diff --git a/src/ifcviewer-web/CMakeLists.txt b/src/ifcviewer-web/CMakeLists.txt
index b48348c387..f3eecfb032 100644
--- a/src/ifcviewer-web/CMakeLists.txt
+++ b/src/ifcviewer-web/CMakeLists.txt
@@ -104,7 +104,7 @@ target_link_options(IfcViewerWeb PRIVATE
     # EMSCRIPTEN_KEEPALIVE alone keeps the symbols in the binary but doesn't
     # add them to Module. ccall lets shell.html pass a JS string (the ?model
     # URL) to load_sidecar_from_url_c without manual heap marshalling.
-    "-sEXPORTED_FUNCTIONS=['_main','_raf_tick_c','_load_sidecar_from_blob_c','_load_sidecar_from_url_c','_ifcv_on_range_done','_ifcv_source_ready','_ifcv_chunks_resident_c','_ifcv_chunks_total_c']"
+    "-sEXPORTED_FUNCTIONS=['_main','_raf_tick_c','_load_sidecar_from_source_c','_clear_scene_c','_ifcv_on_range_done','_ifcv_chunks_resident_c','_ifcv_chunks_total_c']"
     # ccall: shell.html passes the ?model URL string to load_sidecar_from_url_c.
     # HEAPU8: lets tooling/tests read the wasm heap size (e.g. to verify a large
     # sidecar streams by range instead of loading whole). Standard, zero-cost.
diff --git a/src/ifcviewer-web/main_web.cpp b/src/ifcviewer-web/main_web.cpp
index 031ceb6803..b1a1b7f029 100644
--- a/src/ifcviewer-web/main_web.cpp
+++ b/src/ifcviewer-web/main_web.cpp
@@ -191,32 +191,23 @@ extern "C" EMSCRIPTEN_KEEPALIVE void raf_tick_c(void* user) {
     }
 }
 
-// Called from shell.html's file-browse handler after it has stashed the
-// picked File on Module.__ifcvFile. Replaces whatever is currently loaded
-// (the embedded sample on first use, or a prior pick) with the new sidecar.
-// Byte-range (#88): the whole file is NOT copied into the wasm heap — the
-// metadata is read via Blob.slice and chunk bytes stream per-chunk, so a
-// 500 MB sidecar stays in the browser File object. Asynchronous: this
-// returns immediately and the model frames itself from the JS completion
-// callback. Exported to JS via EXPORTED_FUNCTIONS in CMakeLists.txt.
-extern "C" EMSCRIPTEN_KEEPALIVE void load_sidecar_from_blob_c() {
+// Stream a sidecar from a registered JS byte-source and APPEND it to the scene
+// (federation). shell.html registers the source first — a picked File or a
+// remote URL, sized up front — into Module.__ifcvSources[source_id], then calls
+// this. Byte-range: the file is never copied whole into the wasm heap; metadata
+// is read via ranges and chunks stream per-chunk, so a 500 MB sidecar stays in
+// the File / on the server. Asynchronous; the model frames itself from the JS
+// completion callback. Call clear_scene_c first to replace instead of append.
+extern "C" EMSCRIPTEN_KEEPALIVE void load_sidecar_from_source_c(int source_id) {
     if (!g_app || !g_app->ready) return;
-
-    // resetScene drops the previous model's GPU resources so a fresh load
-    // replaces rather than accumulates (loadSidecar* appends).
-    g_app->core.resetScene();
-    g_app->core.loadSidecarFromBlobWeb();
+    g_app->core.loadSidecarMetadataWeb(source_id, "source");
 }
 
