What Is a 203 Status Code?

The 203 Status Code, officially called Non-Authoritative Information, is part of the HTTP/1.1 specification. It indicates that the server has successfully processed a client’s request, but the returned payload has been transformed or modified by an intermediary — like a proxy or cache — before reaching the client.

This means the client did not receive the exact response from the origin server but got a version that was altered along the way. For example, the proxy might have changed the format, stripped sensitive data, or served cached data.
For SEO specialists and developers alike, understanding the 203 response is crucial because it affects data integrity, caching, and troubleshooting — all critical elements for web performance and search engine optimization.

Differences Between 200, 203, and 204 Status Codes

To understand the 203 Status Code, you need to see how it compares to two commonly seen HTTP responses: 200 OK and 204 No Content. Here’s a clear breakdown:

What Does Each Status Code Mean?

Status Code	Meaning	Cacheable?	Example Use Case
200 OK	The request succeeded; the response is direct from the origin server.	Yes	Client requests a page, and the server responds with the expected content.
203 Non-Authoritative Information	The request succeeded, but the response has been modified by an intermediary (like a proxy or cache).	Yes	Proxy adjusts or filters the origin server’s response before sending it to the client.
204 No Content	The request succeeded, but there is no payload to return (empty body).	Yes	A server accepts a request (like updating a record) but doesn’t need to send any content back.

Related Resources

What is a 101 Status Code?
What is a 102 Status Code?
What is a 200 Status Code?
What is a 201 Status Code?
What is a 202 Status Code?
What is a 208 Status Code?
See All Status Code

SEO Services

Other Services

Insights

Blogs

💡 Key Takeaways

200 = success + unaltered
203 = success + modified
204 = success + no content

For SEO professionals, knowing whether a server is returning authoritative or non-authoritative data can affect how you handle caching, validation, and data integrity checks.

Example Request and 203 Response (With Code Snippets)

Let’s walk through a real-world example where a client sends a request, and the server responds with a 203 Non-Authoritative Information status.

Client Request Example

GET /api/data HTTP/1.1
Host: example.com
Accept: application/json

Server Response Example (203)

HTTP/1.1 203 Non-Authoritative Information
Date: Sat, 31 May 2025 12:00:00 GMT
Content-Type: application/json
Warning: 214 Transformation Applied

{
“id”: 789,
“name”: “Alex Johnson”,
“email”: “alex.johnson@example.com”
}

What’s Happening Here?

The proxy or intermediary modified the response before delivering it.
The Warning header (214 Transformation Applied) tells the client that the content might not match the origin server’s response.
This can occur if a CDN serves cached content, a proxy strips sensitive fields, or compression/translation happens mid-route.

Common Causes of a 203 Status Code

A 203 Non-Authoritative Information response is not an error — but it’s a signal that the content you receive has been altered along the path. Here’s why this typically happens:

🔹 1️⃣ Proxy Transformation

A proxy server modifies the original response before passing it to the client.
Example: Compressing images, translating text, or filtering out sensitive data.

🔹 2️⃣ Cached or Stale Data

A CDN or cache layer serves an older or adjusted version of the resource, even if the origin server has a newer one.
Example: Serving cached blog content while the backend updates.

🔹 3️⃣ Content Negotiation

The intermediary adjusts the response based on headers like Accept-Language or Accept-Encoding.
Example: Serving content in the client’s preferred language or compressed format.

🔹 4️⃣ Security or Compliance Filters

A firewall or intermediary applies security policies, such as masking sensitive fields, before sending data downstream.
Example: Removing personally identifiable information (PII) from API responses.

👉 Why It Matters:
For SEO and development, understanding these causes helps ensure you deliver accurate, up-to-date, and authoritative content, which impacts both user experience and search engine trust.

How to Troubleshoot a 203 Status Code

While a 203 response is typically informational (not an error), there are cases when you need to investigate, especially if you’re seeing unexpected behavior, stale data, or missing content.

Step 1: Inspect the Headers

Look for HTTP headers like:

Warning → explains the transformation (214 Transformation Applied)
Via, X-Cache, X-Forwarded-For → shows the intermediary (proxy, CDN, etc.)

These clues tell you which system modified the response.

Step 2: Compare Against Origin Server

Send a direct request (bypassing the proxy if possible) to check if the content differs. Tools like:

curl -v
Postman
Browser DevTools (Network tab)

help you compare responses.

Step 3: Check Proxy or CDN Configurations

Sometimes, transformations are intentional (compression, localization) — but sometimes, they’re misconfigured. Check:

CDN rules
Proxy filters
Security gateways

Step 4: Test With Cache-Control Headers

Add headers like Cache-Control: no-cache or Pragma: no-cache to force fresh responses and bypass stale caches.

Step 5: Coordinate With Server Admins

If transformations are unexpected or breaking your application, collaborate with:

CDN providers
Proxy managers
Backend teams

to trace and resolve the issue.

Real-World Use Cases and Developer Insights

Understanding where and why a 203 status code appears helps developers, SEO specialists, and site owners handle it effectively. Here are some common scenarios:

💡 1️⃣ Content Delivery Networks (CDNs)

CDNs like Cloudflare or Akamai often serve cached or optimized versions of a resource, resulting in a 203 response when the delivered content differs from the origin server.
Example: A global site delivering region-specific or language-adjusted pages.

💡 2️⃣ Reverse Proxies

Systems like NGINX or HAProxy modify responses to improve performance or security.
Example: Stripping sensitive headers, compressing large payloads, or applying rate limits.

💡 3️⃣ Security Gateways

Firewalls or DLP (data loss prevention) systems may mask certain sensitive information (like user IDs, emails) before sending data to the client, triggering a 203.
Example: A healthcare portal masking patient data in API responses.

💡 4️⃣ SEO and Performance Impact

If the modified (non-authoritative) content differs significantly, it can affect:

SEO crawlers receiving inconsistent data.
Users seeing outdated or incomplete pages.
Analytics tools reporting inaccurate metrics.

For SEO teams, it’s essential to ensure that search engines and users receive the same authoritative data when necessary.

Frequently Asked Questions (FAQ)

🔹 What does a 203 status code mean?

A 203 status code means the server successfully processed the request, but the response was modified by an intermediary, like a proxy or cache. The client receives a transformed version of the content, which may differ from the original server’s response.

🔹 How is 203 different from 200?

While a 200 status code means a successful response directly from the origin server, a 203 status indicates that an intermediary (like a proxy) has modified the payload. Both signal success, but only 200 guarantees an unaltered response.

🔹 Can a 203 response be cached?

Yes! By default, 203 responses are cacheable unless the server’s cache-control headers specify otherwise. However, since the content may differ from the origin, caching needs to be carefully managed to avoid serving outdated or altered data.

🔹 When should developers worry about 203 responses?

Developers should pay attention to 203 responses when content integrity is crucial, such as in financial, medical, or legal applications. If data accuracy is critical, they should ensure the client receives authoritative responses (200) and investigate any unexpected 203s.

🔹 How do you troubleshoot a 203 status?

Start by inspecting response headers for clues about intermediaries. Compare the payload against the origin server. Check proxy, CDN, or cache configurations and collaborate with infrastructure teams to ensure the response aligns with what the client needs.