hono: add JSON request body size limit

Author: Theodor N. Engøy

packages/middleware/hono/README.md

@@ -38,6 +38,14 @@ const app = createMcpHonoApp();
 app.all('/mcp', c => transport.handleRequest(c.req.raw, { parsedBody: c.get('parsedBody') }));
 ```
 
+### Options
+
+`createMcpHonoApp({ ... })` supports:
+
+- `host` (default: `127.0.0.1`): used for localhost DNS rebinding protection behavior
+- `allowedHosts` (optional): explicit allowed hostnames
+- `maxBodyBytes` (default: `1_000_000`): maximum JSON request body size parsed by the built-in middleware
+
 ### Host header validation (DNS rebinding protection)
 
 ```ts

packages/middleware/hono/src/hono.ts

@@ -3,6 +3,39 @@ import { Hono } from 'hono';
 
 import { hostHeaderValidation, localhostHostValidation } from './middleware/hostHeaderValidation.js';
 
+const DEFAULT_MAX_BODY_BYTES = 1_000_000; // 1MB
+
+async function readRequestTextWithLimit(req: Request, maxBytes: number): Promise<string> {
+    const body = req.body;
+    if (!body) return '';
+
+    const reader = body.getReader();
+    const chunks: Uint8Array[] = [];
+    let total = 0;
+
+    while (true) {
+        const { value, done } = await reader.read();
+        if (done) break;
+        if (!value) continue;
+
+        total += value.byteLength;
+        if (total > maxBytes) {
+            void reader.cancel().catch(() => {});
+            throw new Error('payload_too_large');
+        }
+        chunks.push(value);
+    }
+
+    const out = new Uint8Array(total);
+    let offset = 0;
+    for (const c of chunks) {
+        out.set(c, offset);
+        offset += c.byteLength;
+    }
+
+    return new TextDecoder().decode(out);
+}
+
 /**
  * Options for creating an MCP Hono application.
  */
@@ -22,6 +55,14 @@ export interface CreateMcpHonoAppOptions {
      * to restrict which hostnames are allowed.
      */
     allowedHosts?: string[];
+
+    /**
+     * Maximum JSON request body size in bytes.
+     * Used by the built-in JSON parsing middleware for basic DoS resistance.
+     *
+     * @default 1_000_000 (1 MB)
+     */
+    maxBodyBytes?: number;
 }
 
 /**
@@ -39,7 +80,7 @@ export interface CreateMcpHonoAppOptions {
  * @returns A configured Hono application
  */
 export function createMcpHonoApp(options: CreateMcpHonoAppOptions = {}): Hono {
-    const { host = '127.0.0.1', allowedHosts } = options;
+    const { host = '127.0.0.1', allowedHosts, maxBodyBytes = DEFAULT_MAX_BODY_BYTES } = options;
 
     const app = new Hono();
 
@@ -55,9 +96,26 @@ export function createMcpHonoApp(options: CreateMcpHonoAppOptions = {}): Hono {
             return await next();
         }
 
+        // Fast-path: reject known oversized payloads without reading.
+        const clRaw = c.req.header('content-length') ?? '';
+        const cl = Number(clRaw);
+        if (Number.isFinite(cl) && cl > maxBodyBytes) {
+            return c.text('Payload too large', 413);
+        }
+
+        // Parse from a clone so we don't consume the original request stream.
+        let text: string;
+        try {
+            text = await readRequestTextWithLimit(c.req.raw.clone(), maxBodyBytes);
+        } catch (error) {
+            if (error instanceof Error && error.message === 'payload_too_large') {
+                return c.text('Payload too large', 413);
+            }
+            return c.text('Invalid JSON', 400);
+        }
+
         try {
-            // Parse from a clone so we don't consume the original request stream.
-            const parsed = await c.req.raw.clone().json();
+            const parsed = JSON.parse(text);
             c.set('parsedBody', parsed);
         } catch {
             // Mirror express.json() behavior loosely: reject invalid JSON.

packages/middleware/hono/test/hono.test.ts

@@ -77,6 +77,19 @@ describe('@modelcontextprotocol/hono', () => {
         expect(await res.json()).toEqual({ a: 1 });
     });
 
+    test('createMcpHonoApp returns 413 on oversized JSON bodies', async () => {
+        const app = createMcpHonoApp({ maxBodyBytes: 10 });
+        app.post('/echo', (c: Context) => c.text('ok'));
+
+        const res = await app.request('http://localhost/echo', {
+            method: 'POST',
+            headers: { Host: 'localhost:3000', 'content-type': 'application/json' },
+            body: JSON.stringify({ a: '0123456789' })
+        });
+        expect(res.status).toBe(413);
+        expect(await res.text()).toBe('Payload too large');
+    });
+
     test('createMcpHonoApp returns 400 on invalid JSON', async () => {
         const app = createMcpHonoApp();
         app.post('/echo', (c: Context) => c.text('ok'));