What Is a 100 Status Code?
The HTTP 100 status code, also known as 100 Continue, is an informational response indicating that the initial part of the client's request has been received and the server is ready to receive the rest. It’s most commonly used when the client sends a request with the Expect: 100-continue header, typically before sending a large payload. This status is part of the 1xx informational response codes and signals that the server is not rejecting the request—yet. Instead, it's a green light to proceed.
Official Source: RFC 7231 – Section 6.2.1
What's the Difference Between 100 and 100 Continue?
The difference between “100” and “100 Continue”:
100 refers to the numeric HTTP status code.
100 Continue is the full HTTP response line that includes the status code and reason phrase.
Example:
HTTP/1.1 100 Continue
Here:
100= status codeContinue= reason phrase
Both refer to the same response, but “100 Continue” is the full version used in actual HTTP communication.
| Term | Meaning | Where You’ll See It |
|---|---|---|
100 | Numeric status code only | In code, documentation, dev tools |
100 Continue | Full HTTP response line with reason | In server responses and logs |
In short: they mean the same thing, but “100 Continue” gives the full context during real-time HTTP communication.
Explanation
Example: When a client sends a large upload request with Expect: 100-continue, the server may respond like this:
HTTP/1.1 100 Continue
This response means:
100= Status codeContinue= Text explaining what the status code meansHTTP/1.1= Protocol version
Summary
Use “100” when referring to the status code itself (e.g., in documentation, programming constants).
Use “100 Continue” when referencing the full server response as it appears on the wire.
- 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
How the 100 Continue Status Code Works
The 100 Continue response is part of a two-phase request process:
Initial Request: The client sends request headers with
Expect: 100-continue, asking the server if it’s okay to send the body (e.g., a large file).Interim Response: The server replies with
HTTP/1.1 100 Continueif it’s willing to accept the full request.Request Body Sent: The client sends the full payload.
Final Response: The server processes the full request and replies with a definitive status code (e.g., 200 OK, 401 Unauthorized, etc.).
Analogy: Think of it like asking, “Can I send this over?” and waiting for the other side to say, “Yes, go ahead.”
Real-World Example of 100 Continue
Best Practices for 100 Continue
To use the 100 Continue status code effectively and efficiently, keep these tips in mind:
Use only when necessary
Apply 100 Continue for large or resource-heavy requests. Skip it for simple, lightweight transactions.Respond promptly
Servers should immediately send the 100 Continue response if the headers and method look valid.Include proper headers
Clearly define what’s expected from the client. Use the correct HTTP headers to streamline communication.Avoid redundant 100 responses
One interim status per request is sufficient. Repeating it can cause confusion and inefficiency.Follow up with a final status
After processing the request, always return a final status code (e.g., 200, 400, 500) to close the loop.
Code References for 100 Continue
Here’s how different frameworks and languages represent the 100 status code:
Ruby on Rails:
:continueGo:
http.StatusContinueSymfony (PHP):
Response::HTTP_CONTINUEPython 2:
httplib.CONTINUEPython 3+:
http.client.CONTINUEPython 3.5+:
http.HTTPStatus.CONTINUE.NET:
HttpStatusCode.ContinueRust:
http::StatusCode::CONTINUE
Apache (Java): org.apache.hc.core5.http.HttpStatus.SC_CONTINUE
Angular: @angular/common/http/HttpStatusCode.Continue
Troubleshooting: Final Status Code Fails After 100 Continue?
If your final response (e.g., 200 OK) fails after a 100 Continue, the issue may lie in:
Incorrect request formatting
Authorization errors
Incorrect or missing resource paths
Server-side processing issues
How to troubleshoot:
Double-check request headers and formatting
Ensure the requested resource exists
Confirm user permissions and authentication
Review server-side error logs or validation checks
Additional resources
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