diff --git a/frontend/src/components/editor/FileExplorer.tsx b/frontend/src/components/editor/FileExplorer.tsx
index 235dc573..57e33901 100644
--- a/frontend/src/components/editor/FileExplorer.tsx
+++ b/frontend/src/components/editor/FileExplorer.tsx
@@ -4,6 +4,7 @@ import { useEditorStore } from '../../store/useEditorStore';
 import { useSimulatorStore } from '../../store/useSimulatorStore';
 import type { BoardKind } from '../../types/board';
 import { BOARD_KIND_LABELS } from '../../types/board';
+import { importVlxFile, VlxParseError } from '../../utils/vlxFile';
 import './FileExplorer.css';
 
 // SVG icons — same style as EditorToolbar (stroke-based, 16x16)
@@ -93,6 +94,25 @@ const IcoSave = () => (
   </svg>
 );
 
+const IcoOpen = () => (
+  // Folder with an "open / upload arrow" — matches Save visually (both
+  // are project-IO actions) but points the opposite way to signal load.
+  <svg
+    width="22"
+    height="22"
+    viewBox="0 0 24 24"
+    fill="none"
+    stroke="currentColor"
+    strokeWidth="2"
+    strokeLinecap="round"
+    strokeLinejoin="round"
+  >
+    <path d="M22 19a2 2 0 0 1-2 2H4a2 2 0 0 1-2-2V5a2 2 0 0 1 2-2h5l2 3h9a2 2 0 0 1 2 2z" />
+    <polyline points="12 11 12 17" />
+    <polyline points="9 14 12 11 15 14" />
+  </svg>
+);
+
 const IcoChevron = ({ open }: { open: boolean }) => (
   <svg
     width="12"
@@ -152,6 +172,33 @@ interface FileExplorerProps {
 }
 
 export const FileExplorer: React.FC<FileExplorerProps> = ({ onSaveClick, onNewClick }) => {
+  // Hidden <input type="file"> we trigger via ref when the user clicks
+  // the Open .vlx button. Kept outside React state so the change event
+  // can fire repeatedly even if the user picks the same file twice.
+  const fileInputRef = useRef<HTMLInputElement | null>(null);
+  const handleOpenVlxClick = useCallback(() => {
+    fileInputRef.current?.click();
+  }, []);
+  const handleVlxFilePicked = useCallback(async (e: React.ChangeEvent<HTMLInputElement>) => {
+    const file = e.target.files?.[0];
+    // Reset so picking the SAME file again later still fires onchange.
+    e.target.value = '';
+    if (!file) return;
+    if (
+      !window.confirm(
+        'Load this .vlx file? Your current workspace will be replaced. This cannot be undone.',
+      )
+    ) {
+      return;
+    }
+    try {
+      await importVlxFile(file);
+    } catch (err) {
+      const msg = err instanceof VlxParseError ? err.message : (err as Error).message;
+      window.alert(`Could not load .vlx file:\n\n${msg}`);
+    }
+  }, []);
+
   const { t } = useTranslation();
   const {
     fileGroups,
@@ -281,6 +328,20 @@ export const FileExplorer: React.FC<FileExplorerProps> = ({ onSaveClick, onNewCl
           >
             <IcoNewWorkspace />
           </button>
+          <button
+            className="file-explorer-save-btn"
+            title="Open .vlx file"
+            onClick={handleOpenVlxClick}
+          >
+            <IcoOpen />
+          </button>
+          <input
+            ref={fileInputRef}
+            type="file"
+            accept=".vlx,application/json"
+            onChange={handleVlxFilePicked}
+            style={{ display: 'none' }}
+          />
           <button
             className="file-explorer-save-btn"
             title={t('editor.fileExplorer.saveProject')}
diff --git a/frontend/src/lib/proSaveAction.ts b/frontend/src/lib/proSaveAction.ts
index 06d75c1c..fcf0780a 100644
--- a/frontend/src/lib/proSaveAction.ts
+++ b/frontend/src/lib/proSaveAction.ts
@@ -7,43 +7,54 @@
  * in the pro overlay. The OSS editor exposes a stable `triggerSaveAction()`
  * that:
  *
- *   - In OSS without an overlay → no-op (in Phase 4 this becomes the
- *     entry point for the .vlx Export dialog so anonymous users still
- *     have a way to persist work).
- *   - With the pro overlay loaded → dispatches to the registered impl,
- *     which inspects the auth store and opens SaveProjectModal (logged
- *     in) or LoginPromptModal (anonymous).
+ *   - In OSS without an overlay → downloads a portable `.vlx` snapshot
+ *     of the current workspace. The user picks where it goes (browser
+ *     save dialog) and can re-load it later via the "Open .vlx" button.
+ *     This is Phase 4 of the OSS split — gives self-hosters durable
+ *     project persistence without requiring a DB or auth.
+ *   - With the pro overlay loaded → installSaveActionImpl() overrides
+ *     the default. The overlay's impl inspects the auth store and opens
+ *     SaveProjectModal (logged in) or LoginPromptModal (anonymous).
  *
  * Impl receives no arguments and returns nothing. State lives in the
- * overlay's React tree (modal open/close, project data) — this registry
+ * caller's React tree (modal open/close, project data) — this registry
  * is just the doorbell.
  */
 