-// Called from shell.html (e.g. a ?model=URL query param) to stream a sidecar
-// hosted at `url` via HTTP Range requests — the same per-chunk byte-range path
-// as the local File load, but the bytes come off the network instead of a
-// Blob. Asynchronous; the model frames itself once metadata lands. Exported
-// to JS via EXPORTED_FUNCTIONS in CMakeLists.txt.
-extern "C" EMSCRIPTEN_KEEPALIVE void load_sidecar_from_url_c(const char* url) {
-    if (!g_app || !g_app->ready || !url) return;
+// Drop all loaded models (used by shell.html to replace the embedded sample /
+// a prior federation before loading a fresh set).
+extern "C" EMSCRIPTEN_KEEPALIVE void clear_scene_c() {
+    if (!g_app || !g_app->ready) return;
     g_app->core.resetScene();
-    g_app->core.loadSidecarFromUrlWeb(url);
 }
 
 // Streaming progress for the loading bar (shell.html polls these each frame).
diff --git a/src/ifcviewer-web/shell.html b/src/ifcviewer-web/shell.html
index 8254d22b15..fc1d14343a 100644
--- a/src/ifcviewer-web/shell.html
+++ b/src/ifcviewer-web/shell.html
@@ -21,10 +21,13 @@
   #status.error { background: rgba(120,30,30,.85); color: #fff; }
   /* Errors re-expand and re-opaque even after the ready-collapse. */
   #status.ready.error { max-height: 28vh; opacity: 1; }
-  #open-btn { position: fixed; top: 8px; right: 12px; z-index: 10;
+  #open-btn, #add-btn { position: fixed; top: 8px; z-index: 10;
     background: #2b6cb0; color: #fff; border: none; padding: 6px 12px;
     border-radius: 4px; font-size: 12px; cursor: pointer; }
+  #open-btn { right: 12px; }
+  #add-btn  { right: 120px; background: #2d3748; }
   #open-btn:hover { background: #3182ce; }
+  #add-btn:hover  { background: #3b465c; }
   #file-input { display: none; }
   /* Streaming loading bar: a thin top progress strip + a centred caption.
      Shown only while a network/file model streams; hidden once resident. */
@@ -42,8 +45,9 @@
 <canvas id="viewer-canvas" width="1280" height="800"></canvas>
 <div id="progress"><div id="progress-fill"></div></div>
 <div id="progress-text"></div>
+<button id="add-btn" title="Add file(s) to the current scene (federation)">Add</button>
 <button id="open-btn">Open .ifcview…</button>
-<input id="file-input" type="file" accept=".ifcview">
+<input id="file-input" type="file" accept=".ifcview" multiple>
 <div id="status">Starting…</div>
 <script>
   // Emscripten Module hook: route stderr to the status overlay so any
@@ -93,10 +97,16 @@
           // ?model=URL streams a remote sidecar via HTTP Range once the app
           // is live (one-shot). Same-origin needs no CORS; cross-origin URLs
           // require the host to send CORS + Accept-Ranges headers.
-          if (!urlLoadTried && modelUrl && Module.ccall) {
+          if (!urlLoadTried && modelUrl && Module._load_sidecar_from_source_c) {
             urlLoadTried = true;
             window.beginLoadProgress();
-            Module.ccall('load_sidecar_from_url_c', null, ['string'], [modelUrl]);
+            registerUrlSource(modelUrl).then(function(sid) {
+              Module._clear_scene_c();                  // replace the embedded sample
+              Module._load_sidecar_from_source_c(sid);
+            }).catch(function(e) {
+              statusEl.textContent += 'url load failed: ' + e + '\n';
+              statusEl.classList.add('error');
+            });
           }
           window.updateLoadProgress();
           Module._raf_tick_c(Module._app_ptr);
@@ -111,6 +121,33 @@
     statusEl.classList.add('error');
   }
 
