diff --git a/tests/unit/infrastructure/adapters/common/test_doi_helpers.py b/tests/unit/infrastructure/adapters/common/test_doi_helpers.py
new file mode 100644
index 0000000000..4ccf163473
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+++ b/tests/unit/infrastructure/adapters/common/test_doi_helpers.py
@@ -0,0 +1,43 @@
+"""Unit tests for DOI transport helpers."""
+
+import pytest
+
+from bioetl.infrastructure.adapters.common.doi_helpers import strip_doi_transport_prefix
+
+
+@pytest.mark.parametrize(
+    ("input_doi", "expected_output"),
+    [
+        # Bare DOIs (no change expected)
+        ("10.1038/s41586-020-2649-2", "10.1038/s41586-020-2649-2"),
+        ("10.1016/j.cell.2021.02.021", "10.1016/j.cell.2021.02.021"),
+        # HTTP URL formats
+        ("http://doi.org/10.1038/s41586-020-2649-2", "10.1038/s41586-020-2649-2"),
+        ("https://doi.org/10.1038/s41586-020-2649-2", "10.1038/s41586-020-2649-2"),
+        ("http://dx.doi.org/10.1038/s41586-020-2649-2", "10.1038/s41586-020-2649-2"),
+        ("https://dx.doi.org/10.1038/s41586-020-2649-2", "10.1038/s41586-020-2649-2"),
+        # DOI: prefix (case insensitive according to implementation)
+        ("doi:10.1038/s41586-020-2649-2", "10.1038/s41586-020-2649-2"),
+        ("DOI:10.1038/s41586-020-2649-2", "10.1038/s41586-020-2649-2"),
+        ("DoI:10.1038/s41586-020-2649-2", "10.1038/s41586-020-2649-2"),
+        # With whitespace
+        ("  10.1038/s41586-020-2649-2  ", "10.1038/s41586-020-2649-2"),
+        ("  doi:10.1038/s41586-020-2649-2  ", "10.1038/s41586-020-2649-2"),
+    ],
+)
+def test_strip_doi_transport_prefix(input_doi: str, expected_output: str) -> None:
+    """Test that strip_doi_transport_prefix correctly removes prefixes."""
+    assert strip_doi_transport_prefix(input_doi) == expected_output
+
+
+def test_strip_doi_transport_prefix_allow_uppercase_prefix_ignored() -> None:
+    """Test that the allow_uppercase_prefix argument is safely ignored."""
+    # It should strip "DOI:" even if allow_uppercase_prefix=False
+    assert (
+        strip_doi_transport_prefix("DOI:10.123/abc", allow_uppercase_prefix=False)
+        == "10.123/abc"
+    )
+    assert (
+        strip_doi_transport_prefix("DOI:10.123/abc", allow_uppercase_prefix=True)
+        == "10.123/abc"
+    )