Test tool errors and input schemas

An MCP tool can report an expected application error as a normal call result. When schema validation is enabled, M3 can also reject test arguments that do not match the tool's advertised input schema before sending the call.

Requirements

Use a Python 3.10+ project prepared with m3 setup. Download the example shipping_server.py and save it as shipping_server.py in the project root. It includes the shipping schema and a tool that returns an MCP error result.

Check a tool result and reject invalid input

Save this complete test file as tests/test_errors_schemas.py:

import sys
from pathlib import Path

import pytest

from m3 import MCPTestKit, ModelValidationError, StdioServer

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


def test_assert_a_tool_error_result() -> None:
    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("always_fails")

    assert result.is_error is True
    assert result.content[0]["text"] == "expected example failure"


def test_validate_schema_and_reject_invalid_arguments() -> None:
    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, validate_schemas=True) as client:
        valid = client.call_tool("shipping_quote", {"weight_kg": 2, "zone": "local"})
        assert valid.structured_content == {"amount": 9.0, "currency": "USD"}
        with pytest.raises(
            ModelValidationError, match="tool JSON Schema validation failed"
        ):
            client.call_tool("shipping_quote", {"weight_kg": -1, "zone": "local"})

Run from the project root:

m3 test -- tests/test_errors_schemas.py

The first test asserts an MCP result with is_error=True; the call itself returns normally. Schema mismatch raises ModelValidationError because validate_schemas=True. Without that option, the client does not perform this local input-schema check. A server can still reject input for its own reasons. Continue to test a tool contract.