diff --git a/backend/src/cyclone/reconcile.py b/backend/src/cyclone/reconcile.py
new file mode 100644
index 0000000..e175eca
--- /dev/null
+++ b/backend/src/cyclone/reconcile.py
@@ -0,0 +1,112 @@
+"""Pure reconciliation functions.
+
+These functions take lists of objects (ORM or stand-in dataclasses) and
+return intent objects describing what should change. They do not own a
+database session; the caller (cyclone.store) persists the intents.
+
+Match algorithm:
+- Group claims by patient_control_number.
+- For each remit, find a candidate claim with the same PCN.
+- Prefer the claim whose service_date_from is closest to the remit's
+  service_date, within +/- window_days.
+- If the remit has no service_date, fall back to the first claim for
+  that PCN (in input order).
+- Skip claims that already have matched_remittance_id set.
+- is_reversal flag is propagated to the Match.
+"""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+from datetime import date
+from typing import Iterable, Optional, Protocol
+
+
+WINDOW_DAYS_DEFAULT = 7
+
+
+class _ClaimLike(Protocol):
+    id: str
+    patient_control_number: str
+    service_date_from: Optional[date]
+    matched_remittance_id: Optional[str]
+
+
+class _RemitLike(Protocol):
+    id: str
+    payer_claim_control_number: str
+    service_date: Optional[date]
+    is_reversal: bool
+
+
+@dataclass
+class Match:
+    claim: _ClaimLike
+    remittance: _RemitLike
+    strategy: str
+    is_reversal: bool
+
+
+def match(
+    claims: Iterable[_ClaimLike],
+    remittances: Iterable[_RemitLike],
+    *,
+    window_days: int = WINDOW_DAYS_DEFAULT,
+) -> list[Match]:
+    """Pair each remit with the closest eligible claim. Returns Match intents."""
+    claims_list = list(claims)
+    remits_list = list(remittances)
+
+    # Eligible = no current match, has a PCN.
+    eligible = [c for c in claims_list if c.patient_control_number and not c.matched_remittance_id]
+    by_pcn: dict[str, list[_ClaimLike]] = {}
+    for c in eligible:
+        by_pcn.setdefault(c.patient_control_number, []).append(c)
+
+    matches: list[Match] = []
+    used_claim_ids: set[str] = set()
+
+    for r in remits_list:
+        candidates = by_pcn.get(r.payer_claim_control_number, [])
+        candidates = [c for c in candidates if c.id not in used_claim_ids]
+        if not candidates:
+            continue
+
+        chosen = _pick_claim(candidates, r.service_date, window_days)
+        if chosen is None:
+            continue
+
+        matches.append(Match(
+            claim=chosen,
+            remittance=r,
+            strategy="auto",
+            is_reversal=r.is_reversal,
+        ))
+        used_claim_ids.add(chosen.id)
+
+    return matches
+
+
+def _pick_claim(
+    candidates: list[_ClaimLike],
+    remit_service_date: Optional[date],
+    window_days: int,
+) -> Optional[_ClaimLike]:
+    """Choose the claim with the closest service_date_from within the window."""
+    if remit_service_date is None:
+        return candidates[0]  # no date info → first claim for that PCN
+
+    best: tuple[int, _ClaimLike] | None = None
+    for c in candidates:
+        if c.service_date_from is None:
+            continue
+        delta = abs((c.service_date_from - remit_service_date).days)
+        if delta > window_days:
+            continue
+        if best is None or delta < best[0]:
+            best = (delta, c)
+
+    if best is not None:
+        return best[1]
+    # Remit has a date but no candidate was within the window → no match.
+    return None
diff --git a/backend/tests/test_reconcile.py b/backend/tests/test_reconcile.py
new file mode 100644
index 0000000..53fa857
--- /dev/null
+++ b/backend/tests/test_reconcile.py
@@ -0,0 +1,114 @@
+"""Tests for the pure reconciliation functions.
+
+These tests use lightweight stand-in objects (namedtuples) rather than
+SQLAlchemy ORM instances, so the reconcile module has zero DB dependency.
