Authentication & Sessions

API Key Setup

FIX API keys use the same key pair as the REST API: Ed25519 (recommended) or 2048-bit RSA. Generate a key pair and register the public key in your account profile, or generate one there. The API Key ID (UUID) is your SenderCompID.

# Ed25519
openssl genpkey -algorithm ed25519 -out kalshi-fix.key
openssl pkey -in kalshi-fix.key -pubout -out kalshi-fix.pub

# RSA
openssl genpkey -algorithm RSA -pkeyopt rsa_keygen_bits:2048 -out kalshi-fix.key
openssl rsa -in kalshi-fix.key -pubout -out kalshi-fix.pub

Logon (35=A)

The initiator sends a Logon message. The acceptor responds with either a Logon (success) or Logout (failure).

Do not send SenderSubID<50> on Logon. Kalshi rejects it with a Logout; send operator identity per message instead — on NewOrderSingle, OrderCancelRequest, or OrderCancelReplaceRequest.

Required Fields

Tag	Name	Description	Value
98	EncryptMethod	Method of encryption	None<0>
96	RawData	Client logon message signature	Base64 encoded signature
1137	DefaultApplVerID	Default application version	FIX50SP2<9>

Optional Fields

Tag	Name	Description	Default
141	ResetSeqNumFlag	Reset sequence numbers on logon. Must be Y for KalshiNR and KalshiDC.	N
108	HeartbeatInt	Heartbeat interval in seconds, must be >= 3.	30
8013	CancelOrdersOnDisconnect	Cancel orders on disconnection	N
20126	ListenerSession	Listen-only session. KalshiNR/KalshiRT only, requires SkipPendingExecReports=Y.	N
20200	MessageRetentionPeriod	How long session messages are stored for retransmission, max 72 hours. KalshiRT only.	24
21005	UseDollars	Fixed-point dollar pricing flag. Margin sessions always use fixed-point dollar pricing; clients should treat this as always enabled.	Always on
21011	SkipPendingExecReports	Skip `PENDING_NEW` / `PENDING_REPLACE` / `PENDING_CANCEL` execution reports	N
21012	UseExpiredOrdStatus	Emit `Expired<C>` for expiry-style system cancellations instead of `Canceled<4>`	N
21007	EnableIocCancelReport	Partially filled IOC orders produce a cancel report	N
21008	PreserveOriginalOrderQty	`OrderQty` tag 38 always reflects original order quantity across states	N

Signature Generation

The RawData field must contain a base64-encoded signature of the pre-hash string: Ed25519 for Ed25519 keys, RSA-PSS with SHA-256 for RSA keys.

PreHashString = SendingTime + SOH + MsgType + SOH + MsgSeqNum + SOH + SenderCompID + SOH + TargetCompID

The SendingTime in the PreHashString must match exactly the value in field 52 of the Logon message. SendingTime must be within 30 seconds of the server’s current time, or the message will be rejected with SessionRejectReason<373>=10.

from base64 import b64encode
from cryptography.hazmat.primitives import serialization

private_key = serialization.load_pem_private_key(open('kalshi-fix.key', 'rb').read(), password=None)
sending_time, msg_type, msg_seq_num = "20230809-05:28:18.035", "A", "1"
sender_comp_id = "your-fix-api-key-uuid"
target_comp_id = "KalshiNR"

msg_string = chr(1).join([sending_time, msg_type, msg_seq_num, sender_comp_id, target_comp_id])
raw_data_value = b64encode(private_key.sign(msg_string.encode('utf-8'))).decode('utf-8')

from base64 import b64encode
from Cryptodome.Signature import pss
from Cryptodome.Hash import SHA256
from Cryptodome.PublicKey import RSA

private_key = RSA.import_key(open('kalshi-fix.key').read().encode('utf-8'))

sending_time = "20230809-05:28:18.035"
msg_type = "A"
msg_seq_num = "1"
sender_comp_id = "your-fix-api-key-uuid"
target_comp_id = "KalshiNR"

msg_string = chr(1).join([
    sending_time, msg_type, msg_seq_num,
    sender_comp_id, target_comp_id
])

msg_hash = SHA256.new(msg_string.encode('utf-8'))
signature = pss.new(private_key).sign(msg_hash)
raw_data_value = b64encode(signature).decode('utf-8')

Heartbeat & Sequence Numbers

Behavior	Detail
Default heartbeat interval	30 seconds
Missed heartbeat	Connection terminates if heartbeat response not received within interval
Sequence number lower than expected	Connection terminated
Sequence number higher than expected	Recoverable with ResendRequest (KalshiRT only)

Message Retransmission

Message retransmission (ResendRequest, SequenceReset) is only supported on KalshiRT. ResetSeqNumFlag<141> must always be Y on KalshiNR and KalshiDC. The drop copy session provides an alternative way to query for missed execution reports. For a real-time streaming feed, see Listener Sessions.

ResendRequest (35=2)

KalshiRT only. Lookback window controlled by MessageRetentionPeriod.

Tag	Name	Description
7	BeginSeqNo	Lower bound (inclusive)
16	EndSeqNo	Upper bound (inclusive)

Logout (35=5)

Either side may initiate a Logout. The counterparty responds with a Logout, and the transport connection is terminated. If CancelOrdersOnDisconnect=Y was set on Logon, all open orders are canceled.

Overview

REST

WebSockets

FIX

API Key Setup

Logon (35=A)

Required Fields

Optional Fields

Signature Generation

Heartbeat & Sequence Numbers

Message Retransmission

ResendRequest (35=2)

Logout (35=5)

​API Key Setup

​Logon (35=A)

​Required Fields

​Optional Fields

​Signature Generation

​Heartbeat & Sequence Numbers

​Message Retransmission

​ResendRequest (35=2)

​Logout (35=5)

API Key Setup

Logon (35=A)

Required Fields

Optional Fields

Signature Generation

Heartbeat & Sequence Numbers

Message Retransmission

ResendRequest (35=2)

Logout (35=5)