fix(api): gracefully handle express body-parser 4xx client errors

.jules/bolt.md

@@ -196,3 +196,11 @@ In highly trafficked endpoints like `/v1/chat/completions`, destructuring proper
 
 Action:
 Replaced the object destructuring and `|| {}` fallback with direct property access after an explicit check of `req.body`. Replaced the `for...of` loop with a classic `for` loop caching the `messages.length` in a local variable `messagesLen` before the loop. This reduces garbage collection pressure and significantly improves execution speed on hot paths.
+
+2026-04-30 — Handle Express body-parser Client Errors
+Learning: In Express.js applications using `body-parser` or `express.json()`, failing to explicitly handle specific 4xx client errors (such as `charset.unsupported`, `encoding.unsupported`, and `request.aborted`) causes them to fall through to the global error handler, resulting in 500 Internal Server Error responses and log spam, presenting a minor DoS risk.
+Action: Updated the global error handler in `src/index.js` to explicitly intercept these error `.type` properties and return proper 415 or 400 JSON responses.
+
+2026-04-30 — Avoid Modifying Express Router Internals in Tests
+Learning: When writing isolated unit tests using `supertest`, attempting to mutate `app._router.stack` to dynamically inject routes before global error handlers fails because `app._router` is lazily initialized and can be undefined at module load time.
+Action: Test error handlers by constructing an isolated `mockApp` using `express()` that mirrors the production routing logic rather than mutating the internals of the exported `app`.

src/index.js

@@ -121,6 +121,9 @@ app.post('/v1/chat/completions', jsonParser, (req, res) => {
   res.status(200).send(payload);
 });
 
+const ERROR_UNSUPPORTED_MEDIA_TYPE = Buffer.from(JSON.stringify({ error: 'Unsupported media type' }));
+const ERROR_BAD_REQUEST = Buffer.from(JSON.stringify({ error: 'Bad request' }));
+
 // Handle invalid JSON gracefully
 app.use((err, req, res, next) => {
   if (res.headersSent) {
@@ -134,6 +137,14 @@ app.use((err, req, res, next) => {
     res.setHeader('Content-Type', 'application/json; charset=utf-8');
     return res.status(413).send(ERROR_PAYLOAD_TOO_LARGE);
   }
+  if (err.type === 'charset.unsupported' || err.type === 'encoding.unsupported') {
+    res.setHeader('Content-Type', 'application/json; charset=utf-8');
+    return res.status(415).send(ERROR_UNSUPPORTED_MEDIA_TYPE);
+  }
+  if (err.type === 'request.aborted') {
+    res.setHeader('Content-Type', 'application/json; charset=utf-8');
+    return res.status(400).send(ERROR_BAD_REQUEST);
+  }
   next(err);
 });
 

tests/api_robustness.test.js

@@ -1,6 +1,7 @@
 const { test } = require('node:test');
 const assert = require('node:assert');
 const request = require('supertest');
+const express = require('express');
 const { app } = require('../src/index.js');
 
 test('POST /v1/chat/completions handles undefined req.body (e.g. non-JSON content-type)', async () => {
@@ -13,6 +14,53 @@ test('POST /v1/chat/completions handles undefined req.body (e.g. non-JSON conten
   assert.strictEqual(res.body.error, 'Missing or invalid model');
 });
 
+test('POST /v1/chat/completions handles unsupported charset gracefully', async () => {
+  const res = await request(app)
+    .post('/v1/chat/completions')
+    .set('Content-Type', 'application/json; charset=weird')
+    .send('{}');
+  assert.strictEqual(res.status, 415);
+  assert.strictEqual(res.body.error, 'Unsupported media type');
+});
+
+test('POST /v1/chat/completions handles unsupported encoding gracefully', async () => {
+  const res = await request(app)
+    .post('/v1/chat/completions')
+    .set('Content-Type', 'application/json')
+    .set('Content-Encoding', 'weird')
+    .send('{}');
+  assert.strictEqual(res.status, 415);
+  assert.strictEqual(res.body.error, 'Unsupported media type');
+});
+
+test('POST /v1/chat/completions handles aborted requests gracefully', async () => {
+  // Test the error handler explicitly without dealing with express routing internals.
+  // We can inject a route to throw the aborted error, but we need the global error
+  // handler to handle it. Since we can't easily alter the app stack to bypass the 404 handler,
+  // we will construct a mock app and use the exact same logic that we added to index.js.
+  // In a real environment express initialization of the _router might be deferred.
+  const mockApp = express();
+
+  mockApp.post('/trigger-aborted', (req, res, next) => {
+    const err = new Error('request aborted');
+    err.status = 400;
+    err.type = 'request.aborted';
+    next(err);
+  });
+
+  mockApp.use((err, req, res, next) => {
+    if (err.type === 'request.aborted') {
+      res.setHeader('Content-Type', 'application/json; charset=utf-8');
+      return res.status(400).send(Buffer.from(JSON.stringify({ error: 'Bad request' })));
+    }
+    next(err);
+  });
+
+  const res = await request(mockApp).post('/trigger-aborted').send({});
+  assert.strictEqual(res.status, 400);
+  assert.strictEqual(res.body.error, 'Bad request');
+});
+
 test('POST /v1/chat/completions handles payload too large gracefully', async () => {
   const largeString = 'a'.repeat(11 * 1024 * 1024); // 11mb, which is > 10mb limit
   const res = await request(app)