Read a finalized trace

Use the operation result for the immediate response and the finalized trace for the complete observed execution.

Requirements

Start from the project in Your first MCP test. This example uses the same server fixture and needs no credentials.

Complete test

Save as tests/test_trace.py:

import sys
from pathlib import Path

import pytest

from m3 import MCPTestKit, StdioServer
from m3.types import ExecutionOutcome

pytestmark = pytest.mark.m3(suite_name="shipping")


def test_shipping_trace():
    root = Path(__file__).parents[1]
    server = StdioServer(
        name="shipping",
        command=sys.executable,
        args=(str(root / "shipping_server.py"),),
        cwd=str(root),
    )
    with MCPTestKit(env={}) as kit, kit.direct(server) as client:
        result = client.call_tool("shipping_quote", {"weight_kg": 2, "zone": "local"})
        assert result.structured_content == {"amount": 9.0, "currency": "USD"}

    trace = client.final_trace
    assert trace is not None
    view = trace.view()
    assert view.outcome is ExecutionOutcome.COMPLETED
    call = view.tool_calls[0]
    assert call.tool.value == "shipping_quote"
    assert call.arguments.value == {"weight_kg": 2, "zone": "local"}
    assert call.wire.state.value == "observed"

Run it from the project root:

m3 test -- tests/test_trace.py

The result assertion checks the server response. The trace assertions check the call M3 observed. Moving client.final_trace inside the with block is an error because the trace is not final yet.

Next, open the saved run or read about evidence availability.