+  // --- Byte-source registry (multi-file federation) -------------------------
+  // Each model streams from its own source: a picked File (Blob.slice) or a
+  // remote URL (HTTP Range), registered here and read lazily by the wasm side
+  // via Module.__ifcvSources[id]. URLs are sized up front (HEAD, else a 0-0
+  // Range's Content-Range) so the loader can bound its reads.
+  Module.__ifcvSources = Module.__ifcvSources || [];
+  function registerFileSource(file) {
+    var sid = Module.__ifcvSources.length;
+    Module.__ifcvSources.push({ file: file, url: null, size: file.size });
+    return sid;
+  }
+  function registerUrlSource(url) {
+    return fetch(url, { method: 'HEAD' }).then(function(resp) {
+      var len = resp.ok ? parseInt(resp.headers.get('Content-Length') || '0', 10) : 0;
+      if (len > 0) return len;
+      return fetch(url, { headers: { Range: 'bytes=0-0' } }).then(function(r2) {
+        var cr = r2.headers.get('Content-Range');   // "bytes 0-0/12345"
+        return cr ? parseInt(cr.split('/')[1] || '0', 10) : 0;
+      });
+    }).then(function(size) {
+      if (!size) throw new Error('could not size ' + url + ' (need HEAD or Range)');
+      var sid = Module.__ifcvSources.length;
+      Module.__ifcvSources.push({ file: null, url: url, size: size });
+      return sid;
+    });
+  }
+
   // --- Streaming loading bar ------------------------------------------------
   // Driven by the C-side progress exports (resident/total chunks) + the
   // bytes-downloaded counter the EM_JS range reader maintains. Shown only for
@@ -156,20 +193,30 @@
   // ever cross into wasm. No drag-drop: that needs an X11 drag source (a
   // file manager), which a minimal WM may not provide; the native file
   // chooser this button opens is WM-independent.
+  // "Open" replaces the scene with the picked file(s); "Add" appends them to
+  // the current scene (federation). Multiple files can be picked at once. Each
+  // File is registered as its own byte-source (kept alive in __ifcvSources for
+  // lazy Blob.slice reads) and streamed independently.
   var openBtn = document.getElementById('open-btn');
+  var addBtn = document.getElementById('add-btn');
   var fileInput = document.getElementById('file-input');
-  openBtn.addEventListener('click', function() { fileInput.click(); });
+  var pendingMode = 'replace';
+  openBtn.addEventListener('click', function() { pendingMode = 'replace'; fileInput.click(); });
+  addBtn.addEventListener('click',  function() { pendingMode = 'add';     fileInput.click(); });
   fileInput.addEventListener('change', function(ev) {
-    var f = ev.target.files && ev.target.files[0];
-    if (!f) return;
-    if (!Module._load_sidecar_from_blob_c) {
+    var files = ev.target.files;
+    if (!files || !files.length) return;
+    if (!Module._load_sidecar_from_source_c) {
       statusEl.textContent += 'viewer not ready yet — wait for WebGPU init\n';
       return;
     }
     try {
-      Module.__ifcvFile = f;          // kept alive for lazy Blob.slice reads
+      if (pendingMode === 'replace') Module._clear_scene_c();
       window.beginLoadProgress();
-      Module._load_sidecar_from_blob_c();
+      for (var i = 0; i < files.length; i++) {
+        var sid = registerFileSource(files[i]);
+        Module._load_sidecar_from_source_c(sid);
+      }
     } catch (e) {
       statusEl.textContent += 'load failed: ' + e + '\n';
       statusEl.classList.add('error');
diff --git a/src/ifcviewer-web/tests/smoke.spec.mjs b/src/ifcviewer-web/tests/smoke.spec.mjs
index 864dfbc5f5..d575ddafad 100644
--- a/src/ifcviewer-web/tests/smoke.spec.mjs
+++ b/src/ifcviewer-web/tests/smoke.spec.mjs
@@ -167,7 +167,7 @@ test('loads a user-picked sidecar through the Blob.slice byte-range path', async
   // side to confirm the blob load landed (logged to stderr → console).
   const samplePath = resolve(__dirname, '..', 'sample.ifcview');
   const loaded = page.waitForEvent('console', {
-    predicate: (m) => /loaded sidecar \(blob:/.test(m.text()),
+    predicate: (m) => /loaded sidecar \(source/.test(m.text()),
     timeout: 15_000,
   });
   await page.locator('#file-input').setInputFiles(samplePath);
@@ -208,7 +208,7 @@ test('streams a remote sidecar over HTTP Range (?model= URL backend)', async ({
 
