From 985116fdc8961e656cd9d3027debcd1a6237d6c2 Mon Sep 17 00:00:00 2001
From: Adam Malczewski <github@tradam.dev>
Date: Sat, 5 Sep 2026 01:20:28 +0900
Subject: feat(layout): plan 02 — provider consumption + layout WS +
 localStorage migration
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

---
 .dispatch/transport-contract.reference.md   |  11 +
 backend-handoff.md                          |  56 ++++
 src/adapters/ws/index.ts                    |   8 +
 src/adapters/ws/logic.test.ts               |  35 +++
 src/adapters/ws/logic.ts                    |  26 ++
 src/app/App.svelte                          |  39 +--
 src/app/App.test.ts                         |  57 +++-
 src/app/store.svelte.ts                     |  45 +++
 src/core/wire/conformance.ts                |   2 +
 src/features/layout/adapter/http.test.ts    | 136 +++++++++
 src/features/layout/adapter/http.ts         | 117 ++++++++
 src/features/layout/index.ts                |   3 +
 src/features/layout/logic/migration.test.ts |  72 +++++
 src/features/layout/logic/migration.ts      |  76 +++++
 src/features/layout/store.svelte.ts         | 270 ++++++++++++++++++
 src/features/layout/store.test.ts           | 420 ++++++++++++++++++++++++++++
 16 files changed, 1337 insertions(+), 36 deletions(-)
 create mode 100644 src/features/layout/adapter/http.test.ts
 create mode 100644 src/features/layout/adapter/http.ts
 create mode 100644 src/features/layout/logic/migration.test.ts
 create mode 100644 src/features/layout/logic/migration.ts
 create mode 100644 src/features/layout/store.svelte.ts
 create mode 100644 src/features/layout/store.test.ts

diff --git a/.dispatch/transport-contract.reference.md b/.dispatch/transport-contract.reference.md
index f220327..304bd0b 100644
--- a/.dispatch/transport-contract.reference.md
+++ b/.dispatch/transport-contract.reference.md
@@ -8,6 +8,17 @@
 > **Orchestrator:** SNAPSHOT of `transport-contract@0.24.0` (MCP status + computers + provider concurrency + cancel queued message). Regenerate whenever
 > it changes.
 >
+> **2026-09-04 delta (tiled layout plan 02 — ADDITIVE, no version bump):** the layout-provider contract ships. NEW
+> WS message: `LayoutChangedMessage` (`{ type: "layout"; workspaceId: string; tree: LayoutTree }` — server→client
+> broadcast after every successful layout write; the FULL tree, decision 7; a global fan-out and CLIENTS FILTER by
+> their active workspace). `LayoutTree` is re-exported from `@dispatch/layout-contract`. NEW HTTP routes (owned by the
+> ACTIVE layout provider — the bundled standard `layout-chat` plugin — consumed reactively; while NO provider is active
+> every route 404s, the frontend's no-provider signal): `GET /workspaces/:id/layout` (`{ tree }` — read seeds a fresh
+> workspace), `PUT /workspaces/:id/layout` (body `{ tree }` → 200 `{ tree }` / 400 `{ error }` structural-invalidity),
+> `DELETE /workspaces/:id/layout` (re-seed on next read; returns the fresh seed). The FE consumes these via
+> `features/layout` (`adapter/http.ts` + `store.svelte.ts`); the `layout` message is parsed in `adapters/ws/logic.ts`
+> and dispatched to `store.layout.onServerTree` (non-active workspaces ignored).
+>
 > **2026-09-03 delta (plugin system — plans 05/06, NO version bump verified against the worktree):** the
 > plugin-manager + per-workspace enablement contract ships. NEW WS messages: `WorkspaceSetMessage`
 > (`{ type: "workspace.set"; workspaceId }` — client→server; the server records the connection's workspace
diff --git a/backend-handoff.md b/backend-handoff.md
index a67fac9..3c71bc3 100644
--- a/backend-handoff.md
+++ b/backend-handoff.md
@@ -5,6 +5,62 @@
 > Per AGENTS.md "Backend seam": FE contract changes are edited + consumers updated here; BACKEND
 > contract changes are REPORTED UP here, and on the new version the FE re-pins the `file:` dep,
 > re-mirrors `.dispatch/*.reference.md`, and updates consumers. NEVER edit the backend repo.
+>
+> **NOTE (tiled-layout worktree, 2026-09-04):** plans 01–04 of the layout series are being
+> implemented in BOTH repos of this worktree by one task (branch `feature/tiled-layout`), so the
+> "never edit the backend repo" rule is superseded by that task's scope — the backend contract
+> changes below were made in the same branch and the FE consumes them via the `file:` deps
+> (dists rebuilt). Listed here for the record.
+
+## Milestone: Layout plan 02 — provider consumption + layout WS + localStorage migration
+
+**Slice status:** COMPLETE + committed (both repos, `feature/tiled-layout`).
+
+### What the FE built
+- `features/layout/adapter/http.ts` — `createLayoutHttp(httpBase, fetchImpl)`: typed
+  `GET/PUT/DELETE /workspaces/:id/layout` with the 404-as-signal mapping (`notFound`) +
+  malformed-200 degrade; tested against a scripted fetch edge.
+- `features/layout/logic/migration.ts` — the §2.7 one-way best-effort migration decision
+  (`migrationTree`/`storedPanelViews`/`sidePanelViews`): import the stored
+  `dispatch.sidebar.views` selection ONLY when the provider's tree is the untouched
+  DEFAULT_LAYOUT seed; the key is then never read again (bytes stay).
+- `features/layout/store.svelte.ts` — `createLayoutStore(deps)`: owns the ACTIVE workspace's
+  tree; probes on boot / workspace switch / `extensions-changed` (§2.6); WS `layout` push via
+  `onServerTree` (non-active workspaces ignored); edits PUT the WHOLE tree lightly debounced
+  (300ms) with rollback to the last server tree on failure (a PUT 404 flips to no-provider mode
+  and KEEPS the edit — ephemeral, no rollback, no warning beyond the once-per-absence log);
+  collapse state NOT touched (`dispatch.sidebar.open` unchanged).
+- `adapters/ws/logic.ts` parses `layout` (`{ type, workspaceId, tree }`, light shape checks);
+  `adapters/ws/index.ts` dispatches via `onLayout`; `core/wire/conformance.ts` exhaustive
+  guard extended.
+- `app/store.svelte.ts` — the app store owns the `LayoutStore` instance + exposes
+  `store.layout`; wires `onLayout` → `layoutStore.onServerTree`, the boot probe
+  (`{ boot: true }` arms the §2.7 migration), `onWorkspaceChanged` in `setActiveWorkspace`, and
+  a re-probe in `onExtensionsChanged`.
+- `app/App.svelte` — the tree is now `store.layout.tree` (was: local `$state` seeded from
+  localStorage); edits flow to `layout.onTreeChange`; the `dispatch.sidebar.views` READ (boot
+  seed) and WRITE (edit write-back) are RETIRED — its only reader is the one-way migration.
+- **App tests updated:** the tests that pre-seeded `dispatch.sidebar.views` to make a sidebar
+  view visible now drive the layout through the provider edge (`fakeFetchWithLayout([...])`
+  serving `GET /workspaces/:id/layout`); the Extensions-view test is async (the tree arrives
+  via the provider GET).
+
+### Pinned contract versions + mirrors
+- `@dispatch/transport-contract` (worktree dist rebuilt): consumed with the plan-02 types
+  (`LayoutChangedMessage` + the `layout` union member; `LayoutTree` re-exported from
+  `@dispatch/layout-contract`). Mirror `.dispatch/transport-contract.reference.md` updated with
+  a **2026-09-04 delta note**.
+
+### Findings / raises for the backend
+1. **Outward events emitter is a noop in the composition root** (host-bin `createNoopEvents`):
+   the plan-02 WS push therefore follows the LIVE `extensions-changed` path instead — a typed
+   event hook (`layoutChanged` in `@dispatch/layout-provider-contract`) that transport-ws
+   subscribes to and broadcasts verbatim. If a generic `host.events` → WS bridge ever ships,
+   the kit's `emitChange` dep is the single wiring point to switch.
+2. **§2.4 double-provide semantics VERIFIED from kernel source** (`kernel/src/bus/bus.ts`):
+   `provideService` on an already-provided handle THROWS ("Only one provider per handle is
+   allowed") → first-provider-wins; the host converts a throwing `activate()` into the
+   disabled-with-reason grammar. No kit wrapper needed (the plan's conditional).
 
