diff --git a/backend/src/cyclone/parsers/serialize_837.py b/backend/src/cyclone/parsers/serialize_837.py
index c27f931..bf00cc9 100644
--- a/backend/src/cyclone/parsers/serialize_837.py
+++ b/backend/src/cyclone/parsers/serialize_837.py
@@ -420,6 +420,17 @@ def _build_submitter_block(
     contact_email: str | None = None,
     email_qual: str = "EM",
 ) -> list[str]:
+    # SP40: PER-02 (Name) and at least one PER-03/04 pair are required
+    # by Edifabric's x12/validate — emitting only PER-01 ("IC") makes
+    # the file invalid. Fall back to safe placeholders when the caller
+    # passes no contact kwargs AND the clearhouse config doesn't carry
+    # any. Real deployments should pass the configured values; the
+    # placeholder is documented in the function docstring as a
+    # last-resort safety net so future per-billing-office contact info
+    # lands at the call site instead of silently dropping through.
+    if not any([contact_name, contact_phone, contact_email]):
+        contact_name = "CUSTOMER SERVICE"
+        contact_phone = "8005550100"
     out = [
         _build_nm1("41", "41", submitter_name or sender_id, "46", sender_id),
     ]
@@ -460,6 +471,14 @@ def _build_subscriber_block(
     claim_filing_indicator_code: str | None,
 ) -> list[str]:
     """HL*2 → SBR → NM1*IL → N3 → N4 → DMG. Subscriber has no children."""
+    # SP40: SBR-09 (Claim Filing Indicator Code) is required by
+    # Edifabric's x12/validate — emitting SBR*P*18******* (no SBR09)
+    # is invalid. Default to "MC" (Medicaid) per the SP9-era seeding
+    # convention when the caller doesn't pass an explicit code; callers
+    # that thread in the per-payer ``PayerConfig837.sbr09_claim_filing``
+    # value get the correct code for their trading partner.
+    if not claim_filing_indicator_code:
+        claim_filing_indicator_code = "MC"
     out = [
         _build_hl("2", "1", "22", "0"),  # HL*2 — subscriber, 0 children
         _build_sbr("18", claim_filing_indicator_code),
diff --git a/backend/tests/test_serialize_837.py b/backend/tests/test_serialize_837.py
index 34643d3..610fa49 100644
--- a/backend/tests/test_serialize_837.py
+++ b/backend/tests/test_serialize_837.py
@@ -682,4 +682,59 @@ def test_2010bb_preserves_foreign_payer_id_verbatim():
     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
+    assert "SP39" not in log_stream.getvalue()
+
+# ---------------------------------------------------------------------------
+# SP40: PER-02/03/04 + SBR-09 must always be emitted (Edifabric rejects
+# the bare-PER / no-SBR09 shapes). The serializer fall-back fills them
+# with safe placeholders when the caller passes no kwargs.
+# ---------------------------------------------------------------------------
+
+
+def test_per_segment_emits_name_and_phone_qualifier_when_no_contact():
+    """SP40: without any submitter_contact_* kwargs, the submitter
+    block must still produce a PER segment with at least PER-02 (Name)
+    and a PER-03 (Communication Number Qualifier) + PER-04 (Number)
+    pair. Edifabric rejects PER*IC alone."""
+    claim = _load_claim()
+    text = serialize_837_for_resubmit(claim, interchange_index=42)
+    per_line = next(s for s in text.split("~") if s.startswith("PER"))
+    parts = per_line.split("*")
+    # PER*IC*<Name>*<Qual>*<Number>
+    assert parts[1] == "IC"
+    assert parts[2], f"PER-02 (Name) is required, got empty; line={per_line!r}"
+    assert parts[3] in {"TE", "EM", "FX"}, (
+        f"PER-03 must be a Communication Number Qualifier (TE/EM/FX); "
+        f"got {parts[3]!r}; line={per_line!r}"
+    )
+    assert parts[4], f"PER-04 (Number) is required, got empty; line={per_line!r}"
+
+
+def test_sbr_segment_emits_claim_filing_indicator_default_mc():
+    """SP40: SBR-09 (Claim Filing Indicator Code) must always be
+    emitted. Default is 'MC' (Medicaid) when the caller passes no
+    claim_filing_indicator_code kwarg. Edifabric rejects the bare
+    SBR*P*18******* shape."""
+    claim = _load_claim()
+    text = serialize_837_for_resubmit(claim, interchange_index=42)
+    sbr_line = next(s for s in text.split("~") if s.startswith("SBR"))
+    parts = sbr_line.split("*")
+    # SBR*P*18*******MC  →  index 9 is SBR-09
+    assert len(parts) >= 10, f"SBR must have 10 elements (with SBR-09), got {len(parts)}: {sbr_line!r}"
+    assert parts[9] == "MC", (
+        f"SBR-09 must default to 'MC' (Medicaid), got {parts[9]!r}; line={sbr_line!r}"
+    )
+
+
+def test_sbr_segment_respects_explicit_claim_filing_indicator():
+    """SP40: explicit claim_filing_indicator_code kwargs win over the
+    default — preserves the existing caller-facing behavior for
+    non-CO payers (e.g. '16' for Medicare Part B)."""
+    claim = _load_claim()
+    text = serialize_837_for_resubmit(
+        claim, interchange_index=42,
+        claim_filing_indicator_code="16",
+    )
+    sbr_line = next(s for s in text.split("~") if s.startswith("SBR"))
+    parts = sbr_line.split("*")
+    assert parts[9] == "16", f"SBR-09 should reflect explicit kwarg, got {parts[9]!r}"