdx = actual.findIndex((m) => m.role === 'tool' && m.tool_call_id === 'shot')
+		expect(imageIdx).toBeGreaterThan(resultIdx)
+	})
+
+	it('does not repeat a preamble the parser has already pushed', async () => {
+		const leavePage = stubHidingPage()
+		const manager = createManager()
+		const saveChat = vi.spyOn(manager.historyManager, 'saveChat').mockResolvedValue(undefined)
+		const preamble = 'Let me look that up.'
+
+		mocks.runChatLoop.mockImplementationOnce(async (config: any) => {
+			// A text-then-tool-call turn in parser order: the preamble is flushed when
+			// the tool call starts, pushed when the message completes, and only then
+			// do the tools run — the long window a checkpoint is most likely to land in.
+			manager.currentReply = preamble
+			config.callbacks.onMessageEnd()
+			config.addedMessages.push(
+				{ role: 'assistant' as const, content: preamble },
+				{
+					role: 'assistant' as const,
+					content: '',
+					tool_calls: [
+						{ id: 't1', type: 'function' as const, function: { name: 'do_thing', arguments: '{}' } }
+					]
+				},
+				{ role: 'tool' as const, tool_call_id: 't1', content: 'ok' }
+			)
+			leavePage.forEach((fn) => fn())
+			return {
+				addedMessages: config.addedMessages,
+				tokenUsage: { prompt: 0, completion: 0, total: 0 },
+				hitMaxIterations: false
+			}
+		})
+
+		await manager.sendRequest({ instructions: 'look something up' })
+
+		const [, actual] = saveChat.mock.calls.find(
+			([, messages]) => messages.length > 1
+		) as unknown as [DisplayMessage[], ChatCompletionMessageParam[]]
+		// Reading flushed text back would show the preamble twice on reload; the
+		// transcript already holds it, so the checkpoint must take it from there.
+		expect(actual.filter((m) => m.content === preamble)).toHaveLength(1)
+	})
+
+	it('checkpoints a live reply that repeats an earlier segment verbatim', async () => {
+		const leavePage = stubHidingPage()
+		const manager = createManager()
+		const saveChat = vi.spyOn(manager.historyManager, 'saveChat').mockResolvedValue(undefined)
+		const repeated = 'Let me check that.'
+
+		mocks.runChatLoop.mockImplementationOnce(async (config: any) => {
+			// The model said the same sentence before its tool call as it is saying
+			// after it — the staleness heuristic must not read the second one as a
+			// duplicate of the first and drop it.
+			config.addedMessages.push(
+				{
+					role: 'assistant' as const,
+					content: repeated,
+					tool_calls: [
+						{ id: 't1', type: 'function' as const, function: { name: 'do_thing', arguments: '{}' } }
+					]
+				},
+				{ role: 'tool' as const, tool_call_id: 't1', content: 'ok' }
+			)
+			manager.currentReply = repeated
+			leavePage.forEach((fn) => fn())
+			return {
+				addedMessages: config.addedMessages,
+				tokenUsage: { prompt: 0, completion: 0, total: 0 },
+				hitMaxIterations: false
+			}
+		})
+
+		await manager.sendRequest({ instructions: 'check the thing' })
+
+		const [display, actual] = saveChat.mock.calls.find(
+			([, messages]) => messages.length > 1
+		) as unknown as [DisplayMessage[], ChatCompletionMessageParam[]]
+		expect(actual[actual.length - 1]).toMatchObject({ role: 'assistant', content: repeated })
+		expect(display[display.length - 1]).toMatchObject({ role: 'assistant', content: repeated })
+	})
+
+	it('checkpoints a turn to history when the page is hidden mid-generation', async () => {
+		const leavePage = stubHidingPage()
+		const manager = createManager()
+		const saveChat = vi.spyOn(manager.historyManager, 'saveChat').mockResolvedValue(undefined)
+		const toolCall = (id: string, name: string) => ({
+			role: 'assistant' as const,
+			content: '',
+			tool_calls: [{ id, type: 'function' as const, function: { name, arguments: '{}' } }]
+		})
+
+		mocks.runChatLoop.mockImplementationOnce(async (config: any) => {
+			// One completed round-trip, then a call still waiting on the user.
