diff --git a/workbench/nextjs-turbopack/app/api/e2e-h2-flag/route.ts b/workbench/nextjs-turbopack/app/api/e2e-h2-flag/route.ts
new file mode 100644
index 0000000000..56eb345278
--- /dev/null
+++ b/workbench/nextjs-turbopack/app/api/e2e-h2-flag/route.ts
@@ -0,0 +1,21 @@
+/**
+ * Repro-branch probe. Proves that `WORKFLOW_H2_MULTIPLEX`, set in this app's
+ * `vercel.json` `env` block, actually reaches the deployed function's *runtime*
+ * environment — which is where `@workflow/world-vercel` reads it, lazily, in
+ * `createEventsDispatcher()` (packages/world-vercel/src/http-client.ts).
+ *
+ * Without this probe a flag-off storm that measures no change is ambiguous
+ * between "multiplexing is not the cause" and "the flag never arrived".
+ */
+export const dynamic = 'force-dynamic';
+
+export function GET() {
+  return Response.json({
+    WORKFLOW_H2_MULTIPLEX: process.env.WORKFLOW_H2_MULTIPLEX ?? null,
+    // Mirrors `h2MultiplexEnabled()` in world-vercel's http-client.
+    multiplexEnabled: process.env.WORKFLOW_H2_MULTIPLEX !== '0',
+    // 'vercel.json' if the deployment supplied it, 'instrumentation' if the
+    // fallback in instrumentation.ts had to.
+    source: process.env.WORKFLOW_H2_MULTIPLEX_SOURCE ?? null,
+  });
+}
diff --git a/workbench/nextjs-turbopack/instrumentation.ts b/workbench/nextjs-turbopack/instrumentation.ts
index bd66985a97..ed7d636e0f 100644
--- a/workbench/nextjs-turbopack/instrumentation.ts
+++ b/workbench/nextjs-turbopack/instrumentation.ts
@@ -1,5 +1,21 @@
 import { registerOTel } from '@vercel/otel';
 
+// Repro branch only. `vercel.json`'s `env` block is the documented way to give
+// the deployed functions a runtime variable, but Vercel marks it deprecated, so
+// this is the belt to that suspenders: if the block were ignored, a storm here
+// would measure stock main and read as "multiplexing is not the cause".
+// `@workflow/world-vercel` reads the variable lazily in
+// `createEventsDispatcher()`, which always runs after instrumentation, so
+// setting it here reaches the same code path.
+// Recorded so the probe route can also tell us whether the deprecated
+// `vercel.json` block still works — that decides where the real mitigation
+// advice should point if #3190 turns out to own the regression.
+process.env.WORKFLOW_H2_MULTIPLEX_SOURCE =
+  process.env.WORKFLOW_H2_MULTIPLEX === undefined
+    ? 'instrumentation'
+    : 'vercel.json';
+process.env.WORKFLOW_H2_MULTIPLEX ??= '0';
+
 export function register() {
   registerOTel({
     serviceName: 'nextjs-turbopack',
diff --git a/workbench/nextjs-turbopack/vercel.json b/workbench/nextjs-turbopack/vercel.json
index aa4f892410..cc7e0a3f9f 100644
--- a/workbench/nextjs-turbopack/vercel.json
+++ b/workbench/nextjs-turbopack/vercel.json
@@ -1,6 +1,7 @@
 {
   "env": {
-    "WORKFLOW_PUBLIC_MANIFEST": "1"
+    "WORKFLOW_PUBLIC_MANIFEST": "1",
+    "WORKFLOW_H2_MULTIPLEX": "0"
   },
   "regions": [
     "iad1",