diff --git a/backend/src/app.ts b/backend/src/app.ts
index 617e428e..7192b745 100644
--- a/backend/src/app.ts
+++ b/backend/src/app.ts
@@ -14,6 +14,7 @@ import { sandboxMiddleware } from "./middleware/sandbox.middleware.js";
 import { globalRateLimiter, healthRateLimiter } from "./middleware/rate-limiter.middleware.js";
 import { metricsMiddleware } from "./middleware/metrics.middleware.js";
 import { requestIdMiddleware } from "./middleware/requestId.js";
+import { bigIntSafeJsonMiddleware } from "./lib/serialize.js";
 import v1Routes from "./routes/v1/index.js";
 import healthRoutes from "./routes/health.routes.js";
 import metricsRoutes from "./routes/metrics.routes.js";
@@ -116,6 +117,10 @@ const BULK_JSON_PATHS = [
 app.use(BULK_JSON_PATHS, express.json({ limit: "1mb" }));
 app.use(express.json({ limit: "100kb" }));
 
+// BigInt-safe JSON responses (Issue #1493): Prisma bigint columns must be
+// emitted as decimal strings, never throw in res.json().
+app.use(bigIntSafeJsonMiddleware);
+
 // Sandbox mode detection (before versioning)
 app.use(sandboxMiddleware);
 
diff --git a/backend/src/lib/serialize.ts b/backend/src/lib/serialize.ts
new file mode 100644
index 00000000..b309dd3c
--- /dev/null
+++ b/backend/src/lib/serialize.ts
@@ -0,0 +1,58 @@
+/**
+ * BigInt-safe JSON serialization for API responses (Issue #1493).
+ *
+ * Prisma maps `bigint` columns (stream ids, i128 amounts) to JavaScript
+ * `BigInt`, which `JSON.stringify` cannot encode — it throws
+ * `TypeError: Do not know how to serialize a BigInt`. Relying on a global
+ * `BigInt.prototype.toJSON` patch is fragile because it only takes effect when
+ * the module installing it happens to be imported, so this module provides an
+ * explicit replacer plus an Express middleware that applies it to every
+ * `res.json()` call, including error bodies.
+ */
+import type { NextFunction, Request, Response } from "express";
+
+/**
+ * `JSON.stringify` replacer that encodes `bigint` values as decimal strings.
+ *
+ * Strings are used rather than numbers because u64/i128 values routinely exceed
+ * `Number.MAX_SAFE_INTEGER`, where a number would silently lose precision. All
+ * other values are returned unchanged.
+ */
+export function bigIntSafeReplacer(_key: string, value: unknown): unknown {
+  return typeof value === "bigint" ? value.toString() : value;
+}
+
+/**
+ * `JSON.stringify` with BigInt support. Returns `undefined` (matching
+ * `JSON.stringify`) for values that cannot be serialized, such as a bare
+ * `undefined`.
+ */
+export function stringifyJson(value: unknown): string | undefined {
+  return JSON.stringify(value, bigIntSafeReplacer);
+}
+
+/**
+ * Express middleware that routes every `res.json()` call through
+ * `stringifyJson`, so a BigInt anywhere in the response body — including inside
+ * nested objects and arrays — is emitted as a decimal string instead of
+ * throwing.
+ *
+ * Mount this on the app before the routers (and before `errorHandler`) so that
+ * every controller response and every error payload is covered.
+ */
+export function bigIntSafeJsonMiddleware(
+  _req: Request,
+  res: Response,
+  next: NextFunction,
+): void {
+  const jsonWithBigIntSupport = (body?: unknown): Response => {
+    const json = stringifyJson(body);
+    if (!res.getHeader("Content-Type")) {
+      res.setHeader("Content-Type", "application/json; charset=utf-8");
+    }
+    return res.send(json);
+  };
+
+  res.json = jsonWithBigIntSupport as Response["json"];
+  next();
+}
diff --git a/backend/tests/serialize.test.ts b/backend/tests/serialize.test.ts
new file mode 100644
index 00000000..e90d1e8d
--- /dev/null
+++ b/backend/tests/serialize.test.ts
@@ -0,0 +1,79 @@
+import { describe, it, expect } from 'vitest';
+import express from 'express';
+import request from 'supertest';
+import {
+  bigIntSafeJsonMiddleware,
+  bigIntSafeReplacer,
+  stringifyJson,
+} from '../src/lib/serialize.js';
+
+describe('BigInt-safe serializer (#1493)', () => {
+  it('encodes a top-level BigInt as a decimal string', () => {
+    expect(stringifyJson({ streamId: 123456789012345678901234567890n })).toBe(
+      '{"streamId":"123456789012345678901234567890"}',
+    );
+  });
+
+  it('serializes nested objects and arrays containing BigInts', () => {
+    const payload = {
+      streams: [
+        { streamId: 42n, meta: { depositedAmount: 9007199254740993n } },
+        { streamId: 7n, withdrawable: [1n, 2n, 3n] },
+      ],
+      total: 2,
+    };
+
+    const parsed = JSON.parse(stringifyJson(payload)!);
+
+    expect(parsed.streams[0].streamId).toBe('42');
+    expect(parsed.streams[0].meta.depositedAmount).toBe('9007199254740993');
+    expect(parsed.streams[1].withdrawable).toEqual(['1', '2', '3']);
+    expect(parsed.total).toBe(2);
+  });
+
+  it('leaves plain values untouched', () => {
+    const payload = {
+      address: 'GABC',
+      amount: 1.5,
+      active: true,
+      pausedAt: null,
+      tags: ['a', 1],
+    };
+
+    expect(JSON.parse(stringifyJson(payload)!)).toEqual(payload);
+  });
+
+  it('does not depend on a global BigInt.prototype.toJSON patch', () => {
+    // `stream-id.ts` installs such a patch as a side effect, but this module
+    // must work even when the patch was never applied (the module was not
+    // imported by any earlier request handler).
+    expect((BigInt.prototype as unknown as { toJSON?: unknown }).toJSON).toBeUndefined();
+    expect(stringifyJson({ value: 7n })).toBe('{"value":"7"}');
+  });
+
+  it('returns undefined for un-serializable input, like JSON.stringify', () => {
+    expect(stringifyJson(undefined)).toBeUndefined();
+  });
+
+  it('passes unknown keys through the replacer unchanged', () => {
+    expect(bigIntSafeReplacer('key', 'str')).toBe('str');
+    expect(bigIntSafeReplacer('key', 5n)).toBe('5');
+  });
+
+  it('routes every res.json() call through the serializer', async () => {
+    const app = express();
+    app.use(bigIntSafeJsonMiddleware);
+    app.get('/big-int', (_req, res) => {
+      res.json({ streamId: 9007199254740993n, nested: [{ value: 5n }] });
+    });
+
+    const response = await request(app).get('/big-int');
+
+    expect(response.status).toBe(200);
+    expect(response.headers['content-type']).toContain('application/json');
+    expect(response.body).toEqual({
+      streamId: '9007199254740993',
+      nested: [{ value: '5' }],
+    });
+  });
+});