feat: restore tab layout + in-flight chunks on browser reopen; agents keep running in background

Implements the 'background-running agents + restore-layout-on-reopen'
feature. Full design and parallel-implementation plan in
`plan-bg-restore.md`; Gemini code review (SHIP verdict, no findings) in
`report.md`.

User-visible behaviors:

  1. Browser-close keeps agents alive. If an agent is mid-stream when
     the browser closes / reloads / loses the network, it continues
     processing on the backend. (This was already the case in code —
     agents run fire-and-forget in app.ts:77-79 — but it was previously
     pointless because the UI never restored the tab to receive the
     output.)
  2. Layout restore on browser reopen. Every tab that existed at the
     time the window was closed is restored, in original `position`
     order, with full persisted message history. Tabs whose agents
     finished while disconnected appear with the completed message.
     Tabs whose agents are still running appear streaming live — the
     in-flight assistant message is reconstructed from the backend's
     in-memory `currentChunks` (sent over the wire on connect) and
     accumulates new deltas as they arrive.
  3. Explicit tab-close cancels + forgets. Clicking the X still
     cancels the agent (existing `stopTab` in DELETE /tabs/:id) and
     archives the row (`is_open = 0`), so it is not restored. No
     change to that path.

The gap that the implementation closes: previously, App.svelte:onMount
unconditionally called `createNewTab()` with a fresh UUID, ignoring
every existing row in the `tabs` table. Every browser open was a
clean slate. The DB had the conversation history but no way for the
UI to discover it.

Implementation:

  • New `TabStatusSnapshot` interface in
    packages/core/src/types/index.ts (auto-exported via existing
    `export * from "./types"`):
        interface TabStatusSnapshot {
            status: AgentStatus;
            currentChunks?: Chunk[];          // present iff running
            currentAssistantId?: string;      // present iff running
        }
  • `agent-manager.ts:getAllStatuses()` rewritten to return
    `Record<string, TabStatusSnapshot>` (was
    `Record<string, AgentStatus>`). For running tabs only, attaches a
    defensive shallow copy of `tabAgent.currentChunks` (the live
    streaming array the per-message loop appends to) plus the DB id
    of the in-flight assistant message. The defensive copy is the
    consumer's to mutate. Idle / error tabs get `{ status }` only.
    `GET /status` and the WS `onOpen` snapshot both pick up the new
    shape automatically — neither call site changed.
  • Frontend mirror of `TabStatusSnapshot` in
    packages/frontend/src/lib/types.ts; `AgentEvent.statuses` variant
    updated to use `Record<string, TabStatusSnapshot>`.
  • New `hydrateFromBackend()` on the tab store
    (packages/frontend/src/lib/tabs.svelte.ts). Sequence on app
    mount:
        1. Bail with 0 if `tabs.length > 0` (hot-reload idempotency).
        2. GET /tabs           → list of `is_open=1` rows in
                                  `position` order.
        3. GET /status         → in-flight TabStatusSnapshot map.
        4. GET /tabs/:id/messages for each tab in parallel via
           Promise.all → persisted ChatMessage[].
        5. Build the Tab objects, splicing the snapshot's live
           chunks into the in-flight assistant message for every
           running tab (two paths: merge into the existing DB row
           with matching id, or append a fresh in-flight message
           if no row matches).
        6. `tabs = restored; activeTabId = restored[0]?.id ?? null;`
    Every fetch is wrapped in try/catch so one tab's failure can't
    destroy the whole restore pass.
  • WS `statuses` handler in `tabs.svelte.ts:handleEvent` rewritten
    for the new shape. Still fires `reloadTabMessagesFromApi` on the
    desync case (frontend thinks running, backend says idle — the
    pre-existing recovery path is preserved). When backend says
    running, seeds in-flight chunks into the assistant message
    matching `snap.currentAssistantId` (creating it if needed).
    When backend says non-running, clears `isStreaming` on the
    previous in-flight message and nulls `currentAssistantId`.
  • `App.svelte:onMount` now awaits `tabStore.hydrateFromBackend()`
    before deciding whether to fall back to `createNewTab()`.
    Fallback condition is the doubly-defensive
    `restored === 0 && tabStore.tabs.length === 0`. `wsClient.connect()`
    fires in parallel with hydration — the resulting WS `statuses`
    event is per-tab idempotent against the hydrated state, so there
    is no race even if it arrives mid-hydration.