 ## Milestone: Plugin Manager UI — FE scope (plan 07, feature/plugin-system)
 
diff --git a/src/adapters/ws/index.ts b/src/adapters/ws/index.ts
index 3a6a97d..3713c8c 100644
--- a/src/adapters/ws/index.ts
+++ b/src/adapters/ws/index.ts
@@ -5,6 +5,7 @@ import type {
   ConversationOpenMessage,
   ConversationStatusChangedMessage,
   ExtensionsChangedMessage,
+  LayoutChangedMessage,
   WsClientMessage,
 } from "@dispatch/transport-contract";
 import type { SurfaceServerMessage } from "@dispatch/ui-contract";
@@ -34,6 +35,11 @@ export interface SurfaceSocketOptions {
    * workspace-filtered surface catalog itself).
    */
   onExtensionsChanged?: (msg: ExtensionsChangedMessage) => void;
+  /**
+   * Broadcast after every successful layout write (plan 02 §2.5): the FULL
+   * tree for one workspace. The consumer filters by its active workspace.
+   */
+  onLayout?: (msg: LayoutChangedMessage) => void;
   onReopen?: () => void;
   socketFactory?: (url: string) => WebSocketLike;
 }
@@ -84,6 +90,8 @@ export function createSurfaceSocket(opts: SurfaceSocketOptions): SurfaceSocketHa
           opts.onConversationCompacted?.(msg as ConversationCompactedMessage);
         } else if (msg.type === "extensions-changed") {
           opts.onExtensionsChanged?.(msg as ExtensionsChangedMessage);
+        } else if (msg.type === "layout") {
+          opts.onLayout?.(msg as LayoutChangedMessage);
         } else {
           opts.onMessage(msg as SurfaceServerMessage);
         }
diff --git a/src/adapters/ws/logic.test.ts b/src/adapters/ws/logic.test.ts
index c4ec7e3..a7d30a7 100644
--- a/src/adapters/ws/logic.test.ts
+++ b/src/adapters/ws/logic.test.ts
@@ -463,3 +463,38 @@ describe("parseServerMessage — extensions-changed (plan 05)", () => {
     expect(parseServerMessage(JSON.stringify({ type: "extensions-changed" }))).toBeNull();
   });
 });
+
+describe("parseServerMessage — layout (plan 02)", () => {
+  const TREE = { version: 1, root: { kind: "leaf", view: "chat" } };
+
+  it("parses the broadcast (workspaceId + full tree)", () => {
+    const result = parseServerMessage(
+      JSON.stringify({ type: "layout", workspaceId: "ws-a", tree: TREE }),
+    );
+    expect(result).toEqual({ type: "layout", workspaceId: "ws-a", tree: TREE });
+  });
+
+  it("rejects a missing/invalid workspaceId", () => {
+    expect(parseServerMessage(JSON.stringify({ type: "layout", tree: TREE }))).toBeNull();
+    expect(parseServerMessage(JSON.stringify({ type: "layout", workspaceId: 7, tree: TREE }))).toBeNull();
+  });
+
+  it("rejects a missing/invalid tree (wrong version, missing root, non-object)", () => {
+    expect(parseServerMessage(JSON.stringify({ type: "layout", workspaceId: "ws-a" }))).toBeNull();
+    expect(
+      parseServerMessage(
+        JSON.stringify({ type: "layout", workspaceId: "ws-a", tree: { version: 2, root: {} } }),
+      ),
+    ).toBeNull();
+    expect(
+      parseServerMessage(
+        JSON.stringify({ type: "layout", workspaceId: "ws-a", tree: { version: 1 } }),
+      ),
+    ).toBeNull();
+    expect(parseServerMessage(JSON.stringify({ type: "layout", workspaceId: "ws-a", tree: 7 }))).toBeNull();
+  });
+
+  it("an unknown type still parses to null (the layout addition doesn't loosen the guard)", () => {
+    expect(parseServerMessage(JSON.stringify({ type: "layout.unknown" }))).toBeNull();
+  });
+});
diff --git a/src/adapters/ws/logic.ts b/src/adapters/ws/logic.ts
index 696655b..6cbc559 100644
--- a/src/adapters/ws/logic.ts
+++ b/src/adapters/ws/logic.ts
@@ -5,6 +5,7 @@ import type {
   ConversationOpenMessage,
   ConversationStatusChangedMessage,
   ExtensionsChangedMessage,
+  LayoutChangedMessage,
   WsClientMessage,
   WsServerMessage,
 } from "@dispatch/transport-contract";
@@ -26,6 +27,7 @@ const VALID_SERVER_TYPES = new Set([
   "conversation.statusChanged",
   "conversation.compacted",
   "extensions-changed",
+  "layout",
 ]);
 
 /** Serialize a client message to a JSON string for the wire. */
@@ -180,6 +182,30 @@ export function parseServerMessage(data: string): WsServerMessage | null {
       };
       return msg;
     }
+    case "layout": {
+      // Layout-provider broadcast (plan 02 §2.5): the FULL tree for ONE
+      // workspace (decision 7 — no patch protocol). Light shape checks here
+      // (the tree's structural validation is the backend's write-time job);
+      // the store filters by its active workspace.
+      if (typeof parsed.workspaceId !== "string") return null;
+      const tree = parsed.tree;
+      if (
+        tree === null ||
+        typeof tree !== "object" ||
+        Array.isArray(tree) ||
+        (tree as { version?: unknown }).version !== 1 ||
+        typeof (tree as { root?: unknown }).root !== "object" ||
+        (tree as { root?: unknown }).root === null
+      ) {
+        return null;
+      }
+      const msg: LayoutChangedMessage = {
+        type: "layout",
+        workspaceId: parsed.workspaceId,
+        tree: tree as LayoutChangedMessage["tree"],
+      };
+      return msg;
+    }
     default:
       return null;
   }
diff --git a/src/app/App.svelte b/src/app/App.svelte
index b7eb783..9eaba9c 100644
--- a/src/app/App.svelte
+++ b/src/app/App.svelte
@@ -10,18 +10,10 @@
 <script lang="ts">
   import type { LayoutTree } from "@dispatch/layout-contract";
   import { untrack } from "svelte";
-  import { createLocalStore } from "../adapters/local-storage";
-  import {
-    createCollapseStore,
-    firstCollapsibleNodeId,
-    firstPanelsNode,
-    listPanelViews,
-    TileRoot,
-  } from "../features/layout";
+  import { createCollapseStore, firstCollapsibleNodeId, TileRoot } from "../features/layout";
   import { RunModal, type HeartbeatRunView } from "../features/heartbeat";
   import { SystemPromptBuilder } from "../features/system-prompt";
   import type { AppStore } from "./store.svelte";
-  import { seedLayout } from "../features/layout";
   import type { AppViewPorts, ChatViewportBridge } from "./view-ports";
   import ErrorModal from "./ErrorModal.svelte";
 
@@ -33,27 +25,18 @@
   // closed). Holds a snapshot run view; the modal re-mounts per run (keyed).
   let heartbeatRun = $state<HeartbeatRunView | null>(null);
 
-  // --- The layout tree: seeded from DEFAULT_LAYOUT (the compatibility tree
-  // that reproduces today's layout) with the sidebar selection overlaid from
-  // localStorage — today's exact boot behavior (unknown/removed kinds dropped;
-  // nothing valid stored → the tabs+model defaults).
-  const sidebarStore = createLocalStore<readonly string[]>("dispatch.sidebar.views", {
-    storage: untrack(() => store.storage),
-  });
-  let tree = $state<LayoutTree>(
-    seedLayout(sidebarStore.load(), listPanelViews().map((v) => v.id)),
-  );
+  // --- The layout tree (plan 02): the ACTIVE workspace's tile tree, served
+  // by the backend's layout provider plugin when one is active (probed on
+  // boot, on workspace switch, and on extensions-changed; `layout` WS pushes
+  // re-render live) and the built-in DEFAULT_LAYOUT otherwise (no-provider
+  // mode: edits are ephemeral, logged once, no error UI). The store owns the
+  // whole lifecycle; the shell just renders `tree` and forwards edits.
+  // `dispatch.sidebar.views` is RETIRED (D-L4): its only reader is the store's
+  // one-way §2.7 migration.
+  const tree = $derived(store.layout.tree);
 
-  // Tile edits flow up from the panels; the OWNER decides persistence. The
-  // panels selection is written back to the SAME key in the SAME format
-  // (`dispatch.sidebar.views` — a JSON array of view ids) so the storage stays
-  // byte-compatible with today (plan 02 migrates it).
   function handleTreeChange(next: LayoutTree): void {
-    tree = next;
-    const panels = firstPanelsNode(next);
-    if (panels !== null) {
-      sidebarStore.save(panels.node.panels.map((p) => p.view));
-    }
+    store.layout.onTreeChange(next);
   }
 
   // --- Sidebar collapse: renderer chrome state (per-node-id open/closed),
diff --git a/src/app/App.test.ts b/src/app/App.test.ts
index 69b181d..57e966f 100644
--- a/src/app/App.test.ts
+++ b/src/app/App.test.ts
@@ -1,4 +1,7 @@
 import type { SetCwdRequest, WsServerMessage } from "@dispatch/transport-contract";
+import type { LayoutTree } from "@dispatch/layout-contract";
+import { DEFAULT_LAYOUT } from "@dispatch/layout-contract";
+import { overlaySidePanels } from "../features/layout";
 import type { SurfaceServerMessage } from "@dispatch/ui-contract";
 import { render, screen } from "@testing-library/svelte";
 import userEvent from "@testing-library/user-event";
@@ -62,6 +65,12 @@ function fakeFetchImpl(): typeof fetch {
         status: 200,
       });
     }
+    if (url.endsWith("/layout")) {
+      // No layout provider in these tests: the plan-02 no-provider signal.
+      return new Response(JSON.stringify({ error: "No layout provider is active" }), {
+        status: 404,
+      });
+    }
     if (url.includes("/conversations?status=")) {
       return new Response(JSON.stringify({ conversations: [] }), { status: 200 });
     }
@@ -82,6 +91,35 @@ function fakeFetchImpl(): typeof fetch {
   };
 }
 
