diff --git a/backend/src/cyclone/db.py b/backend/src/cyclone/db.py
index 508b89a..b65135d 100644
--- a/backend/src/cyclone/db.py
+++ b/backend/src/cyclone/db.py
@@ -221,6 +221,13 @@ class Batch(Base):
     totals_json: Mapped[Optional[dict]] = mapped_column(JSONText, nullable=True)
     validation_json: Mapped[Optional[dict]] = mapped_column(JSONText, nullable=True)
     raw_result_json: Mapped[Optional[dict]] = mapped_column(JSONText, nullable=True)
+    # SP37 Task 2: source 837's ST02 (transaction set control number).
+    # Populated from ``Envelope.transaction_set_control_number`` by
+    # ``store.write.add_record`` for 837P batches; NULL for 835 batches
+    # (the column is an 837P-specific join key for 999 AK2 resolution).
+    # Migration 0020 adds the column additively; no backfill required for
+    # pre-existing rows that lack the value.
+    transaction_set_control_number: Mapped[Optional[str]] = mapped_column(String(32), nullable=True)
 
     claims: Mapped[list["Claim"]] = relationship(
         back_populates="batch", cascade="all, delete-orphan"
diff --git a/backend/src/cyclone/parsers/models.py b/backend/src/cyclone/parsers/models.py
index 2f3090e..f4b8917 100644
--- a/backend/src/cyclone/parsers/models.py
+++ b/backend/src/cyclone/parsers/models.py
@@ -120,6 +120,13 @@ class Envelope(_Base):
     implementation_guide: str | None = None
     # SP3 P1 T2: BHT06 transaction type code (was: transaction_set_purpose_code, which is BHT02).
     transaction_type_code: str | None = None
+    # SP37 Task 2: X12 ST02 (transaction set control number). Distinct
+    # from ``control_number`` above, which is the ISA13 interchange
+    # control number. 999 acks echo ST02 back as AK201, so this is the
+    # join key that lets ``add_record``'s batch row round-trip back to
+    # its source 837. Populated only by the 837P parser today; other
+    # parsers share this class but leave the field None.
+    transaction_set_control_number: str | None = None
 
 
 class BatchSummary(_Base):
diff --git a/backend/src/cyclone/parsers/parse_837.py b/backend/src/cyclone/parsers/parse_837.py
index 1953d49..5f44d3a 100644
--- a/backend/src/cyclone/parsers/parse_837.py
+++ b/backend/src/cyclone/parsers/parse_837.py
@@ -83,6 +83,12 @@ def _build_envelope(segments: list[list[str]], input_file: str = "") -> tuple[En
             except (IndexError, ValueError) as exc:
                 log.warning("Could not parse BHT date: %s", exc)
         elif seg[0] == "ST" and envelope is not None:
+            # SP37 Task 2: capture ST02 (transaction set control number).
+            # 999 acks echo this back as AK201, so this is what makes the
+            # batch row joinable once the 999 ingests. Distinct from ISA13
+            # (which is already on ``control_number``).
+            if len(seg) > 2:
+                envelope = envelope.model_copy(update={"transaction_set_control_number": seg[2].strip()})
             if len(seg) > 3:
                 envelope = envelope.model_copy(update={"implementation_guide": seg[3]})
     return envelope, summary
diff --git a/backend/src/cyclone/store/write.py b/backend/src/cyclone/store/write.py
index e4b05a1..8da42d6 100644
--- a/backend/src/cyclone/store/write.py
+++ b/backend/src/cyclone/store/write.py
@@ -77,6 +77,17 @@ def add_record(record: BatchRecord, *, event_bus=None) -> None:
             totals_json=None,
             validation_json=None,
             raw_result_json=json.loads(record.result.model_dump_json()),
+            # SP37 Task 2: mirror the parsed 837's ST02 onto the batch
+            # row so 999 AK201 set_control_numbers can resolve back via
+            # Pass 1. The ``getattr`` chain handles the 835 path: the
+            # shared ``Envelope`` class is used by both 837P and 835
+            # parsers, but only ``parse_837`` populates this field — for
+            # 835 records it stays ``None`` and the column is NULL.
+            transaction_set_control_number=getattr(
+                getattr(record.result, "envelope", None),
+                "transaction_set_control_number",
+                None,
+            ),
         )
         s.add(batch_row)
 