What was NOT done (deliberately, deferred to wishlist):
  • Pre-existing inconsistency: core `AgentStatus` includes
    "waiting_for_key" but frontend `TabStatusSnapshot.status` uses
    only the existing 3-state pattern ("idle" | "running" | "error").
    Not introduced here; mirrored the existing precedent.
  • Restored tabs use defaults for `reasoningEffort`, `agentSlug`,
    `agentScope`, `agentModels`, `workingDirectory` — these are not
    in the DB `tabs` schema. Future schema expansion.
  • Per-delta DB flushing — not needed; the in-memory snapshot
    covers the gap between flushAssistant calls.
  • LocalStorage cache of tab ids — backend DB is the source of truth.

Process notes:
  • Implemented via parallel programmer subagents (flash agents were
    requested but unavailable in this environment — substituted with
    "programmer" agents, which share the "reads a plan, implements a
    single step" charter). Backend (Segment A: getAllStatuses + 5
    tests) and frontend (Segment B: types + hydrateFromBackend +
    statuses handler + onMount + 8 tests) ran disjoint-file-ownership
    in parallel.
  • Gemini code review (yolo mode for tool access, explicit
    prompt-level write restriction to `report.md` only) returned a
    SHIP verdict with no findings against the plan.
  • Self-review surfaced one followup gap that Gemini's earlier
    plan-mode pass also caught: no explicit test for
    `/tabs/:id/messages` failure isolation. Added a test covering
    both HTTP-500 and network-error variants alongside a healthy
    tab, asserting per-tab failures don't destroy the whole restore.

Tests:
  • api/tests/agent-manager.test.ts: +5 (snapshot empty record,
    idle-tab field omission, running-tab field inclusion, defensive
    copy invariant, omits chunks for running tab with null
    currentChunks). 31 total (was 26).
  • frontend/tests/chat-store.test.ts: +9 (restore-with-messages,
    in-flight seeding, /tabs failure → 0 returned, empty /tabs
    array, idempotency when tabs already exist, idle-status when
    /status omits, running-snapshot statuses handler seeding,
    idle-snapshot statuses handler clearing, per-tab failure
    isolation across HTTP-500 and network-error). 44 total (was 35).

Totals: 243 tests across 3 packages all green; typecheck clean on
core + api + frontend; biome clean across 124 files.

2 files changed, 232 insertions, 12 deletions

diff --git a/packages/frontend/src/lib/tabs.svelte.ts b/packages/frontend/src/lib/tabs.svelte.ts
index 1d60e0b..326d288 100644
--- a/packages/frontend/src/lib/tabs.svelte.ts
+++ b/packages/frontend/src/lib/tabs.svelte.ts
@@ -18,6 +18,7 @@ import type {
 	LogEntry,
 	PermissionPrompt,
 	QueuedMessage,
+	TabStatusSnapshot,
 	TaskItem,
 } from "./types.js";
 import { wsClient } from "./ws.svelte.js";
@@ -399,6 +400,167 @@ export function createTabStore() {
 		}
 	}
 
