diff --git a/backend/src/cyclone/reissue/__init__.py b/backend/src/cyclone/reissue/__init__.py
new file mode 100644
index 0000000..1daa64b
--- /dev/null
+++ b/backend/src/cyclone/reissue/__init__.py
@@ -0,0 +1,43 @@
+"""cyclone.reissue — offline 837P re-emission workflow.
+
+Pure functions for parsing raw 837P files into ``ClaimOutput`` Pydantic
+models and re-emitting one IG-correct single-claim X12 file per claim.
+No Click imports inside :mod:`cyclone.reissue.core`; the CLI wrapper at
+:mod:`cyclone.cli` is a thin shell over these functions.
+
+Public surface:
+
+- :func:`parse_inputs` — parse every ``*.x12`` / ``*.txt`` / ``*.edi``
+  under a directory; tolerates per-file failures and skips claims with
+  hard validation errors.
+- :func:`emit_outputs` — write one X12 per claim to ``output_dir``
+  using HCPF-spec filenames; per-claim 1 ms timestamp offsets guarantee
+  unique filenames within a batch.
+- :func:`write_summary_sidecar` — write a ``_serialize_summary.json``
+  sidecar with one row per emitted file (operator audit trail).
+- :func:`zip_outputs` — zip the per-claim X12 files into a single flat
+  archive with a ``testzip()`` integrity check.
+- :func:`ig_correctness_check` — verify the serializer's
+  ``PATIENT_LOOP_DEFAULT_INCLUDED`` is ``False`` (the IG-correct
+  shape for SBR02 == "18" claims). See
+  ``docs/superpowers/specs/2026-07-08-cyclone-reissue-claims-design.md``
+  for the SP24 rationale.
+
+See `scripts/reissue_claims.py` for the deprecation shim; the
+canonical entry point is ``cyclone reissue-claims`` (SP24).
+"""
+from cyclone.reissue.core import (
+    emit_outputs,
+    ig_correctness_check,
+    parse_inputs,
+    write_summary_sidecar,
+    zip_outputs,
+)
+
+__all__ = [
+    "parse_inputs",
+    "emit_outputs",
+    "write_summary_sidecar",
+    "zip_outputs",
+    "ig_correctness_check",
+]
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diff --git a/backend/src/cyclone/reissue/core.py b/backend/src/cyclone/reissue/core.py
new file mode 100644
index 0000000..f1c8c10
--- /dev/null
+++ b/backend/src/cyclone/reissue/core.py
@@ -0,0 +1,237 @@
+"""SP24 — pure functions for the offline 837P reissue workflow.
+
+The functions here are deliberately Click-free so the CLI layer at
+:mod:`cyclone.cli` stays a thin shell, and so unit tests can exercise
+the behaviour without :class:`click.testing.CliRunner`.
+
+Workflow contract:
+
+    >>> claims, errors = parse_inputs(input_dir, payer_config)
+    >>> written = emit_outputs(claims, payer_config=..., output_dir=..., ...)
+    >>> zip_outputs(written, zip_path)
+
+``ig_correctness_check`` is the regression guard for the SP24 fix.
+The serializer's ``PATIENT_LOOP_DEFAULT_INCLUDED`` constant MUST be
+``False``; flipping it back to ``True`` would re-introduce the
+Edifabric 999 rejection on every SBR02 == "18" claim.
+"""
+from __future__ import annotations
+
+import json
+import logging
+import zipfile
+from datetime import datetime, timedelta
+from pathlib import Path
+from zoneinfo import ZoneInfo
+
+from cyclone.edi.filenames import build_outbound_filename
+from cyclone.parsers.models import ClaimOutput
+from cyclone.parsers.parse_837 import parse as parse_837_text
+from cyclone.parsers.serialize_837 import (
+    PATIENT_LOOP_DEFAULT_INCLUDED,
+    serialize_837,
+)
+
+_log = logging.getLogger(__name__)
+
+
+def ig_correctness_check(*, logger: logging.Logger | None = None) -> bool:
+    """Return True iff the serializer's IG-correct patient-loop default is in place.