+			config.addedMessages.push(
+				toolCall('t1', 'write_script'),
+				{ role: 'tool', tool_call_id: 't1', content: 'created' },
+				toolCall('t2', 'test_run_script')
+			)
+			config.callbacks.setToolStatus('t2', {
+				content: 'Waiting for confirmation...',
+				isLoading: true,
+				needsConfirmation: true
+			})
+			leavePage.forEach((fn) => fn())
+			const message = { role: 'assistant' as const, content: 'done' }
+			config.addedMessages.push({ role: 'tool', tool_call_id: 't2', content: 'ran' }, message)
+			return {
+				addedMessages: config.addedMessages,
+				tokenUsage: { prompt: 0, completion: 0, total: 0 },
+				hitMaxIterations: false
+			}
+		})
+
+		await manager.sendRequest({ instructions: 'write and run a script' })
+
+		const [display, actual] = saveChat.mock.calls.find(
+			([, messages]) => messages.length > 1
+		) as unknown as [DisplayMessage[], ChatCompletionMessageParam[]]
+		// Both steps are kept, and the unfinished t2 call gets a synthesized result:
+		// leaving it dangling would make the next request 400, dropping it would lose
+		// the step the reader can still see on the card below.
+		expect(actual.map((m) => m.role)).toEqual(['user', 'assistant', 'tool', 'assistant', 'tool'])
+		expect(actual[actual.length - 1]).toMatchObject({
+			tool_call_id: 't2',
+			content: expect.stringContaining('Interrupted')
+		})
+		// Its card is kept but settled, so reopening the chat doesn't restore a
+		// confirmation prompt with nothing behind it.
+		expect(display.find((m) => m.role === 'tool' && m.tool_call_id === 't2')).toMatchObject({
+			isLoading: false,
+			needsConfirmation: false,
+			error: 'Interrupted'
+		})
+		// The turn itself is untouched by the checkpoint and still commits in full.
+		expect(manager.messages.map((m) => m.role)).toEqual([
+			'user',
+			'assistant',
+			'tool',
+			'assistant',
+			'tool',
+			'assistant'
+		])
+	})
+
+	it('stops checkpointing once the turn commits, so the transcript is never doubled', async () => {
+		const leavePage = stubHidingPage()
+		const manager = createManager()
+		// The turn-end save is where the race lives: the outcome branch has already
+		// merged the turn into `manager.messages` and is awaiting this call, so a
+		// checkpoint landing here would append the same messages a second time.
+		let leaveDuringFinalSave: () => void = () => {}
+		const saveChat = vi
+			.spyOn(manager.historyManager, 'saveChat')
+			.mockImplementation(async () => leaveDuringFinalSave())
+
+		mocks.runChatLoop.mockImplementationOnce(async (config: any) => {
+			const collected = [
+				{
+					role: 'assistant' as const,
+					content: '',
+					tool_calls: [
+						{
+							id: 't1',
+							type: 'function' as const,
+							function: { name: 'write_script', arguments: '{}' }
+						}
+					]
+				},
+				{ role: 'tool' as const, tool_call_id: 't1', content: 'created' },
+				{ role: 'assistant' as const, content: 'done' }
+			]
+			config.addedMessages.push(...collected)
+			leaveDuringFinalSave = () => leavePage.forEach((fn) => fn())
+			return {
+				addedMessages: collected,
+				tokenUsage: { prompt: 0, completion: 0, total: 0 },
+				hitMaxIterations: false
+			}
+		})
+
+		await manager.sendRequest({ instructions: 'write a script' })
+		await Promise.resolve()
+
+		// A duplicated transcript repeats t1, which providers reject outright — so
+		// no persisted call may carry the same tool_call_id twice.