+"""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+from datetime import date
+from typing import Optional
+
+import pytest
+
+from cyclone import reconcile
+
+
+# --- Stand-in types -----------------------------------------------------------
+
+
+@dataclass
+class FakeClaim:
+    id: str
+    patient_control_number: str
+    service_date_from: Optional[date]
+    state: str = "submitted"
+    matched_remittance_id: Optional[str] = None
+
+
+@dataclass
+class FakeRemit:
+    id: str
+    payer_claim_control_number: str
+    service_date: Optional[date]
+    is_reversal: bool = False
+
+
+# --- match() ------------------------------------------------------------------
+
+
+def test_match_same_pcn_within_window_matches():
+    claims = [FakeClaim("CLM-1", "PCN-A", date(2026, 6, 1))]
+    remits = [FakeRemit("CLP-1", "PCN-A", date(2026, 6, 2))]
+    matches = reconcile.match(claims, remits, window_days=7)
+    assert len(matches) == 1
+    assert matches[0].claim.id == "CLM-1"
+    assert matches[0].remittance.id == "CLP-1"
+    assert matches[0].strategy == "auto"
+
+
+def test_match_dates_outside_window_unmatched():
+    claims = [FakeClaim("CLM-1", "PCN-A", date(2026, 6, 1))]
+    remits = [FakeRemit("CLP-1", "PCN-A", date(2026, 7, 1))]  # 30d apart
+    matches = reconcile.match(claims, remits, window_days=7)
+    assert matches == []
+
+
+def test_match_different_pcn_unmatched():
+    claims = [FakeClaim("CLM-1", "PCN-A", date(2026, 6, 1))]
+    remits = [FakeRemit("CLP-1", "PCN-B", date(2026, 6, 1))]
+    matches = reconcile.match(claims, remits)
+    assert matches == []
+
+
+def test_match_multiple_claims_same_pcn_picks_closest_date():
+    claims = [
+        FakeClaim("CLM-1", "PCN-A", date(2026, 5, 1)),
+        FakeClaim("CLM-2", "PCN-A", date(2026, 6, 1)),  # closer
+    ]
+    remits = [FakeRemit("CLP-1", "PCN-A", date(2026, 6, 2))]
+    matches = reconcile.match(claims, remits)
+    assert len(matches) == 1
+    assert matches[0].claim.id == "CLM-2"
+
+
+def test_match_multiple_remits_same_pcn_each_gets_own_claim():
+    claims = [
+        FakeClaim("CLM-1", "PCN-A", date(2026, 6, 1)),
+        FakeClaim("CLM-2", "PCN-A", date(2026, 6, 15)),
+    ]
+    remits = [
+        FakeRemit("CLP-1", "PCN-A", date(2026, 6, 2)),
+        FakeRemit("CLP-2", "PCN-A", date(2026, 6, 16)),
+    ]
+    matches = reconcile.match(claims, remits)
+    assert len(matches) == 2
+    claim_ids = {m.claim.id for m in matches}
+    assert claim_ids == {"CLM-1", "CLM-2"}
+
+
+def test_match_skips_already_matched_claims():
+    claims = [FakeClaim("CLM-1", "PCN-A", date(2026, 6, 1), matched_remittance_id="OLD")]
+    remits = [FakeRemit("CLP-1", "PCN-A", date(2026, 6, 1))]
+    matches = reconcile.match(claims, remits)
+    assert matches == []
+
+
+def test_match_falls_back_to_first_claim_when_no_service_date():
+    """When the remit has no service_date, match the first claim for that PCN."""
+    claims = [
+        FakeClaim("CLM-1", "PCN-A", None),
+        FakeClaim("CLM-2", "PCN-A", None),
+    ]
+    remits = [FakeRemit("CLP-1", "PCN-A", None)]
+    matches = reconcile.match(claims, remits)
+    assert len(matches) == 1
+    assert matches[0].claim.id == "CLM-1"
+
+
+def test_match_reversal_flag_propagates():
+    claims = [FakeClaim("CLM-1", "PCN-A", date(2026, 6, 1))]
+    remits = [FakeRemit("CLP-1", "PCN-A", date(2026, 6, 1), is_reversal=True)]
+    matches = reconcile.match(claims, remits)
+    assert matches[0].is_reversal is True