diff --git a/package.json b/package.json
index 55d8affa..f485d2e5 100644
--- a/package.json
+++ b/package.json
@@ -20,17 +20,22 @@
     "preset": "ts-jest",
     "testEnvironment": "node",
     "roots": [
-      "<rootDir>/src"
+      "<rootDir>/src",
+      "<rootDir>/tests"
     ],
     "testMatch": [
-      "**/__tests__/**/*.test.ts"
+      "**/__tests__/**/*.test.ts",
+      "**/tests/**/*.test.ts"
     ],
     "moduleFileExtensions": [
       "ts",
       "js",
       "json"
     ],
-    "clearMocks": true
+    "clearMocks": true,
+    "transform": {
+      "^.+\\.tsx?$": ["ts-jest", { "tsconfig": "tsconfig.test.json" }]
+    }
   },
   "dependencies": {
     "@prisma/client": "^5.10.0",
diff --git a/src/api.ts b/src/api.ts
index a55fd8b7..cc3dea74 100644
--- a/src/api.ts
+++ b/src/api.ts
@@ -441,16 +441,20 @@ export function createApp(): express.Application {
 
         // Add data rows
         for (const transfer of result.transfers) {
-          const displayAmount = toDisplayAmount(transfer.amount);
+          const t = transfer as Record<string, unknown>;
+          const displayAmount = toDisplayAmount(String(t.amount ?? "0"));
+          const closedAt = t.ledgerClosedAt instanceof Date
+            ? t.ledgerClosedAt
+            : new Date(String(t.ledgerClosedAt ?? 0));
           csvLines.push(
             formatCSVRow([
-              transfer.ledgerClosedAt.toISOString(),
-              transfer.eventType,
-              transfer.fromAddress || "",
-              transfer.toAddress || "",
+              closedAt.toISOString(),
+              t.eventType,
+              t.fromAddress || "",
+              t.toAddress || "",
               displayAmount,
-              transfer.contractId,
-              transfer.ledger,
+              t.contractId,
+              t.ledger,
             ])
           );
         }
diff --git a/src/db.ts b/src/db.ts
index dfa1a924..2a9bc125 100644
--- a/src/db.ts
+++ b/src/db.ts
@@ -2,6 +2,17 @@ import { PrismaClient, Prisma } from "@prisma/client";
 import type { NftTransferRecord, NftMetadataPayload } from "./ingester/nft";
 import { decodeCursor, encodeCursor, parseODataFilter, parseODataSelect, projectRecord } from "./lib/odata";
 
+const STROOPS = 10_000_000n;
+
+function toDisplayAmount(amount: string): string {
+  const raw = BigInt(amount);
+  const abs = raw < 0n ? -raw : raw;
+  const integer = abs / STROOPS;
+  const remainder = abs % STROOPS;
+  const sign = raw < 0n ? "-" : "";
+  return `${sign}${integer}.${String(remainder).padStart(7, "0")}`;
+}
+
 // ─── Singleton Prisma client ──────────────────────────────────────────────────
 // Re-use one connection pool across the process.
 const globalForPrisma = globalThis as unknown as { prisma?: PrismaClient };
diff --git a/tests/integration/reorg.test.ts b/tests/integration/reorg.test.ts
new file mode 100644
index 00000000..e647639e
--- /dev/null
+++ b/tests/integration/reorg.test.ts
@@ -0,0 +1,222 @@
+/**
+ * Integration test: reorg simulation + deduplication (#79)
+ *
+ * Validates that the ingestion layer is fully idempotent under replay
+ * conditions:
+ *
+ *   1. Normal ingest — a batch of Horizon ledger pages is processed once.
+ *   2. Replay (full duplicate) — the exact same batch is fed a second time;
+ *      no new rows should be inserted.
+ *   3. Skipped-ledger gap + resume — simulates a reorg where ledger N is
+ *      missed on the first pass, then the range including N is replayed;
+ *      only the previously-missing record should be added.
+ *
+ * @prisma/client is mocked at the PrismaClient constructor level so the real
+ * upsertTransfers function is exercised with an in-memory dedup store that
+ * mirrors the Postgres @unique(eventId) constraint.
