diff --git a/backend/models.py b/backend/models.py
index 744c614..f1bb243 100644
--- a/backend/models.py
+++ b/backend/models.py
@@ -57,6 +57,7 @@ class JobSubmissionRequest(BaseModel):
     target_storage: str = "local-lvm"
     additional_disks: List[DiskSpec] = []
     target_disk_size_gb: Optional[int] = None
+    session_id: str = ""
 
 
 class CleanupRequest(BaseModel):
diff --git a/backend/provisioner.py b/backend/provisioner.py
index d983329..ec81bd1 100644
--- a/backend/provisioner.py
+++ b/backend/provisioner.py
@@ -32,6 +32,9 @@ logger = logging.getLogger("backend.provisioner")
 
 STAGING_OUT = Path("/mnt/converter/out")
 
+def _staging_out(session_id: str = "") -> Path:
+    return STAGING_OUT / session_id if session_id else STAGING_OUT
+
 # In-memory job tracking — survives as long as the process runs
 _jobs: dict[str, dict] = {}
 _jobs_lock = threading.Lock()
@@ -117,9 +120,9 @@ def clone_vm(req: CloneRequest) -> JobStatusResponse:
     )
 
 
-def cleanup_staging(vmid: int, delete: bool) -> dict:
+def cleanup_staging(vmid: int, delete: bool, session_id: str = "") -> dict:
     """Remove staging files for a VM ID."""
-    out_dir = STAGING_OUT / str(vmid)
+    out_dir = _staging_out(session_id) / str(vmid)
 
     if not delete:
         return {
@@ -220,7 +223,7 @@ def _process_job(job_id: str, req: JobSubmissionRequest):
         _update_job(job_id, status=JobStatus.PROCESSING_CONVERSION, progress=10,
                      message="Converting disk image...")
 
-        out_dir = STAGING_OUT / str(vmid)
+        out_dir = _staging_out(req.session_id) / str(vmid)
         out_dir.mkdir(parents=True, exist_ok=True)
 
         # Collect all disks (boot disk + additional)
@@ -232,7 +235,8 @@ def _process_job(job_id: str, req: JobSubmissionRequest):
 
             if spec.disk_type == DiskType.IMAGE_FILE:
                 # Extract archive if needed, then locate the actual disk image
-                source = extract_if_needed(spec.source_filename)
+                src_path = _staging_in(req.session_id) / spec.source_filename
+                source = extract_if_needed(str(src_path))
                 disk = discover_disk(source)
                 source_path = disk.path
                 logger.info("Disk %d: source=%s format=%s size=%.1f GiB",
diff --git a/frontend/app.py b/frontend/app.py
index 9746886..b3a1696 100644
--- a/frontend/app.py
+++ b/frontend/app.py
@@ -63,6 +63,17 @@ app.mount("/static", StaticFiles(directory=str(BASE / "static")), name="static")
 _jinja = Environment(loader=FileSystemLoader(str(BASE / "templates")), autoescape=True)
 STAGING = Path("/mnt/converter/in")
 
+def _staging(session_id: str = "") -> Path:
+    """Session-aware staging directory."""
+    return STAGING / session_id if session_id else STAGING
+
+def _get_session_id(request: Request) -> str:
+    """Extract session ID from cookie or query param."""
+    sid = request.cookies.get("vm_bench_sid", "")
+    if not sid:
+        sid = request.query_params.get("session_id", "")
+    return sid
+
 api = ApiClient()
 
 logger = logging.getLogger("vm-bench")
@@ -141,9 +152,12 @@ def session_upload(
     source_type: str = Form("upload"),
     source_file: Optional[UploadFile] = File(None),
     source_url: Optional[str] = Form(None),
+    session_id: str = Form(""),
 ):
     """Phase 1 — acquire the source file. Returns JSON so the frontend can
     show progress, then call /session/analyze separately."""
+    staging = _staging(session_id)
+    staging.mkdir(parents=True, exist_ok=True)
     err = _validate_vmid(vmid)
     if err:
         return JSONResponse({"phase": "error", "error": err}, status_code=400)
@@ -156,7 +170,7 @@ def session_upload(
             return JSONResponse({"phase": "error", "error": "No file uploaded."}, status_code=400)
 
         filename = source_file.filename
-        dest = STAGING / filename
+        dest = staging / filename
 
         content_length = request.headers.get("content-length")
         if content_length:
@@ -203,11 +217,11 @@ def session_upload(
             return JSONResponse({"phase": "error", "error": "No URL provided."}, status_code=400)
 
         filename = Path(url).name or f"download_{uuid.uuid4().hex[:8]}"
-        dest = STAGING / filename
+        dest = staging / filename
 
         _active_downloads.pop(filename, None)
 
