Test FIX from Claude, ChatGPT, or Cursor

You can now ask your AI assistant for a FIX test session and get one. FIXSIM publishes an MCP server, so any assistant that supports remote MCP servers (Claude, Claude Code, Cursor, ChatGPT, VS Code) can create a session on the free Developer Sandbox, hand you the connection details, and then read the wire for you: what your engine sent, what came back, and why an order was rejected.

No account, no API key, no install. The sandbox tools are open to everyone. FIXSIM customers add their API key and the same assistant drives their own instances.

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Connect in 60 Seconds

The server URL is the same everywhere:

https://mcp.fixsim.com/mcp

Claude (web or desktop): Customize, then Connectors, then Add custom connector. Paste the URL, choose authentication None, and add it. Enable it from the plus menu in a chat.

Claude Code:

claude mcp add --transport http fixsim https://mcp.fixsim.com/mcp

Cursor or VS Code (mcp.json):

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

ChatGPT and other assistants: add it as a custom remote MCP connector using the same URL, with no authentication.

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Three Prompts to Try

1. "Create a FIX 4.4 test session for me and tell me what to put in tags 49 and 56." The assistant calls the sandbox, then gives you host, port, SenderCompID, and TargetCompID for your engine's config.
2. "Has my engine logged on yet? Show me the messages so far." It polls the session status and message log and reads the logon exchange back to you, tag by tag.
3. "Fill the first order I sent and then explain the execution report." It acts as the exchange, and then walks you through ExecType, OrdStatus, LastPx, CumQty, and LeavesQty in the 35=8 that came back.

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What the Assistant Can Do

Sandbox tools (free, no key)	What happens
sandbox_create_session	Provisions a FIX session: host, port, CompIDs, expiry. FIX 4.0, 4.2, 4.4, or 5.0 SP2.
sandbox_get_status	Is your engine connected? Connected, waiting, or disabled.
sandbox_get_messages	The full message log, raw and parsed, in both directions.
sandbox_get_orders	Working orders your engine sent in.
sandbox_act_on_order	Fill, partial fill, reject, or cancel an order. FIXSIM sends the execution report.
sandbox_set_autofill	Fill every incoming order automatically.

Every response the assistant sees is real FIX from a real engine. When it explains a reject, it is reading the actual 35=3 or 35=8 that FIXSIM sent, not a guess.

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For FIXSIM Customers

Add your FIXSIM API key and the same connector reaches your own instances: list sessions, read inbound orders and execution reports with their raw FIX and audit trail, act on orders, send orders, or send any raw FIX message. It can only see the instances your key is entitled to.

In Claude Code, pass the key as a header:

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

In Cursor or VS Code, add a headers object with x-api-key to the entry above. In the Claude connector dialog, add x-api-key under Request headers.

Prefer that the assistant only reads? Use https://mcp.fixsim.com/mcp/readonly: same tools, but anything that would send a FIX message is refused. A typical prompt: "On my TESTSELL instance, why is the AAPL order on TESTSELL2 in status 4?" The assistant pulls the order and its execution chain and explains the tags.

The REST API behind the connector is included in Pro and Enterprise plans and during the free trial.

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Limits and Privacy

• Sandbox: one session at a time, 100 application messages per day, 4-hour sessions. Nothing persists.
• Your API key is forwarded to FIXSIM on each call and never stored by the connector.
• The connector is open source: github.com/gdrumma/fixsim-mcp. Run your own copy if you prefer.

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Prefer scripts to assistants? See Test FIX Protocol with curl. For the endpoints the connector calls, see the REST API user guide.