Skip persisted tool_progress entries during session replay

apps/cli/ai/sessions/replay.ts

@@ -56,7 +56,12 @@ export function replaySessionHistory( ui: AiChatUI, entries: SessionEntry[] ): v
 			}
 
 			if ( isStudioCustomEntryOfType( entry, 'studio.tool_progress' ) ) {
-				if ( entry.data ) ui.setLoaderMessage( entry.data.message );
+				// Tool progress is ephemeral UI state (loader text) with no value
+				// when rehydrating history: `finishReplay()` clears the loader at
+				// the end anyway. Replaying it one entry at a time is the bottleneck
+				// behind the "Resuming session…" hang on sessions that persisted
+				// tens of thousands of progress ticks, so skip it during replay.
+				// See https://github.com/Automattic/studio/issues/3865
 				continue;
 			}
 

apps/cli/ai/sessions/tests/replay.test.ts

@@ -0,0 +1,84 @@
+import { describe, expect, it, vi } from 'vitest';
+import { replaySessionHistory } from '../replay';
+import type { SessionEntry } from '@earendil-works/pi-coding-agent';
+import type { AiChatUI } from 'cli/ai/ui';
+
+function createUiSpy() {
+	return {
+		prepareForReplay: vi.fn(),
+		setReplayTimestamp: vi.fn(),
+		setActiveSite: vi.fn(),
+		beginAgentTurn: vi.fn(),
+		addUserMessage: vi.fn(),
+		endAgentTurn: vi.fn(),
+		setLoaderMessage: vi.fn(),
+		showAgentQuestion: vi.fn(),
+		renderToolResults: vi.fn(),
+		handleEvent: vi.fn(),
+		finishReplay: vi.fn(),
+	};
+}
+
+function toolProgress( message: string, timestamp: string ): SessionEntry {
+	return {
+		type: 'custom',
+		customType: 'studio.tool_progress',
+		timestamp,
+		data: { message },
+	} as SessionEntry;
+}
+
+describe( 'replaySessionHistory', () => {
+	it( 'does not replay persisted studio.tool_progress entries', () => {
+		const ui = createUiSpy();
+		const entries: SessionEntry[] = Array.from( { length: 5000 }, ( _, i ) =>
+			toolProgress(
+				`progress ${ i }`,
+				`2026-06-01T00:00:${ String( i % 60 ).padStart( 2, '0' ) }.000Z`
+			)
+		);
+
+		replaySessionHistory( ui as unknown as AiChatUI, entries );
+
+		expect( ui.setLoaderMessage ).not.toHaveBeenCalled();
+		expect( ui.prepareForReplay ).toHaveBeenCalledTimes( 1 );
+		expect( ui.finishReplay ).toHaveBeenCalledTimes( 1 );
+	} );
+
+	it( 'replays the conversation while ignoring interleaved progress', () => {
+		const ui = createUiSpy();
+		const entries: SessionEntry[] = [
+			{
+				type: 'custom',
+				customType: 'studio.user_prompt',
+				timestamp: '2026-06-01T00:00:00.000Z',
+				data: { source: 'prompt', text: 'Create a landing page' },
+			} as SessionEntry,
+			toolProgress( 'Reading files…', '2026-06-01T00:00:01.000Z' ),
+			toolProgress( 'Running WP-CLI…', '2026-06-01T00:00:02.000Z' ),
+			{
+				type: 'message',
+				timestamp: '2026-06-01T00:00:03.000Z',
+				message: {
+					role: 'assistant',
+					content: [ { type: 'text', text: 'Done.' } ],
+				},
+			} as SessionEntry,
+			{
+				type: 'custom',
+				customType: 'studio.turn_closed',
+				timestamp: '2026-06-01T00:00:04.000Z',
+				data: { status: 'success' },
+			} as SessionEntry,
+		];
+
+		replaySessionHistory( ui as unknown as AiChatUI, entries );
+
+		expect( ui.setLoaderMessage ).not.toHaveBeenCalled();
+		expect( ui.addUserMessage ).toHaveBeenCalledWith( 'Create a landing page' );
+		expect( ui.beginAgentTurn ).toHaveBeenCalledTimes( 1 );
+		expect( ui.handleEvent ).toHaveBeenCalledTimes( 1 );
+		expect( ui.endAgentTurn ).toHaveBeenCalledTimes( 1 );
+		expect( ui.finishReplay ).toHaveBeenCalledTimes( 1 );
+	} );
+} );