+/**
+ * A fetch whose layout provider is ACTIVE and serves the given sidebar
+ * panels (plan 02): `GET /workspaces/:id/layout` returns DEFAULT_LAYOUT with
+ * the views overlaid; `PUT` echoes the tree (200). Tests that need specific
+ * sidebar views visible drive the layout through THIS edge — the pre-seeded
+ * `dispatch.sidebar.views` localStorage is only read by the one-way §2.7
+ * migration now.
+ */
+function fakeFetchWithLayout(views: readonly string[]): typeof fetch {
+  const inner = fakeFetchImpl();
+  return async (input: string | URL | Request, init?: RequestInit): Promise<Response> => {
+    const url = typeof input === "string" ? input : input instanceof URL ? input.href : input.url;
+    if (url.endsWith("/layout")) {
+      const method = init?.method ?? "GET";
+      if (method === "PUT" && typeof init?.body === "string") {
+        const body = JSON.parse(init.body) as { tree: LayoutTree };
+        return new Response(JSON.stringify({ tree: body.tree }), { status: 200 });
+      }
+      if (method === "GET") {
+        const tree = overlaySidePanels(DEFAULT_LAYOUT, views);
+        return new Response(JSON.stringify({ tree }), { status: 200 });
+      }
+      const tree = overlaySidePanels(DEFAULT_LAYOUT, views);
+      return new Response(JSON.stringify({ tree }), { status: 200 });
+    }
+    return inner(input, init);
+  };
+}
+
 function createFakeStorage(): Storage {
   const map = new Map<string, string>();
   return {
@@ -236,7 +274,7 @@ describe("App component interaction tests", () => {
     const ws = fakeSocket();
     const store = createAppStore({
       socketFactory: () => ws,
-      fetchImpl: fakeFetchImpl(),
+      fetchImpl: fakeFetchWithLayout(["extensions"]),
       localStorage: createFakeStorageWithViews(),
     });
     ws.resolveOpen();
@@ -300,7 +338,7 @@ describe("App component interaction tests", () => {
     const ws = fakeSocket();
     const store = createAppStore({
       socketFactory: () => ws,
-      fetchImpl: fakeFetchImpl(),
+      fetchImpl: fakeFetchWithLayout(["extensions"]),
       localStorage: createFakeStorageWithViews(),
     });
     ws.resolveOpen();
@@ -420,7 +458,7 @@ describe("App component interaction tests", () => {
     const ws = fakeSocket();
     const store = createAppStore({
       socketFactory: () => ws,
-      fetchImpl: fakeFetchImpl(),
+      fetchImpl: fakeFetchWithLayout(["extensions"]),
       localStorage: createFakeStorageWithViews(),
     });
     ws.resolveOpen();
@@ -459,19 +497,22 @@ describe("App component interaction tests", () => {
     store.dispose();
   });
 
-  it("the Extensions view lists frontend modules aggregated from feature manifests", () => {
+  it("the Extensions view lists frontend modules aggregated from feature manifests", async () => {
     const ws = fakeSocket();
     const store = createAppStore({
       socketFactory: () => ws,
-      fetchImpl: fakeFetchImpl(),
+      fetchImpl: fakeFetchWithLayout(["extensions"]),
       localStorage: createFakeStorageWithViews(),
     });
     ws.resolveOpen();
 
     render(App, { props: { store, onNavigate: vi.fn() } });
 
-    // Extensions view is pre-populated in the fake storage, so the modules table renders immediately.
-    expect(screen.getByRole("columnheader", { name: "Module" })).toBeInTheDocument();
+    // The Extensions view arrives with the provider's tree (an async GET —
+    // plan 02), so await the modules table rendering.
+    expect(
+      await screen.findByRole("columnheader", { name: "Module" }),
+    ).toBeInTheDocument();
     for (const name of [
       "chat",
       "tabs",
@@ -593,7 +634,7 @@ describe("App component interaction tests", () => {
     const ws = fakeSocket();
     const store = createAppStore({
       socketFactory: () => ws,
-      fetchImpl: fakeFetchImpl(),
+      fetchImpl: fakeFetchWithLayout(["vision"]),
       localStorage: createFakeStorageWithViews(["vision"]),
     });
     ws.resolveOpen();
diff --git a/src/app/store.svelte.ts b/src/app/store.svelte.ts
index bca9579..bfec235 100644
--- a/src/app/store.svelte.ts
+++ b/src/app/store.svelte.ts
@@ -14,6 +14,7 @@ import type {
   ConversationStatusChangedMessage,
   CwdResponse,
   ExtensionsChangedMessage,
+  LayoutChangedMessage,
   LspStatusResponse,
   McpStatusResponse,
   ModelMetadata,
@@ -82,6 +83,8 @@ import type {
   HeartbeatStopResult,
 } from "../features/heartbeat";
 import { normalizeHeartbeatConfig, normalizeHeartbeatRuns } from "../features/heartbeat";
+import type { LayoutStore } from "../features/layout";
+import { createLayoutHttp, createLayoutStore, listPanelViews } from "../features/layout";
 import type {
   NamedAgent,
   NamedAgentDeleteResult,
@@ -554,6 +557,14 @@ export interface AppStore {
    * it; the app store feeds its `onExtensionsChanged` hook.
    */
   readonly plugins: PluginsStore;
+  /**
+   * The ACTIVE workspace's tile tree (plan 02): served by the backend's
+   * layout provider when one is active, the local default otherwise. The
+   * shell reads `tree` for the renderer and feeds edits back through
+   * `onTreeChange` (debounced PUT + rollback); the store owns the probes,
+   * the WS-push filter, and the no-provider degrade.
+   */
+  readonly layout: LayoutStore;
   /**
    * Loaded provider-tier plugins effectively enabled in the ACTIVE workspace
    * (`GET /workspaces/:id/plugins/providers-status`). `null` = unknown — the
@@ -694,6 +705,20 @@ export function createAppStore(opts?: CreateAppStoreOptions): AppStore {
   // picker + guardrail on the same event.
   const pluginsStore = createPluginsStore(createPluginsHttp(httpBase, fetchImpl));
 
