diff --git a/tools/celestia-node-fiber/adapter_test.go b/tools/celestia-node-fiber/adapter_test.go
index af3e42161..7b5de1915 100644
--- a/tools/celestia-node-fiber/adapter_test.go
+++ b/tools/celestia-node-fiber/adapter_test.go
@@ -211,7 +211,18 @@ func TestListen_FiltersFibreOnlyAndEmitsEvent(t *testing.T) {
 			return ch, nil
 		},
 	}
-	a := cnfiber.FromModules(&fakeFibre{}, blob, 0)
+
+	// Listen now issues a Download per v2 blob to recover the original
+	// payload size for BlobEvent.DataSize (see Listen's doc comment).
+	// Feed a deterministic payload back; the test asserts DataSize
+	// matches its length.
+	originalPayload := []byte("this is the original blob payload")
+	fibre := &fakeFibre{
+		downloadFn: func(_ context.Context, _ appfibre.BlobID) (*fibreapi.GetBlobResult, error) {
+			return &fibreapi.GetBlobResult{Data: originalPayload}, nil
+		},
+	}
+	a := cnfiber.FromModules(fibre, blob, 0)
 	events, err := a.Listen(context.Background(), namespaceBytes())
 	require.NoError(t, err)
 	require.Equal(t, ns, seenNs)
@@ -224,7 +235,8 @@ func TestListen_FiltersFibreOnlyAndEmitsEvent(t *testing.T) {
 		expectedID := appfibre.NewBlobID(0, expectedCommit)
 		require.Equal(t, block.FiberBlobID(expectedID), ev.BlobID)
 		require.Equal(t, uint64(42), ev.Height)
-		require.Equal(t, uint64(v2Lib.DataLen()), ev.DataSize)
+		require.Equal(t, uint64(len(originalPayload)), ev.DataSize,
+			"DataSize must match the original payload length resolved via Download")
 	case <-time.After(time.Second):
 		t.Fatal("timed out waiting for blob event")
 	}
diff --git a/tools/celestia-node-fiber/listen.go b/tools/celestia-node-fiber/listen.go
index 6ba5c87f1..7c666622a 100644
--- a/tools/celestia-node-fiber/listen.go
+++ b/tools/celestia-node-fiber/listen.go
@@ -2,6 +2,7 @@ package celestianodefiber
 
 import (
 	"context"
+	"errors"
 	"fmt"
 
 	appfibre "github.com/celestiaorg/celestia-app/v8/fibre"
@@ -16,6 +17,14 @@ import (
 // bridge node for the given namespace and forwards only share-version-2
 // (Fibre) blobs as BlobEvents. PFB blobs (v0/v1) sharing the namespace are
 // dropped so consumers see a pure Fibre event stream.
+//
+// DataSize on emitted events is the original payload byte length — matching
+// the fibermock contract ev-node consumers code against. The v2 share only
+// carries (fibre_blob_version + commitment), so the real size isn't derivable
+// from the subscription alone; Listen therefore performs a Download per event
+// to recover the size before forwarding. This adds one FSP round-trip per
+// blob. If that cost becomes material we can expose an opt-out mode, but for
+// now correctness over latency.
 func (a *Adapter) Listen(ctx context.Context, namespace []byte) (<-chan block.FiberBlobEvent, error) {
 	ns, err := toV0Namespace(namespace)
 	if err != nil {
@@ -26,14 +35,14 @@ func (a *Adapter) Listen(ctx context.Context, namespace []byte) (<-chan block.Fi
 		return nil, fmt.Errorf("subscribing to blob stream: %w", err)
 	}
 	out := make(chan block.FiberBlobEvent, a.listenChannelSz)
-	go forwardFibreBlobs(ctx, sub, out)
+	go a.forwardFibreBlobs(ctx, sub, out)
 	return out, nil
 }
 
 // forwardFibreBlobs drains a blob.SubscriptionResponse stream and emits a
 // BlobEvent per share-version-2 blob. The output channel is closed when the
 // subscription closes or ctx is cancelled.
-func forwardFibreBlobs(
+func (a *Adapter) forwardFibreBlobs(
 	ctx context.Context,
 	sub <-chan *blob.SubscriptionResponse,
 	out chan<- block.FiberBlobEvent,
@@ -53,10 +62,13 @@ func forwardFibreBlobs(
 				if b == nil || !b.IsFibreBlob() {
 					continue
 				}
-				event, err := fibreBlobToEvent(b.Blob, height)
+				event, err := a.fibreBlobToEvent(ctx, b.Blob, height)
 				if err != nil {
-					// Skip a malformed v2 blob rather than kill the
-					// subscription. The server should not produce these.
+					// Skip a malformed or un-fetchable v2 blob rather than
+					// kill the subscription. Most likely causes: the v2
+					// payload was garbage-collected from FSPs, or the
+					// download was cancelled. Either way the consumer has
+					// no actionable signal for this single blob.
 					continue
 				}
 				select {
@@ -82,17 +94,17 @@ func resolveHeight(resp *blob.SubscriptionResponse) uint64 {
 }
 
