diff --git a/docs/REQUEST_SIZE_PROTECTION.md b/docs/REQUEST_SIZE_PROTECTION.md
new file mode 100644
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+++ b/docs/REQUEST_SIZE_PROTECTION.md
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+# 🛡️ API Request Body Size Protection
+
+This document details the request payload size protection middleware for NotifyChain HTTP services (Issue #688).
+
+---
+
+## 1. Motivation & Threat Mitigation
+
+To protect API servers against memory exhaustion attacks, slowloris buffer flooding, and DoS from arbitrarily large JSON payloads, strict size limits are enforced before requests reach downstream controllers.
+
+---
+
+## 2. Protection Mechanics
+
+1. **Header Inspection**: If `Content-Length` exceeds `maxSizeBytes` (default: 1MB / 1,048,576 bytes), the request is rejected immediately with HTTP `413 Payload Too Large` without reading the body.
+2. **Streaming Termination**: For chunked transfers without pre-declared lengths, incoming stream data chunks are counted in real-time. If the threshold is passed, the stream is paused, event listeners are cleaned up, and HTTP `413` is returned immediately.
+
+---
+
+## 3. Response Format
+
+```json
+{
+  "error": "PAYLOAD_TOO_LARGE",
+  "message": "Request body exceeds maximum allowed size of 1048576 bytes.",
+  "maxSizeBytes": 1048576,
+  "declaredSizeBytes": 5242880
+}
+```
diff --git a/listener/src/api/request-size-limiter.test.ts b/listener/src/api/request-size-limiter.test.ts
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+++ b/listener/src/api/request-size-limiter.test.ts
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+import { EventEmitter } from 'events';
+import { Request, Response, NextFunction } from 'express';
+import {
+  createRequestSizeLimiter,
+  DEFAULT_MAX_REQUEST_SIZE_BYTES,
+} from './request-size-limiter';
+
+describe('API Request Size Protection (Issue #688)', () => {
+  const createMockReqRes = (contentLength?: string) => {
+    const req = Object.assign(new EventEmitter(), {
+      headers: contentLength ? { 'content-length': contentLength } : {},
+      pause: jest.fn(),
+    }) as unknown as Request;
+
+    const res = {
+      status: jest.fn().mockReturnThis(),
+      json: jest.fn().mockReturnThis(),
+    } as unknown as Response;
+
+    const next = jest.fn() as NextFunction;
+
+    return { req, res, next };
+  };
+
+  test('allows requests within size limits', () => {
+    const middleware = createRequestSizeLimiter({ maxSizeBytes: 1024 });
+    const { req, res, next } = createMockReqRes('500');
+
+    middleware(req, res, next);
+
+    expect(next).toHaveBeenCalledTimes(1);
+    expect(res.status).not.toHaveBeenCalled();
+  });
+
+  test('rejects oversized Content-Length header with 413 Payload Too Large', () => {
+    const middleware = createRequestSizeLimiter({ maxSizeBytes: 1024 });
+    const { req, res, next } = createMockReqRes('2048');
+
+    middleware(req, res, next);
+
+    expect(next).not.toHaveBeenCalled();
+    expect(res.status).toHaveBeenCalledWith(413);
+    expect(res.json).toHaveBeenCalledWith(
+      expect.objectContaining({
+        error: 'PAYLOAD_TOO_LARGE',
+        maxSizeBytes: 1024,
+        declaredSizeBytes: 2048,
+      })
+    );
+  });
+
+  test('aborts stream when streaming chunked data exceeds limit', () => {
+    const middleware = createRequestSizeLimiter({ maxSizeBytes: 100 });
+    const { req, res, next } = createMockReqRes();
+
+    middleware(req, res, next);
+    expect(next).toHaveBeenCalled();
+
+    // Emit chunks exceeding 100 bytes
+    req.emit('data', Buffer.alloc(60));
+    expect(res.status).not.toHaveBeenCalled();
+
+    req.emit('data', Buffer.alloc(60)); // total = 120 > 100
+    expect(res.status).toHaveBeenCalledWith(413);
+    expect(res.json).toHaveBeenCalledWith(
+      expect.objectContaining({
+        error: 'PAYLOAD_TOO_LARGE',
+        receivedBytes: 120,
+      })
+    );
+  });
+});
diff --git a/listener/src/api/request-size-limiter.ts b/listener/src/api/request-size-limiter.ts
new file mode 100644
index 00000000..da8d7853
--- /dev/null
+++ b/listener/src/api/request-size-limiter.ts
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+/**
+ * API Request Size Protection Middleware (Issue #688)
+ *
+ * Enforces strict byte-size limits on incoming HTTP request payloads,
+ * rejecting oversized bodies with HTTP 413 Payload Too Large and aborting streams.
+ */
+
+import { Request, Response, NextFunction } from 'express';
+
+export interface RequestSizeLimiterOptions {
+  /** Maximum allowable request body size in bytes (default: 1MB = 1,048,576 bytes). */
+  maxSizeBytes?: number;
+}
+
+export const DEFAULT_MAX_REQUEST_SIZE_BYTES = 1024 * 1024; // 1 MB
+
+/**
+ * Creates Express middleware enforcing request body size limits.
+ */
+export function createRequestSizeLimiter(options: RequestSizeLimiterOptions = {}) {
+  const maxBytes = options.maxSizeBytes || DEFAULT_MAX_REQUEST_SIZE_BYTES;
+
+  return (req: Request, res: Response, next: NextFunction): void => {
+    // 1. Fast-path check Content-Length header if present
+    const contentLength = req.headers['content-length'];
+    if (contentLength) {
+      const declaredSize = parseInt(contentLength, 10);
+      if (!isNaN(declaredSize) && declaredSize > maxBytes) {
+        res.status(413).json({
+          error: 'PAYLOAD_TOO_LARGE',
+          message: `Request body exceeds maximum allowed size of ${maxBytes} bytes.`,
+          maxSizeBytes: maxBytes,
+          declaredSizeBytes: declaredSize,
+        });
+        return;
+      }
+    }
+
+    // 2. Stream chunk inspection for chunked transfer-encoding
+    let receivedBytes = 0;
+    let limitExceeded = false;
+
+    const onData = (chunk: Buffer | string): void => {
+      if (limitExceeded) return;
+
+      receivedBytes += typeof chunk === 'string' ? Buffer.byteLength(chunk) : chunk.length;
+
+      if (receivedBytes > maxBytes) {
+        limitExceeded = true;
+
+        // Clean up listeners and destroy incoming stream to prevent memory buffering
+        req.removeListener('data', onData);
+        req.pause();
+
+        res.status(413).json({
+          error: 'PAYLOAD_TOO_LARGE',
+          message: `Streaming payload exceeded maximum size limit of ${maxBytes} bytes.`,
+          maxSizeBytes: maxBytes,
+          receivedBytes,
+        });
+      }
+    };
+
+    req.on('data', onData);
+
+    // Continue to next handler if size check passes
+    next();
+  };
+}