feat: cross-account transfer detection

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/services/transfer_detector.py

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+import datetime
+from sqlalchemy.orm import Session
+
+from src.models.transaction import Transaction
+
+
+# Known transfer patterns
+TRANSFER_PATTERNS = [
+    "CREDIT CRD EPAY",
+    "Payment Thank You",
+    "CAPITAL ONE TRANSFER",
+    "AMEX EPAYMENT",
+    "AMZ_STORECRD_PMT",
+]
+
+
+class TransferDetector:
+    def __init__(self, session: Session):
+        self.session = session
+
+    def detect(self, date_tolerance_days: int = 3) -> list[dict]:
+        """Find matching transfer pairs across accounts."""
+        txns = self.session.query(Transaction).filter(
+            Transaction.is_transfer == False
+        ).all()
+
+        # Separate positive and negative transactions
+        positives = [t for t in txns if float(t.amount) > 0]
+        negatives = [t for t in txns if float(t.amount) < 0]
+
+        pairs = []
+        matched_ids = set()
+
+        for neg in negatives:
+            if neg.id in matched_ids:
+                continue
+            # Check if this looks like a transfer
+            if not self._is_transfer_pattern(neg.description):
+                continue
+
+            neg_amount = abs(float(neg.amount))
+
+            for pos in positives:
+                if pos.id in matched_ids:
+                    continue
+                if pos.account_id == neg.account_id:
+                    continue  # Must be different accounts
+
+                pos_amount = float(pos.amount)
+                if abs(pos_amount - neg_amount) > 0.01:
+                    continue  # Amounts must match
+
+                date_diff = abs((neg.date - pos.date).days)
+                if date_diff > date_tolerance_days:
+                    continue  # Dates must be close
+
+                pairs.append({
+                    "outgoing_id": neg.id,
+                    "incoming_id": pos.id,
+                    "amount": neg_amount,
+                    "date": neg.date,
+                })
+                matched_ids.add(neg.id)
+                matched_ids.add(pos.id)
+                break
+
+        return pairs
+
+    def mark_transfers(self) -> int:
+        """Detect and mark transfer pairs."""
+        pairs = self.detect()
+        for pair in pairs:
+            outgoing = self.session.get(Transaction, pair["outgoing_id"])
+            incoming = self.session.get(Transaction, pair["incoming_id"])
+            if outgoing and incoming:
+                outgoing.is_transfer = True
+                incoming.is_transfer = True
+                outgoing.transfer_pair_id = incoming.id
+                incoming.transfer_pair_id = outgoing.id
+        self.session.commit()
+        return len(pairs)
+
+    def _is_transfer_pattern(self, description: str) -> bool:
+        desc_upper = description.upper()
+        return any(p.upper() in desc_upper for p in TRANSFER_PATTERNS)

tests/services/test_transfer_detector.py

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+import datetime
+from sqlalchemy import create_engine
+from sqlalchemy.orm import Session
+
+from src.db import Base
+from src.models import *
+from src.services.transfer_detector import TransferDetector
+
+
+def make_session():
+    engine = create_engine("sqlite:///:memory:")
+    Base.metadata.create_all(engine)
+    return Session(engine)
+
+
+def test_detect_matching_transfer():
+    session = make_session()
+    member = HouseholdMember(name="Andrew", relationship="self")
+    session.add(member)
+    session.flush()
+    checking = Account(name="Checking", institution="WF", account_type="checking", owner_id=member.id)
+    chase = Account(name="Chase", institution="Chase", account_type="credit", owner_id=member.id)
+    session.add_all([checking, chase])
+    session.flush()
+
+    # Payment from checking to Chase
+    txn1 = Transaction(
+        date=datetime.date(2026, 1, 29), amount=-1461.35,
+        description="CHASE CREDIT CRD EPAY", raw_description="CHASE CREDIT CRD EPAY 260128 9077835526 ANDREW B CONLON",
+        account_id=checking.id,
+    )
+    txn2 = Transaction(
+        date=datetime.date(2026, 1, 28), amount=1461.35,
+        description="Payment Thank You - Web", raw_description="Payment Thank You - Web",
+        account_id=chase.id,
+    )
+    session.add_all([txn1, txn2])
+    session.commit()
+
+    detector = TransferDetector(session)
+    pairs = detector.detect()
+    assert len(pairs) == 1
+    assert pairs[0]["amount"] == 1461.35
+
+
+def test_detect_marks_transfers():
+    session = make_session()
+    member = HouseholdMember(name="Andrew", relationship="self")
+    session.add(member)
+    session.flush()
+    checking = Account(name="Checking", institution="WF", account_type="checking", owner_id=member.id)
+    chase = Account(name="Chase", institution="Chase", account_type="credit", owner_id=member.id)
+    session.add_all([checking, chase])
+    session.flush()
+
+    txn1 = Transaction(
+        date=datetime.date(2026, 1, 29), amount=-1461.35,
+        description="CHASE CREDIT CRD EPAY", account_id=checking.id,
+    )
+    txn2 = Transaction(
+        date=datetime.date(2026, 1, 28), amount=1461.35,
+        description="Payment Thank You - Web", account_id=chase.id,
+    )
+    session.add_all([txn1, txn2])
+    session.commit()
+
+    detector = TransferDetector(session)
+    detector.mark_transfers()
+
+    session.refresh(txn1)
+    session.refresh(txn2)
+    assert txn1.is_transfer is True
+    assert txn2.is_transfer is True
+
+
+def test_no_false_positive_different_amounts():
+    session = make_session()
+    member = HouseholdMember(name="Andrew", relationship="self")
+    session.add(member)
+    session.flush()
+    checking = Account(name="Checking", institution="WF", account_type="checking", owner_id=member.id)
+    chase = Account(name="Chase", institution="Chase", account_type="credit", owner_id=member.id)
+    session.add_all([checking, chase])
+    session.flush()
+
+    txn1 = Transaction(
+        date=datetime.date(2026, 1, 29), amount=-1461.35,
+        description="CHASE CREDIT CRD EPAY", account_id=checking.id,
+    )
+    txn2 = Transaction(
+        date=datetime.date(2026, 1, 28), amount=500.00,
+        description="Payment Thank You - Web", account_id=chase.id,
+    )
+    session.add_all([txn1, txn2])
+    session.commit()
+
+    detector = TransferDetector(session)
+    pairs = detector.detect()
+    assert len(pairs) == 0