diff --git a/tests/test_edge_sync_s3_store.py b/tests/test_edge_sync_s3_store.py
new file mode 100644
index 00000000..2999121d
--- /dev/null
+++ b/tests/test_edge_sync_s3_store.py
@@ -0,0 +1,79 @@
+from __future__ import annotations
+
+import io
+import sys
+
+import pytest
+
+from core.sync.object_store import ObjectNotFoundError, ObjectStoreError
+from core.sync.s3_store import S3ObjectStore
+
+
+class FakeS3Client:
+    def __init__(self) -> None:
+        self.objects: dict[tuple[str, str], tuple[bytes, str]] = {}
+
+    def put_object(self, *, Bucket: str, Key: str, Body: bytes, ContentType: str) -> dict:
+        self.objects[(Bucket, Key)] = (Body, ContentType)
+        return {"ETag": '"fake-etag"'}
+
+    def get_object(self, *, Bucket: str, Key: str) -> dict:
+        try:
+            data, _ = self.objects[(Bucket, Key)]
+        except KeyError as exc:
+            raise FakeNoSuchKey("not found") from exc
+        return {"Body": io.BytesIO(data)}
+
+    def head_object(self, *, Bucket: str, Key: str) -> dict:
+        if (Bucket, Key) not in self.objects:
+            raise FakeNoSuchKey("not found")
+        return {}
+
+
+class FakeNoSuchKey(Exception):
+    response = {"Error": {"Code": "NoSuchKey"}}
+
+
+def test_s3_adapter_uses_injected_client_without_boto3() -> None:
+    client = FakeS3Client()
+    store = S3ObjectStore(bucket="bucket", client=client)
+
+    meta = store.put_bytes("key", b"payload", content_type="application/octet-stream")
+
+    assert meta.key == "key"
+    assert meta.size_bytes == len(b"payload")
+    assert meta.content_type == "application/octet-stream"
+    assert meta.etag == "fake-etag"
+    assert store.exists("key")
+    assert store.get_bytes("key") == b"payload"
+
+
+def test_s3_adapter_reports_missing_objects() -> None:
+    store = S3ObjectStore(bucket="bucket", client=FakeS3Client())
+
+    assert not store.exists("missing")
+    with pytest.raises(ObjectNotFoundError):
+        store.get_bytes("missing")
+
+
+def test_s3_adapter_requires_bucket() -> None:
+    with pytest.raises(ValueError, match="bucket is required"):
+        S3ObjectStore(bucket="", client=FakeS3Client())
+
+
+def test_s3_adapter_requires_key() -> None:
+    store = S3ObjectStore(bucket="bucket", client=FakeS3Client())
+
+    with pytest.raises(ValueError, match="key is required"):
+        store.put_bytes("", b"payload", content_type="application/octet-stream")
+    with pytest.raises(ValueError, match="key is required"):
+        store.get_bytes("")
+    with pytest.raises(ValueError, match="key is required"):
+        store.exists("")
+
+
+def test_s3_adapter_lazy_imports_boto3_only_when_constructed_without_client(monkeypatch) -> None:
+    monkeypatch.setitem(sys.modules, "boto3", None)
+
+    with pytest.raises(ObjectStoreError, match="boto3 is not installed"):
+        S3ObjectStore(bucket="bucket")