+import { triggerDownloadVlx } from '../utils/vlxFile';
+import { useProjectStore } from '../store/useProjectStore';
+
 let _impl: (() => void) | null = null;
 
 export function installSaveActionImpl(impl: (() => void) | null): void {
   _impl = impl;
 }
 
+function defaultSaveAction(): void {
+  // OSS fallback: dump the workspace to a `.vlx` file the user downloads.
+  // Use the loaded project's name if there is one (slug/identity tracking
+  // doesn't require auth — useProjectStore stays in OSS).
+  const proj = useProjectStore.getState().currentProject;
+  const name = proj?.slug ?? proj?.id ?? undefined;
+  const filename = triggerDownloadVlx({ name });
+  // eslint-disable-next-line no-console
+  console.info(`[oss] downloaded workspace as ${filename}`);
+}
+
 export function triggerSaveAction(): void {
-  if (_impl) {
-    try {
-      _impl();
-    } catch (err) {
-      // eslint-disable-next-line no-console
-      console.warn('[oss] save-action impl threw:', err);
-    }
-  } else {
+  const impl = _impl ?? defaultSaveAction;
+  try {
+    impl();
+  } catch (err) {
     // eslint-disable-next-line no-console
-    console.info(
-      '[oss] No save handler is installed. Builds without the pro overlay ' +
-        'will gain a local .vlx export in Phase 4 of the OSS split.',
-    );
+    console.warn('[oss] save-action impl threw:', err);
   }
 }
 
