FIXSIM

io.github.GammaThreeTradingPublished by GitHub account GammaThreeTradingv0.1.2Updated Oct 24, 2026

Test FIX from Claude, ChatGPT, or Cursor: free sandbox FIX sessions or your own FIXSIM instances.

VerifiedStreamable HTTPWeb executableFinanceDeveloper Tools

Overview

AI-generated overview

Hosted MCP server for FIXSIM that lets an assistant create FIX test sessions, inspect FIX message logs and orders, and simulate exchange responses.

What you can do
  • Create, inspect and end free anonymous sandbox FIX sessions, including connection cards and expiry
  • Check sandbox connection status and read the FIX message log, working orders and session details
  • Play the exchange on sandbox orders: fill, partial fill, reject or cancel, or turn on automatic fills
  • List FIXSIM instances, sessions, securities, inbound and outbound orders and execution reports
  • Act on orders sent into FIXSIM (ack, fill, reject, cancel, expire, restate) and send orders or cancels out
  • Send arbitrary raw FIX application messages on a session
When to use it
Useful when testing a FIX trading engine or order flow and you want to drive sessions, inspect raw FIX traffic and simulate counterparty responses from a chat client instead of a separate tool.
Requirements
Remote Streamable HTTP endpoint; no local runtime needed. The sandbox tools need no account or key. The fixsim_ tools need a FIXSIM API key sent as the x-api-key header and access to entitled instances. Self-hosting requires .NET 8.
Before you install
The fixsim_ tools send real FIX messages and change order state on your FIXSIM instances, so use the read-only endpoint if you only want to look. Your API key is forwarded to FIXSIM on each call. Sandbox sessions are anonymous and capped (4 hours, 100 application messages per day).

Installation

In SourceWeft

  1. Open FIXSIM in the dashboard and add it to a workspace.
  2. Enable the server for the chats that should use its tools.

Web executable via Streamable HTTP. Remote servers run from the web runtime once configured in a workspace.

Use with an agent

Paste this prompt into Claude Code, Codex, Cursor or another agent — or into SourceWeft chat. The agent reads this server's install guide, shows you its publisher, endpoint and sign-in, and adds it to your client once you agree. Keys never go into a command.

Read https://sourceweft.com/mcp/io.github.GammaThreeTrading%2Ffixsim/install.md and connect the MCP server it describes to my client. Before changing any config, show me its publisher, endpoint and sign-in method and wait for my OK. Never put an API key in a command or in a file I would commit; ask me where it should come from.

Read the install guide the agent follows

Other MCP clients

Add this to your client's mcpServers config.

{
  "mcpServers": {
    "fixsim": {
      "type": "http",
      "url": "https://mcp.fixsim.com/mcp"
    }
  }
}

README

fixsim-mcp

An MCP server for FIXSIM, the hosted FIX protocol simulator. Add it to Claude, Cursor, VS Code, or ChatGPT and test FIX from a conversation:

"Create a FIX 4.4 test session for me, then tell me when my engine has logged on and show me the first order it sent."

Two tool families live on one endpoint:

ToolsWhoAuth
sandbox_*anyonenone. Free, anonymous, 4-hour sessions on the Developer Sandbox
fixsim_*FIXSIM customersyour FIXSIM API key, sent as an x-api-key header

Connect

Hosted endpoint: https://mcp.fixsim.com/mcp (read-only variant: https://mcp.fixsim.com/mcp/readonly).

Claude Code

bash
claude mcp add --transport http fixsim https://mcp.fixsim.com/mcp --header "x-api-key: YOUR_FIXSIM_API_KEY"

Leave off --header if you only want the free sandbox tools.

Cursor / VS Code (mcp.json)

json
{  "mcpServers": {    "fixsim": {      "url": "https://mcp.fixsim.com/mcp",      "headers": { "x-api-key": "YOUR_FIXSIM_API_KEY" }    }  }}

Claude.ai / Claude Desktop: Customize → Connectors → Add custom connector → URL https://mcp.fixsim.com/mcp. Choose authentication None for the sandbox. To use your own instances, add a request header x-api-key (header support in the connector dialog is rolling out; if you do not see it, use Claude Code or Cursor for now).

Tools

Sandbox (no auth)

ToolWhat it does
sandbox_create_sessionProvision a free FIX session. Returns host, port, SenderCompID (your tag 49), TargetCompID (tag 56), BeginString, expiry.
sandbox_get_statusIs your engine connected? connected / waiting / disabled.
sandbox_get_messagesThe FIX message log, raw and parsed. Pass afterSeq to poll for new messages.
sandbox_get_ordersWorking orders your engine sent in.
sandbox_act_on_orderPlay the exchange: fill, partial, reject, cancel.
sandbox_set_autofillFill every incoming order automatically.
sandbox_get_sessionRe-fetch the connection card.
sandbox_end_sessionEnd the session early.