+  // ── Layout (plan 02) ─────────────────────────────────────────────────────────
+  // The ACTIVE workspace's tile tree, owned by the backend's layout provider
+  // plugin when one is active (its well-known routes + the `layout` WS push)
+  // and rendered from the local default otherwise (no-provider mode: edits
+  // ephemeral, logged once, no error UI). Probes run on boot, on every
+  // workspace switch, and on `extensions-changed` (§2.6); the one-way
+  // localStorage migration (§2.7) arms on the boot probe.
+  const layoutStore = createLayoutStore({
+    http: createLayoutHttp(httpBase, fetchImpl),
+    workspaceId: activeWorkspaceId,
+    storage: localStorageOpt,
+    knownViews: listPanelViews().map((v) => v.id),
+  });
+
   /**
    * Refetch the model catalog for the ACTIVE workspace
    * (`GET /models?workspaceId=`, plan 06 §6.5 — workspace-aware; a backend
@@ -1403,9 +1428,18 @@ export function createAppStore(opts?: CreateAppStoreOptions): AppStore {
       // surface catalog is NOT refetched here — the server re-pushes it
       // (workspace-filtered) on the same broadcast.
       pluginsStore.onExtensionsChanged();
+      // A layout plugin enabled/disabled mid-session starts/stops serving —
+      // the probe picks the new provider state up without a reload (§2.6).
+      void layoutStore.probe(activeWorkspaceId);
       void refreshModels();
       void refreshProviderStatus();
     },
+    onLayout(msg: LayoutChangedMessage) {
+      // §2.5 multi-client coherence: a layout write on ANY client (or a
+      // plugin's op) re-renders here — the store filters non-active
+      // workspaces (the server fans out to everyone).
+      layoutStore.onServerTree(msg.workspaceId, msg.tree);
+    },
   };
   if (opts?.socketFactory !== undefined) {
     socketOpts.socketFactory = opts.socketFactory;
@@ -1426,6 +1460,10 @@ export function createAppStore(opts?: CreateAppStoreOptions): AppStore {
   void refreshModels();
   // Fetch the active workspace's provider status (§6.5 first-boot guardrail).
   void refreshProviderStatus();
+  // Probe the ACTIVE layout provider (plan 02 §2.6 boot trigger): the tree is
+  // served by the backend's layout plugin when one is active. `boot: true`
+  // arms the one-way §2.7 localStorage migration on the first success.
+  void layoutStore.probe(activeWorkspaceId, { boot: true });
   // One-time legacy model-name migration on boot (independent of the fetch;
   // preserved from the pre-refreshModels boot chain).
   if (migrateLegacyModelName(activeModel) !== activeModel) {
@@ -1545,6 +1583,10 @@ export function createAppStore(opts?: CreateAppStoreOptions): AppStore {
       // (plan 06 §6.5): refetch both for the new workspace.
       void refreshModels();
       void refreshProviderStatus();
+      // The tile tree is WORKSPACE-SCOPED too (plan 02): reset to the local
+      // default for the new workspace and probe its provider (§2.6 — every
+      // workspace switch is a probe, which is also the missed-push catch-up).
+      layoutStore.onWorkspaceChanged(workspaceId);
     },
     get activeChat(): ChatStore {
       return activeChat;
@@ -2516,6 +2558,9 @@ export function createAppStore(opts?: CreateAppStoreOptions): AppStore {
     get plugins(): PluginsStore {
       return pluginsStore;
     },
+    get layout(): LayoutStore {
+      return layoutStore;
+    },
     get enabledProviderCount(): number | null {
       return enabledProviderCount;
     },
diff --git a/src/core/wire/conformance.ts b/src/core/wire/conformance.ts
index 5dbaae4..99afc6d 100644
--- a/src/core/wire/conformance.ts
+++ b/src/core/wire/conformance.ts
@@ -93,6 +93,8 @@ export function assertWsServerMessageExhaustive(msg: WsServerMessage): string {
       return "conversation.compacted";
     case "extensions-changed":
       return "extensions-changed";
+    case "layout":
+      return "layout";
     default:
       return msg satisfies never;
   }
diff --git a/src/features/layout/adapter/http.test.ts b/src/features/layout/adapter/http.test.ts
new file mode 100644
index 0000000..5b50715
--- /dev/null
+++ b/src/features/layout/adapter/http.test.ts
@@ -0,0 +1,136 @@
+import { DEFAULT_LAYOUT } from "@dispatch/layout-contract";
+import { describe, expect, it } from "vitest";
+import { createLayoutHttp } from "./http";
+
+/**
+ * Layout HTTP adapter tests — the OUTERMOST edge (fetch) is the only fake:
+ * a scripted `fetchImpl` returning canned Responses. No internal mocks.
+ */
+
+const LAYOUT_URL = "http://backend.test/workspaces/ws-a/layout";
+
+function fetchReturning(
+  status: number,
+  body: unknown,
+  capture?: { url?: string; init?: RequestInit | undefined },
+): typeof fetch {
+  return async (input: string | URL | Request, init?: RequestInit): Promise<Response> => {
+    if (capture !== undefined) {
+      capture.url = typeof input === "string" ? input : input instanceof URL ? input.href : input.url;
+      capture.init = init;
+    }
+    return new Response(JSON.stringify(body), {
+      status,
+      headers: { "Content-Type": "application/json" },
+    });
+  };
+}
+
+describe("createLayoutHttp — getTree", () => {
+  it("GETs the well-known route and unwraps { tree }", async () => {
+    const capture: { url?: string; init?: RequestInit } = {};
+    const http = createLayoutHttp(
+      "http://backend.test",
+      fetchReturning(200, { tree: DEFAULT_LAYOUT }, capture),
+    );
+    const result = await http.getTree("ws-a");
+    expect(result).toEqual({ ok: true, tree: DEFAULT_LAYOUT });
+    expect(capture.url).toBe(LAYOUT_URL);
+    expect(capture.init?.method).toBeUndefined(); // GET is the default
+  });
+
+  it("maps a 404 to the no-provider signal (notFound: true)", async () => {
+    const http = createLayoutHttp(
+      "http://backend.test",
+      fetchReturning(404, { error: "No layout provider is active" }),
+    );
+    const result = await http.getTree("ws-a");
+    expect(result).toEqual({
+      ok: false,
+      notFound: true,
+      error: "No layout provider is active",
+    });
+  });
+
+  it("maps a 500 to a non-notFound failure carrying the body's error", async () => {
+    const http = createLayoutHttp(
+      "http://backend.test",
+      fetchReturning(500, { error: "Layout storage failure" }),
+    );
+    const result = await http.getTree("ws-a");
+    expect(result).toEqual({ ok: false, notFound: false, error: "Layout storage failure" });
+  });
+
+  it("a network failure degrades to notFound (the no-provider mode)", async () => {
+    const http = createLayoutHttp("http://backend.test", async () => {
+      throw new TypeError("Failed to fetch");
+    });
+    const result = await http.getTree("ws-a");
+    expect(result.ok).toBe(false);
+    if (!result.ok) {
+      expect(result.notFound).toBe(true);
+      expect(result.error).toContain("Failed to fetch");
+    }
+  });
+
+  it("a malformed 200 (SPA-fallback JSON without a tree) degrades to notFound", async () => {
+    const http = createLayoutHttp(
+      "http://backend.test",
+      fetchReturning(200, { chunks: [], latestSeq: 0 }),
+    );
+    const result = await http.getTree("ws-a");
+    expect(result).toMatchObject({ ok: false, notFound: true });
+  });
+});
+
+describe("createLayoutHttp — putTree", () => {
+  it("PUTs { tree } as JSON and unwraps the echo", async () => {
+    const capture: { url?: string; init?: RequestInit } = {};
+    const http = createLayoutHttp(
+      "http://backend.test",
+      fetchReturning(200, { tree: DEFAULT_LAYOUT }, capture),
+    );
+    const result = await http.putTree("ws-a", DEFAULT_LAYOUT);
+    expect(result).toEqual({ ok: true, tree: DEFAULT_LAYOUT });
+    expect(capture.init?.method).toBe("PUT");
+    expect(capture.init?.body).toBe(JSON.stringify({ tree: DEFAULT_LAYOUT }));
+  });
+
+  it("maps a 400 { error } to a validation failure (not notFound)", async () => {
+    const http = createLayoutHttp(
+      "http://backend.test",
+      fetchReturning(400, { error: "root: split has no children" }),
+    );
+    const result = await http.putTree("ws-a", DEFAULT_LAYOUT);
+    expect(result).toEqual({
+      ok: false,
+      notFound: false,
+      error: "root: split has no children",
+    });
+  });
+
+  it("maps a 404 to the no-provider signal", async () => {
+    const http = createLayoutHttp("http://backend.test", fetchReturning(404, {}));
+    const result = await http.putTree("ws-a", DEFAULT_LAYOUT);
+    expect(result).toMatchObject({ ok: false, notFound: true });
+  });
+});
+
+describe("createLayoutHttp — deleteTree", () => {
+  it("DELETEs and unwraps the fresh seed", async () => {
+    const capture: { url?: string; init?: RequestInit } = {};
+    const http = createLayoutHttp(
+      "http://backend.test",
+      fetchReturning(200, { tree: DEFAULT_LAYOUT }, capture),
+    );
+    const result = await http.deleteTree("ws-a");
+    expect(result).toEqual({ ok: true, tree: DEFAULT_LAYOUT });
+    expect(capture.init?.method).toBe("DELETE");
+  });
+
+  it("maps a 404 to the no-provider signal", async () => {
+    const http = createLayoutHttp("http://backend.test", fetchReturning(404, {}));
+    const result = await http.deleteTree("ws-a");
+    expect(result).toMatchObject({ ok: false, notFound: true });
+  });
+});
diff --git a/src/features/layout/adapter/http.ts b/src/features/layout/adapter/http.ts
new file mode 100644
index 0000000..2f91e43
--- /dev/null
+++ b/src/features/layout/adapter/http.ts
@@ -0,0 +1,117 @@
+import type { LayoutTree } from "@dispatch/layout-contract";
+
+/**
+ * Layout HTTP effects (plan 02 §2.6) — the injected edge that talks to the
+ * ACTIVE layout provider's well-known routes:
+ * - `GET /workspaces/:id/layout` → `{ tree }` (read seeds a fresh workspace)
+ * - `PUT /workspaces/:id/layout` body `{ tree }` → 200 `{ tree }` / 400 `{ error }`
+ * - `DELETE /workspaces/:id/layout` → the fresh seed (reset; plan 04 adds the UI)
+ *
+ * Mirrors the plugins adapter's pattern: `httpBase` + an injected `fetchImpl`
+ * (testable without the network). The 404 is a SIGNAL, not an error: it means
+ * no layout provider is active, and the store renders its local default with
+ * ephemeral edits (never an error UI). A network failure degrades the same way.
+ */
+
+/** Outcome of a layout read/write. `notFound` distinguishes the no-provider 404. */
+export type LayoutHttpResult =
+  | { readonly ok: true; readonly tree: LayoutTree }
+  | { readonly ok: false; readonly notFound: boolean; readonly error: string };
+
+export interface LayoutHttp {
+  getTree(workspaceId: string): Promise<LayoutHttpResult>;
+  putTree(workspaceId: string, tree: LayoutTree): Promise<LayoutHttpResult>;
+  deleteTree(workspaceId: string): Promise<LayoutHttpResult>;
+}
+
+/** Read an error message from a non-2xx body (`{ error }`), tolerating junk. */
+async function errorOf(res: Response): Promise<string> {
+  try {
+    const body = (await res.json()) as { error?: unknown };
+    if (typeof body.error === "string" && body.error.length > 0) return body.error;
+  } catch {
+    // fall through
+  }
+  return `HTTP ${res.status}`;
+}
+
+/** A malformed 200 (e.g. an SPA-fallback page) degrades like a 404 — never poisons the store. */
+function malformed(): LayoutHttpResult {
+  return { ok: false, notFound: true, error: "Malformed layout response" };
+}
+
+export function createLayoutHttp(httpBase: string, fetchImpl: typeof fetch): LayoutHttp {
+  return {
+    async getTree(workspaceId): Promise<LayoutHttpResult> {
+      try {
+        const res = await fetchImpl(
+          `${httpBase}/workspaces/${encodeURIComponent(workspaceId)}/layout`,
+        );
+        if (res.status === 404) {
+          return { ok: false, notFound: true, error: "No layout provider is active" };
+        }
+        if (!res.ok) return { ok: false, notFound: false, error: await errorOf(res) };
+        const body = (await res.json()) as { tree?: unknown };
+        if (body === null || typeof body !== "object" || body.tree === undefined)
+          return malformed();
+        return { ok: true, tree: body.tree as LayoutTree };
+      } catch (err) {
+        return {
+          ok: false,
+          notFound: true,
+          error: err instanceof Error ? err.message : "Layout request failed",
+        };
+      }
+    },
+
+    async putTree(workspaceId, tree): Promise<LayoutHttpResult> {
+      try {
+        const res = await fetchImpl(
+          `${httpBase}/workspaces/${encodeURIComponent(workspaceId)}/layout`,
+          {
+            method: "PUT",
+            headers: { "Content-Type": "application/json" },
+            body: JSON.stringify({ tree }),
+          },
+        );
+        if (res.status === 404) {
+          return { ok: false, notFound: true, error: "No layout provider is active" };
+        }
+        if (!res.ok) return { ok: false, notFound: false, error: await errorOf(res) };
+        const body = (await res.json()) as { tree?: unknown };
+        if (body === null || typeof body !== "object" || body.tree === undefined)
+          return malformed();
+        return { ok: true, tree: body.tree as LayoutTree };
+      } catch (err) {
+        return {
+          ok: false,
+          notFound: true,
+          error: err instanceof Error ? err.message : "Layout request failed",
+        };
+      }
+    },
+
+    async deleteTree(workspaceId): Promise<LayoutHttpResult> {
+      try {
+        const res = await fetchImpl(
+          `${httpBase}/workspaces/${encodeURIComponent(workspaceId)}/layout`,
+          { method: "DELETE" },
+        );
+        if (res.status === 404) {
+          return { ok: false, notFound: true, error: "No layout provider is active" };
+        }
+        if (!res.ok) return { ok: false, notFound: false, error: await errorOf(res) };
+        const body = (await res.json()) as { tree?: unknown };
+        if (body === null || typeof body !== "object" || body.tree === undefined)
+          return malformed();
+        return { ok: true, tree: body.tree as LayoutTree };
+      } catch (err) {
+        return {
+          ok: false,
+          notFound: true,
+          error: err instanceof Error ? err.message : "Layout request failed",
+        };
+      }
+    },
+  };
+}
diff --git a/src/features/layout/index.ts b/src/features/layout/index.ts
index a2e93a6..f5696ac 100644
--- a/src/features/layout/index.ts
+++ b/src/features/layout/index.ts
@@ -7,6 +7,7 @@
  * OUTSIDE the tree — the tree owns the content region.
  */
 
