diff --git a/src/server/services/lake/__tests__/replay.test.ts b/src/server/services/lake/__tests__/replay.test.ts
index 5c597682..6a84b726 100644
--- a/src/server/services/lake/__tests__/replay.test.ts
+++ b/src/server/services/lake/__tests__/replay.test.ts
@@ -298,6 +298,56 @@ describe("nextReplayBatch", () => {
     expect(result).toMatchObject({ status: "COMPLETED", done: true, replayedEvents: BigInt(10) });
   });
 
+  it("marks the job FAILED with a reason when the drained replay served fewer than totalEvents (NF-6)", async () => {
+    // Estimated 10 events at create, but the window drains after only 3 (short
+    // read) → cumulative 3 < 10 → FAILED with a reason, not a silent COMPLETED.
+    prismaMock.replayJob.findFirst.mockResolvedValue(
+      jobFixture({ status: "RUNNING", replayedEvents: BigInt(0), totalEvents: BigInt(10), startedAt: FROM }) as never,
+    );
+    lakeQueryMock.mockResolvedValueOnce([lakeEvent("a"), lakeEvent("b"), lakeEvent("c")]);
+    prismaMock.replayJob.updateMany.mockResolvedValue({ count: 1 } as never);
+
+    const result = await nextReplayBatch({ orgId: ORG, targetPipelineId: "tgt", batchSize: 5 });
+
+    expect(prismaMock.replayJob.updateMany).toHaveBeenCalledWith(
+      expect.objectContaining({
+        data: expect.objectContaining({
+          status: "FAILED",
+          replayedEvents: BigInt(3),
+          completedAt: expect.any(Date),
+          error: expect.stringContaining("3 of 10"),
+        }),
+      }),
+    );
+    expect(result).toMatchObject({ status: "FAILED", done: true, replayedEvents: BigInt(3) });
+    // The final partial batch is still handed back — those events are real lake rows.
+    expect(result?.events).toHaveLength(3);
+  });
+
+  it("marks COMPLETED and clears the error when the drained replay met or exceeded totalEvents (NF-6)", async () => {
+    // totalEvents was under-counted at create (the lake grew afterwards); the
+    // window drains at 3 >= 2 → COMPLETED, with `error` explicitly cleared.
+    prismaMock.replayJob.findFirst.mockResolvedValue(
+      jobFixture({ status: "RUNNING", replayedEvents: BigInt(0), totalEvents: BigInt(2), startedAt: FROM }) as never,
+    );
+    lakeQueryMock.mockResolvedValueOnce([lakeEvent("a"), lakeEvent("b"), lakeEvent("c")]);
+    prismaMock.replayJob.updateMany.mockResolvedValue({ count: 1 } as never);
+
+    const result = await nextReplayBatch({ orgId: ORG, targetPipelineId: "tgt", batchSize: 5 });
+
+    expect(prismaMock.replayJob.updateMany).toHaveBeenCalledWith(
+      expect.objectContaining({
+        data: expect.objectContaining({
+          status: "COMPLETED",
+          replayedEvents: BigInt(3),
+          completedAt: expect.any(Date),
+          error: null,
+        }),
+      }),
+    );
+    expect(result).toMatchObject({ status: "COMPLETED", done: true, replayedEvents: BigInt(3) });
+  });
+
   it("completes immediately on an empty window (totalEvents 0)", async () => {
     prismaMock.replayJob.findFirst.mockResolvedValue(
       jobFixture({ status: "PENDING", replayedEvents: BigInt(0), totalEvents: BigInt(0) }) as never,
diff --git a/src/server/services/lake/replay.ts b/src/server/services/lake/replay.ts
index 84b1194a..c69397e7 100644
--- a/src/server/services/lake/replay.ts
+++ b/src/server/services/lake/replay.ts
@@ -103,7 +103,8 @@ export interface ReplayBatch {
   /** Cursor after this batch (BigInt — total events served so far). */
   replayedEvents: bigint;
   totalEvents: bigint;
-  /** True once the window is drained and the job flipped to COMPLETED. */
+  /** True once the window is drained and the job reached a terminal state —
+   *  COMPLETED on a full replay, FAILED on a short one (see nextReplayBatch). */
   done: boolean;
   events: ReplayEvent[];
 }
@@ -331,7 +332,8 @@ export async function createReplayJob(args: {
 /**
  * Agent-pull primitive. Find the org's oldest active (PENDING|RUNNING) job for
  * `targetPipelineId`, serve the next bounded window, advance the cursor, and
- * flip status: PENDING→RUNNING on the first pull, →COMPLETED once drained.
+ * flip status: PENDING→RUNNING on the first pull, then on drain →COMPLETED when
+ * the full `totalEvents` were served or →FAILED on a shortfall (NF-6).
  * Returns `null` when no active job exists (the route answers 204).
  *
  * The cursor read+advance happen in one org transaction, and the advancing
@@ -378,12 +380,31 @@ export async function nextReplayBatch(args: {
     // estimate from create time, and trusting it would silently drop events if
     // the window were still receiving writes. The cost is one final empty pull.
     const done = fetched < batchSize;
-    const status: ReplayStatus = done ? REPLAY_STATUS.COMPLETED : REPLAY_STATUS.RUNNING;
+    // NF-6: validate completion. A drained replay is COMPLETED only when it
+    // served at least the `totalEvents` counted at create time; a shortfall
+    // means events that existed at create time never made it into the replay
+    // (a lake gap, TTL eviction, or a window that lost rows) — so the job is
+    // FAILED with a reason instead of a silent partial COMPLETED. An over-count
+    // (the lake grew after create) still satisfies `>=` and completes cleanly.
+    let status: ReplayStatus;
+    let failureReason: string | null = null;
+    if (!done) {
+      status = REPLAY_STATUS.RUNNING;
+    } else if (replayedEvents >= job.totalEvents) {
+      status = REPLAY_STATUS.COMPLETED;
+    } else {
+      status = REPLAY_STATUS.FAILED;
+      failureReason = `Replay drained after ${replayedEvents} of ${job.totalEvents} expected events`;
+    }
     const now = new Date();
 
     const data: Prisma.ReplayJobUpdateManyMutationInput = { status, replayedEvents };
     if (!job.startedAt) data.startedAt = now;
-    if (done) data.completedAt = now;
+    if (done) {
+      data.completedAt = now;
+      // Stamp the failure reason on a short replay; null clears it on a clean finish.
+      data.error = failureReason;
+    }
 
     // Guard on the job still being active: if it was cancelled/completed
     // concurrently, do not resurrect it or hand out the batch.