HTTP 101 Switching Protocols vs 400 Bad Request
HTTP 101 (Switching Protocols) is a 1xx Informational response, while 400 (Bad Request) is a 4xx Client Error response. 101 indicates that the server understands the Upgrade header field request and indicates which protocol it is switching to. In contrast, 400 means that the server cannot process the request due to malformed syntax, invalid request message framing, or deceptive request routing.
Description
The server understands the Upgrade header field request and indicates which protocol it is switching to.
When You See It
When upgrading from HTTP/1.1 to WebSocket, or to HTTP/2.
How to Fix
This is normal behavior during protocol upgrades. Ensure your client supports the target protocol.
Description
The server cannot process the request due to malformed syntax, invalid request message framing, or deceptive request routing.
When You See It
When sending malformed JSON, missing required fields, or invalid query parameters.
How to Fix
Check the request body format, validate all required fields, and ensure proper encoding.
Key Differences
101 is a 1xx Informational response, while 400 is a 4xx Client Error response.
HTTP 101: The server understands the Upgrade header field request and indicates which protocol it is switching to.
HTTP 400: The server cannot process the request due to malformed syntax, invalid request message framing, or deceptive request routing.
You encounter 101 when when upgrading from HTTP/1.1 to WebSocket, or to HTTP/2.
You encounter 400 when when sending malformed JSON, missing required fields, or invalid query parameters.
When to Use Which
For 101 (Switching Protocols): This is normal behavior during protocol upgrades. Ensure your client supports the target protocol. For 400 (Bad Request): Check the request body format, validate all required fields, and ensure proper encoding.