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:

Tools	Who	Auth
sandbox_*	anyone	none. Free, anonymous, 4-hour sessions on the Developer Sandbox
fixsim_*	FIXSIM customers	your 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

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)

{
  "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)

Tool	What it does
sandbox_create_session	Provision a free FIX session. Returns host, port, SenderCompID (your tag 49), TargetCompID (tag 56), BeginString, expiry.
sandbox_get_status	Is your engine connected? connected / waiting / disabled.
sandbox_get_messages	The FIX message log, raw and parsed. Pass afterSeq to poll for new messages.
sandbox_get_orders	Working orders your engine sent in.
sandbox_act_on_order	Play the exchange: fill, partial, reject, cancel.
sandbox_set_autofill	Fill every incoming order automatically.
sandbox_get_session	Re-fetch the connection card.
sandbox_end_session	End 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)

Tool	What it does
fixsim_list_instances	Instances your key can access. Start here.
fixsim_list_sessions, fixsim_get_session	Sessions on an instance: FIX version, CompIDs, role, logged-in state, sequence numbers.
fixsim_list_orders_in, fixsim_get_order_in	Orders your system sent into FIXSIM, with raw FIX and audit trail.
fixsim_list_executions_out	Execution reports FIXSIM sent you (acks, fills, rejects). Filter by ClOrdID.
fixsim_list_orders_out, fixsim_list_executions_in	The reverse direction, when FIXSIM is the order sender.
fixsim_list_securities	Symbols and prices on an instance.
fixsim_act_on_orders	Ack, fill, partially fill, reject, cancel, expire… orders your system sent in.
fixsim_send_order	Have FIXSIM send you a New Order Single.
fixsim_cancel_order_out	Have FIXSIM cancel an order it sent.
fixsim_send_raw_fix	Send 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.

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

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

Key	Default	Meaning
FixSim:BaseUrl	https://portal.fixsim.com/	The FIXSIM site whose /v1 API the fixsim_* tools call
FixSim:PerKeyCallsPerMinute	120	Per-key rate limit at this server
Sandbox:Enabled	true	Register the sandbox_* tools
Sandbox:BaseUrl	https://sandbox.fixsim.com/	The Developer Sandbox web app
Sandbox:CallerKey	empty	Shared 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