-/** Whether an implementation has been installed. Lets UI conditionally
- * show the Save button — without an impl, clicking it does nothing useful. */
+/** Whether a custom (overlay) implementation has been installed. The OSS
+ * default always works, so callers don't usually need this. Kept as a
+ * hook for UIs that want to label the button differently in OSS vs Pro
+ * (e.g. "Download .vlx" vs "Save project"). */
 export function hasSaveActionImpl(): boolean {
   return _impl !== null;
 }
diff --git a/frontend/src/utils/vlxFile.ts b/frontend/src/utils/vlxFile.ts
new file mode 100644
index 00000000..8cdaf11a
--- /dev/null
+++ b/frontend/src/utils/vlxFile.ts
@@ -0,0 +1,231 @@
+/**
+ * .vlx file format — portable project export/import for OSS Velxio.
+ *
+ * Phase 4 of the OSS / pro split. The OSS image has no auth, no DB, no
+ * server-side project persistence. The user's work is otherwise ephemeral
+ * (lost on tab refresh). `.vlx` is a single-file JSON snapshot that
+ * round-trips everything the server-side Save flow captures:
+ *
+ *   {
+ *     "format": "velxio-project",
+ *     "version": 1,
+ *     "exportedAt": "ISO timestamp",
+ *     "name": "project name (optional)",
+ *     "boards": [...],
+ *     "fileGroups": { "<groupId>": [{ name, content }, ...] },
+ *     "components": [...],
+ *     "wires": [...],
+ *     "activeBoardId": "..." | null
+ *   }
+ *
+ * Reading: `parseVlxFile(File) → payload` validates and returns a shape
+ *  directly consumable by `useSimulatorStore.loadProjectState(...)`.
+ *
+ * Writing: `buildVlxBlob()` snapshots the current store state into a
+ *  `Blob`, ready to feed an `<a download>` link.
+ *
+ * The format is INTENTIONALLY identical to the server's POST/PUT body
+ * for `/api/projects/`, so a pro user can export-from-Pro / import-into-
+ * OSS (and vice-versa) without surprises.
+ */
+
+import type { BoardInstance } from '../types/board';
+import type { Component } from '../types/component';
+import type { Wire } from '../types/wire';
+import { useEditorStore } from '../store/useEditorStore';
+import { useSimulatorStore } from '../store/useSimulatorStore';
+
+const VLX_FORMAT = 'velxio-project';
+const VLX_VERSION = 1;
+
+export interface VlxPayload {
+  format: typeof VLX_FORMAT;
+  version: number;
+  exportedAt: string;
+  name?: string;
+  boards: Array<{
+    id: string;
+    boardKind: string;
+    x: number;
+    y: number;
+    activeFileGroupId: string;
+    languageMode?: string;
+    serialBaudRate?: number;
+  }>;
+  fileGroups: Record<string, Array<{ name: string; content: string }>>;
+  components: Component[];
+  wires: Wire[];
+  activeBoardId: string | null;
+}
+
+function serialisableBoard(b: BoardInstance) {
+  return {
+    id: b.id,
+    boardKind: b.boardKind,
+    x: b.x,
+    y: b.y,
+    activeFileGroupId: b.activeFileGroupId,
+    languageMode: b.languageMode,
+    serialBaudRate: b.serialBaudRate,
+  };
+}
+
+/**
+ * Snapshot the current editor + simulator state into a VlxPayload object.
+ * Pure function: no side effects.
+ */
+export function buildVlxPayload(opts: { name?: string } = {}): VlxPayload {
+  const sim = useSimulatorStore.getState();
+  const editor = useEditorStore.getState();
+
+  // Only persist file groups that are actually referenced by a board.
+  // Stray groups left over from deleted boards don't need to round-trip.
+  const referencedGroupIds = new Set(sim.boards.map((b) => b.activeFileGroupId));
+  const fileGroups: VlxPayload['fileGroups'] = {};
+  for (const gid of referencedGroupIds) {
+    fileGroups[gid] = (editor.fileGroups[gid] ?? []).map((f) => ({
+      name: f.name,
+      content: f.content,
+    }));
+  }
+
+  return {
+    format: VLX_FORMAT,
+    version: VLX_VERSION,
+    exportedAt: new Date().toISOString(),
+    name: opts.name,
+    boards: sim.boards.map(serialisableBoard),
+    fileGroups,
+    components: sim.components,
+    wires: sim.wires,
+    activeBoardId: sim.activeBoardId,
+  };
+}
+
+/** Build a Blob (MIME: application/json) carrying the current state. */
+export function buildVlxBlob(opts: { name?: string } = {}): Blob {
+  const payload = buildVlxPayload(opts);
+  const json = JSON.stringify(payload, null, 2);
