diff --git a/apps/api/src/routes/features.ts b/apps/api/src/routes/features.ts
index 4abfe22f..7bbfff29 100644
--- a/apps/api/src/routes/features.ts
+++ b/apps/api/src/routes/features.ts
@@ -202,9 +202,32 @@ export async function registerFeatureRoutes(app: FastifyInstance): Promise<void>
             duration_ms: Date.now() - installStartTime,
           });
         } else {
-          const errorDetail =
-            lastStderrLines.filter((l) => !l.startsWith("{")).join("\n") || stdoutBuffer.trim();
-          const errorMsg = errorDetail || `Install failed with exit code ${code}`;
+          // Extract the structured error from Python's fail() function first.
+          // fail() writes {"error": "..."} to stderr — prefer this over raw lines.
+          let errorMsg: string | undefined;
+          for (let i = lastStderrLines.length - 1; i >= 0; i--) {
+            const line = lastStderrLines[i];
+            if (line.startsWith("{")) {
+              try {
+                const parsed = JSON.parse(line) as Record<string, unknown>;
+                if (typeof parsed.error === "string") {
+                  errorMsg = parsed.error;
+                  break;
+                }
+              } catch {
+                // Not valid JSON
+              }
+            }
+          }
+          if (!errorMsg) {
+            const meaningful = lastStderrLines.filter(
+              (l) => !l.startsWith("{") && !l.includes("pthread_setaffinity_np"),
+            );
+            errorMsg =
+              meaningful.join("\n") ||
+              stdoutBuffer.trim() ||
+              `Install failed with exit code ${code}`;
+          }
           setInstallProgress(bundleId, null, errorMsg);
           updateSingleFileProgress({ jobId, phase: "failed", percent: 0, error: errorMsg });
         }
diff --git a/apps/web/src/stores/features-store.ts b/apps/web/src/stores/features-store.ts
index 6cad7106..fb3f0bc9 100644
--- a/apps/web/src/stores/features-store.ts
+++ b/apps/web/src/stores/features-store.ts
@@ -191,7 +191,10 @@ export const useFeaturesStore = create<FeaturesState>((set, get) => {
       });
 
       try {
-        const result = await apiPost<{ jobId: string }>(`/v1/admin/features/${bundleId}/install`);
+        const result = await apiPost<{ jobId: string }>(
+          `/v1/admin/features/${bundleId}/install`,
+          {},
+        );
         listenToProgress(bundleId, result.jobId);
       } catch (err) {
         const installing = { ...get().installing };
@@ -209,7 +212,7 @@ export const useFeaturesStore = create<FeaturesState>((set, get) => {
 
     uninstallBundle: async (bundleId: string) => {
       try {
-        await apiPost(`/v1/admin/features/${bundleId}/uninstall`);
+        await apiPost(`/v1/admin/features/${bundleId}/uninstall`, {});
         await refreshBundles();
       } catch (err) {
         set({
diff --git a/packages/ai/python/install_feature.py b/packages/ai/python/install_feature.py
index c8228c43..f88975f7 100644
--- a/packages/ai/python/install_feature.py
+++ b/packages/ai/python/install_feature.py
@@ -277,6 +277,45 @@ def download_url_model(model: dict, models_dir: str) -> None:
     os.rename(tmp_path, final_path)
 
 
+_matting_registered = False
+
+
+def _register_birefnet_matting() -> None:
+    """Register the custom BiRefNet-matting ONNX session.
+
+    This model is not built into rembg — it must be registered before
+    calling new_session("birefnet-matting").  The same registration is
+    done in remove_bg.py (runtime) and download_models.py (build-time).
+    """
+    global _matting_registered
+    if _matting_registered:
+        return
+    _matting_registered = True
+
+    import pooch
+    from rembg.sessions import sessions_class
+    from rembg.sessions.birefnet_general import BiRefNetSessionGeneral
+
+    class BiRefNetMattingSession(BiRefNetSessionGeneral):
+        @classmethod
+        def download_models(cls, *args, **kwargs):
+            fname = f"{cls.name(*args, **kwargs)}.onnx"
+            pooch.retrieve(
+                "https://github.com/ZhengPeng7/BiRefNet/releases/download/v1/BiRefNet-matting-epoch_100.onnx",
+                None,
+                fname=fname,
+                path=cls.u2net_home(*args, **kwargs),
+                progressbar=True,
+            )
+            return os.path.join(cls.u2net_home(*args, **kwargs), fname)
+
+        @classmethod
+        def name(cls, *args, **kwargs):
+            return "birefnet-matting"
+
+    sessions_class.append(BiRefNetMattingSession)
+
+
 def download_rembg_session(model: dict, models_dir: str) -> None:
     """Download a rembg model by initializing a session."""
     args = model.get("args", [])
@@ -291,6 +330,7 @@ def download_rembg_session(model: dict, models_dir: str) -> None:
     os.environ["U2NET_HOME"] = u2net_dir
 
     from rembg import new_session
+    _register_birefnet_matting()
     new_session(model_name)
 
 
diff --git a/tests/unit/web/zustand-stores.test.ts b/tests/unit/web/zustand-stores.test.ts
index a1e6f4d1..f79bd059 100644
--- a/tests/unit/web/zustand-stores.test.ts
+++ b/tests/unit/web/zustand-stores.test.ts
@@ -1966,7 +1966,7 @@ describe("useFeaturesStore", () => {
 
     await useFeaturesStore.getState().uninstallBundle("ai-rembg");
 
-    expect(mockApiPost).toHaveBeenCalledWith("/v1/admin/features/ai-rembg/uninstall");
+    expect(mockApiPost).toHaveBeenCalledWith("/v1/admin/features/ai-rembg/uninstall", {});
   });
 
   it("uninstallBundle sets error on failure", async () => {
@@ -2207,8 +2207,8 @@ describe("useFeaturesStore", () => {
 
     await useFeaturesStore.getState().reinstallBundle("ai-rembg");
 
-    expect(mockApiPost).toHaveBeenCalledWith("/v1/admin/features/ai-rembg/uninstall");
-    expect(mockApiPost).toHaveBeenCalledWith("/v1/admin/features/ai-rembg/install");
+    expect(mockApiPost).toHaveBeenCalledWith("/v1/admin/features/ai-rembg/uninstall", {});
+    expect(mockApiPost).toHaveBeenCalledWith("/v1/admin/features/ai-rembg/install", {});
   });
 
   // -- EventSource progress handling ----------------------------------------
@@ -2452,7 +2452,7 @@ describe("useFeaturesStore", () => {
     expect(useFeaturesStore.getState().installAllActive).toBe(false);
     expect(useFeaturesStore.getState().queued).toEqual([]);
     // Only not-installed bundle should have been installed
-    expect(mockApiPost).toHaveBeenCalledWith("/v1/admin/features/ai-rembg/install");
+    expect(mockApiPost).toHaveBeenCalledWith("/v1/admin/features/ai-rembg/install", {});
   });
 
   it("installAll skips bundles that are already installed", async () => {