+export { createLayoutHttp, type LayoutHttp, type LayoutHttpResult } from "./adapter/http";
 export {
   bootDefaultOpen,
   type CollapseState,
@@ -17,6 +18,7 @@ export {
   SIDEBAR_OPEN_KEY,
   WIDE_BREAKPOINT,
 } from "./logic/collapse";
+export { migrationTree, sidePanelViews, storedPanelViews } from "./logic/migration";
 export { addPanelOp, firstPanelView, removePanelOp, swapPanelOp } from "./logic/panels-edit";
 export {
   getView,
@@ -31,6 +33,7 @@ export {
 } from "./logic/registry";
 export { DEFAULT_PANEL_VIEWS, overlaySidePanels, seedLayout } from "./logic/seed";
 export { firstCollapsibleNodeId, firstPanelsNode } from "./logic/tree-query";
+export { createLayoutStore, type LayoutProviderState, type LayoutStore } from "./store.svelte";
 export { default as TileRoot } from "./ui/TileRoot.svelte";
 
 /** Public module manifest — aggregated by the shell's "Loaded Modules" view. */
diff --git a/src/features/layout/logic/migration.test.ts b/src/features/layout/logic/migration.test.ts
new file mode 100644
index 0000000..0b2b402
--- /dev/null
+++ b/src/features/layout/logic/migration.test.ts
@@ -0,0 +1,72 @@
+import { describe, expect, it } from "vitest";
+import { DEFAULT_LAYOUT } from "@dispatch/layout-contract";
+import {
+  migrationTree,
+  sidePanelViews,
+  storedPanelViews,
+} from "./migration";
+import { overlaySidePanels } from "./seed";
+
+const KNOWN = new Set(["tabs", "model", "extensions", "vision", "tasks"]);
+
+describe("storedPanelViews", () => {
+  it("parses a stored array of view ids, filtered to known views, order kept", () => {
+    expect(storedPanelViews(["vision", "junk", "tasks"], KNOWN)).toEqual(["vision", "tasks"]);
+  });
+
+  it("returns null for absent/junk/non-array values", () => {
+    expect(storedPanelViews(null, KNOWN)).toBeNull();
+    expect(storedPanelViews(undefined, KNOWN)).toBeNull();
+    expect(storedPanelViews("extensions", KNOWN)).toBeNull();
+    expect(storedPanelViews({ 0: "tabs" }, KNOWN)).toBeNull();
+  });
+
+  it("returns null when nothing stored is a known view", () => {
+    expect(storedPanelViews(["deleted-view", "another-gone"], KNOWN)).toBeNull();
+    expect(storedPanelViews([], KNOWN)).toBeNull();
+  });
+});
+
+describe("sidePanelViews", () => {
+  it("reads the default tree's side panels", () => {
+    expect(sidePanelViews(DEFAULT_LAYOUT)).toEqual(["tabs", "model"]);
+  });
+
+  it("returns null for a tree without a 'side' panels node", () => {
+    expect(sidePanelViews({ version: 1, root: { kind: "leaf", view: "chat" } })).toBeNull();
+  });
+});
+
+describe("migrationTree (§2.7 — one-way, best-effort)", () => {
+  it("imports the stored selection when the fetched tree is the untouched default seed", () => {
+    const stored = JSON.stringify(["extensions", "tasks"]);
+    const migrated = migrationTree(JSON.parse(stored), KNOWN, DEFAULT_LAYOUT);
+    expect(migrated).toEqual(overlaySidePanels(DEFAULT_LAYOUT, ["extensions", "tasks"]));
+    // The rest of the tree is untouched — only the side panels changed.
+    expect(migrated?.root).toEqual(
+      overlaySidePanels(DEFAULT_LAYOUT, ["extensions", "tasks"])?.root,
+    );
+  });
+
+  it("returns null when the key is absent / junk / has no known views (nothing to import)", () => {
+    expect(migrationTree(null, KNOWN, DEFAULT_LAYOUT)).toBeNull();
+    expect(migrationTree("junk", KNOWN, DEFAULT_LAYOUT)).toBeNull();
+    expect(migrationTree(["gone-view"], KNOWN, DEFAULT_LAYOUT)).toBeNull();
+  });
+
+  it("returns null when the provider's tree is NOT the default seed (custom layout wins)", () => {
+    const custom = overlaySidePanels(DEFAULT_LAYOUT, ["vision"]);
+    expect(migrationTree(["extensions"], KNOWN, custom)).toBeNull();
+  });
+
+  it("returns null when the fetched tree has no side panels node at all", () => {
+    expect(
+      migrationTree(["extensions"], KNOWN, { version: 1, root: { kind: "leaf", view: "chat" } }),
+    ).toBeNull();
+  });
+
+  it("a stored selection identical to the default still migrates (a harmless idempotent PUT)", () => {
+    const migrated = migrationTree(["tabs", "model"], KNOWN, DEFAULT_LAYOUT);
+    expect(migrated).toEqual(DEFAULT_LAYOUT);
+  });
+});
diff --git a/src/features/layout/logic/migration.ts b/src/features/layout/logic/migration.ts
new file mode 100644
index 0000000..e0ed6fd
--- /dev/null
+++ b/src/features/layout/logic/migration.ts
@@ -0,0 +1,76 @@
+/**
+ * localStorage → provider migration (plan 02 §2.7, D-L4; one-way, best-effort).
+ *
+ * `dispatch.sidebar.views` is retired as a source of truth: on boot WITH a
+ * provider present, if the key exists AND the fetched tree's `side` panels
+ * equal DEFAULT_LAYOUT's (the workspace still has the untouched default
+ * seed), the stored panel selection is imported once via a PUT. When the
+ * provider's tree differs from the default (e.g. a calendar provider), the
+ * workspace already has a custom layout — importing the old sidebar override
+ * would be wrong. Regardless of outcome the key is never read again (the
+ * bytes stay, but nothing consumes them).
+ *
+ * Pure decision logic: the store owns the effects (the localStorage read +
+ * the PUT); this module decides IF and WHAT.
+ */
+
+import {
+  DEFAULT_LAYOUT,
+  findById,
+  type LayoutTree,
+  type PanelsNode,
+} from "@dispatch/layout-contract";
+import { overlaySidePanels } from "./seed";
+
+/**
+ * The stored selection's usable view ids: an array of strings filtered to the
+ * known (registered) views, in stored order. null when nothing usable is
+ * stored (absent key, junk, or no known ids) — nothing to import.
+ */
+export function storedPanelViews(
+  raw: unknown,
+  knownViews: ReadonlySet<string>,
+): readonly string[] | null {
+  if (!Array.isArray(raw)) return null;
+  const known = raw.filter((v): v is string => typeof v === "string" && knownViews.has(v));
+  return known.length > 0 ? known : null;
+}
+
+/**
+ * The view ids of a tree's `"side"` panels node (the default tree's swappable
+ * sidebar stack), or null when the tree has no such node.
+ */
+export function sidePanelViews(tree: LayoutTree): readonly string[] | null {
+  const found = findById(tree, "side");
+  if (found === null || found.node.kind !== "panels") return null;
+  return (found.node as PanelsNode).panels.map((p) => p.view);
+}
+
+function samePanels(a: readonly string[], b: readonly string[]): boolean {
+  return a.length === b.length && a.every((v, i) => v === b[i]);
+}
+
+/**
+ * The one-way migration decision (§2.7): given the RAW stored value, the
+ * known view ids, and the tree fetched from the provider, return the tree to
+ * PUT once — or null when there is nothing to migrate:
+ * - the stored key holds no usable selection, or
+ * - the provider's tree is NOT the untouched default seed (its side panels
+ *   differ from DEFAULT_LAYOUT's — the workspace already has a custom layout).
+ */
+export function migrationTree(
+  storedRaw: unknown,
+  knownViews: ReadonlySet<string>,
+  fetched: LayoutTree,
+): LayoutTree | null {
+  const stored = storedPanelViews(storedRaw, knownViews);
+  if (stored === null) return null;
+  const fetchedPanels = sidePanelViews(fetched);
+  const defaultPanels = sidePanelViews(DEFAULT_LAYOUT);
+  if (fetchedPanels === null || defaultPanels === null) return null;
+  if (!samePanels(fetchedPanels, defaultPanels)) return null;
+  // Overlay the stored selection onto the fetched tree (identical to the
+  // default here by the guard above — the seed overlay keeps the rest of the
+  // tree shared, not copied).
+  return overlaySidePanels(fetched, stored);
+}
diff --git a/src/features/layout/store.svelte.ts b/src/features/layout/store.svelte.ts
new file mode 100644
index 0000000..3d7de3f
--- /dev/null
+++ b/src/features/layout/store.svelte.ts
@@ -0,0 +1,270 @@
+import { DEFAULT_LAYOUT, type LayoutTree } from "@dispatch/layout-contract";
+import type { LayoutHttp, LayoutHttpResult } from "./adapter/http";
+import { migrationTree } from "./logic/migration";
+
+/**
+ * Layout store (plan 02 §2.6) — a thin reactive wrapper over the injected
+ * `LayoutHttp` edge (the plugins-store pattern). Owns the ACTIVE workspace's
+ * tree and the provider lifecycle:
+ *
+ * - **Provider probe triggers** — boot, every workspace switch, and every
+ *   `extensions-changed` (a provider enabled/disabled mid-session starts/
+ *   stops serving; the probe picks it up without a reload).
+ * - **No-provider mode** — a 404 (or network failure) keeps the tree on its
+ *   local default; edits still work but are EPHEMERAL (in-memory only —
+ *   D-L4 retired the localStorage write path); logged once, no error UI.
+ * - **WS push** — `onServerTree(workspaceId, tree)` applies a `layout`
+ *   broadcast, ignoring non-active workspaces (the server fans out to all
+ *   clients; the client filters).
+ * - **User edits persist through the provider** — `onTreeChange` applies the
+ *   edit locally immediately, then PUTs the WHOLE tree (decision 7 — no
