diff --git a/benchmarks/cpbench/writers/__init__.py b/benchmarks/cpbench/writers/__init__.py
new file mode 100644
index 000000000..2152fb990
--- /dev/null
+++ b/benchmarks/cpbench/writers/__init__.py
@@ -0,0 +1,8 @@
+from .parse_json import flatten, load_json
+from .sheets_writer import SheetsWriter
+
+__all__ = [
+    "SheetsWriter",
+    "flatten",
+    "load_json",
+]
diff --git a/benchmarks/cpbench/writers/cli.py b/benchmarks/cpbench/writers/cli.py
new file mode 100644
index 000000000..e5f5bf50c
--- /dev/null
+++ b/benchmarks/cpbench/writers/cli.py
@@ -0,0 +1,246 @@
+"""Append a CPBench result JSON file to a Google Sheet, or preview what would be written.
+
+The aggregated and per-seed blocks go to separate worksheets. Rows are appended, so
+re-running a config adds rows rather than replacing them.
+"""
+
+from __future__ import annotations
+
+import argparse
+import sys
+from typing import Any
+
+from .parse_json import flatten, load_json
+from .sheets_writer import SheetsWriter
+
+_AGGREGATED_WORKSHEET = "Aggregated Worksheet"
+_PER_SEED_WORKSHEET = "Per-seed Results"
+
+
+def _format_value(value: Any) -> str:
+    if value is None:
+        return ""
+    if isinstance(value, dict):
+        return ", ".join(f"{k}={_format_value(v)}" for k, v in value.items())
+    if isinstance(value, (list, tuple)):
+        return "[" + ", ".join(_format_value(v) for v in value) + "]"
+    return str(value)
+
+
+def _render_table(header: list[Any], rows: list[list[Any]]) -> str:
+    columns = [_format_value(column) for column in header]
+    cells = [[_format_value(value) for value in row] for row in rows]
+
+    widths = [len(column) for column in columns]
+    for row in cells:
+        for index, cell in enumerate(row):
+            if index < len(widths):
+                widths[index] = max(widths[index], len(cell))
+
+    def line(values: list[str]) -> str:
+        padded = [value.ljust(widths[i]) for i, value in enumerate(values[: len(widths)])]
+        return "  ".join(padded).rstrip()
+
+    body = [line(columns)] + [line(row) for row in cells]
+    rule = "-" * max(len(text) for text in body)
+
+    return "\n".join([body[0], rule] + body[1:])
+
+
+def _metadata_rows(
+    header: list[Any], values: list[Any], width: int = 70
+) -> list[list[str]]:
+    import textwrap
+
+    rows = []
+    for field, value in zip(header, values):
+        chunks = textwrap.wrap(_format_value(value), width=width) or [""]
+        rows.append([_format_value(field), chunks[0]])
+        rows.extend([["", chunk] for chunk in chunks[1:]])
+
+    return rows
+
+
+def _render_section(title: str, body: str) -> str:
+    rule = "=" * max(len(title), 40)
+
+    return f"\n{rule}\n{title}\n{rule}\n{body}"
+
+
+def _clip(text: str, width: int) -> str:
+    return text if len(text) <= width else text[: max(width - 3, 1)] + "..."
+
+
+def _target_block(
+    worksheet_name: str,
+    header: list[Any],
+    rows: list[list[Any]],
+    sample: int,
+    width: int,
+) -> str:
+    columns = [_format_value(column) for column in header]
+    lines = [
+        f"worksheet  {worksheet_name}",
+        _clip(f"columns    {len(columns)}  ({', '.join(columns)})", width),
+        f"rows       {len(rows)}",
+    ]
+
+    if not rows:
+        lines.append("")
+        lines.append("  nothing to append")
+
+        return "\n".join(lines)
+
+    preview = _render_table(header, rows[:sample]).splitlines()
+    lines.append("")
+    lines.extend("  " + _clip(line, width - 2) for line in preview)
+
+    remaining = len(rows) - sample
+    if remaining > 0:
+        lines.append(f"  ... {remaining} more row{'' if remaining == 1 else 's'}")
+
+    return "\n".join(lines)
+
+
+def print_write_preview(
+    data: dict[str, tuple[list[Any], list[Any]]],
+    worksheets: dict[str, str] | None = None,
+    sample: int = 3,
+    width: int | None = None,
+) -> None:
+    if width is None:
+        import shutil
+
+        width = shutil.get_terminal_size((100, 24)).columns
+
+    worksheets = worksheets or {}
+
+    metadata_header, metadata_values = data.get("metadata", ([], []))
+    metadata_table = (
+        _render_table(["Field", "Value"], _metadata_rows(metadata_header, metadata_values))
+        if metadata_header
+        else "(no data)"
+    )
+    print(_render_section("METADATA", metadata_table))
+
+    for section in ("aggregated", "per_seed"):
+        header, rows = data.get(section, ([], []))
+        worksheet_name = worksheets.get(section) or section
+        print(
+            _render_section(
+                f"WRITE PREVIEW: {section}",
+                _target_block(worksheet_name, header, rows, sample, width),
+            )
+        )
+
+
+def _sheet_value(value: Any) -> Any:
+    """Keep scalars as they are; flatten anything a cell cannot hold to text."""
