# http.client.CannotSendHeader

- **ID:** `python/http-client-cannotsendheader`
- **Domain:** python
- **Category:** protocol_error
- **Verification:** official
- **Fix Rate:** 80%

## Root Cause

Attempt to send a header after request body has started being sent.

## Version Compatibility

| Version | Status | Introduced | Deprecated |
|---------|--------|------------|------------|
| 3.x | active | — | — |