+  return new Blob([json], { type: 'application/json' });
+}
+
+/** Sanitise a filename: keep letters, digits, dashes, dots, underscores. */
+function safeFilename(name?: string): string {
+  const base = (name ?? 'velxio-project').trim() || 'velxio-project';
+  const cleaned = base
+    .replace(/[^a-zA-Z0-9._-]+/g, '-')
+    .replace(/^-+|-+$/g, '')
+    .slice(0, 80);
+  return `${cleaned || 'velxio-project'}.vlx`;
+}
+
+/**
+ * Trigger a browser download of the current state as `<name>.vlx`.
+ * Returns the filename actually used (for UI feedback).
+ */
+export function triggerDownloadVlx(opts: { name?: string } = {}): string {
+  const blob = buildVlxBlob(opts);
+  const filename = safeFilename(opts.name);
+  const url = URL.createObjectURL(blob);
+  const a = document.createElement('a');
+  a.href = url;
+  a.download = filename;
+  document.body.appendChild(a);
+  a.click();
+  // The browser starts the download immediately. Revoke the URL on the
+  // next tick so the click handler has time to settle before GC.
+  setTimeout(() => {
+    document.body.removeChild(a);
+    URL.revokeObjectURL(url);
+  }, 0);
+  return filename;
+}
+
+/** Thrown by parseVlxFile when the file isn't a valid .vlx payload. */
+export class VlxParseError extends Error {
+  constructor(message: string) {
+    super(message);
+    this.name = 'VlxParseError';
+  }
+}
+
+function isPlainObject(value: unknown): value is Record<string, unknown> {
+  return typeof value === 'object' && value !== null && !Array.isArray(value);
+}
+
+/**
+ * Validate that `data` is shaped like a VlxPayload. Throws VlxParseError
+ * on mismatch with a human-readable reason. Keep the checks defensive —
+ * users may edit .vlx files by hand or feed us a wrong file by accident.
+ */
+function validatePayload(data: unknown): VlxPayload {
+  if (!isPlainObject(data)) {
+    throw new VlxParseError('File is not a JSON object.');
+  }
+  if (data.format !== VLX_FORMAT) {
+    throw new VlxParseError(
+      `Not a Velxio project file (expected format="${VLX_FORMAT}", got ${JSON.stringify(
+        data.format,
+      )}).`,
+    );
+  }
+  if (typeof data.version !== 'number') {
+    throw new VlxParseError('Missing or invalid "version" field.');
+  }
+  if (data.version > VLX_VERSION) {
+    throw new VlxParseError(
+      `This file uses .vlx format version ${data.version}, but this Velxio supports up to v${VLX_VERSION}. Update Velxio to open it.`,
+    );
+  }
+  if (!Array.isArray(data.boards)) {
+    throw new VlxParseError('Missing or invalid "boards" array.');
+  }
+  if (!isPlainObject(data.fileGroups)) {
+    throw new VlxParseError('Missing or invalid "fileGroups" object.');
+  }
+  if (!Array.isArray(data.components)) {
+    throw new VlxParseError('Missing or invalid "components" array.');
+  }
+  if (!Array.isArray(data.wires)) {
+    throw new VlxParseError('Missing or invalid "wires" array.');
+  }
+  return data as unknown as VlxPayload;
+}
+
+/**
+ * Read a File object (from a `<input type="file">` change event or a
+ * drop), parse it as JSON, validate, and return the payload. Throws
+ * VlxParseError on any failure.
+ */
+export async function parseVlxFile(file: File): Promise<VlxPayload> {
+  let text: string;
+  try {
+    text = await file.text();
+  } catch (err) {
+    throw new VlxParseError(`Could not read file: ${(err as Error).message}`);
+  }
+  let parsed: unknown;
+  try {
+    parsed = JSON.parse(text);
+  } catch (err) {
+    throw new VlxParseError(`Invalid JSON: ${(err as Error).message}`);
+  }
+  return validatePayload(parsed);
+}
+
+/**
+ * Convenience wrapper: parse the file AND load its contents into the
+ * simulator stores via `loadProjectState`. Returns the parsed payload
+ * so the caller can show a confirmation toast or similar.
+ */
+export async function importVlxFile(file: File): Promise<VlxPayload> {
+  const payload = await parseVlxFile(file);
+  useSimulatorStore.getState().loadProjectState({
+    boards: payload.boards as unknown as BoardInstance[],
+    fileGroups: payload.fileGroups,
+    components: payload.components,
+    wires: payload.wires,
+    activeBoardId: payload.activeBoardId,
+  });
+  return payload;
+}