   // Wait for the C side to confirm the URL-sourced load landed.
   const loaded = page.waitForEvent('console', {
-    predicate: (m) => /loaded sidecar \(net:/.test(m.text()),
+    predicate: (m) => /loaded sidecar \(source/.test(m.text()),
     timeout: 20_000,
   });
   await page.goto('/IfcViewerWeb.html?model=/sample.ifcview');
diff --git a/src/ifcviewer/ModelGpuData.h b/src/ifcviewer/ModelGpuData.h
index 272bff4b51..e7fcb25737 100644
--- a/src/ifcviewer/ModelGpuData.h
+++ b/src/ifcviewer/ModelGpuData.h
@@ -287,6 +287,11 @@ struct ModelGpuData {
     // so driveStreamingLoads routes this model through the async web path
     // instead of the MEMFS sync read.
     bool        streaming_from_web = false;
+    // Web analog of streaming_file_path: which registered JS byte-source
+    // (Module.__ifcvSources[id] = a picked File or a remote URL) this model's
+    // chunk + deferred reads pull from. Lets several federated models stream
+    // from different files at once, mirroring the desktop per-model path.
+    int         web_source_id = 0;
 
     // v15 deferred property metadata (web, on-demand). The IFC element tree
     // (elements + string_table — names/GUIDs/hierarchy, for UI/picking, never
diff --git a/src/ifcviewer/ViewportCore.cpp b/src/ifcviewer/ViewportCore.cpp
index 8927960a30..e08a7ce200 100644
--- a/src/ifcviewer/ViewportCore.cpp
+++ b/src/ifcviewer/ViewportCore.cpp
@@ -3203,22 +3203,22 @@ std::uint32_t ViewportCore::loadSidecarFromPath(const std::string& path) {
 
 namespace {
 
-// Active byte-source size in bytes, or 0 if none. Bounds the metadata tail
-// read (index-section end .. EOF). The source is either a picked File
-// (Module.__ifcvFile, local) or a remote URL whose total length was resolved
-// up front (Module.__ifcvUrlSize). The File takes precedence if both are set.
-EM_JS(double, ifcvFileSize, (void), {
-    if (Module["__ifcvFile"] && Module["__ifcvFile"].size) return Module["__ifcvFile"].size;
-    if (Module["__ifcvUrl"]) return Module["__ifcvUrlSize"] || 0;
-    return 0;
+// Byte-source registry (multi-file federation). Module.__ifcvSources[id] is
+// { file: File|null, url: string|null, size: number } — a picked File or a
+// remote URL, registered + sized by shell.html before the load. Several models
+// can stream from different sources at once (the web analog of the desktop
+// per-model streaming_file_path). Size of source `sid`, or 0 if unknown.
+EM_JS(double, ifcvSourceSize, (int sid), {
+    var s = Module["__ifcvSources"] && Module["__ifcvSources"][sid];
+    return s ? (s.size || 0) : 0;
 });
 
