PolicyEngine
diff --git a/‎pyproject.toml‎
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diff --git a/‎src/microplex_us/pipelines/us.py‎
Lines changed: 204 additions & 0 deletions b/‎src/microplex_us/pipelines/us.py‎
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@@ -35,6 +35,10 @@ policyengine = [
     "microimpute==1.15.1 ; python_full_version >= '3.12' and python_full_version < '3.15'",
     "policyengine-us==1.715.2; python_version >= '3.11' and python_version < '3.15'",
     "spm-calculator>=0.3.1",
+    # Standalone tax-unit construction engine (the extraction of eCPS's
+    # tax-unit logic), used by the PolicyEngine pipeline to reconstruct tax
+    # units from CPS-like person frames (issue #113).
+    "microunit>=0.1.0",
 ]
 
 [project.urls]
 
@@ -6262,12 +6262,198 @@ def _build_policyengine_tax_units(
         tax_units = pd.DataFrame(tax_unit_rows)
         return tax_units, person_rows
 
+    # Raw CPS ASEC columns that ``microunit.construct_tax_units`` consumes to
+    # reconstruct tax units. ``microunit`` is the standalone extraction of
+    # eCPS's tax-unit logic (issue #113); it is *source-agnostic* and expects
+    # this normalized CPS-like contract rather than microplex's collapsed
+    # ``relationship_to_head`` coding. We only delegate when the frame actually
+    # carries these columns, so the delegation is behavior-preserving on
+    # today's frames (which do not carry them) and only becomes active once an
+    # upstream change threads CPS columns through to entity construction.
+    _MICROUNIT_REQUIRED_CPS_COLUMNS = (
+        "PH_SEQ",
+        "A_LINENO",
+        "A_AGE",
+        "A_MARITL",
+        "A_SPOUSE",
+        "PEPAR1",
+        "PEPAR2",
+        "A_EXPRRP",
+    )
+
+    def _build_policyengine_tax_units_via_microunit(
+        self,
+        persons: pd.DataFrame,
+        *,
+        start_tax_unit_id: int = 0,
+    ) -> tuple[pd.DataFrame, pd.DataFrame, set[Any]] | None:
+        """Reconstruct tax units by delegating to ``microunit`` (issue #113).
+
+        This is the *reconstruction-from-scratch* path. The authoritative-ID
+        path (#112, :meth:`_build_policyengine_tax_units_from_existing_ids`) is
+        handled separately and is never routed here.
+
+        Delegation only happens when ``persons`` carries the raw CPS columns in
+        :attr:`_MICROUNIT_REQUIRED_CPS_COLUMNS`. ``microunit``'s logic genuinely
+        depends on marital status (``A_MARITL``), spouse/parent line pointers
+        (``A_SPOUSE``/``PEPAR1``/``PEPAR2``) and the CPS relationship recode
+        (``A_EXPRRP``); microplex's reconstruction-stage frame collapses
+        relationship into a 0/1/2/3 coding and drops the pointer columns, so a
+        *faithful* mapping is not possible from that frame. Rather than fabricate
+        microunit inputs (which would silently change behavior), we return
+        ``None`` when the columns are absent and let the caller fall back to the
+        legacy role-flag reconstruction.
+
+        .. warning::
+            ``microunit`` *is* eCPS's tax-unit construction. Routing microplex
+            through it makes microplex's constructed tax units **converge toward
+            eCPS's**. Any loss change from enabling this delegation is an
+            *entity-convergence* effect and must be interpreted as such, not as
+            a quality improvement. See issue #113.
+
+        Returns the same ``(tax_units, person_rows, households)`` triple shape as
+        :meth:`_build_policyengine_tax_units_from_role_flags`, or ``None`` to
+        defer to the caller's fallback.
+        """
+        if "person_id" not in persons.columns or "household_id" not in persons.columns:
+            return None
+        if not set(self._MICROUNIT_REQUIRED_CPS_COLUMNS).issubset(persons.columns):
+            return None
+
+        # Imported lazily to match this module's optional-dependency convention:
+        # ``microunit`` ships in the ``policyengine`` extra, and the base test
+        # suite must import this module without that extra installed.
+        from microunit import POLICYENGINE_MODE, construct_tax_units
+
+        # microunit keys its CPS-style frame on (PH_SEQ, A_LINENO); resetting the
+        # index keeps row order so the returned per-person TAX_ID and role align
+        # positionally back onto person_rows.
+        person_rows = persons.reset_index(drop=True).copy()
+        person_assignments, tax_unit = construct_tax_units(
+            person_rows.copy(),
+            year=self._microunit_reference_year(person_rows),
+            mode=POLICYENGINE_MODE,
+        )
+
+        tax_id = pd.to_numeric(person_assignments["TAX_ID"], errors="coerce")
+        person_rows["tax_unit_id"] = (
+            tax_id.to_numpy() + int(start_tax_unit_id)
+        ).astype(np.int64)
+        # microunit emits an authoritative per-person HEAD/SPOUSE/DEPENDENT role;
+        # use it directly for the filer/dependent split rather than re-deriving
+        # from the (possibly absent) collapsed relationship_to_head coding.
+        person_rows["_microunit_role"] = [
+            self._decode_microunit_bytes(role)
+            for role in person_assignments["tax_unit_role_input"].tolist()
+        ]
+
+        # microunit emits the canonical filing-status vocabulary already, but
+        # normalize defensively so this path can never diverge from the legacy
+        # paths if microunit ever changes its spelling/casing.
+        filing_status_by_unit = {
+            int(row_tax_id) + int(start_tax_unit_id): (
+                self._normalize_policyengine_filing_status(
+                    self._decode_microunit_bytes(filing_value)
+                )
+            )
+            for row_tax_id, filing_value in zip(
+                tax_unit["TAX_ID"].tolist(),
+                tax_unit["filing_status_input"].tolist(),
+                strict=True,
+            )
+        }
+
+        tax_unit_rows: list[dict[str, Any]] = []
+        for unit_id, unit_persons in person_rows.groupby("tax_unit_id", sort=False):
+            ordered = unit_persons.sort_values(
+                ["_microunit_role", "age", "person_id"],
+                ascending=[True, False, True],
+            ).reset_index(drop=True)
+            is_filer = ordered["_microunit_role"].isin(["HEAD", "SPOUSE"])
+            filer_ids = [
+                int(person_id) for person_id in ordered.loc[is_filer, "person_id"]
+            ]
+            dependent_ids = [
+                int(person_id) for person_id in ordered.loc[~is_filer, "person_id"]
+            ]
+            if not filer_ids:
+                filer_ids = [int(ordered.iloc[0]["person_id"])]
+                dependent_ids = [
+                    int(person_id)
+                    for person_id in ordered["person_id"].tolist()
+                    if int(person_id) not in filer_ids
+                ]
+            tax_unit_rows.append(
+                {
+                    "tax_unit_id": int(unit_id),
+                    "household_id": int(ordered.iloc[0]["household_id"]),
+                    "filing_status": filing_status_by_unit.get(int(unit_id), "SINGLE"),
+                    "member_ids": [
+                        int(person_id) for person_id in ordered["person_id"]
+                    ],
+                    "filer_ids": filer_ids,
+                    "dependent_ids": dependent_ids,
+                    "n_dependents": len(dependent_ids),
+                    "total_income": float(
+                        pd.to_numeric(ordered.get("income", 0.0), errors="coerce")
+                        .fillna(0.0)
+                        .sum()
+                    ),
+                    "tax_liability": 0.0,
+                    **self._aggregate_policyengine_tax_unit_input_columns(ordered),
+                }
+            )
+
+        if not tax_unit_rows:
+            return None
+
+        households = set(person_rows["household_id"].drop_duplicates().tolist())
+        person_rows = person_rows.drop(columns=["_microunit_role"], errors="ignore")
+        return pd.DataFrame(tax_unit_rows), person_rows, households
+
+    @staticmethod
+    def _decode_microunit_bytes(value: Any) -> str:
+        """Decode a ``microunit`` bytes-typed status/role into an upper string."""
+        if isinstance(value, bytes):
+            return value.decode()
+        return str(value)
+
+    def _microunit_reference_year(self, persons: pd.DataFrame) -> int:
+        """Year passed to ``microunit`` for its dependency income thresholds.
+
+        Prefers an explicit ``year``/``tax_year`` column when the frame carries
+        one; otherwise falls back to the pipeline's configured reference year so
+        the only year-dependent behavior (the qualifying-relative gross income
+        limit) matches the rest of the pipeline. TODO(#113): thread the dataset
+        reference year through entity construction explicitly.
+        """
+        for column in ("year", "tax_year"):
+            if column in persons.columns:
+                values = pd.to_numeric(persons[column], errors="coerce").dropna()
+                if not values.empty:
+                    return int(values.iloc[0])
+        configured = getattr(self.config, "reference_year", None)
+        if configured is not None:
+            return int(configured)
+        return 2024
+
     def _build_policyengine_tax_units_from_role_flags(
         self,
         persons: pd.DataFrame,
         *,
         start_tax_unit_id: int = 0,
     ) -> tuple[pd.DataFrame, pd.DataFrame, set[Any]] | None:
+        # Issue #113: when the frame carries microunit's CPS-style input
+        # columns, delegate the reconstruction to microunit. Otherwise fall
+        # through to the legacy role-flag reconstruction below (the current
+        # production path, since these columns are not yet threaded through).
+        microunit_result = self._build_policyengine_tax_units_via_microunit(
+            persons,
+            start_tax_unit_id=start_tax_unit_id,
+        )
+        if microunit_result is not None:
+            return microunit_result
+
         role_columns = {
             "is_tax_unit_head",
             "is_tax_unit_spouse",
@@ -7326,6 +7512,16 @@ def _coerce_policyengine_status_code(self, value: Any) -> int | None:
         return int(numeric)
 
