304 vs 200: Not Modified vs OK
304 and 200 can look similar in logs, but they tell clients, crawlers, and API consumers different things.
| Aspect | HTTP 304 — Not Modified | HTTP 200 — OK |
|---|---|---|
| Definition | Issued in response to a conditional GET (If-None-Match or If-Modified-Since). The server confirms the cached version is still valid, saving bandwidth by not resending the body. | The server successfully processed the request and is returning the requested resource or result. This is the most common status code in normal web traffic. |
| Plain-language summary | The resource has not changed since the version the client already has cached. The server sends no body — the client uses its cached copy. This is the conditional request success response, triggered by If-None-Match or If-Modified-Since request headers. | The server received the request, processed it, and is returning the result. This is the expected response for most GET, POST, PUT, and PATCH requests. If you see 200 in your logs, the connection and request lifecycle completed without error — though the business logic in the response body may still contain application-level errors depending on the API design. |
| When to use | You do not "choose" to return 304 — it is issued automatically when the server honors a conditional GET. Implement 304 support by: setting ETag or Last-Modified headers on cacheable resources, validating the If-None-Match / If-Modified-Since headers in your server handler, and returning 304 (no body) when the resource is unchanged. | Return 200 when the request succeeded and there is a body to return. For successful creation use 201. For successful deletion or updates with no body use 204. Avoid wrapping error messages inside 200 responses — it breaks client error handling and monitoring dashboards. |
| Client behavior | Client uses its cached copy of the resource. No new body is received. The response may include updated header fields (e.g., Cache-Control, Expires) to refresh the cache metadata. Browsers update cache freshness information and render the cached version. | Clients display the response body as-is. Browsers render the page. API clients parse the JSON body. No retry is attempted. If caching headers are present, the response may be stored. |
| Caching behavior | The core of HTTP conditional caching. Saves bandwidth by confirming the cached version is still valid. Works with ETags (content hash) or Last-Modified timestamps. Proxies and CDNs also honor these and may serve 304s from their own caches without hitting the origin. | Cacheable by default if Cache-Control, Expires, or Last-Modified headers are present. Without explicit cache headers, client behavior varies. Set Cache-Control: no-store for dynamic API responses, or appropriate max-age for stable resources. |
| SEO / crawler impact | Search crawlers interpret 304 (redirect-codes) for indexation and link equity accordingly. | Search crawlers interpret 200 (success) for indexation and link equity accordingly. |
| API / backend impact | API clients branching on 304 expect Not Modified semantics. | API clients branching on 200 expect OK semantics. |
| Safe to retry? | Follow redirect, then retry original intent | Only after fixing the underlying cause |
Common real-world scenarios
When you see HTTP 304
High 304 rates on static assets (JS, CSS, images) are healthy and expected — this is efficient caching working correctly. Unexpected 304s on API endpoints that should always return fresh data indicate ETag generation is too conservative. Watch for: 304s on endpoints where the client always expects fresh data, and missing 304 support on cacheable static assets (wastes bandwidth).
When you see HTTP 200
High-volume 200s are normal. Watch for latency spikes on 200 responses — they indicate slow processing before the successful response. Alert on p99 latency, not just 5xx rate. Application errors returned inside 200 bodies (common in older RPC-style APIs) are invisible to standard APM dashboards unless you parse the body.
Decision rule
Use 304 when the response should communicate not modified behavior; use 200 when ok is the accurate protocol signal.
A frequent mistake is swapping 304 and 200 for convenience; that causes client retry bugs, incorrect cache signals, and misleading monitoring data.
Use 304 when the correct protocol signal is Not Modified. Use 200 when the correct signal is OK. Returning either code for the wrong reason breaks client expectations, cache behavior, and monitoring accuracy.
FAQ
What is the biggest difference between 304 and 200?
304 communicates Not Modified, while 200 communicates OK. Choosing the right one keeps clients and intermediaries predictable.
Do 304 and 200 have SEO or caching impact?
Yes. Search engines and caches interpret status classes differently. Use each code according to its semantics to avoid accidental indexing, stale responses, or crawl inefficiency.
Can APIs safely return 304 instead of 200?
Only when it matches contract semantics. API clients often branch logic by exact code, so swapping them can break retries, auth handling, or user-facing errors.
Full guides
HTTP 304 Not Modified — full guide · HTTP 200 OK — full guide · All comparisons · HTTP 304 status reference · HTTP 200 status reference