 // fibreBlobToEvent reconstructs the Fibre BlobID (version byte + 32-byte
-// commitment) from a share-version-2 libshare.Blob and wraps it as a
-// BlobEvent.
+// commitment) from a share-version-2 libshare.Blob, downloads the blob to
+// determine the original payload size, and wraps everything as a BlobEvent.
 //
-// DataSize caveat: a v2 share carries only (fibre_blob_version + commitment),
-// not the original blob payload, so b.DataLen() is the on-chain share size
-// (a fixed constant), not the user-facing "how big is this blob" number
-// that ev-node's fibermock and its consumers typically expect. Reporting
-// the true payload size requires an on-chain query against x/fibre's
-// PaymentPromise keyed by commitment. Tracked as a follow-up; for now we
-// report the share size so the field is non-zero.
-func fibreBlobToEvent(b *libshare.Blob, height uint64) (block.FiberBlobEvent, error) {
+// The Download is what makes DataSize accurate. Without it we would have to
+// either report the v2 share size (wrong — misleads consumers) or zero
+// (lossy). See the Listen doc for the cost / correctness rationale.
+func (a *Adapter) fibreBlobToEvent(
+	ctx context.Context,
+	b *libshare.Blob,
+	height uint64,
+) (block.FiberBlobEvent, error) {
 	version, err := b.FibreBlobVersion()
 	if err != nil {
 		return block.FiberBlobEvent{}, err
@@ -110,9 +122,18 @@ func fibreBlobToEvent(b *libshare.Blob, height uint64) (block.FiberBlobEvent, er
 	var c appfibre.Commitment
 	copy(c[:], commit)
 	id := appfibre.NewBlobID(uint8(version), c)
+
+	res, err := a.fibre.Download(ctx, id)
+	if err != nil {
+		return block.FiberBlobEvent{}, fmt.Errorf("resolving payload size via Download: %w", err)
+	}
+	if res == nil {
+		return block.FiberBlobEvent{}, errors.New("fibre.Download returned nil result while resolving payload size")
+	}
+
 	return block.FiberBlobEvent{
 		BlobID:   block.FiberBlobID(id),
 		Height:   height,
-		DataSize: uint64(b.DataLen()),
+		DataSize: uint64(len(res.Data)),
 	}, nil
 }
diff --git a/tools/celestia-node-fiber/testing/showcase_test.go b/tools/celestia-node-fiber/testing/showcase_test.go
index 5c7ec1962..929e540fb 100644
--- a/tools/celestia-node-fiber/testing/showcase_test.go
+++ b/tools/celestia-node-fiber/testing/showcase_test.go
@@ -131,18 +131,16 @@ func TestShowcase(t *testing.T) {
 		}
 	}
 
-	// Every event must carry a real block height. DataSize is
-	// intentionally not asserted against the payload length: the v2 share
-	// contains only (fibre_blob_version + commitment), not the original
-	// payload bytes, so b.DataLen() — what the adapter reports today — is
-	// always the fixed share-data size, not len(payload). Fixing that
-	// needs a PaymentPromise chain lookup; tracked as a TODO on the
-	// adapter's Listen path.
+	// Every event must carry the right DataSize and a non-zero block
+	// height. DataSize matches the original payload length because the
+	// adapter's Listen issues a Download per event to recover it (see
+	// listen.go). A silent byte truncation anywhere upstream would
+	// surface here before we even get to the Download round-trip.
 	for key, ev := range seen {
 		require.Greater(t, ev.Height, uint64(0),
 			"BlobEvent %s must carry a real block height", key)
-		require.Greater(t, ev.DataSize, uint64(0),
-			"BlobEvent %s must report a non-zero DataSize", key)
+		require.Equal(t, uint64(len(expected[key])), ev.DataSize,
+			"BlobEvent %s DataSize must match original payload length", key)
 	}
 
 	// Round-trip every blob through Download and diff bytes. Walking