diff --git a/package-lock.json b/package-lock.json
index 09e5180..6efd141 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -8483,6 +8483,7 @@
         "ajv": "8.20.0",
         "ajv-formats": "3.0.1",
         "express": "5.2.1",
+        "express-rate-limit": "^8.2.1",
         "jose": "^6.2.3"
       },
       "devDependencies": {
diff --git a/packages/shared/package.json b/packages/shared/package.json
index 0775226..460ceb3 100644
--- a/packages/shared/package.json
+++ b/packages/shared/package.json
@@ -34,6 +34,7 @@
     "ajv": "8.20.0",
     "ajv-formats": "3.0.1",
     "express": "5.2.1",
+    "express-rate-limit": "^8.2.1",
     "jose": "^6.2.3"
   }
 }
diff --git a/packages/shared/src/dispatch.ts b/packages/shared/src/dispatch.ts
index 583810f..f1a2da5 100644
--- a/packages/shared/src/dispatch.ts
+++ b/packages/shared/src/dispatch.ts
@@ -91,7 +91,15 @@ export interface DispatchDeps {
 // One Ajv instance for the process. Schemas are JSON Schema (draft-07 / the
 // OpenAPI 3.1 subset our tools use). `strict: false` because tool authors use
 // vocabulary (e.g. `description`) liberally; we only need structural validation.
-const ajv = new Ajv({ allErrors: true, strict: false, coerceTypes: false });
+//
+// `allErrors: false` (the default) is deliberate and security-relevant: the
+// input is UNTRUSTED, and `allErrors: true` makes ajv enumerate every schema
+// violation, which an attacker can weaponize into CPU/memory exhaustion by
+// sending a large/deeply-nested payload that fails many constraints at once
+// (CodeQL js/resource-exhaustion). Short-circuiting on the first error caps the
+// work per request; the 400 still names the first failing path, which is enough
+// for a caller to fix their input.
+const ajv = new Ajv({ allErrors: false, strict: false, coerceTypes: false });
 addFormats(ajv);
 
 const validatorCache = new WeakMap<object, ValidateFunction>();
diff --git a/packages/shared/src/http.test.ts b/packages/shared/src/http.test.ts
index 5c7dfc4..65c0e97 100644
--- a/packages/shared/src/http.test.ts
+++ b/packages/shared/src/http.test.ts
@@ -16,7 +16,7 @@ import { NoopAuditLogger } from './audit.js';
 import { InMemoryRateLimiter } from './rate-limit.js';
 import type { DispatchDeps } from './dispatch.js';
 
-function build() {
+function build(opts: { edgeRateLimit?: { windowMs: number; limit: number } } = {}) {
   const registry = new ToolRegistry();
   registry.register(
     defineTool<{ id: string }, { id: string; ok: boolean }>({
@@ -71,6 +71,7 @@ function build() {
     deps,
     mcpInfo: { name: 'sh-mcp-test', version: '0.0.1' },
     openApi: { info: { title: 'Test', version: '0.0.1' }, servers: [{ url: 'http://x' }] },
+    edgeRateLimit: opts.edgeRateLimit,
   });
 }
 
@@ -145,6 +146,25 @@ describe('createApp routes', () => {
     expect(res.status).toBe(429);
   });
 
+  it('edge rate limiter — throttles by IP BEFORE auth (429 on an unauthenticated flood)', async () => {
+    // limit 2/window. Unauthenticated requests would normally 401, but the edge
+    // limiter sits in front of `authenticate`, so the 3rd request is throttled
+    // (429), not 401 — proving floods are capped before any JWT verification.
+    const app = build({ edgeRateLimit: { windowMs: 60_000, limit: 2 } });
+    expect((await request(app).post('/tools/lookup_thing').send({ id: '1' })).status).toBe(401);
+    expect((await request(app).post('/tools/lookup_thing').send({ id: '2' })).status).toBe(401);
+    const res = await request(app).post('/tools/lookup_thing').send({ id: '3' });
+    expect(res.status).toBe(429);
+    expect(res.body).toEqual({ error: 'rate_limited' });
+  });
+
+  it('edge rate limiter — also fronts /mcp', async () => {
+    const app = build({ edgeRateLimit: { windowMs: 60_000, limit: 1 } });
+    expect((await request(app).post('/mcp').send({})).status).toBe(401);
+    const res = await request(app).post('/mcp').send({});
+    expect(res.status).toBe(429);
+  });
+
   it('POST /mcp — 401 without a token', async () => {
     const res = await request(build())
       .post('/mcp')
diff --git a/packages/shared/src/http.ts b/packages/shared/src/http.ts
index 9a4d40b..883f677 100644
--- a/packages/shared/src/http.ts
+++ b/packages/shared/src/http.ts
@@ -18,6 +18,7 @@
  */
 
