diff --git a/src/switchyard/sync_worker.py b/src/switchyard/sync_worker.py
index 47e0a7d..3731007 100644
--- a/src/switchyard/sync_worker.py
+++ b/src/switchyard/sync_worker.py
@@ -14,6 +14,14 @@
 log = logger.bind(component="sync")
 
 
+class SyncMissingBlobsError(Exception):
+    """Raised when blobs referenced by a manifest are not available locally."""
+
+    def __init__(self, missing: list[str]) -> None:
+        self.missing = missing
+        super().__init__(f"Missing {len(missing)} blob(s): {', '.join(d[:19] for d in missing)}")
+
+
 async def sync_one(
     marker: SyncMarker,
     storage: Storage,
@@ -39,17 +47,11 @@ async def sync_one(
 
     body, content_type = result
 
-    # Parse manifest to extract layer digests
-    digests = _extract_blob_digests(body)
+    # Collect all blobs and child manifests, checking for missing blobs before
+    # pushing anything upstream.
+    all_blobs: list[str] = _extract_blob_digests(body)
+    children: list[tuple[str, bytes, str]] = []
 
-    # Push each blob that doesn't exist upstream
-    for digest in digests:
-        if await storage.has_blob(digest):
-            await upstream.push_blob_streaming(name, digest, storage.stream_blob(digest))
-        else:
-            log.warning("Blob {} referenced by manifest but missing locally", digest[:19])
-
-    # For image indexes, push child manifests before the index itself
     for child_digest in _extract_child_manifests(body):
         child = await storage.get_manifest(name, child_digest)
         if child is None:
@@ -59,23 +61,31 @@ async def sync_one(
             )
             continue
         child_body, child_ct = child
+        children.append((child_digest, child_body, child_ct))
+        all_blobs.extend(_extract_blob_digests(child_body))
+
+    missing = [d for d in all_blobs if not await storage.has_blob(d)]
+    if missing:
+        for digest in missing:
+            log.warning("Blob {} referenced by manifest but missing locally", digest[:19])
+        raise SyncMissingBlobsError(missing)
 
-        # Push blobs referenced by the child manifest
-        for blob_digest in _extract_blob_digests(child_body):
-            if await storage.has_blob(blob_digest):
-                await upstream.push_blob_streaming(
-                    name, blob_digest, storage.stream_blob(blob_digest)
-                )
-            else:
-                log.warning("Blob {} referenced by child manifest but missing locally", blob_digest[:19])
+    # Push blobs
+    pushed_blobs: set[str] = set()
+    for digest in all_blobs:
+        if digest not in pushed_blobs:
+            await upstream.push_blob_streaming(name, digest, storage.stream_blob(digest))
+            pushed_blobs.add(digest)
 
+    # Push child manifests before the index
+    for child_digest, child_body, child_ct in children:
         await upstream.push_manifest(name, child_digest, child_body, child_ct)
 
-    # Push the manifest
+    # Push the top-level manifest
     await upstream.push_manifest(name, reference, body, content_type)
 
     await queue.mark_done(marker)
-    log.info("Synced {name}:{ref} ({n} blobs)", name=name, ref=reference, n=len(digests))
+    log.info("Synced {name}:{ref} ({n} blobs)", name=name, ref=reference, n=len(pushed_blobs))
 
 
 def _extract_blob_digests(manifest_body: bytes) -> list[str]:
diff --git a/tests/test_sync_worker.py b/tests/test_sync_worker.py
index 2fa7ce6..246cd2d 100644
--- a/tests/test_sync_worker.py
+++ b/tests/test_sync_worker.py
@@ -6,12 +6,18 @@
 import json
 from pathlib import Path
 
+import pytest
 import respx
 from httpx import Response
 
 from switchyard.storage import Storage
 from switchyard.sync_queue import SyncQueue
-from switchyard.sync_worker import _extract_blob_digests, _extract_child_manifests, sync_one
+from switchyard.sync_worker import (
+    SyncMissingBlobsError,
+    _extract_blob_digests,
+    _extract_child_manifests,
+    sync_one,
+)
 from switchyard.upstream import UpstreamClient
 
 BASE = "https://central:5000"
@@ -243,3 +249,36 @@ def _record_manifest_put(request: respx.MockRequest) -> Response:
 
     remaining = await queue.list_pending()
     assert len(remaining) == 0
+
+
+@respx.mock
+async def test_sync_one_fails_when_child_manifest_blobs_missing(tmp_path: Path) -> None:
+    """Sync must fail when a child manifest references blobs not stored locally."""
+    storage = Storage(str(tmp_path))
+    await storage.init()
+    queue = SyncQueue(str(tmp_path))
+    await queue.init()
+
+    # Store a child manifest referencing a blob we DON'T store locally
+    missing_blob = "sha256:deadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeef"
+    child_body = _make_manifest([missing_blob])
+    child_ct = "application/vnd.oci.image.manifest.v1+json"
+    child_digest = f"sha256:{hashlib.sha256(child_body).hexdigest()}"
+    await storage.store_manifest("myapp", child_digest, child_body, child_ct)
+
+    # Store an image index referencing the child
+    index_body = _make_index([child_digest])
+    index_ct = "application/vnd.oci.image.index.v1+json"
+    await storage.store_manifest("myapp", "latest", index_body, index_ct)
+
+    await queue.enqueue("myapp", "latest")
+    pending = await queue.list_pending()
+
+    upstream = UpstreamClient(BASE)
+    with pytest.raises(SyncMissingBlobsError):
+        await sync_one(pending[0], storage, queue, upstream)
+    await upstream.close()
+
+    # Marker should NOT be cleared (sync failed)
+    remaining = await queue.list_pending()
+    assert len(remaining) == 1
diff --git a/uv.lock b/uv.lock
index 9539af3..ec800bb 100644
--- a/uv.lock
+++ b/uv.lock
@@ -340,7 +340,7 @@ wheels = [
 
 [[package]]
 name = "switchyard"
-version = "0.1.7"
+version = "0.1.8"
 source = { editable = "." }
 dependencies = [
     { name = "granian", extra = ["uvloop"] },