diff --git a/backend/src/cyclone/parsers/serialize_837.py b/backend/src/cyclone/parsers/serialize_837.py
index 6ec6b7e..c27f931 100644
--- a/backend/src/cyclone/parsers/serialize_837.py
+++ b/backend/src/cyclone/parsers/serialize_837.py
@@ -45,6 +45,7 @@ the prodfile parametrized smoke in
 """
 from __future__ import annotations
 
+import logging
 from datetime import date, datetime
 from decimal import Decimal
 
@@ -484,11 +485,45 @@ def _build_subscriber_block(
 
 
 def _build_payer_block(payer) -> list[str]:
+    name, pid = _normalize_payer_id(payer)
     return [
-        _build_nm1("PR", "PR", payer.name, "PI", payer.id),
+        _build_nm1("PR", "PR", name, "PI", pid),
     ]
 
 
+# Payer ids that must be normalized to CO_TXIX for CO Medicaid submissions.
+# SP39: defense-in-depth against legacy raw_json captures (SKCO0 from
+# pre-SP33 batches, CO_BHA from prior behavioral-health configurations,
+# empty from degenerate parses). Foreign payer IDs are emitted verbatim.
+_NORMALIZE_TO_CO_TXIX_IDS = frozenset({"", "SKCO0", "CO_BHA"})
+_NORMALIZE_TO_CO_TXIX_NAMES = frozenset({"", "COHCPF", "CO_BHA"})
+_CO_TXIX = "CO_TXIX"
+
+_log = logging.getLogger(__name__)
+
+
+def _normalize_payer_id(payer) -> tuple[str, str]:
+    """Return (name, id) normalized so CO Medicaid claims always emit CO_TXIX.
+
+    SP39. Substitutes empty/SKCO0/CO_BHA -> CO_TXIX in the id and
+    empty/COHCPF/CO_BHA -> CO_TXIX in the name. Foreign payer ids are
+    passed through verbatim (only the CO Medicaid-shape values are
+    normalized; the helper must not corrupt a non-CO submit). Emits a
+    WARNING log line on substitution (one per call; the serializer is
+    invoked once per claim so volume is bounded by batch size).
+    """
+    raw_id = (getattr(payer, "id", None) or "").strip()
+    raw_name = (getattr(payer, "name", None) or "").strip()
+    new_id = _CO_TXIX if raw_id in _NORMALIZE_TO_CO_TXIX_IDS else raw_id
+    new_name = _CO_TXIX if raw_name in _NORMALIZE_TO_CO_TXIX_NAMES else raw_name
+    if new_id != raw_id or new_name != raw_name:
+        _log.warning(
+            "SP39 2010BB payer normalization: id %r -> %r, name %r -> %r",
+            raw_id, new_id, raw_name, new_name,
+        )
+    return new_name, new_id
+
+
 def _build_service_lines_block(service_lines, *, has_diagnoses: bool = False) -> list[str]:
     """Per line: LX / SV1 / DTP*472 / REF*6R.
 