+    if value is None or isinstance(value, (str, int, float, bool)):
+        return value
+
+    return _format_value(value)
+
+
+def _with_metadata(
+    metadata: tuple[list[Any], list[Any]],
+    header: list[Any],
+    rows: list[list[Any]],
+) -> tuple[list[Any], list[list[Any]]]:
+    """Prefix every row with the run's metadata so each sheet row stands alone."""
+    metadata_header, metadata_values = metadata
+    prefix = [_sheet_value(value) for value in metadata_values]
+
+    return list(metadata_header) + list(header), [prefix + row for row in rows]
+
+
+def parse_args(argv: list[str] | None = None) -> argparse.Namespace:
+    parser = argparse.ArgumentParser(
+        prog="python -m benchmarks.cpbench.writers.cli",
+        description=__doc__,
+        formatter_class=argparse.RawDescriptionHelpFormatter,
+    )
+    parser.add_argument("--results", required=True, metavar="PATH", help="Result JSON.")
+    parser.add_argument(
+        "--sheet-id",
+        metavar="ID",
+        help="Spreadsheet id. Required unless --preview.",
+    )
+    parser.add_argument(
+        "--per-seed-worksheet",
+        default=_PER_SEED_WORKSHEET,
+        metavar="NAME",
+        help=f'Per-seed worksheet (default: "{_PER_SEED_WORKSHEET}").',
+    )
+    parser.add_argument(
+        "--aggregated-worksheet",
+        default=_AGGREGATED_WORKSHEET,
+        metavar="NAME",
+        help=f'Aggregated worksheet (default: "{_AGGREGATED_WORKSHEET}").',
+    )
+    parser.add_argument(
+        "--credentials",
+        metavar="PATH",
+        help="Service account JSON. Required unless --preview.",
+    )
+    parser.add_argument(
+        "--preview",
+        action="store_true",
+        help="Print what would be written; write nothing.",
+    )
+
+    args = parser.parse_args(argv)
+
+    if not args.preview:
+        missing = [
+            flag
+            for flag, value in (
+                ("--sheet-id", args.sheet_id),
+                ("--credentials", args.credentials),
+            )
+            if not value
+        ]
+        if missing:
+            parser.error(f"{', '.join(missing)} required unless --preview is set.")
+
+    return args
+
+
+def main(argv: list[str] | None = None) -> int:
+    args = parse_args(argv)
+
+    try:
+        data = flatten(load_json(args.results))
+    except ValueError as exc:
+        print(exc, file=sys.stderr)
+        return 1
+
+    worksheets = {
+        "aggregated": args.aggregated_worksheet,
+        "per_seed": args.per_seed_worksheet,
+    }
+
+    if args.preview:
+        print_write_preview(data, worksheets)
+        print("\n(preview only, nothing written)", file=sys.stderr)
+        return 0
+
+    writer = SheetsWriter(args.sheet_id, args.credentials)
+    metadata = data.get("metadata", ([], []))
+    total = 0
+    for section, worksheet_name in worksheets.items():
+        header, rows = data.get(section, ([], []))
+        columns, rows = _with_metadata(metadata, header, rows)
+        appended = writer.append(worksheet_name, rows, columns)
+        print(f"{appended} rows appended to {worksheet_name}")
+        total += appended
+
+    if not total:
+        print("No rows found in the given file.", file=sys.stderr)
+        return 1
+
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())
diff --git a/benchmarks/cpbench/writers/parse_json.py b/benchmarks/cpbench/writers/parse_json.py
new file mode 100644
index 000000000..848a2c587
--- /dev/null
+++ b/benchmarks/cpbench/writers/parse_json.py
@@ -0,0 +1,71 @@
+import json
+from typing import Any
+
+def load_json(path: str) -> list[Any] | dict:
+    try:
+        with open(path, "r", encoding="utf-8") as f:
+            return json.load(f)
+    except FileNotFoundError:
+        raise ValueError(f"File not found: {path}")
+    except json.JSONDecodeError as e:
+        raise ValueError(f"Invalid JSON: {e}") from e
+    except OSError as e:
+        raise ValueError(f"Could not read {path}: {e}") from e
+
+
+def flatten(data: dict[Any, Any]) -> dict[str, tuple[list[Any], list[list[Any]]]]:
+    aggregated = data.pop("aggregated")
+    per_seed = data.pop("per_seed")
+
