Authentication & Sessions

API key creation, logon, session lifecycle, and message retransmission

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
108	HeartbeatInt	Heartbeat interval (seconds)	N > 3
1137	DefaultApplVerID	Default application version	FIX50SP2<9>

Optional Fields

Tag	Name	Description	Default
141	ResetSeqNumFlag	Reset sequence numbers on logon. Must be Y for non-retransmission sessions.	N
8013	CancelOrdersOnDisconnect	Cancel orders on any disconnection (including graceful logout)	N
20126	ListenerSession	Listen-only session. KalshiNR/RT only, requires SkipPendingExecReports=Y.	N
20127	ReceiveSettlementReports	Receive settlement reports. KalshiNR, KalshiRT, and KalshiPT only. Defaults to Y on KalshiPT.	N (Y on KalshiPT)
20200	MessageRetentionPeriod	How long session messages will be stored for retransmission, max of 72 hours. KalshiRT and KalshiPT only.	24
21005	UseDollars	Enable dollar-based price format for prices, including subpenny precision	N
21011	SkipPendingExecReports	Skip PENDING_{NEW|REPLACE|CANCEL} execution reports	N
21012	UseExpiredOrdStatus	Emit Expired<C> (150/39) for expiry-style system cancellations (CloseCancel and OrderExpiryCancel) 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 all states	N
21026	AlwaysEmitNewBeforeTrade	Emit a standalone New<0> execution report before any Trade<F> report when both occur in the same matching cycle	N
21027	SplitCollateralReturn	Include per-trade collateral return breakdown (tags 21030/21031) on Execution Reports	N
21032	UseCapReservation	Reserve event contract daily cap headroom on order entry and replace. FCM members only, event-contract sessions only. Replace requires OrderID<37>, only after that order’s ExecType=New<0> report, one amend per order in flight. RFQ quote acceptances are not cap-reserved.	N
21035	AutoTransfer	Set Y on each Logon to automatically move shared FCM collateral between exchange shards for new orders and replacements. FCM event-contract order-entry sessions (KalshiNR/KalshiRT) only, excluding listeners. Omit ExDestination<100> or use -1. Disable automatic rebalancing while this option is enabled.	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

SendingTime in Signature

The SendingTime in the PreHashString must match exactly the value in field 52 of the Logon message. Format: YYYYMMDD-HH:MM:SS.mmm.

SendingTime must be within 30 seconds of the server’s current time, or the message will be rejected with SessionRejectReason<373>=10.

Python (Ed25519)

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"  # Must match TargetCompID in tag 56

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')

Reconnecting

Use one active FIX connection per API key. If Logon is rejected, inspect Text (58) in the Logout (35=5) response:

• already exists: check for another connection using the same key before retrying.
• rate limit exceeded or FIX logon service busy; retry with backoff: retry with exponential backoff and jitter.
• Authentication, permission, or invalid-field errors: correct the issue before retrying.

Avoid overlapping reconnect attempts. Generate a fresh SendingTime and signature for each attempt, and follow the sequence-number and retransmission rules below.

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, KalshiPT only)

Message Retransmission

Message retransmission (ResendRequest, SequenceReset) is only supported on KalshiRT and KalshiPT.

For all other sessions, ResetSeqNumFlag<141> in the Logon message must always be Y or the Logon will be rejected.

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 and KalshiPT only. Lookback window limited to 24 hours (or up to 72 hours if MessageRetentionPeriod was set on Logon).

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.