-// Read [offset, offset+size) of the active source into dst (which must hold
-// `size` bytes), then call back _ifcv_on_range_done(reqId, ok). Async. Local:
-// Blob.slice. Remote: an HTTP Range request — if a server ignores Range and
+// Read [offset, offset+size) of source `sid` into dst (which must hold `size`
+// bytes), then call back _ifcv_on_range_done(reqId, ok). Async. File:
+// Blob.slice. URL: an HTTP Range request — if a server ignores Range and
 // returns the whole body (200), slice out the requested window so it still
 // works (just without the bandwidth saving).
-EM_JS(void, ifcvReadRangeInto, (int reqId, double offset, double size, void* dst), {
+EM_JS(void, ifcvReadRangeInto, (int sid, int reqId, double offset, double size, void* dst), {
     var deliver = function(ok, buf) {
         if (ok && buf) {
             HEAPU8.set(new Uint8Array(buf), dst);
@@ -3226,17 +3226,16 @@ EM_JS(void, ifcvReadRangeInto, (int reqId, double offset, double size, void* dst
         }
         Module["_ifcv_on_range_done"](reqId, ok ? 1 : 0);
     };
-    var f = Module["__ifcvFile"];
-    if (f) {
-        f.slice(offset, offset + size).arrayBuffer()
+    var s = Module["__ifcvSources"] && Module["__ifcvSources"][sid];
+    if (s && s.file) {
+        s.file.slice(offset, offset + size).arrayBuffer()
             .then(function(buf) { deliver(1, buf); })
             .catch(function(e) { deliver(0, null); });
         return;
     }
-    var url = Module["__ifcvUrl"];
-    if (url) {
+    if (s && s.url) {
         var end = offset + size - 1;
-        fetch(url, { headers: { "Range": "bytes=" + offset + "-" + end } })
+        fetch(s.url, { headers: { "Range": "bytes=" + offset + "-" + end } })
             .then(function(resp) {
                 if (resp.status !== 206 && resp.status !== 200) { deliver(0, null); return; }
                 var full = resp.status === 200;
@@ -3251,32 +3250,11 @@ EM_JS(void, ifcvReadRangeInto, (int reqId, double offset, double size, void* dst
     Module["_ifcv_on_range_done"](reqId, 0);
 });
 
-// Switch the active source to a remote URL and resolve its total byte length
-// (needed to bound the metadata tail read), then call _ifcv_source_ready(ok).
-// Tries a HEAD's Content-Length first, then a 0-0 ranged GET's Content-Range
-// total. Clears any local File so the URL source is used.
-EM_JS(void, ifcvBeginUrlSource, (const char* urlPtr), {
-    var url = UTF8ToString(urlPtr);
-    Module["__ifcvFile"]    = null;
-    Module["__ifcvUrl"]     = url;
-    Module["__ifcvUrlSize"] = 0;
-    var ready = function(ok) { Module["_ifcv_source_ready"](ok ? 1 : 0); };
-    fetch(url, { method: "HEAD" }).then(function(resp) {
-        var len = resp.ok ? parseInt(resp.headers.get("Content-Length") || "0", 10) : 0;
-        if (len > 0) { Module["__ifcvUrlSize"] = len; ready(1); return; }
-        return fetch(url, { headers: { "Range": "bytes=0-0" } }).then(function(r2) {
-            var cr = r2.headers.get("Content-Range");  // "bytes 0-0/12345"
-            var total = cr ? parseInt(cr.split("/")[1] || "0", 10) : 0;
-            Module["__ifcvUrlSize"] = total;
-            ready(total > 0);
-        });
-    }).catch(function(e) { ready(0); });
-});
-
 // One in-flight multi-range read: a sequence of coalesced plans, each read
 // into `scratch` then scattered into `out`. `done(ok, out)` fires once every
 // plan has landed, or on the first failure.
 struct WebRangeRead {
+    int                          source_id = 0;  // Module.__ifcvSources index
     std::vector<SidecarReadPlan> plans;
     std::size_t                  plan_idx = 0;
     std::vector<std::uint8_t>    scratch;
@@ -3287,11 +3265,6 @@ struct WebRangeRead {
 std::unordered_map<int, WebRangeRead> g_web_reads;
 int g_web_read_next = 1;
 
-// The ViewportCore awaiting an async URL-source size resolution. Set by
-// loadSidecarFromUrlWeb; consumed by the ifcv_source_ready callback. One
-// ViewportCore exists on web, so a single slot suffices.
-ViewportCore* g_url_ready_core = nullptr;
-
 // Issue the current plan's Blob.slice, or finish (success) if all plans done.
 void webIssueCurrentPlan(int id) {
     auto it = g_web_reads.find(id);
@@ -3306,7 +3279,7 @@ void webIssueCurrentPlan(int id) {
     }
     const SidecarReadPlan& p = r.plans[r.plan_idx];
     r.scratch.assign(std::size_t(p.read_size), 0);
-    ifcvReadRangeInto(id, double(p.file_offset), double(p.read_size),
+    ifcvReadRangeInto(r.source_id, id, double(p.file_offset), double(p.read_size),
                       r.scratch.data());
 }
 