+		for (const [, messages] of saveChat.mock.calls as unknown as [
+			unknown,
+			ChatCompletionMessageParam[]
+		][]) {
+			const toolCallIds = messages.flatMap((m: any) => m.tool_calls?.map((c: any) => c.id) ?? [])
+			expect(toolCallIds).toEqual([...new Set(toolCallIds)])
+		}
+		expect(manager.messages.map((m) => m.role)).toEqual(['user', 'assistant', 'tool', 'assistant'])
+	})
+
 	it('restores consumed DOM selector chips when a turn is cancelled before output', async () => {
 		const manager = createManager(createInputMock())
 		manager.mode = AIMode.GLOBAL
@@ -1966,6 +2273,61 @@ describe('AIChatManager queued messages', () => {
 		expect(manager.queuedMessage).toBe('')
 	})
 
+	it('refuses to switch conversation while a turn is running', async () => {
+		const manager = createManager(createInputMock())
+		const loadStored = vi.spyOn(manager.historyManager, 'loadPastChat').mockReturnValue({
+			id: 'chat-b',
+			title: 'Chat B',
+			displayMessages: [{ role: 'user', content: 'belongs to chat B', index: 0 }],
+			actualMessages: [{ role: 'user', content: 'belongs to chat B' }],
+			lastModified: 0
+		} as unknown as ReturnType<typeof manager.historyManager.loadPastChat>)
+		vi.spyOn(manager.historyManager, 'saveChat').mockResolvedValue(undefined)
+
+		mocks.runChatLoop.mockImplementationOnce(async (config: any) => {
+			// The user opens History mid-turn. Swapping the transcript in underneath
+			// the turn makes the commit below land on a foreign one — and when the
+			// loaded chat is this one, on top of its own checkpoint.
+			await manager.loadPastChat('chat-b')
+			const message = { role: 'assistant' as const, content: 'done' }
+			config.addedMessages.push(message)
+			return {
+				addedMessages: [message],
+				tokenUsage: { prompt: 0, completion: 0, total: 0 },
+				hitMaxIterations: false
+			}
+		})
+
+		await manager.sendRequest({ instructions: 'belongs to chat A' })
+
+		expect(loadStored).not.toHaveBeenCalled()
+		expect(manager.messages.map((m) => m.content)).toEqual(['belongs to chat A', 'done'])
+	})
+
+	it('refuses to switch conversation before `loading` has risen', async () => {
+		const manager = createManager(createInputMock())
+		const loadStored = vi.spyOn(manager.historyManager, 'loadPastChat').mockReturnValue({
+			id: 'chat-b',
+			title: 'Chat B',
+			displayMessages: [{ role: 'user', content: 'belongs to chat B', index: 0 }],
+			actualMessages: [{ role: 'user', content: 'belongs to chat B' }],
+			lastModified: 0
+		} as unknown as ReturnType<typeof manager.historyManager.loadPastChat>)
+		vi.spyOn(manager.historyManager, 'saveChat').mockResolvedValue(undefined)
+		replyWith('done')
+
+		const sent = manager.sendRequest({ instructions: 'belongs to chat A' })
+		// The send is registered but its attachment upkeep hasn't finished, so
+		// `loading` is still false — the window `sendOrQueue` also guards.
+		expect(manager.loading).toBe(false)
+		expect(manager.sendInFlight).toBe(true)
+		await manager.loadPastChat('chat-b')
+		await sent
+
+		expect(loadStored).not.toHaveBeenCalled()
+		expect(manager.messages.map((m) => m.content)).toEqual(['belongs to chat A', 'done'])
+	})
+
 	it('clears attachments on New chat / load past chat (non-session), keeps them in a session', async () => {
 		const txt = (n: string) => new File(['hello\n'], n, { type: 'text/plain' })
 
diff --git a/frontend/src/lib/components/copilot/chat/chatLoop.test.ts b/frontend/src/lib/components/copilot/chat/chatLoop.test.ts
index a010f1369a..251c2ff858 100644
--- a/frontend/src/lib/components/copilot/chat/chatLoop.test.ts
+++ b/frontend/src/lib/components/copilot/chat/chatLoop.test.ts
@@ -1,7 +1,12 @@
 import { beforeEach, describe, expect, it, vi } from 'vitest'
 import { randomUUID } from '$lib/utils/uuid'
 import type { ChatCompletionMessageParam } from 'openai/resources/chat/completions.mjs'
-import { runChatLoop, truncateToToolPairedPrefix, type ChatLoopConfig } from './chatLoop'
+import {
+	closeInterruptedToolBatch,
+	runChatLoop,
+	truncateToToolPairedPrefix,
+	type ChatLoopConfig
+} from './chatLoop'
 import type { ReasoningProviderModel } from '../reasoningRegistry'
 
 const mocks = vi.hoisted(() => ({
@@ -553,6 +558,25 @@ const tool = (id: string): ChatCompletionMessageParam => ({
 })
 const user = (content: string): ChatCompletionMessageParam => ({ role: 'user', content })
 
+describe('closeInterruptedToolBatch', () => {
+	it('keeps the answered half of a batch still executing, pairing the rest', () => {
+		// The model asked for two tools in one message; the first wrote a script
+		// (a real side effect) and the second is still going. Truncating would drop
+		// both, so the restored chat would not know the script exists.