+ */
+
+import type { TransferRecord } from "../../src/db";
+
+// ── In-memory dedup store (declared before jest.mock hoisting) ───────────────
+// Using a module-scoped object so the factory closure can reference it safely.
+const _store: Map<string, TransferRecord> = new Map();
+const _createManyCalls: Array<{ data: TransferRecord[]; skipDuplicates: boolean }> = [];
+
+// ── Mock @prisma/client ───────────────────────────────────────────────────────
+// By mocking the constructor we intercept the module-level `prisma` singleton
+// created inside src/db.ts, which is what upsertTransfers closes over.
+jest.mock("@prisma/client", () => {
+  return {
+    PrismaClient: jest.fn().mockImplementation(() => ({
+      tokenTransfer: {
+        createMany: jest.fn(async (args: { data: TransferRecord[]; skipDuplicates: boolean }) => {
+          _createManyCalls.push(args);
+
+          let count = 0;
+          for (const record of args.data) {
+            if (!_store.has(record.eventId)) {
+              _store.set(record.eventId, record);
+              count++;
+            }
+            // Duplicate silently skipped — mirrors Postgres UNIQUE constraint
+          }
+          return { count };
+        }),
+      },
+      // Silence other Prisma calls used by db module initialisation
+      $queryRaw: jest.fn().mockResolvedValue([]),
+      $on: jest.fn(),
+    })),
+    Prisma: {
+      // Provide stub values for any Prisma namespace references used in db.ts
+      sql: jest.fn((strings: TemplateStringsArray, ...values: unknown[]) => ({ strings, values })),
+      join: jest.fn((arr: unknown[]) => arr),
+      empty: "",
+    },
+  };
+});
+
+// Import AFTER the mock is registered so db.ts gets the mock PrismaClient
+import { upsertTransfers } from "../../src/db";
+
+// ── Helpers ───────────────────────────────────────────────────────────────────
+
+function makeTransfer(overrides: Partial<TransferRecord> & { eventId: string }): TransferRecord {
+  return {
+    contractId:     "CTEST_CONTRACT_001",
+    eventType:      "transfer",
+    fromAddress:    "GFROM_ADDR_AAAA",
+    toAddress:      "GTO_ADDR_BBBB",
+    amount:         "100000000",
+    ledger:         1000,
+    ledgerClosedAt: new Date("2024-06-01T00:00:00Z"),
+    txHash:         "deadbeef0000",
+    ...overrides,
+  };
+}
+
+/**
+ * Build a Horizon-style ledger page: 2 events per ledger
+ * (eventIds: `<ledger>-0001`, `<ledger>-0002`).
+ */
+function makeLedgerPage(fromLedger: number, toLedger: number): TransferRecord[] {
+  const records: TransferRecord[] = [];
+  for (let ledger = fromLedger; ledger <= toLedger; ledger++) {
+    records.push(
+      makeTransfer({
+        eventId:   `${ledger}-0001`,
+        ledger,
+        txHash:    `tx${ledger}a`,
+      }),
+      makeTransfer({
+        eventId:   `${ledger}-0002`,
+        ledger,
+        txHash:    `tx${ledger}b`,
+        eventType: "mint",
+        fromAddress: null,
+      })
+    );
+  }
+  return records;
+}
+
+// ── Test suite ────────────────────────────────────────────────────────────────
+
+describe("Reorg simulation — ingestion deduplication", () => {
+  beforeEach(() => {
+    _store.clear();
+    _createManyCalls.length = 0;
+  });
+
+  // ── 1. Normal ingest ────────────────────────────────────────────────────────
+  describe("normal ingest (ledgers 1–5)", () => {
+    it("inserts all records on first pass", async () => {
+      const page = makeLedgerPage(1, 5);
+      const inserted = await upsertTransfers(page);
+
+      expect(inserted).toBe(10); // 2 events × 5 ledgers
+      expect(_store.size).toBe(10);
+    });
+
+    it("each record is stored exactly once", async () => {
+      const page = makeLedgerPage(1, 5);
+      await upsertTransfers(page);
+
+      for (const record of page) {
+        expect(_store.has(record.eventId)).toBe(true);
+      }
+    });
+  });
+
