From 2bf1dd47b496a927963ca250a72ba1940ac93841 Mon Sep 17 00:00:00 2001
From: chillaranand <anand21nanda@gmail.com>
Date: Wed, 7 Jan 2026 23:49:24 +0530
Subject: [PATCH] fix(serialize): include protocol prefix for external paths

---
 dvc/repo/experiments/serialize.py             | 10 ++++++--
 tests/unit/repo/experiments/test_serialize.py | 25 +++++++++++++++++++
 2 files changed, 33 insertions(+), 2 deletions(-)
 create mode 100644 tests/unit/repo/experiments/test_serialize.py

diff --git a/dvc/repo/experiments/serialize.py b/dvc/repo/experiments/serialize.py
index 6c9921d4f4..aff5ae3534 100644
--- a/dvc/repo/experiments/serialize.py
+++ b/dvc/repo/experiments/serialize.py
@@ -62,12 +62,18 @@ def from_repo(
 
         params = _gather_params(repo, deps_only=param_deps, on_error="return")
         metrics = _gather_metrics(repo, on_error="return")
+
+        def _get_path(item) -> str:
+            if item.is_in_repo:
+                return relpath(item.fs_path, repo.root_dir)
+            return item.fs.unstrip_protocol(item.fs_path)
+
         return cls(
             rev=rev,
             params=params,
             metrics=metrics,
             deps={
-                relpath(dep.fs_path, repo.root_dir): ExpDep(
+                _get_path(dep): ExpDep(
                     hash=dep.hash_info.value if dep.hash_info else None,
                     size=dep.meta.size if dep.meta else None,
                     nfiles=dep.meta.nfiles if dep.meta else None,
@@ -79,7 +85,7 @@ def from_repo(
                 )
             },
             outs={
-                relpath(out.fs_path, repo.root_dir): ExpOut(
+                _get_path(out): ExpOut(
                     hash=out.hash_info.value if out.hash_info else None,
                     size=out.meta.size if out.meta else None,
                     nfiles=out.meta.nfiles if out.meta else None,
diff --git a/tests/unit/repo/experiments/test_serialize.py b/tests/unit/repo/experiments/test_serialize.py
new file mode 100644
index 0000000000..d977c96e3c
--- /dev/null
+++ b/tests/unit/repo/experiments/test_serialize.py
@@ -0,0 +1,25 @@
+from dvc.repo.experiments.serialize import SerializableExp
+
+
+def test_serialize_external_paths_include_protocol(dvc, mocker):
+    """External paths should include protocol prefix (e.g. s3://)."""
+    mock_dep = mocker.MagicMock(fs_path="bucket/data.csv", is_in_repo=False)
+    mock_dep.fs.unstrip_protocol.return_value = "s3://bucket/data.csv"
+
+    mock_out = mocker.MagicMock(
+        fs_path="bucket/model.pkl", is_in_repo=False, is_metric=False, is_plot=False
+    )
+    mock_out.fs.unstrip_protocol.return_value = "s3://bucket/model.pkl"
+
+    mocker.patch.object(
+        type(dvc.index), "deps", mocker.PropertyMock(return_value=[mock_dep])
+    )
+    mocker.patch.object(
+        type(dvc.index), "outs", mocker.PropertyMock(return_value=[mock_out])
+    )
+    mocker.patch.object(dvc, "get_rev", return_value="abc123")
+
+    result = SerializableExp.from_repo(dvc)
+
+    assert "s3://bucket/data.csv" in result.deps
+    assert "s3://bucket/model.pkl" in result.outs