An OK packet is sent from the server to the client to signal successful completion of a command.

As of MySQL 5.7.5, OK packets are also used to indicate EOF, and EOF packets are deprecated.

if CLIENT_PROTOCOL_41 is set, the packet contains a warning count.

The Payload of an OK Packet
Type	Name	Description
int<1>	header	`0x00` or `0xFE` the OK packet header
int<lenenc>	affected_rows	affected rows
int<lenenc>	last_insert_id	last insert-id
if capabilities & CLIENT_PROTOCOL_41 {
int<2>	status_flags	SERVER_STATUS_flags_enum
int<2>	warnings	number of warnings
} else if capabilities & CLIENT_TRANSACTIONS {
int<2>	status_flags	SERVER_STATUS_flags_enum
}
if capabilities & CLIENT_SESSION_TRACK
string<lenenc>	info	human readable status information
if status_flags & SERVER_SESSION_STATE_CHANGED {
string<lenenc>	session state info	Session State Information
}
} else {
string<EOF>	info	human readable status information
}

These rules distinguish whether the packet represents OK or EOF:

OK: header = 0 and length of packet > 7
EOF: header = 0xfe and length of packet < 9

To ensure backward compatibility between old (prior to 5.7.5) and new (5.7.5 and up) versions of MySQL, new clients advertise the CLIENT_DEPRECATE_EOF flag:

Old clients do not know about this flag and do not advertise it. Consequently, the server does not send OK packets that represent EOF. (Old servers never do this, anyway. New servers recognize the absence of the flag to mean they should not.)
New clients advertise this flag. Old servers do not know this flag and do not send OK packets that represent EOF. New servers recognize the flag and can send OK packets that represent EOF.

Example

OK with CLIENT_PROTOCOL_41. 0 affected rows, last-insert-id was 0, AUTOCOMMIT enabled, 0 warnings. No further info.

07 00 00 02 00 00 00 02 00 00 00

Session State Information

State-change information is sent in the OK packet as a array of state-change blocks which are made up of:

Layout of Session State Information
Type	Name	Description
int<1>	type	type of data. See enum_session_state_type
string<lenenc>	data	data of the changed session info

Interpretation of the data field depends on the type value:

SESSION_TRACK_SYSTEM_VARIABLES

Type	Name	Description
string<lenenc>	name	name of the changed system variable
string<lenenc>	value	value of the changed system variable

Example:

After a SET autocommit = OFF statement:

00 00 0f1 0a 61 75 74 6f 63 6f 6d 6d 69 74 03 4f 46 46

....autocommit.OFF

classic_protocol::status::pos::autocommit

constexpr value_type autocommit

Definition: classic_protocol_constants.h:151

SESSION_TRACK_SCHEMA

Type	Name	Description
string<lenenc>	name	name of the changed schema

Example:

After a USE test statement:

01 00 05 04 74 65 73 74

...test

SESSION_TRACK_STATE_CHANGE

A flag byte that indicates whether session state changes occurred. This flag is represented as an ASCII value.

Type	Name	Description
string<lenenc>	is_tracked	`0x31` ("1") if state tracking got enabled.

Example:

After a SET SESSION session_track_state_change = 1 statement:

03 02 00 01 31

...1