+  // ── 2. Full replay — same dataset fed twice ─────────────────────────────────
+  describe("full replay (same dataset ingested twice)", () => {
+    it("inserts 0 new records on the second pass", async () => {
+      const page = makeLedgerPage(1, 5);
+      const firstPass = await upsertTransfers(page);
+      expect(firstPass).toBe(10);
+
+      const secondPass = await upsertTransfers(page);
+      expect(secondPass).toBe(0);
+    });
+
+    it("store size is unchanged after replay", async () => {
+      const page = makeLedgerPage(1, 5);
+      await upsertTransfers(page);
+      const before = _store.size;
+
+      await upsertTransfers(page);
+      expect(_store.size).toBe(before);
+    });
+
+    it("no duplicate records exist after replay", async () => {
+      const page = makeLedgerPage(1, 5);
+      await upsertTransfers(page);
+      await upsertTransfers(page);
+
+      expect(_store.size).toBe(10); // exactly 10 unique events
+    });
+  });
+
+  // ── 3. Skipped-ledger gap + resume ─────────────────────────────────────────
+  describe("skipped-ledger gap simulation", () => {
+    it("detects missing ledger after the first pass", async () => {
+      // First pass: ledgers 1–3 and 5 (ledger 4 missing — simulates reorg gap)
+      await upsertTransfers([...makeLedgerPage(1, 3), ...makeLedgerPage(5, 5)]);
+
+      expect(_store.has("4-0001")).toBe(false);
+      expect(_store.has("4-0002")).toBe(false);
+    });
+
+    it("backfills the missing ledger without duplicating existing records", async () => {
+      await upsertTransfers([...makeLedgerPage(1, 3), ...makeLedgerPage(5, 5)]);
+      const sizeAfterFirst = _store.size; // 8 records — no ledger 4
+
+      // Resume: replay full range including the missing ledger 4
+      const newlyInserted = await upsertTransfers(makeLedgerPage(1, 5));
+
+      expect(newlyInserted).toBe(2);                   // only ledger 4's 2 events
+      expect(_store.size).toBe(sizeAfterFirst + 2);    // 10 total
+      expect(_store.has("4-0001")).toBe(true);
+      expect(_store.has("4-0002")).toBe(true);
+    });
+
+    it("no missing data after resume — all 5 ledgers are present", async () => {
+      await upsertTransfers([...makeLedgerPage(1, 3), ...makeLedgerPage(5, 5)]);
+      await upsertTransfers(makeLedgerPage(1, 5));
+
+      for (const record of makeLedgerPage(1, 5)) {
+        expect(_store.has(record.eventId)).toBe(true);
+      }
+    });
+
+    it("system remains consistent after aggressive overlapping replays", async () => {
+      await upsertTransfers(makeLedgerPage(1, 3));
+      await upsertTransfers(makeLedgerPage(3, 5)); // ledger 3 overlaps
+      await upsertTransfers(makeLedgerPage(1, 5)); // full range replay
+
+      expect(_store.size).toBe(10); // exactly 2 events × 5 ledgers — no extras
+    });
+  });
+
+  // ── 4. upsertTransfers contract ────────────────────────────────────────────
+  describe("upsertTransfers contract", () => {
+    it("always passes skipDuplicates:true to Prisma createMany", async () => {
+      await upsertTransfers(makeLedgerPage(1, 1));
+
+      expect(_createManyCalls).toHaveLength(1);
+      expect(_createManyCalls[0].skipDuplicates).toBe(true);
+    });
+
+    it("returns 0 and skips Prisma call for empty input", async () => {
+      const result = await upsertTransfers([]);
+      expect(result).toBe(0);
+      expect(_createManyCalls).toHaveLength(0);
+    });
+
+    it("returns the count of newly inserted records", async () => {
+      const inserted = await upsertTransfers(makeLedgerPage(10, 12));
+      expect(inserted).toBe(6); // 2 events × 3 ledgers
+    });
+  });
+});
diff --git a/tsconfig.test.json b/tsconfig.test.json
new file mode 100644
index 00000000..efc81b36
--- /dev/null
+++ b/tsconfig.test.json
@@ -0,0 +1,8 @@
+{
+  "extends": "./tsconfig.json",
+  "compilerOptions": {
+    "rootDir": ".",
+    "noEmit": true
+  },
+  "include": ["src/**/*", "tests/**/*"]
+}