diff --git a/packages/cli/src/telemetry/events.ts b/packages/cli/src/telemetry/events.ts
index c6273b2de..88196e247 100644
--- a/packages/cli/src/telemetry/events.ts
+++ b/packages/cli/src/telemetry/events.ts
@@ -53,7 +53,8 @@ export interface RenderObservabilityTelemetryPayload {
   captureDePreRouterWorkers?: number;
   captureDeSelfVerifyFallback?: boolean;
   captureDeFallbackReason?: string;
-  /** Non-DE parallel-streaming router outcome ("screenshot" | "beginframe"). */
+  /** Non-DE parallel-streaming router outcome ("screenshot" | "beginframe" —
+   * routed; "eligible_off" — would route but the kill switch is off). */
   captureParallelStream?: string;
   observabilityExtractVideoCount?: number;
   observabilityExtractedVideoCount?: number;
diff --git a/packages/cli/src/telemetry/renderObservability.test.ts b/packages/cli/src/telemetry/renderObservability.test.ts
index 8b5b3c99f..0a64cc866 100644
--- a/packages/cli/src/telemetry/renderObservability.test.ts
+++ b/packages/cli/src/telemetry/renderObservability.test.ts
@@ -91,6 +91,13 @@ describe("renderObservabilityTelemetryPayload — non-DE parallel-stream router"
     expect(payload.captureParallelStream).toBe("beginframe");
   });
 
+  it("maps the passive eligible_off cohort-sizing signal", () => {
+    const payload = renderObservabilityTelemetryPayload(
+      makeSummary({ captureParallelStream: "eligible_off" }),
+    );
+    expect(payload.captureParallelStream).toBe("eligible_off");
+  });
+
   it("stays undefined when the router never fired", () => {
     const payload = renderObservabilityTelemetryPayload(makeSummary({}));
     expect(payload.captureParallelStream).toBeUndefined();
diff --git a/packages/producer/src/services/render/observability.ts b/packages/producer/src/services/render/observability.ts
index ca542532e..c180f0481 100644
--- a/packages/producer/src/services/render/observability.ts
+++ b/packages/producer/src/services/render/observability.ts
@@ -70,11 +70,14 @@ export interface RenderCaptureObservability {
   dePreRouterWorkers?: number;
   /**
    * Non-DE parallel-streaming router outcome (HF_CAPTURE_PARALLEL_STREAM):
-   * set when a multi-worker screenshot/BeginFrame render was routed through
-   * the interleaved streaming encoder instead of the parallel disk path.
-   * The value is the capture mode that streamed. Absent = not routed.
+   * "screenshot" | "beginframe" — the render passed every gate AND the kill
+   * switch was on, so it was routed through the interleaved streaming encoder
+   * (the value is the capture mode that streamed); "eligible_off" — the render
+   * passed every gate EXCEPT the kill switch (passive cohort-sizing signal for
+   * the default-off soak: how many renders WOULD route if enabled). Absent =
+   * ineligible regardless of the switch.
    */
-  captureParallelStream?: "screenshot" | "beginframe";
+  captureParallelStream?: "screenshot" | "beginframe" | "eligible_off";
   protocolTimeoutMs?: number;
   pageNavigationTimeoutMs?: number;
   playerReadyTimeoutMs?: number;
diff --git a/packages/producer/src/services/renderOrchestrator.ts b/packages/producer/src/services/renderOrchestrator.ts
index 8328bfeba..74dcb3d17 100644
--- a/packages/producer/src/services/renderOrchestrator.ts
+++ b/packages/producer/src/services/renderOrchestrator.ts
@@ -2245,13 +2245,17 @@ export async function executeRenderJob(
     // Non-DE parallel-streaming router — see shouldStreamParallelCapture.
     // Mutually exclusive with the DE inversion/router above by construction
     // (both DE predicates require useDrawElement; this requires its negation).
-    const captureParallelStreamEligible = shouldStreamParallelCapture({
-      routerEnabled: process.env.HF_CAPTURE_PARALLEL_STREAM === "true",
+    const captureParallelStreamRouterEnabled = process.env.HF_CAPTURE_PARALLEL_STREAM === "true";
+    const captureParallelStreamArgs = {
       workerCount,
       useDrawElement: cfg.useDrawElement,
       outputFormat,
       streamingOk: shouldUseStreamingEncode(cfg, outputFormat, 1, job.duration),
       layeredOrEffectRoute: hasHdrContent || compiled.hasShaderTransitions,
+    };
+    const captureParallelStreamEligible = shouldStreamParallelCapture({
+      routerEnabled: captureParallelStreamRouterEnabled,
+      ...captureParallelStreamArgs,
     });
     if (captureParallelStreamEligible) {
       captureParallelStreamForced = true;
@@ -2273,6 +2277,13 @@ export async function executeRenderJob(
         "worker_resolution",
         `parallel ${captureParallelStream} capture routed to streaming`,
       );
+    } else if (shouldStreamParallelCapture({ routerEnabled: true, ...captureParallelStreamArgs })) {
+      // The kill switch is the ONLY failed gate: emit a passive cohort-sizing
+      // signal (capture_parallel_stream = "eligible_off") so the default-off
+      // soak can measure how many fleet renders WOULD route before anyone
+      // enables the flag. Observability-only — no behavior change, no log
+      // noise on the default path.
+      updateCaptureObservability({ captureParallelStream: "eligible_off" });
     }
 
     if (workerCount > 1 && probeSession) {