diff --git a/.gitignore b/.gitignore
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+++ b/.gitignore
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+__pycache__
diff --git a/test_input_file_consistency.py b/test_input_file_consistency.py
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+++ b/test_input_file_consistency.py
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+# SPDX-FileCopyrightText: 2024-present Proxima Fusion GmbH <info@proximafusion.com>
+#
+# SPDX-License-Identifier: MIT
+"""Tests for consistency between main and large_cpp_tests input files.
+
+The large_cpp_tests test_data contains extended versions of the main repo's
+input files, with additional dump_* diagnostic flags. After conversion via
+indata_to_json (which strips non-standard fields), the JSON output must be
+identical.
+
+Run via: pytest src/vmecpp/cpp/vmecpp_large_cpp_tests/test_input_file_consistency.py -v
+"""
+
+import json
+import tempfile
+from pathlib import Path
+
+import pytest
+
+from vmecpp import _util
+
+REPO_ROOT = Path(__file__).parent.parent.parent.parent.parent
+MAIN_TEST_DATA = REPO_ROOT / "src" / "vmecpp" / "cpp" / "vmecpp" / "test_data"
+LARGE_TEST_DATA = (
+    REPO_ROOT / "src" / "vmecpp" / "cpp" / "vmecpp_large_cpp_tests" / "test_data"
+)
+
+
+def _get_input_files(directory: Path) -> set[str]:
+    """Return the set of input.* filenames in the given directory."""
+    return {p.name for p in directory.glob("input.*")}
+
+
+def test_no_extra_input_files_in_large_tests():
+    """Every large tests input file must also exist in the main repo.
+
+    The main repo may contain additional input files that are not (yet) in
+    the large tests repo, but the reverse is not allowed.
+    """
+    main_files = _get_input_files(MAIN_TEST_DATA)
+    large_files = _get_input_files(LARGE_TEST_DATA)
+
+    extra = large_files - main_files
+    assert not extra, (
+        f"Large tests input files not present in main repo: {sorted(extra)}. "
+        f"Add them to vmecpp/test_data/ or remove them from "
+        f"vmecpp_large_cpp_tests/test_data/."
+    )
+
+
+def _discover_large_test_input_files() -> list[str]:
+    """Discover input files in the large tests directory."""
+    return sorted(_get_input_files(LARGE_TEST_DATA))
+
+
+@pytest.mark.parametrize("input_file", _discover_large_test_input_files())
+def test_input_files_produce_identical_json(input_file: str):
+    """Input files in both repos must produce identical JSON after conversion."""
+    with tempfile.TemporaryDirectory() as tmpdir:
+        main_json_path = _util.indata_to_json(
+            MAIN_TEST_DATA / input_file,
+            output_override=Path(tmpdir) / "main.json",
+        )
+        large_json_path = _util.indata_to_json(
+            LARGE_TEST_DATA / input_file,
+            output_override=Path(tmpdir) / "large.json",
+        )
+
+        main_data = json.loads(main_json_path.read_text())
+        large_data = json.loads(large_json_path.read_text())
+
+    assert main_data == large_data, (
+        f"JSON mismatch for {input_file}. "
+        f"Keys only in main: {set(main_data) - set(large_data)}, "
+        f"Keys only in large: {set(large_data) - set(main_data)}"
+    )