@@ -3314,6 +3287,7 @@ void webIssueCurrentPlan(int id) {
 // in input order, then call done(true, bytes). On any failure: done(false,{}).
 // `section_offset` makes the offsets absolute (pass 0 if already absolute).
 void webReadRangesAsync(
+        int source_id,
         std::uint64_t section_offset,
         const std::vector<std::pair<std::uint64_t, std::uint64_t>>& ranges,
         std::function<void(bool, std::vector<std::uint8_t>&&)> done) {
@@ -3321,6 +3295,7 @@ void webReadRangesAsync(
     for (const auto& rg : ranges) total += rg.second;
 
     WebRangeRead r;
+    r.source_id = source_id;
     r.out.assign(std::size_t(total), 0);
     // Coalesce within 1 MB: each Blob.slice is an async round trip, so a
     // generous gap trades a few wasted bytes for far fewer JS hops.
@@ -3360,21 +3335,6 @@ extern "C" EMSCRIPTEN_KEEPALIVE void ifcv_on_range_done(int reqId, int ok) {
     webIssueCurrentPlan(reqId);
 }
 
-// JS callback once a remote URL source's total size has been resolved. Runs
-// the shared metadata bootstrap against the now-active URL source. Exported as
-// _ifcv_source_ready (see ifcviewer-web/CMakeLists.txt).
-extern "C" EMSCRIPTEN_KEEPALIVE void ifcv_source_ready(int ok) {
-    ViewportCore* core = g_url_ready_core;
-    g_url_ready_core = nullptr;
-    if (!core) return;
-    if (!ok) {
-        Log::warn() << "ifcviewer-web: could not resolve remote sidecar size "
-                       "(server must support HEAD or Range)";
-        return;
-    }
-    core->loadSidecarMetadataWeb("net:model.ifcview");
-}
-
 void ViewportCore::beginWebChunkLoad(std::uint32_t model_id, std::size_t chunk_idx) {
     auto it = models_gpu_.find(model_id);
     if (it == models_gpu_.end()) return;
@@ -3382,6 +3342,7 @@ void ViewportCore::beginWebChunkLoad(std::uint32_t model_id, std::size_t chunk_i
     if (chunk_idx >= m.chunks.size()) return;
 
     const StreamingThread::Request req = makeChunkRequest(m, chunk_idx, model_id);
+    const int sid = m.web_source_id;  // which registered byte-source to read from
     const std::uint64_t vsec = req.vertex_section_offset;
     const std::uint64_t isec = req.index_section_offset;
     const std::vector<std::pair<std::uint64_t, std::uint64_t>> v_ranges = req.v_ranges;
@@ -3438,14 +3399,14 @@ void ViewportCore::beginWebChunkLoad(std::uint32_t model_id, std::size_t chunk_i
         host_->requestFrame();
     };
 
-    webReadRangesAsync(vsec, v_ranges,
+    webReadRangesAsync(sid, vsec, v_ranges,
         [join, finish](bool ok, std::vector<std::uint8_t>&& vbytes) {
             join->v_ok = ok;
             join->vbytes = std::move(vbytes);
             join->v_done = true;
             finish();
         });
-    webReadRangesAsync(isec, i_byte_ranges,
+    webReadRangesAsync(sid, isec, i_byte_ranges,
         [join, finish](bool ok, std::vector<std::uint8_t>&& ibytes) {
             join->i_ok = ok;
             join->idx.resize(ibytes.size() / sizeof(std::uint32_t));
@@ -3465,19 +3426,19 @@ void ViewportCore::beginWebChunkLoad(std::uint32_t model_id, std::size_t chunk_i
 // here — they stream per chunk through beginWebChunkLoad. `source_label` is a
 // log/identity tag stored as file_path (chunk reads go through the JS source,
 // not this path).
-void ViewportCore::loadSidecarMetadataWeb(std::string source_label) {
+void ViewportCore::loadSidecarMetadataWeb(int source_id, std::string source_label) {
     if (!device_ || !queue_) {
         Log::warn() << "loadSidecarMetadataWeb: wgpu not initialised";
         return;
     }
-    const double fsize = ifcvFileSize();
+    const double fsize = ifcvSourceSize(source_id);
     if (fsize <= 0.0) {
-        Log::warn() << "loadSidecarMetadataWeb: no active source / zero size";
+        Log::warn() << "loadSidecarMetadataWeb: source " << source_id << " has zero size";
         return;
     }
 
