Building an MCP tool-call test rig with the Python SDK in 2026
You can test an agent's tool-call loop without a model. Write down the calls the model would have made, replay them against your real MCP server over stdio, and assert on what comes back. It runs offline in about two seconds, costs nothing per run, and catches renamed tools and schema drift before a customer does. The model is the last thing you should be faking.
The Function Call Flow Simulator I link to below is one I built. I tried five existing playgrounds first and every one wanted an API key before it would render a single tool_use block, which is backwards when the whole point is sketching a flow you haven't paid for yet. Mine runs in the browser, free, no signup, nothing leaves the tab. If you know a better one, tell me and I'll link to it instead.
The goal
Here's the picture I wanted on my screen. One command, one JSON file, six lines of output: each tool call my agent would make during a customer refund, run against the same MCP server that handles production traffic, with a failure line and an exit code of 1 the moment something breaks. No API key. Nothing over the network. Under two seconds.