 import express, { type Express, type Request, type Response, type NextFunction } from 'express';
+import { rateLimit, type RateLimitRequestHandler } from 'express-rate-limit';
 import { StreamableHTTPServerTransport } from '@modelcontextprotocol/sdk/server/streamableHttp.js';
 
 import { AuthError } from './cognito-auth.js';
@@ -43,6 +44,16 @@ export interface CreateAppOptions {
   /** MCP handshake identity + OpenAPI `info`/`servers`. */
   mcpInfo: McpServerInfo;
   openApi: BuildOpenApiOptions;
+  /**
+   * Per-IP edge rate limit for the authenticated routes (`/mcp`, `/tools/:name`).
+   * This is a coarse abuse/DoS guard that runs BEFORE `authProvider.authenticate`
+   * — so it throttles unauthenticated and invalid-token floods before the (more
+   * expensive) JWT verification and dispatch. It complements, and does not
+   * replace, the per-`sub` + per-tool limiter inside `executeTool` (design.md
+   * §7.3). Defaults: 120 requests / 60s / IP. In production the API Gateway/WAF
+   * is the first edge; this is defense-in-depth.
+   */
+  edgeRateLimit?: { windowMs: number; limit: number };
 }
 
 /** Express `Request` augmented with the authenticated context. */
@@ -58,6 +69,21 @@ export function createApp(options: CreateAppOptions): Express {
   const app = express();
   app.use(express.json());
 
+  // --- Per-IP edge rate limiter for the authenticated routes ---
+  // Runs ahead of auth so request floods are throttled before JWT verification
+  // and dispatch. Keyed by client IP (express-rate-limit default). Returns the
+  // same 429 shape adapters use elsewhere; standard RateLimit headers, no legacy.
+  const { windowMs = 60_000, limit = 120 } = options.edgeRateLimit ?? {};
+  const edgeRateLimit: RateLimitRequestHandler = rateLimit({
+    windowMs,
+    limit,
+    standardHeaders: 'draft-7',
+    legacyHeaders: false,
+    handler: (_req: Request, res: Response) => {
+      res.status(429).json({ error: 'rate_limited' });
+    },
+  });
+
   // --- Unauthenticated liveness ---
   app.get('/healthz', (_req: Request, res: Response) => {
     res.status(200).json({ status: 'ok' });
@@ -103,12 +129,12 @@ export function createApp(options: CreateAppOptions): Express {
     await server.connect(transport);
     await transport.handleRequest(req, res, req.body);
   };
-  app.post('/mcp', authenticate, (req, res) => void handleMcp(req, res));
-  app.get('/mcp', authenticate, (req, res) => void handleMcp(req, res));
-  app.delete('/mcp', authenticate, (req, res) => void handleMcp(req, res));
+  app.post('/mcp', edgeRateLimit, authenticate, (req, res) => void handleMcp(req, res));
+  app.get('/mcp', edgeRateLimit, authenticate, (req, res) => void handleMcp(req, res));
+  app.delete('/mcp', edgeRateLimit, authenticate, (req, res) => void handleMcp(req, res));
 
   // --- One-shot OpenAPI tool call ---
-  app.post('/tools/:name', authenticate, (req: AuthedRequest, res: Response) => {
+  app.post('/tools/:name', edgeRateLimit, authenticate, (req: AuthedRequest, res: Response) => {
     const ctx = req.authContext!;
     const rawName = req.params['name'];
     const name = Array.isArray(rawName) ? (rawName[0] ?? '') : (rawName ?? '');