-    webReadRangesAsync(0, {{0, SIDECAR_HEAD_BYTES}},
-        [this, fsize, source_label](bool ok, std::vector<std::uint8_t>&& head) {
+    webReadRangesAsync(source_id, 0, {{0, SIDECAR_HEAD_BYTES}},
+        [this, fsize, source_id, source_label](bool ok, std::vector<std::uint8_t>&& head) {
             std::uint32_t nvb = 0;
             if (!ok || !parseSidecarHead(head.data(), head.size(), nvb)) {
                 Log::warn() << "loadSidecarMetadataWeb: bad sidecar head";
@@ -3486,8 +3447,8 @@ void ViewportCore::loadSidecarMetadataWeb(std::string source_label) {
             const std::uint64_t vsec          = SIDECAR_HEAD_BYTES;
             const std::uint64_t idx_count_off = std::uint64_t(SIDECAR_HEAD_BYTES) + nvb;
 
-            webReadRangesAsync(0, {{idx_count_off, 4}},
-                [this, fsize, nvb, vsec, idx_count_off, source_label]
+            webReadRangesAsync(source_id, 0, {{idx_count_off, 4}},
+                [this, fsize, nvb, vsec, idx_count_off, source_id, source_label]
                 (bool ok2, std::vector<std::uint8_t>&& cnt) {
                     if (!ok2 || cnt.size() < 4) {
                         Log::warn() << "loadSidecarMetadataWeb: short index count";
@@ -3505,8 +3466,9 @@ void ViewportCore::loadSidecarMetadataWeb(std::string source_label) {
                     // render-critical metadata only. The deferred block
                     // (elements/strings) is fetched on demand later — first
                     // paint no longer waits on the property tree.
-                    webReadRangesAsync(0, {{crit_size_off, 8}},
-                        [this, vsec, nvb, isec, num_indices, source_label, crit_size_off, fsize]
+                    webReadRangesAsync(source_id, 0, {{crit_size_off, 8}},
+                        [this, vsec, nvb, isec, num_indices, source_id, source_label,
+                         crit_size_off, fsize]
                         (bool okc, std::vector<std::uint8_t>&& cb) {
                             if (!okc || cb.size() < 8) {
                                 Log::warn() << "loadSidecarMetadataWeb: short critical size";
@@ -3519,8 +3481,8 @@ void ViewportCore::loadSidecarMetadataWeb(std::string source_label) {
                                 Log::warn() << "loadSidecarMetadataWeb: critical block past EOF";
                                 return;
                             }
-                            webReadRangesAsync(0, {{crit_off, crit_bytes}},
-                                [this, vsec, nvb, isec, num_indices, source_label,
+                            webReadRangesAsync(source_id, 0, {{crit_off, crit_bytes}},
+                                [this, vsec, nvb, isec, num_indices, source_id, source_label,
                                  crit_off, crit_bytes, fsize]
                                 (bool ok3, std::vector<std::uint8_t>&& crit) {
                                     if (!ok3) {
@@ -3544,6 +3506,7 @@ void ViewportCore::loadSidecarMetadataWeb(std::string source_label) {
                                     auto mit = models_gpu_.find(mid);
                                     if (mit != models_gpu_.end()) {
                                         mit->second.streaming_from_web    = true;
+                                        mit->second.web_source_id         = source_id;
                                         // Deferred block: [end of critical, EOF).
                                         mit->second.deferred_meta_offset  = crit_off + crit_bytes;
                                         mit->second.deferred_meta_bytes   =
@@ -3560,30 +3523,13 @@ void ViewportCore::loadSidecarMetadataWeb(std::string source_label) {
         });
 }
 