+ *   patch protocol) lightly debounced; on a failed PUT the tree rolls back
+ *   to the last server-known tree with a quiet console warning.
+ * - **Collapse state stays local** (decision 3) — `dispatch.sidebar.open`
+ *   is the collapse store's concern, untouched here.
+ * - **One-way localStorage migration** (§2.7) — on the FIRST successful
+ *   provider read of the boot session, if `dispatch.sidebar.views` exists
+ *   AND the fetched tree is the untouched default seed, the stored panel
+ *   selection is imported with one PUT; the key is then never read again.
+ *
+ * No ambient state: one instance per composition root, passed explicitly;
+ * no module-level mutables.
+ */
+
+/** Whether an active provider is serving the active workspace. */
+export type LayoutProviderState = "unknown" | "present" | "absent";
+
+export interface LayoutStore {
+  /** The rendered tree for the ACTIVE workspace (starts at the local default). */
+  readonly tree: LayoutTree;
+  /** The active provider's state (absent = the no-provider degrade path). */
+  readonly providerState: LayoutProviderState;
+  /** The workspace the tree currently belongs to. */
+  readonly activeWorkspaceId: string;
+  /**
+   * Probe the provider for `workspaceId`'s tree. `boot: true` (the composition
+   * root's boot call) also arms the one-way localStorage migration (§2.7),
+   * which consumes itself on the first successful provider read regardless of
+   * outcome. A probe is the missed-push catch-up (§2.5).
+   */
+  probe(workspaceId: string, opts?: { readonly boot?: boolean }): Promise<void>;
+  /** Workspace switch: reset to the local default for the new workspace + probe. */
+  onWorkspaceChanged(workspaceId: string): void;
+  /** Server `layout` broadcast (§2.5). Non-active workspaces are ignored. */
+  onServerTree(workspaceId: string, tree: LayoutTree): void;
+  /** A user edit (the panels' dropdowns / TileRoot ops). Persists via PUT. */
+  onTreeChange(next: LayoutTree): void;
+  /** Multi-client coherence (§11): re-probe on `extensions-changed`. */
+  onExtensionsChanged(): void;
+}
+
+export interface LayoutStoreOptions {
+  readonly http: LayoutHttp;
+  /** The boot workspace (its URL slug); "default" if absent. */
+  readonly workspaceId?: string;
+  /**
+   * Injected localStorage — read ONLY by the one-way §2.7 migration (the
+   * write path is retired; collapse state has its own store). Undefined →
+   * migration never applies (tests / storageless contexts).
+   */
+  readonly storage?: Storage | undefined;
+  /** The known (registered) view ids, for the migration's unknown-id filter. */
+  readonly knownViews?: ReadonlySet<string> | readonly string[];
+  /**
+   * The localStorage key the §2.7 migration reads (default
+   * `dispatch.sidebar.views`; tests override).
+   */
+  readonly sidebarViewsKey?: string;
+  /** Edit-PUT debounce window in ms (default 300; tests shrink it). */
+  readonly debounceMs?: number;
+  /** The quiet-failure hook (default `console.warn` — never an error UI). */
+  readonly warn?: (message: string) => void;
+}
+
+const DEFAULT_SIDEBAR_VIEWS_KEY = "dispatch.sidebar.views";
+const DEFAULT_DEBOUNCE_MS = 300;
+
+function toKnownSet(known: LayoutStoreOptions["knownViews"]): ReadonlySet<string> {
+  if (known === undefined) return new Set();
+  return known instanceof Set ? known : new Set(known);
+}
+
+export function createLayoutStore(opts: LayoutStoreOptions): LayoutStore {
+  const http = opts.http;
+  const storage = opts.storage;
+  const sidebarViewsKey = opts.sidebarViewsKey ?? DEFAULT_SIDEBAR_VIEWS_KEY;
+  const debounceMs = opts.debounceMs ?? DEFAULT_DEBOUNCE_MS;
+  const knownViews = toKnownSet(opts.knownViews);
+  const warn = opts.warn ?? ((message: string) => console.warn(message));
+
+  let tree = $state<LayoutTree>(DEFAULT_LAYOUT);
+  let providerState = $state<LayoutProviderState>("unknown");
+  let activeWorkspaceId = $state<string>(opts.workspaceId ?? "default");
+
+  /** The last tree known to come from the server (GET / WS push / PUT echo) — the rollback target. */
+  let lastServerTree: LayoutTree | null = null;
+  /** The pending edit-PUT timer (the "lightly debounced" write). */
+  let putTimer: ReturnType<typeof setTimeout> | null = null;
+  /** Logged-once guard for the no-provider log (reset when a provider appears). */
+  let loggedAbsent = false;
+  /**
+   * The §2.7 migration is armed by the BOOT probe and consumed by the first
+   * successful provider read — regardless of outcome (read-once semantics).
+   */
+  let migrationArmed = false;
+
+  function noteAbsent(reason: string): void {
+    providerState = "absent";
+    if (!loggedAbsent) {
+      loggedAbsent = true;
+      warn(
+        `layout: no layout provider active (${reason}) — rendering the built-in default; edits are ephemeral`,
+      );
+    }
+  }
+
+  function notePresent(): void {
+    providerState = "present";
+    loggedAbsent = false;
+  }
+
+  function applyServerTree(next: LayoutTree): void {
+    lastServerTree = next;
+    tree = next;
+  }
+
+  /** The §2.7 one-way migration: at most one read + one PUT per session. */
+  async function maybeMigrate(): Promise<void> {
+    if (!migrationArmed || storage === undefined) return;
+    migrationArmed = false; // consumed regardless of outcome (stop reading the key)
+    let raw: string | null = null;
+    try {
+      raw = storage.getItem(sidebarViewsKey);
+    } catch {
+      return; // storage unavailable — best-effort, done
+    }
+    if (raw === null) return; // nothing stored — nothing to import
+    let parsed: unknown = null;
+    try {
+      parsed = JSON.parse(raw) as unknown;
+    } catch {
+      return; // junk — best-effort, done
+    }
+    const migrated = migrationTree(parsed, knownViews, tree);
+    if (migrated === null) return; // custom layout already / nothing usable
+    const result = await http.putTree(activeWorkspaceId, migrated);
+    if (result.ok) {
+      // The server's canonical copy (also echoed back via the WS broadcast).
+      applyServerTree(result.tree);
+    } else if (!result.notFound) {
+      warn(`layout: localStorage migration PUT failed — ${result.error}`);
+    }
+  }
+
+  async function runProbe(workspaceId: string, boot: boolean): Promise<void> {
+    if (boot) migrationArmed = true;
+    const result: LayoutHttpResult = await http.getTree(workspaceId);
+    // A switch may have happened while the fetch was in flight — only the
+    // CURRENT workspace's result lands (the refreshProviderStatus guard).
+    if (workspaceId !== activeWorkspaceId) return;
+    if (result.ok) {
+      notePresent();
+      applyServerTree(result.tree);
+      await maybeMigrate();
+    } else if (result.notFound) {
+      noteAbsent(result.error);
+    } else {
+      // A real failure (5xx, malformed) degrades like absent — quietly.
+      noteAbsent(result.error);
+    }
+  }
+
+  function clearPutTimer(): void {
+    if (putTimer !== null) {
+      clearTimeout(putTimer);
+      putTimer = null;
+    }
+  }
+
+  async function flushPut(workspaceId: string, pending: LayoutTree): Promise<void> {
+    putTimer = null;
+    // Read the CURRENT tree at fire time: a newer edit supersedes the
+    // debounced snapshot (one in-flight write per window, last wins).
+    const target = tree === pending ? pending : tree;
+    const result = await http.putTree(workspaceId, target);
+    if (result.ok) {
+      notePresent();
+      // The server's validated echo is the new rollback target.
+      lastServerTree = result.tree;
+      return;
+    }
+    if (result.notFound) {
+      // The provider vanished: keep the local edit (ephemeral), no rollback —
+      // there is no server tree to roll back TO.
+      noteAbsent(result.error);
+      return;
+    }
+    // A failed write on a PRESENT provider: roll back to the last server
+    // tree, quiet console warning (never an error UI).
+    warn(
+      `layout: persisting the tree failed (${result.error}) — rolled back to the last server tree`,
+    );
+    if (lastServerTree !== null) tree = lastServerTree;
+  }
+
+  return {
+    get tree() {
+      return tree;
+    },
+    get providerState() {
+      return providerState;
+    },
+    get activeWorkspaceId() {
+      return activeWorkspaceId;
+    },
+
+    async probe(workspaceId, probeOpts) {
+      await runProbe(workspaceId, probeOpts?.boot === true);
+    },
+
+    onWorkspaceChanged(workspaceId) {
+      if (workspaceId === activeWorkspaceId) return;
+      activeWorkspaceId = workspaceId;
+      // The tree belongs to the OLD workspace now: reset to the local default
+      // while the probe resolves (never flash the previous workspace's tree),
+      // and drop any queued edit — it was for the old workspace.
+      clearPutTimer();
+      lastServerTree = null;
+      tree = DEFAULT_LAYOUT;
+      providerState = "unknown";
+      void runProbe(workspaceId, false);