Sandbox limits: 100 application messages per session per day, 4-hour sessions, a handful of sessions per caller.

FIXSIM instances (API key)

ToolWhat it does
fixsim_list_instancesInstances your key can access. Start here.
fixsim_list_sessions, fixsim_get_sessionSessions on an instance: FIX version, CompIDs, role, logged-in state, sequence numbers.
fixsim_list_orders_in, fixsim_get_order_inOrders your system sent into FIXSIM, with raw FIX and audit trail.
fixsim_list_executions_outExecution reports FIXSIM sent you (acks, fills, rejects). Filter by ClOrdID.
fixsim_list_orders_out, fixsim_list_executions_inThe reverse direction, when FIXSIM is the order sender.
fixsim_list_securitiesSymbols and prices on an instance.
fixsim_act_on_ordersAck, fill, partially fill, reject, cancel, expire… orders your system sent in.
fixsim_send_orderHave FIXSIM send you a New Order Single.
fixsim_cancel_order_outHave FIXSIM cancel an order it sent.
fixsim_send_raw_fixSend any raw FIX message on a session.

The /mcp/readonly endpoint exposes the same tools but refuses the ones that send.

Every fixsim_* call is checked against the instances your key is entitled to, and is rate-limited per key (120 calls per minute by default).

Run it yourself

Requires .NET 8.

bash
dotnet run --project src/FixSim.Mcp --urls http://localhost:5199

Configuration (appsettings.json or environment variables such as FixSim__BaseUrl):

KeyDefaultMeaning
FixSim:BaseUrlhttps://portal.fixsim.com/The FIXSIM site whose /v1 API the fixsim_* tools call
FixSim:PerKeyCallsPerMinute120Per-key rate limit at this server
Sandbox:EnabledtrueRegister the sandbox_* tools
Sandbox:BaseUrlhttps://sandbox.fixsim.com/The Developer Sandbox web app
Sandbox:CallerKeyemptyShared secret sent as X-Sandbox-Caller so the sandbox can trust server-to-server calls

The server is stateless (Streamable HTTP), so it scales horizontally behind any load balancer.

How it works

The server is a thin client. fixsim_* tools call the public FIXSIM REST API (/v1/...) with your key; sandbox_* tools call the Developer Sandbox's session API. Nothing is stored here beyond a one-minute cache of which instances a key may access. Your API key is forwarded to FIXSIM on each call and never logged.

License

MIT

Source: README.md at commit 5c8b2fe

Tools

21

fixsim_list_orders_out

List outbound orders

Orders FIXSIM sent OUT on a session (created with fixsim_send_order). Pass clOrdId to fetch one.

ParameterslimitapiKeyclOrdIdsessionrequiredinstancerequired

sandbox_get_status

Check sandbox connection status

Is the user's FIX engine connected to the sandbox session? Returns state: connected, waiting (nobody logged on yet), disabled, or unknown.

ParameterssessionIdrequired

fixsim_send_order

Send a New Order Single

Have FIXSIM send a New Order Single (35=D) OUT on a session to the user's system, so they can test how their system handles inbound orders. Follow up with fixsim_list_orders_out and fixsim_list_executions_in to see what came back.

ParameterssiderequiredpriceapiKeysymbolrequiredclOrdIdrequiredordTyperequiredsessionrequiredinstancerequiredorderQtyrequiredadditionalTags

fixsim_list_orders_in

List inbound orders

Orders the user's trading system sent INTO FIXSIM on a session (FIXSIM is the counterparty). Includes ClOrdID, symbol, side, quantity, remaining quantity, order status and the raw FIX message. Optionally filter by FIX OrdStatus code (tag 39): 0 new, 1 partially filled, 2 filled, 4 canceled, 8 rejected.

ParameterslimitapiKeysessionrequiredinstancerequiredorderStatus

fixsim_list_executions_in

List inbound execution reports

Execution reports the user's system sent INTO FIXSIM on a session (when FIXSIM is the order sender and the user's system is the executing side). Optionally filter by ExecID (tag 17).

ParameterslimitapiKeyexecIdsessionrequiredinstancerequired

fixsim_act_on_orders

Act on inbound orders

Respond to orders the user's system sent into FIXSIM: acknowledge, fully or partially execute, reject, cancel remaining, expire, done-for-day, restate, or accept/reject a pending cancel/replace. FIXSIM emits the matching execution reports on the FIX session. Give one or more ClOrdIDs; for executions supply lastPx and, for a partial, lastQty.

