diff --git a/packages/cli/src/telemetry/events.ts b/packages/cli/src/telemetry/events.ts
index d8c42e6ec03..1e8efe3fb6c 100644
--- a/packages/cli/src/telemetry/events.ts
+++ b/packages/cli/src/telemetry/events.ts
@@ -341,8 +341,8 @@ export function trackRenderComplete(
     catalogUsage?: CatalogUsage;
     workers?: number;
     // Worker auto-sizing provenance (RenderPerfSummary.workerSizing). Answers
-    // "why N workers?" fleet-wide, and validates the advisory per-worker heap
-    // budget before it's enforced (field OOM: 6 auto workers on a 24GB/4GB-heap
+    // "why N workers?" fleet-wide, and reports the per-worker heap budget
+    // that caps auto sizing (field OOM: 6 auto workers on a 24GB/4GB-heap
     // machine — see computeWorkerSizing in @hyperframes/engine).
     workersBoundBy?: string;
     workersCpuBased?: number;
diff --git a/packages/engine/src/services/parallelCoordinator.test.ts b/packages/engine/src/services/parallelCoordinator.test.ts
index 414902db5aa..365ce3ffbca 100644
--- a/packages/engine/src/services/parallelCoordinator.test.ts
+++ b/packages/engine/src/services/parallelCoordinator.test.ts
@@ -1,4 +1,6 @@
-import { describe, it, expect, vi } from "vitest";
+import { cpus, totalmem } from "os";
+import { getHeapStatistics } from "v8";
+import { afterEach, describe, it, expect, vi } from "vitest";
 import {
   calculateOptimalWorkers,
   computeWorkerSizing,
@@ -128,7 +130,43 @@ describe("calculateOptimalWorkers", () => {
   });
 });
 
+vi.mock("os", async (importOriginal) => {
+  const os = await importOriginal<typeof import("os")>();
+  return { ...os, default: os, cpus: vi.fn(os.cpus), totalmem: vi.fn(os.totalmem) };
+});
+vi.mock("v8", async (importOriginal) => {
+  const v8 = await importOriginal<typeof import("v8")>();
+  return { ...v8, default: v8, getHeapStatistics: vi.fn(v8.getHeapStatistics) };
+});
+
 describe("computeWorkerSizing", () => {
+  afterEach(() => {
+    vi.mocked(cpus).mockRestore();
+    vi.mocked(totalmem).mockRestore();
+    vi.mocked(getHeapStatistics).mockRestore();
+  });
+
+  // The field case: a 24GB 14-core Mac, Node's default ~4GB heap, a long 1080x1920 project.
+  it("never auto-picks more workers than the V8 heap can feed", () => {
+    vi.mocked(cpus).mockReturnValue(
+      Array.from({ length: 14 }, () => ({}) as ReturnType<typeof cpus>[number]),
+    );
+    vi.mocked(totalmem).mockReturnValue(24 * 1024 ** 3);
+    vi.mocked(getHeapStatistics).mockReturnValue({
+      heap_size_limit: 4192 * 1024 ** 2,
+    } as ReturnType<typeof getHeapStatistics>);
+    const sizing = computeWorkerSizing(1300, undefined, { concurrency: "auto" });
+    expect(sizing.heapBasedWorkers).toBe(4);
+    expect(sizing.workers).toBe(4);
+    expect(sizing.boundBy).toBe("heap");
+    expect(sizing.exceedsHeapAdvisory).toBe(false);
+    expect(computeWorkerSizing(1300, 6, { concurrency: "auto" }).workers).toBe(6);
+    vi.mocked(getHeapStatistics).mockReturnValue({
+      heap_size_limit: 8192 * 1024 ** 2,
+    } as ReturnType<typeof getHeapStatistics>);
+    expect(computeWorkerSizing(1300, undefined, { concurrency: "auto" }).workers).toBe(5);
+  });
+
   it("matches calculateOptimalWorkers and reports every constraint", () => {
     const config = { concurrency: "auto" as const };
     const sizing = computeWorkerSizing(900, undefined, config);
diff --git a/packages/engine/src/services/parallelCoordinator.ts b/packages/engine/src/services/parallelCoordinator.ts
index 457f22b6317..dc8240af104 100644
--- a/packages/engine/src/services/parallelCoordinator.ts
+++ b/packages/engine/src/services/parallelCoordinator.ts
@@ -141,10 +141,8 @@ const MEMORY_PER_WORKER_MB = 1536;
 const HEAP_RESERVED_MB = 1024;
 // Parent-process V8 heap consumed per worker (protocol buffers + in-flight
 // frame buffers). Derived from the field OOM: 6 workers exhausted a ~4GB
-// default heap ⇒ >~500MB/worker + base. ponytail: advisory-only until the
-// workers_heap_* telemetry added alongside this constant validates the figure
-// — enforcing a guessed budget could silently cut worker counts fleet-wide.
-// TODO(PRINFRA-341): decide enforcement after ~2 weeks of fleet soak.
+// default heap ⇒ >~500MB/worker + base. Caps auto sizing; an explicit
+// `--workers N` is still the operator's call.
 const HEAP_PER_WORKER_MB = 640;
 const MIN_WORKERS = 1;
 const MAX_WORKER_DIAGNOSTIC_LINES = 8;
@@ -275,7 +273,8 @@ export type WorkerSizingBound =
   | "frames"
   | "max_workers"
   | "min_parallel_floor"
-  | "contention";
+  | "contention"
+  | "heap";
 
