FIXSIM
Test FIX from Claude, ChatGPT, or Cursor: free sandbox FIX sessions or your own FIXSIM instances.
Install
Add this to your client's MCP configuration (Claude Desktop: claude_desktop_config.json; Cursor: .cursor/mcp.json; VS Code and others use the same "mcpServers" shape).
claude mcp add --transport http fixsim 'https://mcp.fixsim.com/mcp' --header 'x-api-key: <Optional. Your FIXSIM API key (portal.fixsim.com, API Keys). Leave blank to use only the free sandbox tools.>'{
"mcpServers": {
"fixsim": {
"type": "http",
"url": "https://mcp.fixsim.com/mcp",
"headers": {
"x-api-key": "<Optional. Your FIXSIM API key (portal.fixsim.com, API Keys). Leave blank to use only the free sandbox tools.>"
}
}
}
}Documentation
From the project's README on GitHub, refreshed weekly. Links open on GitHub.
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:5199Configuration (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
Frequently asked questions
What is FIXSIM?
Test FIX from Claude, ChatGPT, or Cursor: free sandbox FIX sessions or your own FIXSIM instances.
How do I install FIXSIM?
It runs as a remote server. In Claude Code: claude mcp add --transport http fixsim 'https://mcp.fixsim.com/mcp' --header 'x-api-key: <Optional. Your FIXSIM API key (portal.fixsim.com, API Keys). Leave blank to use only the free sandbox tools.>'. For other clients, paste the JSON configuration from this page into their MCP settings.
Does FIXSIM need an API key?
Yes. It expects x-api-key, which you set in its configuration.
Is FIXSIM open source?
Yes. Its code is on GitHub under the MIT license.
Who maintains FIXSIM?
It is published by GammaThreeTrading. AI.tn lists it but does not maintain or endorse it.
From the official MCP Registry. Listing is not an endorsement: review a server's code and permissions before giving it access to your data.