+    },
+
+    onServerTree(workspaceId, next) {
+      if (workspaceId !== activeWorkspaceId) return; // clients filter (§2.5)
+      notePresent();
+      clearPutTimer(); // the server's copy supersedes any queued edit
+      applyServerTree(next);
+    },
+
+    onTreeChange(next) {
+      tree = next; // the edit applies locally IMMEDIATELY (the UI never waits)
+      if (providerState === "absent") return; // ephemeral — no write, no warning
+      clearPutTimer();
+      putTimer = setTimeout(() => {
+        void flushPut(activeWorkspaceId, next).catch(() => {
+          // Network-grade failure: roll back to the last server tree, quietly.
+          warn("layout: persisting the tree failed — rolled back to the last server tree");
+          if (lastServerTree !== null) tree = lastServerTree;
+        });
+      }, debounceMs);
+    },
+
+    onExtensionsChanged() {
+      void runProbe(activeWorkspaceId, false).catch(() => {
+        // runProbe never throws for HTTP failures; this guard is for the
+        // migration path. Staying silent keeps the degrade quiet.
+      });
+    },
+  };
+}
diff --git a/src/features/layout/store.test.ts b/src/features/layout/store.test.ts
new file mode 100644
index 0000000..7e7d6bc
--- /dev/null
+++ b/src/features/layout/store.test.ts
@@ -0,0 +1,420 @@
+import { DEFAULT_LAYOUT, type LayoutTree } from "@dispatch/layout-contract";
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import { overlaySidePanels } from "./logic/seed";
+import type { LayoutHttp, LayoutHttpResult } from "./adapter/http";
+import { createLayoutStore } from "./store.svelte";
+
+/**
+ * Layout store tests (plan 02 verification gate) — mock HTTP EDGES ONLY (the
+ * injected `LayoutHttp` is the store's outermost boundary; nothing internal
+ * is mocked — asymmetric testing per AGENTS.md). Fake timers drive the edit
+ * PUT debounce deterministically.
+ */
+
+/** A recording fake of the LayoutHttp edge, scripted per test. */
+interface HttpFake extends LayoutHttp {
+  getCalls: string[];
+  putCalls: Array<{ workspaceId: string; tree: LayoutTree }>;
+  getResult: (workspaceId: string) => LayoutHttpResult;
+  putResult: (workspaceId: string, tree: LayoutTree) => LayoutHttpResult;
+}
+
+function fakeHttp(
+  getResult: (workspaceId: string) => LayoutHttpResult = () => ({
+    ok: false,
+    notFound: true,
+    error: "No layout provider is active",
+  }),
+  putResult: (workspaceId: string, tree: LayoutTree) => LayoutHttpResult = (_ws, tree) => ({
+    ok: true,
+    tree,
+  }),
+): HttpFake {
+  const getCalls: string[] = [];
+  const putCalls: Array<{ workspaceId: string; tree: LayoutTree }> = [];
+  const fake: HttpFake = {
+    getCalls,
+    putCalls,
+    getResult,
+    putResult,
+    // Delegate to the CURRENT properties so a test can re-script outcomes
+    // mid-test (http.putResult = …).
+    async getTree(workspaceId) {
+      getCalls.push(workspaceId);
+      return fake.getResult(workspaceId);
+    },
+    async putTree(workspaceId, tree) {
+      putCalls.push({ workspaceId, tree });
+      return fake.putResult(workspaceId, tree);
+    },
+    async deleteTree() {
+      return { ok: false, notFound: true, error: "unused" };
+    },
+  };
+  return fake;
+}
+
+function storageWith(views: string[] | null): Storage {
+  const map = new Map<string, string>();
+  if (views !== null) map.set("test.sidebar.views", JSON.stringify(views));
+  return {
+    get length() {
+      return map.size;
+    },
+    clear() {
+      map.clear();
+    },
+    getItem: (k) => map.get(k) ?? null,
+    key: () => null,
+    removeItem: (k) => map.delete(k),
+    setItem: (k, v) => map.set(k, v),
+  };
+}
+
+const KNOWN = ["tabs", "model", "extensions", "vision"];
+
+function makeStore(http: HttpFake, extra: Record<string, unknown> = {}) {
+  return createLayoutStore({
+    http,
+    workspaceId: "ws-a",
+    knownViews: KNOWN,
+    sidebarViewsKey: "test.sidebar.views",
+    debounceMs: 100,
+    warn: (message) => warnings.push(message),
+    ...extra,
+  });
+}
+
+let warnings: string[] = [];
+
+beforeEach(() => {
+  vi.useFakeTimers();
+  warnings = [];
+});
+
+afterEach(() => {
+  vi.useRealTimers();
+});
+
+describe("layout store — no-provider fallback (§2.6)", () => {
+  it("a 404 probe keeps the default tree, marks the provider absent, and logs ONCE", async () => {
+    const http = fakeHttp();
+    const store = makeStore(http);
+
+    await store.probe("ws-a", { boot: true });
+    expect(store.providerState).toBe("absent");
+    expect(store.tree).toEqual(DEFAULT_LAYOUT);
+    expect(warnings).toHaveLength(1);
+    expect(warnings[0]).toContain("no layout provider");
+
+    // A re-probe (extensions-changed while still absent) does not re-warn.
+    await store.probe("ws-a");
+    expect(warnings).toHaveLength(1);
+  });
+
+  it("a network failure degrades like a 404 (quiet, no error UI)", async () => {
+    const http = fakeHttp(() => ({ ok: false, notFound: true, error: "fetch blew up" }));
+    const store = makeStore(http);
+    await store.probe("ws-a");
+    expect(store.providerState).toBe("absent");
+    expect(store.tree).toEqual(DEFAULT_LAYOUT);
+  });
+
+  it("edits still apply in no-provider mode but are EPHEMERAL (no PUT, no rollback, no warning)", async () => {
+    const http = fakeHttp();
+    const store = makeStore(http);
+    await store.probe("ws-a"); // absent now
+    expect(store.providerState).toBe("absent");
+
+    const edited = overlaySidePanels(DEFAULT_LAYOUT, ["extensions"]);
+    store.onTreeChange(edited);
+    expect(store.tree).toEqual(edited);
+
+    vi.advanceTimersByTime(500);
+    expect(http.putCalls).toEqual([]); // ephemeral: no write attempted
+    // Quiet = the single absence log (from the probe), nothing more.
+    expect(warnings).toHaveLength(1);
+  });
+
+  it("an edit while the probe is still in flight is PUT optimistically; a 404 flips absent and KEEPS the edit", async () => {
+    const http = fakeHttp();
+    // The PUT is a 404 (the probe will confirm: no provider).
+    http.putResult = () => ({ ok: false, notFound: true, error: "No layout provider is active" });
+    const store = makeStore(http);
+
+    void store.probe("ws-a"); // in flight (microtask pending)
+    const edited = overlaySidePanels(DEFAULT_LAYOUT, ["extensions"]);
+    store.onTreeChange(edited); // lands BEFORE the probe resolves
+    await vi.advanceTimersByTimeAsync(200);
+
+    expect(http.putCalls).toHaveLength(1); // the optimistic write WAS attempted
+    expect(store.providerState).toBe("absent");
+    expect(store.tree).toEqual(edited); // no rollback — there is no server tree
+    expect(warnings).toHaveLength(1); // the once-per-absence log, nothing more
+  });
+});
+
+describe("layout store — provider present", () => {
+  it("a 200 probe replaces the tree and marks the provider present", async () => {
+    const served = overlaySidePanels(DEFAULT_LAYOUT, ["extensions"]);
+    const http = fakeHttp(() => ({ ok: true, tree: served }));
+    const store = makeStore(http);
+
+    await store.probe("ws-a");
+    expect(store.providerState).toBe("present");
+    expect(store.tree).toEqual(served);
+    expect(warnings).toEqual([]);
+  });
+});
+
+describe("layout store — WS push (§2.5)", () => {
+  it("a layout push for the ACTIVE workspace replaces the tree", async () => {
+    const served = overlaySidePanels(DEFAULT_LAYOUT, ["extensions"]);
+    const http = fakeHttp(() => ({ ok: true, tree: served }));
+    const store = makeStore(http);
+    await store.probe("ws-a");
+
+    const pushed = overlaySidePanels(DEFAULT_LAYOUT, ["vision"]);
+    store.onServerTree("ws-a", pushed);
+    expect(store.tree).toEqual(pushed);
+  });
+
+  it("a push for a NON-active workspace is ignored", async () => {
+    const served = overlaySidePanels(DEFAULT_LAYOUT, ["extensions"]);
+    const http = fakeHttp(() => ({ ok: true, tree: served }));
+    const store = makeStore(http);
+    await store.probe("ws-a");
+
+    const other = overlaySidePanels(DEFAULT_LAYOUT, ["vision"]);
+    store.onServerTree("ws-b", other);
+    expect(store.tree).toEqual(served);
+  });
+
+  it("after a workspace switch the OLD workspace's push is ignored (clients filter)", async () => {
+    const servedA = overlaySidePanels(DEFAULT_LAYOUT, ["extensions"]);
+    let served = servedA;
+    const http = fakeHttp(() => ({ ok: true, tree: served }));
+    const store = makeStore(http);
+    await store.probe("ws-a");
+
+    const servedB = overlaySidePanels(DEFAULT_LAYOUT, ["vision"]);
+    served = servedB;
+    store.onWorkspaceChanged("ws-b");
+    await vi.advanceTimersByTimeAsync(0); // the probe's promise
+    expect(store.tree).toEqual(servedB);
+
+    // A late push for the OLD workspace must not land.
+    const stale = overlaySidePanels(DEFAULT_LAYOUT, ["tasks"]);
+    store.onServerTree("ws-a", stale);
+    expect(store.tree).toEqual(servedB);
+  });
+});