-void ViewportCore::loadSidecarFromBlobWeb() {
-    // The picked File is already on Module.__ifcvFile (set by shell.html); the
-    // metadata bootstrap reads it via Blob.slice.
-    loadSidecarMetadataWeb("blob:model.ifcview");
-}
-
-void ViewportCore::loadSidecarFromUrlWeb(std::string url) {
-    if (!device_ || !queue_) {
-        Log::warn() << "loadSidecarFromUrlWeb: wgpu not initialised";
-        return;
-    }
-    // Resolve the URL's total size first (async); ifcv_source_ready then runs
-    // the shared bootstrap once Module.__ifcvUrl/__ifcvUrlSize are populated.
-    g_url_ready_core = this;
-    ifcvBeginUrlSource(url.c_str());
-}
-
 void ViewportCore::loadDeferredMetadataWeb(std::uint32_t model_id,
                                            std::function<void(bool)> done) {
     // On-demand fetch of the v15 deferred block (element tree + string table)
     // for a web-streamed model — the property data a UI needs (tree, selected-
-    // object name, search) but rendering doesn't. Fetches at most once. Valid
-    // only for the currently-loaded model, since the JS byte-source
-    // (Module.__ifcvUrl/File) tracks the latest load.
+    // object name, search) but rendering doesn't. Fetches at most once. Reads
+    // from the model's own registered byte-source, so it works per-model even
+    // with several federated files loaded.
     auto it = models_gpu_.find(model_id);
     if (it == models_gpu_.end()) { if (done) done(false); return; }
     ModelGpuData& m = it->second;
@@ -3592,7 +3538,7 @@ void ViewportCore::loadDeferredMetadataWeb(std::uint32_t model_id,
         if (done) done(true);
         return;
     }
-    webReadRangesAsync(0, {{m.deferred_meta_offset, m.deferred_meta_bytes}},
+    webReadRangesAsync(m.web_source_id, 0, {{m.deferred_meta_offset, m.deferred_meta_bytes}},
         [this, model_id, done](bool ok, std::vector<std::uint8_t>&& buf) {
             auto mit = models_gpu_.find(model_id);
             if (mit == models_gpu_.end()) { if (done) done(false); return; }
diff --git a/src/ifcviewer/ViewportCore.h b/src/ifcviewer/ViewportCore.h
index c4dd303e49..a082a988c4 100644
--- a/src/ifcviewer/ViewportCore.h
+++ b/src/ifcviewer/ViewportCore.h
@@ -362,14 +362,12 @@ public:
     // are read via byte ranges, the streaming model is built, and it is tagged
     // web-sourced so each chunk's vertex/index ranges are pulled lazily.
     // Asynchronous — returns immediately and frames the model from the JS
-    // completion callback. resetScene() first to replace.
-    //
-    //  - Blob:  the picked File on Module.__ifcvFile (Blob.slice).
-    //  - URL:   a remote sidecar (HTTP Range); resolves total size first, then
-    //           runs the shared bootstrap via the ifcv_source_ready callback.
-    void loadSidecarFromBlobWeb();
-    void loadSidecarFromUrlWeb(std::string url);
-    void loadSidecarMetadataWeb(std::string source_label);
+    // completion callback. APPENDS the model (federation); call resetScene()
+    // first to replace. `source_id` indexes the JS byte-source registry
+    // (Module.__ifcvSources[source_id] — a picked File or remote URL, already
+    // sized by shell.html); each federated model streams from its own source.
+    // `source_label` is a log/identity tag.
+    void loadSidecarMetadataWeb(int source_id, std::string source_label);
 
     // On-demand fetch of the v15 deferred property block (element tree + string
     // table) for a web-streamed model — what a UI (object tree / selected-name