diff --git a/backend/tests/test_serialize_837.py b/backend/tests/test_serialize_837.py
index 6b09611..34643d3 100644
--- a/backend/tests/test_serialize_837.py
+++ b/backend/tests/test_serialize_837.py
@@ -593,4 +593,93 @@ def test_serialize_837_for_resubmit_assigns_unique_control_numbers():
     isa_b = next(line for line in text_b.split("~") if line.startswith("ISA"))
     assert isa_a != isa_b
     assert "000000042" in isa_a
-    assert "000000043" in isa_b
\ No newline at end of file
+    assert "000000043" in isa_b
+
+
+# ---------------------------------------------------------------------------
+# SP39: _build_payer_block normalizes legacy payer ids to CO_TXIX
+# ---------------------------------------------------------------------------
+
+
+def _first_pr_segment(text: str) -> str:
+    """Extract the first NM1*PR segment from a serialized 837P document."""
+    for seg in text.split("~"):
+        if seg.startswith("NM1*PR"):
+            return seg
+    raise AssertionError("no NM1*PR segment found in output")
+
+
+def test_2010bb_normalizes_skco0_to_co_txix(caplog):
+    """SP39: legacy SKCO0 + COHCPF must be normalized to CO_TXIX."""
+    import logging
+    claim = _load_claim()
+    claim.payer.id = "SKCO0"
+    claim.payer.name = "COHCPF"
+    with caplog.at_level(logging.WARNING, logger="cyclone.parsers.serialize_837"):
+        text = serialize_837_for_resubmit(claim, interchange_index=1)
+    parts = _first_pr_segment(text).split("*")
+    # parts: NM1, PR, 2, NM103, NM104, NM105, NM106, NM107, NM108, NM109
+    assert parts[8] == "PI", f"NM108 must be PI, got {parts[8]!r}"
+    assert parts[9] == "CO_TXIX", f"NM109 must be CO_TXIX, got {parts[9]!r}"
+    assert parts[3] == "CO_TXIX", f"NM103 must be CO_TXIX, got {parts[3]!r}"
+    assert "SKCO0" not in text, "SKCO0 must not appear in output bytes"
+    assert "COHCPF" not in text, "COHCPF must not appear in output bytes"
+    # WARNING log was emitted
+    assert any(
+        "SP39" in rec.message and "SKCO0" in rec.message
+        for rec in caplog.records
+    ), "expected SP39 substitution WARNING log for SKCO0->CO_TXIX"
+
+
+def test_2010bb_normalizes_empty_payer_id_to_co_txix(caplog):
+    """SP39: empty payer.id (degenerate parse artifact) -> CO_TXIX."""
+    import logging
+    claim = _load_claim()
+    claim.payer.id = ""
+    claim.payer.name = ""
+    with caplog.at_level(logging.WARNING, logger="cyclone.parsers.serialize_837"):
+        text = serialize_837_for_resubmit(claim, interchange_index=1)
+    parts = _first_pr_segment(text).split("*")
+    assert parts[8] == "PI"
+    assert parts[9] == "CO_TXIX"
+    assert parts[3] == "CO_TXIX"
+
+
+def test_2010bb_normalizes_co_bha_to_co_txix(caplog):
+    """SP39: CO_BHA also gets normalized to CO_TXIX (operator policy)."""
+    import logging
+    claim = _load_claim()
+    claim.payer.id = "CO_BHA"
+    claim.payer.name = "CO_BHA"
+    with caplog.at_level(logging.WARNING, logger="cyclone.parsers.serialize_837"):
+        text = serialize_837_for_resubmit(claim, interchange_index=1)
+    parts = _first_pr_segment(text).split("*")
+    assert parts[8] == "PI"
+    assert parts[9] == "CO_TXIX"
+    assert parts[3] == "CO_TXIX"
+    assert "CO_BHA" not in text
+
+
+def test_2010bb_preserves_foreign_payer_id_verbatim():
+    """SP39: helper must NOT corrupt non-CO submits. Foreign payer id
+    is emitted verbatim with no substitution log."""
+    import logging
+    claim = _load_claim()
+    claim.payer.id = "OTHER_PAYER"
+    claim.payer.name = "OTHER PAYER NAME"
+    import io
+    log_stream = io.StringIO()
+    handler = logging.StreamHandler(log_stream)
+    handler.setLevel(logging.WARNING)
+    log = logging.getLogger("cyclone.parsers.serialize_837")
+    log.addHandler(handler)
+    try:
+        text = serialize_837_for_resubmit(claim, interchange_index=1)
+    finally:
+        log.removeHandler(handler)
+    parts = _first_pr_segment(text).split("*")
+    assert parts[8] == "PI"
+    assert parts[9] == "OTHER_PAYER"
+    assert parts[3] == "OTHER PAYER NAME"
+    # No substitution log for a foreign payer id
+    assert "SP39" not in log_stream.getvalue()
\ No newline at end of file