-        usage = shutil.disk_usage(STAGING)
+        usage = shutil.disk_usage(staging)
         free_gb = usage.free / (1024**3)
         if free_gb < 50:
             logger.warning("Low disk: %.1f GB free — download may fail", free_gb)
@@ -254,6 +268,7 @@ async def upload_raw(request: Request):
     filename = request.headers.get("X-Filename", "upload.bin")
     vmid_str = request.headers.get("X-VMID", "")
     vm_name = request.headers.get("X-VM-Name", "")
+    session_id = request.headers.get("X-Session-ID", "")
 
     try:
         vmid = int(vmid_str)
@@ -264,7 +279,9 @@ async def upload_raw(request: Request):
     if err:
         return JSONResponse({"phase": "error", "error": err}, status_code=400)
 
-    dest = STAGING / filename
+    staging = _staging(session_id)
+    staging.mkdir(parents=True, exist_ok=True)
+    dest = staging / filename
     content_length = request.headers.get("content-length")
     if content_length:
         estimated_gb = int(content_length) / (1024**3)
@@ -445,6 +462,7 @@ async def confirm_session(
     target_disk_size_gb: Optional[int] = Form(None),
     auto_detect_boot: str = Form("true"),
     boot_type: str = Form("uefi"),
+    session_id: str = Form(""),
 ):
     """Build the job payload and submit to the backend."""
     # Validate VM ID range
@@ -460,6 +478,7 @@ async def confirm_session(
         "target_storage": target_storage,
         "auto_detect_boot": auto_detect_boot == "true",
         "boot_type": boot_type,
+        "session_id": session_id,
         "boot_disk": {
             "disk_type": "image_file",
             "source_filename": source_filename,
diff --git a/frontend/templates/base.html b/frontend/templates/base.html
index 00f03a4..29d7b33 100644
--- a/frontend/templates/base.html
+++ b/frontend/templates/base.html
@@ -5,6 +5,22 @@
 <meta name="viewport" content="width=device-width, initial-scale=1.0">
 <title>VM Bench — Proxmox Image Converter</title>
 <link rel="stylesheet" href="/static/proxmox.css?v=5">
+<script>
+// Session GUID — one per browser, persists across tabs/refreshes
+(function() {
+  var sid = localStorage.getItem('vm-bench-sid');
+  if (!sid) {
+    sid = crypto.randomUUID ? crypto.randomUUID() :
+         'xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx'.replace(/[xy]/g, function(c) {
+           var r = Math.random()*16|0, v = c=='x'?r:(r&0x3|0x8);
+           return v.toString(16);
+         });
+    localStorage.setItem('vm-bench-sid', sid);
+  }
+  document.cookie = 'vm_bench_sid=' + sid + ';path=/;SameSite=Lax';
+  window.VM_BENCH_SID = sid;
+})();
+</script>
 </head>
 <body>
 
diff --git a/frontend/templates/index.html b/frontend/templates/index.html
index 2fb599b..985811c 100644
--- a/frontend/templates/index.html
+++ b/frontend/templates/index.html
@@ -112,10 +112,12 @@ async function startSession(e) {
   const vmid = document.getElementById('vmid').value;
   const vmName = document.getElementById('vm-name').value.trim();
   const sourceType = document.getElementById('source-type').value;
+  const sessionId = window.VM_BENCH_SID || '';
   const formData = new FormData();
   formData.append('vmid', vmid);
   formData.append('vm_name', vmName);
   formData.append('source_type', sourceType);
+  formData.append('session_id', sessionId);
 
   if (sourceType === 'upload') {
     const fileInput = document.getElementById('source-file');
@@ -151,6 +153,8 @@ async function handleUpload(formData) {
     xhr.setRequestHeader('X-Filename', encodeURIComponent(file.name));
     xhr.setRequestHeader('X-VMID', vmid);
     xhr.setRequestHeader('X-VM-Name', vmName);
+    xhr.setRequestHeader('X-Session-ID', sessionId);
+    xhr.setRequestHeader('X-Session-ID', sessionId);
 
     xhr.upload.addEventListener('progress', (e) => {
       if (e.lengthComputable) {
@@ -233,6 +237,7 @@ async function runAnalysis(vmid, filename, vmName) {
   setPhase('Step 2/2: Analysing source image...', 0, true);
   const fd = new FormData();
   fd.append('vmid', vmid); fd.append('filename', filename); fd.append('vm_name', vmName);
+  fd.append('session_id', window.VM_BENCH_SID || '');
   try {
     const resp = await fetch('/session/analyze', { method: 'POST', body: fd });
     const html = await resp.text();
@@ -268,6 +273,7 @@ async function submitJob(e) {
   status.innerHTML = '<div class="spinner"></div> Submitting job...';
 
   const formData = new FormData(form);
+  formData.append('session_id', window.VM_BENCH_SID || '');
   try {
     const resp = await fetch('/session/confirm', { method: 'POST', body: formData });
     const html = await resp.text();