+    aggregated_header, aggregated_rows = _flatten_rows(aggregated)
+    per_seed_header, per_seed_rows = _flatten_per_seed(per_seed)
+
+    metadata_header, metadata_values = _flatten_metadata(data)
+
+    return {
+        "aggregated" : (aggregated_header, aggregated_rows),
+        "per_seed" : (per_seed_header, per_seed_rows),
+        "metadata" : (metadata_header, metadata_values)
+            }
+
+
+def _flatten_metadata(metadata: dict):
+    header = list(metadata.keys())
+    values = list(metadata.values())
+
+    return header, values
+
+
+def _flatten_rows(rows: list[dict]):
+    if not rows:
+        return [], []
+
+    header = list(rows[0].keys())
+    flattened_rows = []
+
+    for row in rows:
+        flattened_rows.append(list(row.values()))
+
+    return header, flattened_rows
+
+
+def _flatten_per_seed(seed_to_rows: dict[Any, list[dict]]):
+    header, aggregated_flattened_rows = [], []
+
+    for seed, rows in seed_to_rows.items():
+        seed_header, flattened_rows = _flatten_rows(rows)
+
+        if not header:
+            header = seed_header
+
+        aggregated_flattened_rows.extend(flattened_rows)
+
+    return header, aggregated_flattened_rows
+
+
+if __name__ == "__main__":
+    from pprint import pprint
+
+    data = load_json("benchmarks/cpbench/writers/los_dev_4alpha.json")
+    flattened_data = flatten(data)
+    pprint(flattened_data)
\ No newline at end of file
diff --git a/benchmarks/cpbench/writers/sheets_writer.py b/benchmarks/cpbench/writers/sheets_writer.py
new file mode 100644
index 000000000..781d02d2e
--- /dev/null
+++ b/benchmarks/cpbench/writers/sheets_writer.py
@@ -0,0 +1,73 @@
+from __future__ import annotations
+
+from typing import Any
+
+
+class SheetsWriter:
+    def __init__(
+        self,
+        spreadsheet_id: str,
+        credentials_path: str,
+    ) -> None:
+        self.spreadsheet_id = spreadsheet_id
+        self.credentials_path = credentials_path
+
+
+    def _open_worksheet(self, worksheet_name, columns: int = 26) -> Any:
+        try:
+            import gspread
+        except ImportError as exc:
+            raise ImportError(
+                "gspread is required to write to Sheets. Install it with "
+                "'pip install gspread', or use --dry-run to preview rows."
+            ) from exc
+
+        client = gspread.service_account(filename=self.credentials_path)
+        spreadsheet = client.open_by_key(self.spreadsheet_id)
+        try:
+            return spreadsheet.worksheet(worksheet_name)
+        except gspread.WorksheetNotFound:
+            return spreadsheet.add_worksheet(
+                title=worksheet_name, rows=1, cols=max(columns, 26)
+            )
+
+
+    def append(self, worksheet_name, rows: list[list[Any]], columns: list[str]) -> int:
+        if not rows:
+            return 0
+
+        duplicates = {column for column in columns if columns.count(column) > 1}
+        if duplicates:
+            raise ValueError(
+                f"Duplicate columns would collapse into one: {sorted(duplicates)}"
+            )
+
+        worksheet = self._open_worksheet(worksheet_name, len(columns))
+        header = worksheet.row_values(1)
+        merged = header + [column for column in columns if column not in header]
+        # A sheet that predates these columns may be too narrow to hold the header.
+        if len(merged) > worksheet.col_count:
+            worksheet.resize(cols=len(merged))
+        if merged != header:
+            worksheet.update([merged], "A1")
+
+        worksheet.append_rows(
+            [self._align(row, columns, merged) for row in rows],
+            value_input_option="USER_ENTERED",
+        )
+
+        return len(rows)
+
+
+    @staticmethod
+    def _align(row: list[Any], columns: list[str], merged: list[str]) -> list[Any]:
+        """Reorder one row to the worksheet's header, blanking columns it lacks.
+        """
+        if len(row) > len(columns):
+            raise ValueError(
+                f"Row has {len(row)} values but only {len(columns)} columns are named"
+            )
+
+        by_column = dict(zip(columns, row))
+
+        return [by_column.get(column, "") for column in merged]