+	/**
+	 * Hydrate the tab store from the backend on app mount. Restores the
+	 * full list of open tabs (every row with `is_open = 1` in the DB),
+	 * loads each tab's persisted message history, and seeds the in-flight
+	 * assistant message for any tab the backend is currently streaming.
+	 *
+	 * Wire calls:
+	 *   - GET /tabs                       → list of open tabs in `position` order
+	 *   - GET /tabs/:id/messages          → persisted ChatMessage[] for each
+	 *   - GET /status                     → in-flight TabStatusSnapshot map
+	 *
+	 * Failure modes (all log + continue with whatever was successfully
+	 * hydrated; callers fall back to creating a fresh tab if the final
+	 * `tabs` array is empty):
+	 *   - /tabs request fails → no tabs restored
+	 *   - /tabs/:id/messages fails → that tab restored with empty messages
+	 *   - /status fails → tabs restored, in-flight streaming will be
+	 *     lost (will surface as a static "running" status until the next
+	 *     event arrives); harmless because the WS will broadcast `statuses`
+	 *     on reconnect anyway.
+	 *
+	 * Returns the number of tabs hydrated (0 on total failure, ≥1 on
+	 * partial or full success). Caller uses this to decide whether to
+	 * create a fresh tab.
+	 *
+	 * Idempotency: if `tabs.length > 0` when called, returns 0 without
+	 * touching state — the caller already has tabs from elsewhere (e.g.
+	 * a hot-reload that preserved Svelte state).
+	 */
+	async function hydrateFromBackend(): Promise<number> {
+		if (tabs.length > 0) return 0;
+
+		// 1. Fetch the list of open tabs from the DB.
+		let tabRows: Array<{
+			id: string;
+			title: string;
+			keyId?: string | null;
+			modelId?: string | null;
+			parentTabId?: string | null;
+		}> = [];
+		try {
+			const res = await fetch(`${config.apiBase}/tabs`);
+			if (!res.ok) return 0;
+			const data = (await res.json()) as { tabs?: typeof tabRows };
+			tabRows = Array.isArray(data.tabs) ? data.tabs : [];
+		} catch {
+			return 0;
+		}
+
+		if (tabRows.length === 0) return 0;
+
+		// 2. Fetch the in-flight snapshot. Failure is non-fatal.
+		let statusMap: Record<string, TabStatusSnapshot> = {};
+		try {
+			const res = await fetch(`${config.apiBase}/status`);
+			if (res.ok) {
+				const data = (await res.json()) as { statuses?: Record<string, TabStatusSnapshot> };
+				if (data.statuses && typeof data.statuses === "object") {
+					statusMap = data.statuses;
+				}
+			}
+		} catch {
+			// Non-fatal: tabs still restore with idle status.
+		}
+
+		// 3. For each tab, fetch its persisted messages in parallel.
+		const messageFetches = tabRows.map(async (row) => {
+			try {
+				const res = await fetch(`${config.apiBase}/tabs/${row.id}/messages`);
+				if (!res.ok) return { id: row.id, messages: [] as ChatMessage[] };
+				const data = (await res.json()) as {
+					messages?: Array<{ id?: string; role: string; chunks?: Chunk[] }>;
+				};
+				const messages: ChatMessage[] = (data.messages ?? []).map((m) => ({
+					id: m.id ?? generateId(),
+					role: m.role as ChatMessage["role"],
+					chunks: Array.isArray(m.chunks) ? m.chunks : [],
+					isStreaming: false,
+				}));
+				return { id: row.id, messages };
+			} catch {
+				return { id: row.id, messages: [] as ChatMessage[] };
+			}
+		});
+
+		const messagesByTab = new Map<string, ChatMessage[]>();
+		for (const result of await Promise.all(messageFetches)) {
+			messagesByTab.set(result.id, result.messages);
+		}
+
+		// 4. Build the Tab objects, splicing in the in-flight snapshot for
+		//    running tabs.
+		const restored: Tab[] = tabRows.map((row) => {
+			const snap = statusMap[row.id];
+			const messages = messagesByTab.get(row.id) ?? [];
+			const agentStatus: Tab["agentStatus"] = snap?.status ?? "idle";
+
+			let currentAssistantId: string | null = null;
+			let finalMessages = messages;
+
+			if (agentStatus === "running" && snap?.currentAssistantId) {
+				currentAssistantId = snap.currentAssistantId;
+				// Find or create the in-flight assistant message. If the DB
+				// already has a row with this id (the backend appended on
+				// first flush and we picked it up via /tabs/:id/messages),
+				// merge the snapshot chunks on top — the snapshot is the
+				// live source of truth and may have chunks the DB doesn't.
+				// If there's no matching row, append a new in-flight
+				// assistant message holding only the snapshot chunks.
+				const existingIdx = finalMessages.findIndex((m) => m.id === snap.currentAssistantId);
+				if (existingIdx >= 0) {
+					finalMessages = finalMessages.map((m, i) =>
+						i === existingIdx
+							? {
+									...m,
+									chunks: snap.currentChunks ? [...snap.currentChunks] : m.chunks,
+									isStreaming: true,
+								}
+							: m,
+					);
+				} else {
+					finalMessages = [
+						...finalMessages,
+						{
+							id: snap.currentAssistantId,
+							role: "assistant",
+							chunks: snap.currentChunks ? [...snap.currentChunks] : [],
+							isStreaming: true,
+						},
+					];
+				}
+			}
+
+			return {
+				id: row.id,
+				title: row.title,
+				messages: finalMessages,
+				agentStatus,
+				keyId: row.keyId ?? null,
+				modelId: row.modelId ?? null,
+				reasoningEffort: "max",
+				currentAssistantId,
+				tasks: [],
+				injectedSkills: [],
+				parentTabId: row.parentTabId ?? null,
+				persistent: true,
+				agentSlug: null,
+				agentScope: null,
+				agentModels: null,
+				workingDirectory: null,
+				queuedMessages: [],
+			};
+		});
+
+		tabs = restored;
+		// Activate the first restored tab (the list is already ordered by
+		// `position` from the backend).
+		activeTabId = restored[0]?.id ?? null;
+		return restored.length;
+	}
+
 	function handleEvent(event: AgentEvent & { tabId?: string }): void {
 		const tabId = event.tabId;
 
