diff --git a/src/art/utils/s3.py b/src/art/utils/s3.py
index a96acf19..d9fd0262 100644
--- a/src/art/utils/s3.py
+++ b/src/art/utils/s3.py
@@ -322,11 +322,8 @@ async def archive_and_presign_step_url(
         await ensure_bucket_exists(s3_bucket)
         await s3_sync(archive_path, s3_step_path, verbose=verbose, delete=delete)
 
-    # Remove the s3:// prefix to get the key
-    s3_key = s3_step_path.removeprefix("s3://")
-
     # Generate presigned URL with 1 hour expiration
-    cmd = ["aws", "s3", "presign", s3_key, "--expires-in", "3600"]
+    cmd = ["aws", "s3", "presign", s3_step_path, "--expires-in", "3600"]
 
     process = await asyncio.create_subprocess_exec(
         *cmd, stdout=asyncio.subprocess.PIPE, stderr=asyncio.subprocess.PIPE
diff --git a/tests/unit/test_s3_utils.py b/tests/unit/test_s3_utils.py
new file mode 100644
index 00000000..a3e47fc3
--- /dev/null
+++ b/tests/unit/test_s3_utils.py
@@ -0,0 +1,140 @@
+import asyncio
+import importlib
+from pathlib import Path
+import zipfile
+
+import pytest
+
+s3_utils = importlib.import_module("art.utils.s3")
+
+
+class _FakeProcess:
+    def __init__(
+        self,
+        *,
+        returncode: int = 0,
+        stdout: bytes = b"",
+        stderr: bytes = b"",
+    ) -> None:
+        self.returncode = returncode
+        self._stdout = stdout
+        self._stderr = stderr
+
+    async def communicate(self) -> tuple[bytes, bytes]:
+        return self._stdout, self._stderr
+
+
+@pytest.mark.asyncio
+async def test_archive_and_presign_step_url_uses_full_s3_uri(
+    monkeypatch: pytest.MonkeyPatch, tmp_path: Path
+) -> None:
+    checkpoint_path = tmp_path / "checkpoint"
+    checkpoint_path.mkdir()
+    (checkpoint_path / "adapter_config.json").write_text('{"r": 8}', encoding="utf-8")
+
+    upload_calls: list[tuple[str, str, bool, bool]] = []
+    presign_calls: list[tuple[tuple[str, ...], object | None, object | None]] = []
+    bucket_calls: list[str | None] = []
+
+    async def fake_ensure_bucket_exists(
+        s3_bucket: str | None = None, profile: str | None = None
+    ) -> None:
+        assert profile is None
+        bucket_calls.append(s3_bucket)
+
+    async def fake_s3_sync(
+        source: str,
+        destination: str,
+        *,
+        profile: str | None = None,
+        verbose: bool = False,
+        delete: bool = False,
+        exclude: list[s3_utils.ExcludableOption] | None = None,
+    ) -> None:
+        assert profile is None
+        assert exclude is None
+        with zipfile.ZipFile(source) as archive:
+            assert archive.namelist() == ["adapter_config.json"]
+        upload_calls.append((source, destination, verbose, delete))
+
+    async def fake_create_subprocess_exec(*cmd: str, stdout=None, stderr=None):
+        presign_calls.append((cmd, stdout, stderr))
+        return _FakeProcess(stdout=b"https://signed.example.com/model.zip\n")
+
+    monkeypatch.setattr(s3_utils, "ensure_bucket_exists", fake_ensure_bucket_exists)
+    monkeypatch.setattr(s3_utils, "s3_sync", fake_s3_sync)
+    monkeypatch.setattr(
+        s3_utils.asyncio, "create_subprocess_exec", fake_create_subprocess_exec
+    )
+
+    presigned_url = await s3_utils.archive_and_presign_step_url(
+        model_name="demo-model",
+        project="demo-project",
+        step=7,
+        s3_bucket="demo-bucket",
+        prefix="exports",
+        checkpoint_path=str(checkpoint_path),
+    )
+
+    expected_s3_uri = s3_utils.build_s3_zipped_step_path(
+        model_name="demo-model",
+        project="demo-project",
+        step=7,
+        s3_bucket="demo-bucket",
+        prefix="exports",
+    )
+
+    assert presigned_url == "https://signed.example.com/model.zip"
+    assert bucket_calls == ["demo-bucket"]
+    assert len(upload_calls) == 1
+    assert upload_calls[0][1:] == (expected_s3_uri, False, False)
+    assert presign_calls == [
+        (
+            ("aws", "s3", "presign", expected_s3_uri, "--expires-in", "3600"),
+            asyncio.subprocess.PIPE,
+            asyncio.subprocess.PIPE,
+        )
+    ]
+
+
+@pytest.mark.asyncio
+async def test_archive_and_presign_step_url_surfaces_presign_failures(
+    monkeypatch: pytest.MonkeyPatch, tmp_path: Path
+) -> None:
+    checkpoint_path = tmp_path / "checkpoint"
+    checkpoint_path.mkdir()
+    (checkpoint_path / "adapter_model.bin").write_text("weights", encoding="utf-8")
+
+    async def fake_ensure_bucket_exists(
+        s3_bucket: str | None = None, profile: str | None = None
+    ) -> None:
+        return None
+
+    async def fake_s3_sync(
+        source: str,
+        destination: str,
+        *,
+        profile: str | None = None,
+        verbose: bool = False,
+        delete: bool = False,
+        exclude: list[s3_utils.ExcludableOption] | None = None,
+    ) -> None:
+        return None
+
+    async def fake_create_subprocess_exec(*cmd: str, stdout=None, stderr=None):
+        return _FakeProcess(returncode=1, stderr=b"invalid S3 URI")
+
+    monkeypatch.setattr(s3_utils, "ensure_bucket_exists", fake_ensure_bucket_exists)
+    monkeypatch.setattr(s3_utils, "s3_sync", fake_s3_sync)
+    monkeypatch.setattr(
+        s3_utils.asyncio, "create_subprocess_exec", fake_create_subprocess_exec
+    )
+
+    with pytest.raises(RuntimeError, match="invalid S3 URI"):
+        await s3_utils.archive_and_presign_step_url(
+            model_name="demo-model",
+            project="demo-project",
+            step=3,
+            s3_bucket="demo-bucket",
+            checkpoint_path=str(checkpoint_path),
+        )