+		const msgs = [assistantTools('a', 'b'), tool('a')]
+		expect(closeInterruptedToolBatch(msgs, 'stopped')).toEqual([
+			assistantTools('a', 'b'),
+			tool('a'),
+			{ role: 'tool', tool_call_id: 'b', content: 'stopped' }
+		])
+	})
+
+	it('leaves an already-paired transcript alone', () => {
+		const msgs = [assistantTools('a'), tool('a'), assistant('done')]
+		expect(closeInterruptedToolBatch(msgs, 'stopped')).toEqual(msgs)
+	})
+})
+
 describe('truncateToToolPairedPrefix', () => {
 	it('returns an empty array unchanged', () => {
 		expect(truncateToToolPairedPrefix([])).toEqual([])
diff --git a/frontend/src/lib/components/copilot/chat/chatLoop.ts b/frontend/src/lib/components/copilot/chat/chatLoop.ts
index 91dd3ab29c..110e4f1154 100644
--- a/frontend/src/lib/components/copilot/chat/chatLoop.ts
+++ b/frontend/src/lib/components/copilot/chat/chatLoop.ts
@@ -129,6 +129,38 @@ export function truncateToToolPairedPrefix(
 	return messages.slice(0, lastValidLen)
 }
 
+/**
+ * Like `truncateToToolPairedPrefix`, but closes the batch it would have dropped:
+ * every call in the trailing `tool_calls` message still missing a result gets one
+ * saying it was interrupted. A batch's calls execute one at a time, so truncating
+ * it discards steps that already finished — and whose side effects are already
+ * real. For a snapshot that must outlive its turn, keeping them beats a clean cut.
+ */
+export function closeInterruptedToolBatch(
+	messages: ChatCompletionMessageParam[],
+	interruptedContent: string
+): ChatCompletionMessageParam[] {
+	const paired = truncateToToolPairedPrefix(messages)
+	const rest = messages.slice(paired.length)
+	const batch = rest[0]
+	if (batch?.role !== 'assistant' || !batch.tool_calls?.length) return paired
+	// Results for THIS batch only, stopping at the first non-tool message:
+	// anything past a still-pending batch was never valid context.
+	const batchIds = new Set(batch.tool_calls.map((c) => c.id))
+	const answers: ChatCompletionMessageParam[] = []
+	const answered = new Set<string>()
+	for (const m of rest.slice(1)) {
+		if (m.role !== 'tool') break
+		if (!batchIds.has(m.tool_call_id)) continue
+		answers.push(m)
+		answered.add(m.tool_call_id)
+	}
+	const interrupted = [...batchIds]
+		.filter((id) => !answered.has(id))
+		.map((id) => ({ role: 'tool' as const, tool_call_id: id, content: interruptedContent }))
+	return [...paired, batch, ...answers, ...interrupted]
+}
+
 const unsupportedWebSearchCache = new Set<string>()
 const WEB_SEARCH_UNAVAILABLE_STATUS_CODES = new Set([400, 403, 404])
 
diff --git a/frontend/src/lib/components/copilot/chat/shared.ts b/frontend/src/lib/components/copilot/chat/shared.ts
index 543cca7d66..c33a3a61a1 100644
--- a/frontend/src/lib/components/copilot/chat/shared.ts
+++ b/frontend/src/lib/components/copilot/chat/shared.ts
@@ -1024,20 +1024,29 @@ export async function processToolCall<T>({
  * id is already answered by its tool result before this non-tool message. The image
  * parts ride the same `image_url` carrier that the provider converters translate.