@@ -417,25 +579,62 @@ export function createTabStore() {
 				break;
 			}
 			case "statuses": {
-				// WS (re)connect snapshot. For any tab where the frontend thought
-				// we were running but the backend says otherwise, we missed the
-				// finishing events while disconnected — pull the persisted
-				// chunks from the API to recover. Also sync agentStatus and
-				// clear in-flight pointers on every desyncing tab.
+				// WS (re)connect snapshot. The shape was widened to
+				// TabStatusSnapshot (status + optional currentChunks +
+				// optional currentAssistantId) so the frontend can seed
+				// in-flight assistant messages on browser reopen.
 				const backend = event.statuses;
 				for (const t of tabs) {
-					const backendStatus = backend[t.id] ?? "idle";
+					const snap = backend[t.id];
+					const backendStatus = snap?.status ?? "idle";
+
+					// Desync case: frontend thought it was streaming, backend
+					// has already moved on. Pull the persisted chunks so the
+					// final answer shows up.
 					if (t.agentStatus === "running" && backendStatus !== "running") {
 						void reloadTabMessagesFromApi(t.id);
 					}
+
+					// Status alignment.
 					if (t.agentStatus !== backendStatus) {
 						updateTab(t.id, { agentStatus: backendStatus });
 					}
-					if (backendStatus !== "running" && t.currentAssistantId) {
-						// Mark any in-flight assistant message as no-longer-streaming;
-						// `reloadTabMessagesFromApi` (if it ran) will replace the
-						// whole messages array, but if no reload was triggered we
-						// still want streaming flags cleared.
+
+					if (backendStatus === "running") {
+						// Seed the in-flight assistant message from the snapshot.
+						// This handles the "browser just reopened mid-stream"
+						// path: the DB only has chunks up to the last
+						// flushAssistant call, but the snapshot has the live
+						// in-memory currentChunks.
+						if (snap?.currentAssistantId) {
+							const targetId = snap.currentAssistantId;
+							updateTab(t.id, { currentAssistantId: targetId });
+							updateMessages(t.id, (msgs) => {
+								const idx = msgs.findIndex((m) => m.id === targetId);
+								if (idx >= 0) {
+									return msgs.map((m, i) =>
+										i === idx
+											? {
+													...m,
+													chunks: snap.currentChunks ? [...snap.currentChunks] : m.chunks,
+													isStreaming: true,
+												}
+											: m,
+									);
+								}
+								return [
+									...msgs,
+									{
+										id: targetId,
+										role: "assistant",
+										chunks: snap.currentChunks ? [...snap.currentChunks] : [],
+										isStreaming: true,
+									},
+								];
+							});
+						}
+					} else if (t.currentAssistantId) {
+						// Not running: clear streaming flags.
 						updateMessages(t.id, (msgs) =>
 							msgs.map((m) => (m.id === t.currentAssistantId ? { ...m, isStreaming: false } : m)),
 						);
@@ -1303,6 +1502,7 @@ export function createTabStore() {
 		// WS callback uses in production. Not intended for use in
 		// components — they should rely on the WS subscription instead.
 		handleEvent,
+		hydrateFromBackend,
 	};
 }
 
diff --git a/packages/frontend/src/lib/types.ts b/packages/frontend/src/lib/types.ts
index 6051810..22837a0 100644
--- a/packages/frontend/src/lib/types.ts
+++ b/packages/frontend/src/lib/types.ts
@@ -76,13 +76,33 @@ export interface ChatMessage {
 
 export type ConnectionStatus = "connecting" | "connected" | "disconnected";
 
+/**
+ * Mirror of core's `TabStatusSnapshot` (see packages/core/src/types/index.ts).
+ *
+ * Sent on every WS (re)connect and via `GET /status`. The frontend uses
+ * this to:
+ *   - reconcile its in-memory `agentStatus` with the backend's truth
+ *     after a disconnect window;
+ *   - reconstruct the in-flight assistant message for any tab the
+ *     backend is currently streaming, so the user sees the partial
+ *     thinking / text without waiting for the next delta.
+ *
+ * Wire-format symmetry MUST be kept with core. If you change one,
+ * change the other.
+ */
+export interface TabStatusSnapshot {
+	status: "idle" | "running" | "error";
+	currentChunks?: Chunk[];
+	currentAssistantId?: string;
+}
+
 export type AgentEvent =
 	| { type: "status"; status: "idle" | "running" | "error" }
 	// Sent on every WS (re)connect: a snapshot of every tab the backend is
 	// currently tracking and its live status. The frontend uses this to
 	// detect desync after a reconnect (e.g. bun --watch restart killed the
 	// in-flight agent state, frontend missed `done` / `status:idle` events).
-	| { type: "statuses"; statuses: Record<string, "idle" | "running" | "error"> }
+	| { type: "statuses"; statuses: Record<string, TabStatusSnapshot> }
 	| { type: "text-delta"; delta: string }
 	| { type: "reasoning-delta"; delta: string }
 	| { type: "reasoning-end"; metadata?: Record<string, unknown> }