+
+    Per X12 005010X222A1, the 2000C Patient Hierarchical Level
+    (``HL*3 → PAT → NM1*QC``) MUST be absent when ``SBR02 == "18"``
+    (Self-pay, the CO-Medicaid IHSS workflow). The serializer encodes
+    this as a module-level constant ``PATIENT_LOOP_DEFAULT_INCLUDED``
+    in :mod:`cyclone.parsers.serialize_837`; this function checks
+    that constant.
+
+    Args:
+        logger: optional logger for the diagnostic WARNING when the
+            guard fires. Defaults to this module's logger.
+
+    Returns:
+        True if the constant is ``False`` (IG-correct). False
+        otherwise — callers should refuse to emit any X12 in that case.
+    """
+    log = logger or _log
+    if PATIENT_LOOP_DEFAULT_INCLUDED is not False:
+        log.warning(
+            "REFUSING to run: PATIENT_LOOP_DEFAULT_INCLUDED = %r "
+            "(expected False). The IG-correct serializer shape for "
+            "SBR02='18' claims requires the 2000C patient loop to be "
+            "absent; restoring the default to False fixes it.",
+            PATIENT_LOOP_DEFAULT_INCLUDED,
+        )
+        return False
+    return True
+
+
+def parse_inputs(
+    input_dir: Path,
+    payer_config,
+) -> tuple[list[ClaimOutput], list[tuple[Path, str]]]:
+    """Parse every ``*.x12`` / ``*.txt`` / ``*.edi`` under ``input_dir``.
+
+    AppleDouble metadata files (``._foo.x12``, the macOS resource
+    forks that Samba / Finder scatter) are skipped at the glob stage.
+
+    Per-file failures are tolerated: a file that raises during
+    ``parse_837_text`` lands in the second tuple element with the
+    exception class + message. The first tuple element collects every
+    surviving :class:`ClaimOutput`, including those with **warnings**
+    (warnings don't abort). Claims with hard ``validation.errors``
+    are dropped with an error message in the second tuple element.
+
+    Args:
+        input_dir: directory to walk (one level).
+        payer_config: a :class:`PayerConfig` instance passed to the
+            parser — typically ``PayerConfig.co_medicaid()``.
+
+    Returns:
+        ``(claims, errors)`` where ``claims`` is the surviving
+        :class:`ClaimOutput` list and ``errors`` is a list of
+        ``(path, message)`` tuples for each per-file failure.
+    """
+    raw_files: list[Path] = []
+    for ext in ("*.x12", "*.txt", "*.edi"):
+        for p in sorted(input_dir.glob(ext)):
+            # Skip macOS AppleDouble metadata (._<name>) — binary
+            # forks of the resource fork, not actual EDI.
+            if p.name.startswith("._"):
+                continue
+            raw_files.append(p)
+
+    claims: list[ClaimOutput] = []
+    errors: list[tuple[Path, str]] = []
+    for f in raw_files:
+        try:
+            text = f.read_text(encoding="utf-8")
+            result = parse_837_text(text, payer_config, input_file=str(f))
+        except Exception as exc:  # noqa: BLE001 — log + skip
+            errors.append((f, f"{exc.__class__.__name__}: {exc}"))
+            continue
+        if not result.claims:
+            errors.append((f, "no claims parsed (empty CLM loop?)"))
+            continue
+        for c in result.claims:
+            if c.validation.errors:
+                msgs = [f"{i.rule}: {i.message}" for i in c.validation.errors]
+                errors.append((f, f"hard errors in {c.claim_id}: {msgs}"))
+                continue
+            claims.append(c)
+    return claims, errors
+
+
+def emit_outputs(
+    claims: list[ClaimOutput],
+    *,
+    payer_config,
+    output_dir: Path,
+    sender_id: str,
+    receiver_id: str,
+    submitter_name: str,
+    submitter_contact_name: str,
+    submitter_contact_email: str,
+    receiver_name: str,
+) -> list[Path]:
+    """Write one X12 per claim under ``output_dir`` using HCPF-spec filenames.
