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