  */
-export function appendPendingToolImages(
-	messages: ChatCompletionMessageParam[],
-	addedMessages: ChatCompletionMessageParam[],
-	toolCallbacks: ToolCallbacks
-): void {
-	const images = toolCallbacks.takePendingToolImages?.() ?? []
-	if (images.length === 0) return
-	const message: ChatCompletionMessageParam = {
+/** The message that hands tool-produced images to the model, or undefined when
+ *  there are none. Split out so a snapshot of a turn still buffering them can
+ *  build the same message without draining the buffer the live turn still owns. */
+export function pendingToolImagesMessage(
+	images: AttachedImage[]
+): ChatCompletionMessageParam | undefined {
+	if (images.length === 0) return undefined
+	return {
 		role: 'user',
 		content: [
 			{ type: 'text', text: 'Screenshot(s) of the app preview:' },
 			...images.map((img) => dataUrlToImagePart(img.dataUrl))
 		]
 	}
+}
+
+export function appendPendingToolImages(
+	messages: ChatCompletionMessageParam[],
+	addedMessages: ChatCompletionMessageParam[],
+	toolCallbacks: ToolCallbacks
+): void {
+	const message = pendingToolImagesMessage(toolCallbacks.takePendingToolImages?.() ?? [])
+	if (!message) return
 	messages.push(message)
 	addedMessages.push(message)
 }
diff --git a/frontend/src/lib/components/copilot/chat/typewriterReveal.test.ts b/frontend/src/lib/components/copilot/chat/typewriterReveal.test.ts
index b99048c360..1f415fce0a 100644
--- a/frontend/src/lib/components/copilot/chat/typewriterReveal.test.ts
+++ b/frontend/src/lib/components/copilot/chat/typewriterReveal.test.ts
@@ -73,6 +73,26 @@ describe('TypewriterReveal', () => {
 		expect(revealed().length).toBeLessThan(text.length)
 	})
 
+	it('exposes un-revealed text without advancing or ending the reveal', () => {
+		const sched = new FakeScheduler()
+		const { reveal, revealed } = makeReveal(sched)
+		const text = 'x'.repeat(300)
+		reveal.push(text)
+		sched.frame(16)
+		// A hidden tab stops scheduling frames while text keeps arriving, so a
+		// caller that must account for everything received reads `pending` — and
+		// reading it must not paint or stop the animation the way flush() does.
+		expect(revealed() + reveal.pending).toBe(text)
+		expect(reveal.pending.length).toBeGreaterThan(0)
+		const paintedBefore = revealed()
+		expect(reveal.pending).toBe(text.slice(paintedBefore.length))
+		expect(revealed()).toBe(paintedBefore)
+		// Still animating: further frames keep painting, which flush() would have
+		// prevented by ending the reveal.
+		sched.frames(3, 34)
+		expect(revealed().length).toBeGreaterThan(paintedBefore.length)
+	})
+
 	it('preserves text exactly after flush (no loss, no duplication)', () => {
 		const sched = new FakeScheduler()
 		const { reveal, revealed } = makeReveal(sched)
diff --git a/frontend/src/lib/components/copilot/chat/typewriterReveal.ts b/frontend/src/lib/components/copilot/chat/typewriterReveal.ts
index a4c71212f1..0cc1d75ac3 100644
--- a/frontend/src/lib/components/copilot/chat/typewriterReveal.ts
+++ b/frontend/src/lib/components/copilot/chat/typewriterReveal.ts
@@ -87,6 +87,15 @@ export class TypewriterReveal {
 		this.ensureRunning()
 	}
 
+	/** Text that has arrived but has not been revealed yet. Lets a caller that must
+	 *  account for everything the model has sent — not just what has been painted —
+	 *  read it without disturbing the pacing. `flush()` is the wrong tool there: it
+	 *  ends the animation. The gap it covers is unbounded, because a hidden tab
+	 *  pauses the paint loop while text keeps arriving. */
+	get pending(): string {
+		return this.buffer.slice(this.revealed)
+	}
+
 	/** Reveal everything still buffered now and stop. Call before reading the
 	 *  owner's reactive state into committed state, so the read sees the full text. */
 	flush(): void {