+
+    Per-claim 1 ms timestamp offsets on the base datetime guarantee
+    unique filenames within a batch. The output directory is created
+    if it doesn't exist. Claims are sorted by ``claim_id`` before
+    emission so the per-claim filenames line up with the canonical
+    sort order.
+
+    Args:
+        claims: parsed :class:`ClaimOutput` instances (typically the
+            first tuple element from :func:`parse_inputs`).
+        payer_config: a :class:`PayerConfig` — threads
+            ``sbr09_claim_filing`` into the SBR09 segment.
+        output_dir: directory to write the per-claim X12 files into.
+        sender_id, receiver_id, submitter_*, receiver_name: envelope
+            metadata threaded into the ISA/GS/NM1*41/NM1*40 segments.
+
+    Returns:
+        The list of written :class:`Path` instances, in the same
+        order as the sorted claims.
+    """
+    output_dir.mkdir(parents=True, exist_ok=True)
+    # America/Denver is the operator's home tz and the canonical
+    # timezone for HCPF submission timestamps; matching the tz
+    # keeps the 999 round-trip deterministic for the operator's
+    # daily pull.
+    base_ts = datetime.now(tz=ZoneInfo("America/Denver"))
+
+    written: list[Path] = []
+    for i, claim in enumerate(sorted(claims, key=lambda c: c.claim_id), start=1):
+        body = serialize_837(
+            claim,
+            sender_id=sender_id,
+            receiver_id=receiver_id,
+            submitter_name=submitter_name,
+            submitter_contact_name=submitter_contact_name,
+            submitter_contact_email=submitter_contact_email,
+            receiver_name=receiver_name,
+            claim_filing_indicator_code=payer_config.sbr09_claim_filing,
+            interchange_control_number=f"{i:09d}",
+            group_control_number="1",
+        )
+        ts = base_ts + timedelta(milliseconds=i)
+        fname = build_outbound_filename(sender_id, "837P", now_mt=ts)
+        path = output_dir / fname
+        path.write_text(body, encoding="ascii")
+        written.append(path)
+    return written
+
+
+def write_summary_sidecar(
+    claims: list[ClaimOutput],
+    written: list[Path],
+    output_dir: Path,
+) -> Path:
+    """Write ``_serialize_summary.json`` next to the per-claim X12 files.
+
+    Operator-facing sidecar for audit: one row per emitted file with
+    ``claim_id``, ``output_file`` (absolute path), and ``byte_size``.
+    Both ``claims`` and ``written`` are zipped in the same order as
+    ``emit_outputs`` produced them (i.e. sorted by ``claim_id``).
+
+    Returns the path of the written sidecar.
+    """
+    summary = [
+        {
+            "claim_id": c.claim_id,
+            "output_file": str(p),
+            "byte_size": p.stat().st_size,
+        }
+        for c, p in zip(sorted(claims, key=lambda c: c.claim_id), written)
+    ]
+    path = output_dir / "_serialize_summary.json"
+    path.write_text(json.dumps(summary, indent=2), encoding="utf-8")
+    return path
+
+
+def zip_outputs(files: list[Path], zip_path: Path) -> None:
+    """Zip the per-claim X12 files into a single flat archive.
+
+    Uses :class:`zipfile.ZIP_DEFLATED`. After writing, runs
+    :meth:`zipfile.ZipFile.testzip` to verify CRC integrity and
+    raises :class:`RuntimeError` if any entry is corrupt.
+
+    Args:
+        files: the per-claim X12 file paths (typically the return
+            value of :func:`emit_outputs`).
+        zip_path: destination archive path; parent directories are
+            created as needed.
+
+    Raises:
+        RuntimeError: if any entry fails the ``testzip()`` integrity
+            check.
+    """
+    zip_path.parent.mkdir(parents=True, exist_ok=True)
+    with zipfile.ZipFile(zip_path, "w", zipfile.ZIP_DEFLATED) as zf:
+        for p in files:
+            zf.write(p, arcname=p.name)
+    bad = zipfile.ZipFile(zip_path).testzip()
+    if bad is not None:
+        raise RuntimeError(f"zip integrity check failed: {bad} is corrupt")
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