ParametersactionrequiredapiKeylastPxlastQtysessionrequiredclOrdIdsrequiredinstancerequired

sandbox_set_autofill

Set sandbox auto-fill

Turn automatic fills on or off for a sandbox session. On: every order the user sends is filled immediately at the security's last price. Off (default): orders are acknowledged and wait for sandbox_act_on_order.

ParametersenabledrequiredsessionIdrequired

fixsim_cancel_order_out

Send an Order Cancel Request

Have FIXSIM send an Order Cancel Request (35=F) for an order it previously sent out with fixsim_send_order.

ParametersapiKeyclOrdIdrequiredsessionrequiredinstancerequirednewClOrdIdadditionalTags

fixsim_list_sessions

List FIX sessions

List the FIX sessions on an instance with their FIX version, CompIDs, acceptor/initiator role, host/port, enabled and logged-in state, and sequence numbers.

ParametersapiKeyinstancerequired

fixsim_list_executions_out

List outbound execution reports

Execution reports FIXSIM sent OUT on a session (acks, fills, partial fills, rejects, cancels). Pass clOrdId to see the full execution history of one order: ExecType, OrdStatus, LastPx/LastQty, CumQty, LeavesQty, AvgPx and the raw FIX.

ParameterslimitapiKeyclOrdIdsessionrequiredinstancerequired

fixsim_get_session

Get FIX session

Get one FIX session's settings and live state (logged in?, sequence numbers, connection details).

ParametersapiKeysessionrequiredinstancerequired

sandbox_act_on_order

Fill, reject, or cancel a sandbox order

Play the exchange for one order the user sent in: fill (full execution), partial (partial fill), reject, or cancel (cancel the remaining quantity). FIXSIM sends the matching execution report back to the user's engine.

ParametersactionrequiredclOrdIdrequiredsessionIdrequired

fixsim_get_order_in

Get inbound order

Get one inbound order by ClOrdID (tag 11), including its raw FIX message and audit trail. Use this to explain why an order was rejected or what state it is in.

ParametersapiKeyclOrdIdrequiredsessionrequiredinstancerequired

sandbox_get_orders

List sandbox orders

Working orders the user's engine has sent into the sandbox session: ClOrdID, symbol, side, qty, filled, leaves, price, status.

ParameterssessionIdrequired

sandbox_get_messages

Read sandbox FIX message log

The FIX message log for a sandbox session, oldest first: logons, heartbeats, orders in, execution reports out, rejects. Each message has seq, timeUtc, inbound (true = user's engine sent it), msgType, raw FIX, and parsed tag/value fields. Pass afterSeq (the last seq you saw) to fetch only new messages. Use this to explain what happened on the wire.

ParametersafterSeqsessionIdrequired

sandbox_create_session

Create sandbox FIX session

Create a free, anonymous FIX test session on the FIXSIM Developer Sandbox. No account or API key needed. Returns the connection card: host, port, the SenderCompID the user must put in tag 49, the TargetCompID for tag 56, BeginString, and the expiry (sessions last 4 hours, cap 100 application messages per day). The user's FIX engine connects as the initiator; FIXSIM is the acceptor and acknowledges every order. Keep the returned id; every other sandbox tool needs it.

ParametersfixVersion

sandbox_end_session

End sandbox session

End a sandbox session now instead of waiting for it to expire. Frees the slot for the user's IP.

ParameterssessionIdrequired

fixsim_send_raw_fix

Send raw FIX messages

Send one or more raw FIX application messages OUT on a session as given (tag=value pairs separated by | or SOH). FIXSIM supplies the session header fields (BeginString, CompIDs, MsgSeqNum, SendingTime) and the checksum. Use for message types the other tools do not cover.

ParametersapiKeysessionrequiredinstancerequiredfixMessagesrequired

fixsim_list_instances

List FIXSIM instances

List the FIXSIM instances your API key can access. An instance is a hosted FIX engine; each holds one or more FIX sessions. Start here to learn the instance names other tools need.

ParametersapiKey

fixsim_list_securities

List securities

Securities (symbol, last price, CUSIP, ISIN) configured on an instance. FIXSIM prices fills from these.

ParametersapiKeyinstancerequired

sandbox_get_session

Get sandbox session

Re-fetch a sandbox session's connection card by id, including whether auto-fill is on and when it expires.

ParameterssessionIdrequired

Version history

1
  1. v0.1.2LatestSep 16, 2026

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