nautilus-trader-catalog-instrument-precision

name	nautilus-trader-catalog-instrument-precision
description	Fix for "RuntimeError: invalid bar.volume.precision=N did not match instrument.size_precision=M" or similar bar/instrument precision-mismatch errors at BacktestEngine.run() time. Use when: (1) bars come from a ParquetDataCatalog and the instrument is loaded from somewhere else (TestInstrumentProvider, hand-built, hard-coded), (2) eval works but paper fails (or vice versa) on the same data, (3) you've seeded a catalog with one tool and are running the engine with an instrument from another. The fix is to load instruments FROM the catalog so their precisions match the bars stored alongside them.
author	Claude Code
version	1.0.0
date	"2026-05-07T00:00:00.000Z"

nautilus_trader catalog ↔ instrument precision mismatch

Problem

nautilus_trader.persistence.catalog.ParquetDataCatalog stores bars and instruments together. A Bar's volume is a Quantity with a precision field, and the instrument has a size_precision field. The engine's OrderMatchingEngine.process_bar validates that the two match — and aborts the run if they don't:

RuntimeError: invalid bar.volume.precision=5 did not match instrument.size_precision=6

This bites when you load bars from a catalog but use a different source for the instrument — typically TestInstrumentProvider.btcusdt_binance() (size_precision=6) over bars seeded by a script that derived its precision from a different instrument (size_precision=5).

The two ends look identical at the symbol level (BTCUSDT.BINANCE in both cases), but they're literally different objects with different precision metadata. The engine catches this only at run(), not at add_instrument or add_data — so unit tests that don't actually call engine.run() won't catch it.

Trigger conditions

Stack trace at engine.run() includes:

RuntimeError: invalid bar.volume.precision=N did not match instrument.size_precision=M
  in nautilus_trader.backtest.engine.OrderMatchingEngine.process_bar:4645

Common N/M pairs we've seen:

• bars=5, instrument=6 (TestInstrumentProvider's BTCUSDT vs catalog-seeded BTCUSDT)

The error is only at run-time, not at engine setup. Tests that monkeypatch BacktestEngine.run won't catch it.

Solution

Always load instruments from the same source as the bars. If bars come from a catalog, load instruments from that same catalog:

from nautilus_trader.persistence.catalog import ParquetDataCatalog

catalog = ParquetDataCatalog(str(catalog_path))
bars = catalog.bars(
    bar_types=[bar_type_str],
    start=window_start,
    end=window_end,
)

# Load instruments FROM the catalog. Their precision matches the bars.
instruments = list(catalog.instruments())
if not instruments:
    raise RuntimeError(f"No instruments in catalog: {catalog_path}")
for instrument in instruments:
    engine.add_instrument(instrument)

engine.add_data(bars)

Do not mix sources:

# WRONG — bars from catalog, instrument from TestInstrumentProvider
from nautilus_trader.test_kit.providers import TestInstrumentProvider
engine.add_instrument(TestInstrumentProvider.btcusdt_binance())  # precision likely differs
engine.add_data(catalog.bars(...))   # precision encoded by the seed script

If your test/demo path needs to seed a catalog from scratch, derive every precision from the same instrument:

# In your seed script:
instrument = _build_my_instrument()    # one canonical instrument
catalog.write_data([instrument])       # write it BEFORE bars

price_prec = instrument.price_precision
size_prec  = instrument.size_precision  # use these for every Bar/Quantity below

bar = Bar(
    bar_type=bar_type,
    open=Price(..., precision=price_prec),
    high=Price(..., precision=price_prec),
    low=Price(..., precision=price_prec),
    close=Price(..., precision=price_prec),
    volume=Quantity(..., precision=size_prec),  # MUST match instrument.size_precision
    ...
)
catalog.write_data([bar, ...])

Verification

1. Read instruments back from the catalog and confirm:

catalog = ParquetDataCatalog(str(catalog_path))
for inst in catalog.instruments():
    print(f"{inst.id}: size_precision={inst.size_precision}, price_precision={inst.price_precision}")

2. Read one bar and confirm:

bars = catalog.bars(bar_types=[bar_type_str])
b = bars[0]
print(f"volume.precision={b.volume.precision}")
# Must equal the instrument's size_precision above.

3. Run a tiny engine.run() end-to-end. If it completes without RuntimeError, the precisions match.

Notes

• Quantity and Price carry their precision in their type system — it's not just a display concern. The engine's order-matching code uses it to reject inconsistent input.
• TestInstrumentProvider returns instruments with fixed, hardcoded precisions matched to nautilus_trader's own test fixtures. They're great for tests that ALSO use nautilus_trader's test bar fixtures, but they're the wrong choice when bars come from a user-seeded catalog.
• Different exchanges (Binance vs Coinbase vs Kraken) have different conventions for size_precision of "the same" symbol. Don't assume.
• This bug does not show up in unit tests that monkeypatch run() or use canned EvalResult/PaperResult fixtures. Only the e2e or full-run path catches it. Add an e2e gate that exercises engine.run() against real seeded data.

Example — both eval and paper phases load from catalog

# nautilus_competition/eval_phase.py and paper_phase.py — same pattern
def _build_engine(...):
    catalog = ParquetDataCatalog(str(catalog_path))
    bars = catalog.bars(bar_types=[bar_type], start=..., end=...)

    engine = BacktestEngine(config=...)
    engine.add_venue(venue=Venue("BINANCE"), oms_type=..., account_type=...,
                     starting_balances=[Money.from_str("1000 USDT")])

    # Load instruments from catalog — same source as bars; precisions agree.
    for instrument in catalog.instruments():
        engine.add_instrument(instrument)

    engine.add_data(bars)
    return engine

References

• nautilus_trader.model.objects.Quantity.precision — type carries precision; the engine validates compatibility.
• OrderMatchingEngine.process_bar (backtest/engine.cpython-*.so:4645): the validation site that raises the RuntimeError.
• ParquetDataCatalog: stores instruments alongside bars under the same root, retrievable via catalog.instruments() and catalog.bars(...).
• Related: TestInstrumentProvider is a good source for nautilus_trader's own test data, NOT for user-seeded catalogs.