     def _assign_family_and_spm_units(self, persons: pd.DataFrame) -> pd.DataFrame:
+        """Assign family and SPM units, preserving authoritative IDs when present.
+
+        NOT delegated to ``microunit`` in this pass (issue #113). At the pinned
+        commit ``microunit.units.spm.assign_spm_partition`` is documented as "a
+        conservative adapter, not yet the full Census-parity constructor" and is
+        not exported from microunit's public API, and microunit has no
+        family-unit constructor. The authoritative-ID fast path is preserved
+        here. TODO(#113): delegate once microunit grows a Census-parity
+        SPM/family constructor.
+        """
         result = persons.copy()
         preserved_family_ids = self._normalized_complete_existing_group_ids(
             result,
@@ -7399,6 +7595,14 @@ def _assign_marital_units(
         self,
         persons: pd.DataFrame,
     ) -> pd.DataFrame:
+        """Assign marital units, preserving authoritative IDs when present.
+
+        NOT delegated to ``microunit`` in this pass (issue #113): microunit does
+        not construct marital units at the pinned commit (filing status is its
+        only marital-related output; there is no ``construct_marital_units``).
+        The authoritative-ID fast path is preserved here. TODO(#113): revisit if
+        microunit grows marital-unit support.
+        """
         result = persons.copy()
         preserved_marital_unit_ids = self._normalized_complete_existing_group_ids(
             result,