diff --git a/backend/tests/test_batch_txn_set_cn.py b/backend/tests/test_batch_txn_set_cn.py
new file mode 100644
index 0000000..ef68da5
--- /dev/null
+++ b/backend/tests/test_batch_txn_set_cn.py
@@ -0,0 +1,103 @@
+"""SP37 Task 2: add_record populates Batch.transaction_set_control_number.
+
+The column should mirror the source 837 envelope's
+``transaction_set_control_number`` (ST02) so the join-key update in
+Task 3 can resolve 999 ``set_control_number`` (AK201) values back to
+the right batch. The 835 path leaves the column NULL because the
+field is an 837P-specific join key (835 remittances don't need to
+resolve back to a source 837 — they're the response side of the loop).
+"""
+from __future__ import annotations
+
+from datetime import date, datetime, timezone
+from pathlib import Path
+
+from cyclone import db as db_mod
+from cyclone.db import Batch
+from cyclone.parsers.parse_837 import parse as parse_837_text
+from cyclone.parsers.payer import PayerConfig
+from cyclone.store import store as cycl_store
+from cyclone.store.records import BatchRecord837, BatchRecord835
+
+FIXTURE_837 = Path(__file__).parent / "fixtures" / "minimal_837p.txt"
+FIXTURE_835 = Path(__file__).parent / "fixtures" / "minimal_835.txt"
+
+
+def test_add_record_populates_transaction_set_control_number_for_837():
+    """parse_837 → add_record writes the parsed ST02 onto the Batch row.
+
+    The fixture's ST02 (``991102977``) must round-trip into the new
+    ``batches.transaction_set_control_number`` column. This is the
+    join key Task 3's 999-ingest Pass 1 update relies on.
+    """
+    text = FIXTURE_837.read_text()
+    parsed = parse_837_text(text, PayerConfig.co_medicaid())
+
+    record = BatchRecord837(
+        id="b-txn-cn-1",
+        input_filename="minimal_837p.txt",
+        parsed_at=datetime.now(timezone.utc),
+        result=parsed,
+    )
+    cycl_store.add(record)
+
+    with db_mod.SessionLocal()() as s:
+        row = s.get(Batch, "b-txn-cn-1")
+        assert row is not None
+        assert row.transaction_set_control_number == parsed.envelope.transaction_set_control_number
+        # Sanity: the fixture's ST02 must be the parsed-and-stored value.
+        assert row.transaction_set_control_number == "991102977"
+
+
+def test_envelope_model_exposes_transaction_set_control_number():
+    """The Envelope Pydantic model carries the new field with a None default.
+
+    Guards against a regression where someone removes the field from
+    the model — the rest of the chain (parser + ORM + write path)
+    silently degrades to None if the model loses the attribute.
+    """
+    from cyclone.parsers.models import Envelope
+
+    env = Envelope(
+        sender_id="S",
+        receiver_id="R",
+        control_number="000000001",
+        transaction_date=date(2026, 1, 1),
+    )
+    assert env.transaction_set_control_number is None
+
+    env2 = env.model_copy(update={"transaction_set_control_number": "0001"})
+    assert env2.transaction_set_control_number == "0001"
+
+
+def test_add_record_leaves_835_column_null():
+    """835 batches don't carry an ST02 join key; column stays NULL.
+
+    The shared ``Envelope`` class is used by both 837P and 835 parsers,
+    but only ``parse_837`` populates ``transaction_set_control_number``.
+    For 835 records the field is None and ``add_record`` writes NULL
+    to the column — verified end-to-end via a real 835 ingest.
+    """
+    text = FIXTURE_835.read_text()
+    from cyclone.parsers.parse_835 import parse as parse_835_text
+    parsed835 = parse_835_text(text, payer_config=None)  # type: ignore[arg-type]
+
+    # Sanity: the 835 envelope also has the field (shared model class),
+    # but the parser doesn't populate it — so it must be None.
+    assert parsed835.envelope.transaction_set_control_number is None
+
+    record = BatchRecord835(
+        id="b-txn-cn-835-1",
+        input_filename="minimal_835.txt",
+        parsed_at=datetime.now(timezone.utc),
+        result=parsed835,
+    )
+    cycl_store.add(record)
+
+    with db_mod.SessionLocal()() as s:
+        row = s.get(Batch, "b-txn-cn-835-1")
+        assert row is not None
+        assert row.transaction_set_control_number is None
+        # And the kind/kind round-trip is correct (sanity check that
+        # we did exercise the 835 path, not the 837 path).
+        assert row.kind == "835"
\ No newline at end of file