Content Negotiation: Accept and Content-Type Headers

APIs return JSON to all clients regardless of what format they requested, breaking clients that need XML or CBOR, and returning wrong content-type headers causing parse errors.

Inspect the Accept header to determine response format. Set Content-Type to match what was actually sent. Use 406 Not Acceptable when the requested format is unsupported.

app.get('/data', (req, res) => {
  const accept = req.headers.accept || 'application/json';
  const data = getData();

  if (accept.includes('application/json') || accept.includes('*/*')) {
    return res.json(data);
  }
  if (accept.includes('text/csv')) {
    res.setHeader('Content-Type', 'text/csv');
    return res.send(toCsv(data));
  }
  if (accept.includes('application/xml')) {
    res.setHeader('Content-Type', 'application/xml');
    return res.send(toXml(data));
  }
  res.status(406).json({ error: 'Not Acceptable', supported: ['application/json', 'text/csv'] });
});

Content negotiation allows a single endpoint to serve multiple representations without versioning the URL. It's the HTTP-native way to support multiple response formats.