msi_to_osi silently drops offset_window / offset_to_grain on DERIVED metric inputs, producing constant-zero expressions

Start in converters/dbt/src/ossie_dbt/msi_to_osi.py, especially _resolve_derived and the existing CUMULATIVE_SEMANTICS_LOSS handling. Reproduce the year-over-year case from converters/dbt/ and add coverage in tests/test_msi_to_osi.py for an input with offset_window or offset_to_grain. Done means the lost offset is reported as a ConverterIssue and the test verifies the issue.

MSIToOSIConverter._resolve_derived ignores offset_window and offset_to_grain on DERIVED metric inputs. The offset is dropped silently — no ConverterIssue is emitted — so a period-over-period metric converts into an expression that is algebraically constant, and looks like a successful conversion.

Root cause

converters/dbt/src/ossie_dbt/msi_to_osi.py:337-351, _resolve_derived:

expr = metric.type_params.expr or ""
for input_metric in metric.type_params.metrics or []:
    ref = input_metric.alias if input_metric.alias else input_metric.name
    dep_metric = self._lookup_metric(metric_index, input_metric.name, f"DERIVED metric '{metric.name}'")
    input_filter = _merge_filter_sqls(filter_sql, _collect_filter_sql(input_metric.filter))
    resolved = self._resolve_metric_expression(dep_metric, metric_index, cache, input_filter)
    if dep_metric.type in (MetricType.DERIVED, MetricType.RATIO):
        resolved = f"({resolved})"
    expr = re.sub(rf"\b{re.escape(ref)}\b", resolved, expr)

input_metric.alias, .name and .filter are all read; input_metric.offset_window and input_metric.offset_to_grain are not read anywhere in the file. Two inputs referencing the same metric — one current, one offset — therefore resolve to byte-identical SQL, and the difference between them collapses to zero.

The year-over-year shape this breaks is the canonical use of offset_window, and the one in the MetricFlow docs.

Repro (HEAD 88e0011148283302c9a04cd0287e00e0b9d87354), using the repo's own test helpers:

import sys
sys.path.insert(0, "src"); sys.path.insert(0, "."); sys.path.insert(0, "../../python/src")

from ossie_dbt.msi_to_osi import MSIToOSIConverter
from tests.helpers import _manifest, _simple_metric
from metricflow_semantic_interfaces.implementations.elements.measure import PydanticMeasure
from metricflow_semantic_interfaces.implementations.metric import (
    PydanticMetric, PydanticMetricInput, PydanticMetricTimeWindow, PydanticMetricTypeParams,
)
from metricflow_semantic_interfaces.test_utils import default_meta, semantic_model_with_guaranteed_meta
from metricflow_semantic_interfaces.type_enums import AggregationType, MetricType

sm = semantic_model_with_guaranteed_meta(
    name="bookings",
    measures=[PydanticMeasure(name="booking_count", agg=AggregationType.SUM, expr="1")],
)
base = _simple_metric("booking_count", "booking_count")

yoy = PydanticMetric(
    name="bookings_yoy_growth",
    description=None,
    type=MetricType.DERIVED,
    type_params=PydanticMetricTypeParams(
        expr="(this_year - last_year) / NULLIF(last_year, 0) * 100",
        metrics=[
            PydanticMetricInput(name="booking_count", alias="this_year"),
            PydanticMetricInput(name="booking_count", alias="last_year",
                                offset_window=PydanticMetricTimeWindow(count=1, granularity="year")),
        ],
    ),
    filter=None, metadata=default_meta(), config=None,
)

res = MSIToOSIConverter().convert(_manifest(semantic_models=[sm], metrics=[base, yoy]))
out = [m for m in res.output.semantic_model[0].metrics if m.name == "bookings_yoy_growth"][0]
print("EXPRESSION:", out.expression.dialects[0].expression)
print("ISSUES:", res.issues)

Run from converters/dbt/.

Observed:

EXPRESSION: (SUM(1) - SUM(1)) / NULLIF(SUM(1), 0) * 100
ISSUES: []

The expression is algebraically 0 for every input row, and nothing signals that anything was lost.

Expected: either the offset represented in the output, or — since Ossie expressions have no window semantics to represent it with — a ConverterIssue reporting the loss, as CUMULATIVE metrics already do.

Why this is worse than the cumulative case

CUMULATIVE metrics already emit ConverterIssueType.CUMULATIVE_SEMANTICS_LOSS (msi_to_osi.py:95-98), so a consumer can see the semantics didn't survive and decide what to do. offset_window has no equivalent, so the failure mode is different in kind: the cumulative metric comes out approximate, the offset metric comes out wrong, and both look identical to anything downstream.

Concretely, converting through converters/snowflake and creating the semantic view in Snowflake produces a metric that runs successfully and returns 0 for every period — indistinguishable from genuinely flat year-on-year growth. It is the only failure I hit in an end-to-end dbt → Ossie → Snowflake conversion that produced a plausible wrong number rather than an error or an obvious gap.

Suggested fix

At minimum, emit a ConverterIssue when an input metric carries offset_window or offset_to_grain, matching the existing CUMULATIVE_SEMANTICS_LOSS pattern — e.g. OFFSET_SEMANTICS_LOSS. That turns a silent wrong answer into a visible gap, and lets consumers gate on it.

Beyond that it's a spec question rather than a converter one: representing a time-offset input needs something in the Ossie spec to carry it, which I don't think exists today (related to #290's shape, though not the same issue). Happy to raise that separately if it's useful.

tests/test_msi_to_osi.py has DERIVED coverage but no case where an input carries offset_window, which is why this isn't caught.

Happy to send a PR for the ConverterIssue part.