 /**
  * Full provenance of a worker-sizing decision. Threaded into render
@@ -290,9 +289,8 @@ export interface WorkerSizing {
   frameBasedWorkers: number;
   effectiveMaxWorkers: number;
   /**
-   * ADVISORY, not enforced (see HEAP_PER_WORKER_MB): how many workers the
-   * parent process's V8 heap could feed. Compare against `workers` in
-   * telemetry to validate the budget before enforcement.
+   * How many workers the parent process's V8 heap can feed (see
+   * HEAP_PER_WORKER_MB); auto sizing never exceeds it.
    */
   heapBasedWorkers: number;
   /** V8 `heap_size_limit` for the parent process, MB. */
@@ -300,7 +298,7 @@ export interface WorkerSizing {
   totalMemoryMb: number;
   cpuCount: number;
   captureCostMultiplier: number;
-  /** true when the chosen count exceeds the advisory heap budget. */
+  /** true when the chosen count exceeds the heap budget (only an explicit request can). */
   exceedsHeapAdvisory: boolean;
 }
 
@@ -405,6 +403,11 @@ export function computeWorkerSizing(
     }
   }
 
+  if (finalWorkers > heapBasedWorkers) {
+    finalWorkers = heapBasedWorkers;
+    boundBy = "heap";
+  }
+
   return finish(finalWorkers, boundBy, effectiveMaxWorkers);
 }
 
diff --git a/packages/producer/src/services/render/captureCost.ts b/packages/producer/src/services/render/captureCost.ts
index db00538d92b..c7ceb1c7286 100644
--- a/packages/producer/src/services/render/captureCost.ts
+++ b/packages/producer/src/services/render/captureCost.ts
@@ -116,11 +116,8 @@ function combineCaptureCostEstimates(
  * - Auto-sized renders only (`requestedWorkers === undefined`) — the field
  *   failure was auto sizing, and an explicit `--workers N` is the operator's
  *   own call.
- * - Not enforced as a cap yet — the per-worker budget constant is derived
- *   from one field report; the `workers_heap_*` telemetry emitted with the
- *   sizing decides whether to enforce (see the TODO on HEAP_PER_WORKER_MB in
- *   @hyperframes/engine's parallelCoordinator). The message gives the
- *   operator the actionable knobs today.
+ * - Auto sizing is capped at the heap budget (computeWorkerSizing), so the
+ *   warning cannot fire today; it stays for a budget that stops being a cap.
  *
  * Pure so the message shape + firing condition are unit-testable with a
  * synthetic `WorkerSizing` (the real one depends on the host's heap).
diff --git a/packages/producer/src/services/renderOrchestrator.ts b/packages/producer/src/services/renderOrchestrator.ts
index f26bcbf7dae..d590f14c6f5 100644
--- a/packages/producer/src/services/renderOrchestrator.ts
+++ b/packages/producer/src/services/renderOrchestrator.ts
@@ -455,9 +455,8 @@ export interface RenderPerfSummary {
   /**
    * Provenance of the auto worker-sizing decision (undefined when the
    * htmlInCanvas / low-memory pins short-circuited sizing). `boundBy` names
-   * the binding constraint; the heap fields are the advisory budget being
-   * validated by fleet telemetry before enforcement — see
-   * `computeWorkerSizing` in @hyperframes/engine.
+   * the binding constraint; the heap fields are the budget that caps auto
+   * sizing — see `computeWorkerSizing` in @hyperframes/engine.
    */
   workerSizing?: WorkerSizing;
   chunkedEncode: boolean;