HTTP 100 Continue vs 428 Precondition Required
HTTP 100 (Continue) is a 1xx Informational response, while 428 (Precondition Required) is a 4xx Client Error response. 100 indicates that the server has received the request headers and the client should proceed to send the request body. This lets the client know it can continue with the request or abort if the headers were rejected. In contrast, 428 means that the server requires the request to be conditional (e.g., include If-Match header) to prevent lost updates.
Description
The server has received the request headers and the client should proceed to send the request body. This lets the client know it can continue with the request or abort if the headers were rejected.
When You See It
When a client sends an Expect: 100-continue header, the server responds with 100 before the client sends the body.
How to Fix
This is an interim response — no fix needed. The client should continue sending the request body.
Description
The server requires the request to be conditional (e.g., include If-Match header) to prevent lost updates.
When You See It
When an API requires optimistic concurrency control via ETags.
How to Fix
Fetch the resource first to get its ETag, then include If-Match in your update request.
Key Differences
100 is a 1xx Informational response, while 428 is a 4xx Client Error response.
HTTP 100: The server has received the request headers and the client should proceed to send the request body. This lets the client know it can continue with the request or abort if the headers were rejected.
HTTP 428: The server requires the request to be conditional (e.g., include If-Match header) to prevent lost updates.
You encounter 100 when when a client sends an Expect: 100-continue header, the server responds with 100 before the client sends the body.
You encounter 428 when when an API requires optimistic concurrency control via ETags.
When to Use Which
For 100 (Continue): This is an interim response — no fix needed. The client should continue sending the request body. For 428 (Precondition Required): Fetch the resource first to get its ETag, then include If-Match in your update request.