pytest-mcp-assert 0.12.1

pytest plugin for mcp-assert: run MCP server assertions as pytest test items

pytest-mcp-assert

pytest plugin for mcp-assert. Run MCP server assertions as pytest test items.

Install

pip install pytest-mcp-assert

You also need the mcp-assert binary. Any of these:

brew install blackwell-systems/tap/mcp-assert
pip install mcp-assert
npm install -g @blackwell-systems/mcp-assert

Usage

Write assertion YAML files (see mcp-assert docs):

# evals/echo.yaml
name: echo returns the message
server:
  command: my-mcp-server
assert:
  tool: echo
  args:
    message: hello
  expect:
    not_error: true
    contains: ["hello"]
timeout: 30s

Run with pytest:

pytest --mcp-suite evals/

Each YAML file becomes a pytest test item with pass/fail/skip semantics.

Options

Option	pyproject.toml	Description
--mcp-suite DIR	mcp_suite	Directory containing assertion YAML files
--mcp-fixture DIR	mcp_fixture	Fixture directory (substituted for {{fixture}})
--mcp-server CMD		Override server command for all assertions
--mcp-timeout DUR	mcp_timeout	Per-assertion timeout (default: 30s)
--mcp-binary PATH		Path to mcp-assert binary

Configuration

# pyproject.toml
[tool.pytest.ini_options]
mcp_suite = "evals/"
mcp_fixture = "fixtures/"
mcp_timeout = "30s"

Then just run pytest with no extra flags.

How it works

The plugin discovers .yaml files in the suite directory, creates a pytest Item for each one, and calls mcp-assert run --suite <file> --json to execute it. The JSON result is mapped to pytest outcomes:

• status: "pass" becomes a pytest pass
• status: "fail" becomes a pytest failure with the detail as the message
• status: "skip" becomes a pytest skip
• skip: true in the YAML skips the test (for known bugs)

The Go binary handles all MCP protocol interaction. The plugin is a thin bridge.