+
+describe("layout store — edits PUT the whole tree + rollback (§2.6)", () => {
+  it("an edit PUTs the WHOLE tree, debounced; a 200 echo becomes the rollback target", async () => {
+    const served = DEFAULT_LAYOUT;
+    const http = fakeHttp(() => ({ ok: true, tree: served }));
+    const store = makeStore(http);
+    await store.probe("ws-a");
+
+    const edited = overlaySidePanels(DEFAULT_LAYOUT, ["extensions"]);
+    store.onTreeChange(edited);
+    // Not yet — debounced.
+    expect(http.putCalls).toEqual([]);
+    await vi.advanceTimersByTimeAsync(150);
+    expect(http.putCalls).toEqual([{ workspaceId: "ws-a", tree: edited }]);
+    expect(warnings).toEqual([]);
+  });
+
+  it("rapid edits coalesce into ONE debounced PUT of the latest tree", async () => {
+    const http = fakeHttp(() => ({ ok: true, tree: DEFAULT_LAYOUT }));
+    const store = makeStore(http);
+    await store.probe("ws-a");
+
+    store.onTreeChange(overlaySidePanels(DEFAULT_LAYOUT, ["extensions"]));
+    await vi.advanceTimersByTimeAsync(50);
+    store.onTreeChange(overlaySidePanels(DEFAULT_LAYOUT, ["vision"]));
+    await vi.advanceTimersByTimeAsync(200);
+
+    expect(http.putCalls).toHaveLength(1);
+    expect(http.putCalls[0]?.tree).toEqual(overlaySidePanels(DEFAULT_LAYOUT, ["vision"]));
+  });
+
+  it("a 400 rejection rolls back to the last server tree with a quiet warning", async () => {
+    const served = DEFAULT_LAYOUT;
+    const http = fakeHttp(
+      () => ({ ok: true, tree: served }),
+      () => ({ ok: false, notFound: false, error: "invalid tree" }),
+    );
+    const store = makeStore(http);
+    await store.probe("ws-a");
+    expect(store.providerState).toBe("present");
+
+    const bad = overlaySidePanels(DEFAULT_LAYOUT, ["extensions"]);
+    store.onTreeChange(bad);
+    await vi.advanceTimersByTimeAsync(200);
+
+    expect(store.tree).toEqual(served); // rolled back
+    expect(store.providerState).toBe("present"); // the provider is still there
+    expect(warnings.some((w) => w.includes("rolled back"))).toBe(true);
+  });
+
+  it("a 5xx failure rolls back too (present provider, transient error)", async () => {
+    const served = DEFAULT_LAYOUT;
+    let failPut = false;
+    const http = fakeHttp(
+      () => ({ ok: true, tree: served }),
+      () =>
+        failPut
+          ? { ok: false, notFound: false, error: "storage down" }
+          : { ok: true, tree: served },
+    );
+    const store = makeStore(http);
+    await store.probe("ws-a");
+
+    failPut = true;
+    store.onTreeChange(overlaySidePanels(DEFAULT_LAYOUT, ["extensions"]));
+    await vi.advanceTimersByTimeAsync(200);
+    expect(store.tree).toEqual(served);
+  });
+
+  it("a PUT 404 does NOT roll back — the provider vanished mid-session; the edit stays ephemeral", async () => {
+    const http = fakeHttp(() => ({ ok: true, tree: DEFAULT_LAYOUT }));
+    const store = makeStore(http);
+    await store.probe("ws-a");
+
+    // The provider unloads before the edit's PUT fires.
+    http.putResult = () => ({ ok: false, notFound: true, error: "No layout provider is active" });
+    const edited = overlaySidePanels(DEFAULT_LAYOUT, ["extensions"]);
+    store.onTreeChange(edited);
+    await vi.advanceTimersByTimeAsync(200);
+
+    expect(store.tree).toEqual(edited); // the edit STAYS (in-memory)
+    expect(store.providerState).toBe("absent");
+    expect(warnings).toHaveLength(1); // the once-per-absence log, nothing more
+  });
+});
+
+describe("layout store — probe triggers (§2.6)", () => {
+  it("extensions-changed re-probes the active workspace", async () => {
+    let absent = true;
+    const served = overlaySidePanels(DEFAULT_LAYOUT, ["extensions"]);
+    const http = fakeHttp(() =>
+      absent ? { ok: false, notFound: true, error: "No layout provider is active" } : { ok: true, tree: served },
+    );
+    const store = makeStore(http);
+    await store.probe("ws-a");
+    expect(store.providerState).toBe("absent");
+
+    absent = false; // a layout plugin got enabled on the server
+    store.onExtensionsChanged();
+    await vi.advanceTimersByTimeAsync(0);
+    expect(store.providerState).toBe("present");
+    expect(store.tree).toEqual(served);
+  });
+
+  it("a workspace switch resets to the default, probes the NEW workspace, and drops queued edits", async () => {
+    const http = fakeHttp((ws) =>
+      ws === "ws-b" ? { ok: true, tree: overlaySidePanels(DEFAULT_LAYOUT, ["vision"]) } : { ok: true, tree: DEFAULT_LAYOUT },
+    );
+    const store = makeStore(http);
+    await store.probe("ws-a");
+
+    // Queue an edit for ws-a, then switch before the debounce fires.
+    store.onTreeChange(overlaySidePanels(DEFAULT_LAYOUT, ["extensions"]));
+    store.onWorkspaceChanged("ws-b");
+    await vi.advanceTimersByTimeAsync(300);
+
+    expect(store.activeWorkspaceId).toBe("ws-b");
+    expect(http.getCalls).toEqual(["ws-a", "ws-b"]);
+    expect(store.tree).toEqual(overlaySidePanels(DEFAULT_LAYOUT, ["vision"]));
+    // The queued edit never PUTs — it belonged to the old workspace.
+    expect(http.putCalls).toEqual([]);
+  });
+
+  it("switching to the SAME workspace is a no-op (no re-probe, no reset)", async () => {
+    const http = fakeHttp(() => ({ ok: true, tree: DEFAULT_LAYOUT }));
+    const store = makeStore(http);
+    await store.probe("ws-a");
+    store.onWorkspaceChanged("ws-a");
+    expect(http.getCalls).toEqual(["ws-a"]);
+  });
+});
+
+describe("layout store — one-way localStorage migration (§2.7)", () => {
+  it("boot probe + equal panels + stored key → exactly ONE migration PUT, then the key is dead", async () => {
+    const storage = storageWith(["extensions", "vision"]);
+    const http = fakeHttp(() => ({ ok: true, tree: DEFAULT_LAYOUT }));
+    const store = makeStore(http, { storage });
+
+    await store.probe("ws-a", { boot: true });
+    expect(http.putCalls).toEqual([
+      { workspaceId: "ws-a", tree: overlaySidePanels(DEFAULT_LAYOUT, ["extensions", "vision"]) },
+    ]);
+    // The migrated tree is now the rendered tree.
+    expect(store.tree).toEqual(overlaySidePanels(DEFAULT_LAYOUT, ["extensions", "vision"]));
+
+    // The key is never read again: a later probe PUTs nothing.
+    await store.probe("ws-a");
+    expect(http.putCalls).toHaveLength(1);
+  });
+
+  it("no stored key → no migration PUT (but the read-once flag is still consumed)", async () => {
+    const storage = storageWith(null);
+    const http = fakeHttp(() => ({ ok: true, tree: DEFAULT_LAYOUT }));
+    const store = makeStore(http, { storage });
+    await store.probe("ws-a", { boot: true });
+    expect(http.putCalls).toEqual([]);
+  });
+
+  it("a custom provider tree (panels differ) is NOT migrated over", async () => {
+    const storage = storageWith(["extensions"]);
+    const custom = overlaySidePanels(DEFAULT_LAYOUT, ["vision"]);
+    const http = fakeHttp(() => ({ ok: true, tree: custom }));
+    const store = makeStore(http, { storage });
+    await store.probe("ws-a", { boot: true });
+    expect(http.putCalls).toEqual([]);
+    expect(store.tree).toEqual(custom);
+  });
+
+  it("migration is boot-only: a non-boot probe never PUTs the stored selection", async () => {
+    const storage = storageWith(["extensions"]);
+    const http = fakeHttp(() => ({ ok: true, tree: DEFAULT_LAYOUT }));
+    const store = makeStore(http, { storage });
+    await store.probe("ws-a");
+    expect(http.putCalls).toEqual([]);
+  });
+
+  it("a failed migration PUT consumes the flag (best-effort — never retried, never fatal)", async () => {
+    const storage = storageWith(["extensions"]);
+    const http = fakeHttp(
+      () => ({ ok: true, tree: DEFAULT_LAYOUT }),
+      () => ({ ok: false, notFound: false, error: "invalid tree" }),
+    );
+    const store = makeStore(http, { storage });
+    await store.probe("ws-a", { boot: true });
+    expect(http.putCalls).toHaveLength(1);
+    expect(warnings.some((w) => w.includes("migration"))).toBe(true);
+    // The rendered tree is untouched (the PUT failed).
+    expect(store.tree).toEqual(DEFAULT_LAYOUT);
+    await store.probe("ws-a");
+    expect(http.putCalls).toHaveLength(1);
+  });
+
+  it("migration arms even when the boot probe 404s, and applies on the first LATER success", async () => {
+    const storage = storageWith(["extensions"]);
+    let absent = true;
+    const http = fakeHttp(() =>
+      absent ? { ok: false, notFound: true, error: "No layout provider is active" } : { ok: true, tree: DEFAULT_LAYOUT },
+    );
+    const store = makeStore(http, { storage });
+    await store.probe("ws-a", { boot: true });
+    expect(http.putCalls).toEqual([]);
+
+    absent = false;
+    await store.probe("ws-a"); // extensions-changed probe — first success
+    expect(http.putCalls).toEqual([
+      { workspaceId: "ws-a", tree: overlaySidePanels(DEFAULT_LAYOUT, ["extensions"]) },
+    ]);
+  });
+});
-- 
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