diff --git a/src/tracks/BackgroundEffectTransformer.test.ts b/src/tracks/BackgroundEffectTransformer.test.ts
index 32ab17c3a..9eacef13d 100644
--- a/src/tracks/BackgroundEffectTransformer.test.ts
+++ b/src/tracks/BackgroundEffectTransformer.test.ts
@@ -76,10 +76,14 @@ describe("BackgroundEffectTransformer", () => {
       .spyOn(FilesetResolver, "forVisionTasks")
       .mockResolvedValue(served);
 
-    await new BackgroundEffectTransformer(
+    const wasmBaseUrl = vi.fn(() => "https://cdn.example.org/mediapipe");
+    const transformer = new BackgroundEffectTransformer(
       { backgroundDisabled: true },
-      { wasmBaseUrl: "https://cdn.example.org/mediapipe" },
-    ).init({} as VideoTransformerInitOptions);
+      { wasmBaseUrl },
+    );
+    expect(wasmBaseUrl).not.toHaveBeenCalled();
+
+    await transformer.init({} as VideoTransformerInitOptions);
 
     expect(resolve).toHaveBeenCalledWith("https://cdn.example.org/mediapipe");
     expect(create.mock.calls[0][0]).toBe(served);
diff --git a/src/tracks/BackgroundEffectTransformer.ts b/src/tracks/BackgroundEffectTransformer.ts
index 959e1aa55..6025afdbc 100644
--- a/src/tracks/BackgroundEffectTransformer.ts
+++ b/src/tracks/BackgroundEffectTransformer.ts
@@ -36,8 +36,10 @@ export interface BackgroundEffectTransformerSources {
   /**
    * Where to fetch MediaPipe's runtime from instead of the bundled copy: a
    * directory holding its `vision_wasm_internal.js` and `.wasm` files.
+   * Consulted only when the runtime is about to load, so a transformer can be
+   * built before the deployment's config has arrived.
    */
-  wasmBaseUrl?: string;
+  wasmBaseUrl?: () => string | undefined;
 }
 
 /**
@@ -94,10 +96,11 @@ export class BackgroundEffectTransformer extends BackgroundTransformer {
       inputElement: inputVideo,
     });
 
+    const wasmBaseUrl = this.sources.wasmBaseUrl?.();
     const fileset =
-      this.sources.wasmBaseUrl === undefined
+      wasmBaseUrl === undefined
         ? bundledFileset
-        : await FilesetResolver.forVisionTasks(this.sources.wasmBaseUrl);
+        : await FilesetResolver.forVisionTasks(wasmBaseUrl);
     this.imageSegmenter = await ImageSegmenter.createFromOptions(fileset, {
       baseOptions: {
         modelAssetPath,
diff --git a/src/tracks/BackgroundEffectsContext.tsx b/src/tracks/BackgroundEffectsContext.tsx
index 68168e007..6831deefd 100644
--- a/src/tracks/BackgroundEffectsContext.tsx
+++ b/src/tracks/BackgroundEffectsContext.tsx
@@ -50,7 +50,9 @@ export function createAppBackgroundEffects(
 ): BackgroundEffects {
   const transformer = new BackgroundEffectTransformer(
     { backgroundDisabled: true },
-    { wasmBaseUrl: Config.get().background_effects?.mediapipe_wasm_url },
+    {
+      wasmBaseUrl: () => Config.get().background_effects?.mediapipe_wasm_url,
+    },
   );
   return createBackgroundEffects(scope, {
     supported: supportsBackgroundProcessors(),