Create E2e Test

Purpose

Use this skill when adding an end-to-end integration test for a new or existing MCP tool or feature. It encodes the exact conventions, infrastructure, and file structure that all existing e2e tests follow.

Project Context

• MCP Server: Rust binary built with cargo build --release
• Test framework: Rust integration tests via cargo test --test e2e
• Test location: tests/e2e/
• Entry point: tests/e2e/main.rs — contains #[test] wrapper functions and shared setup
• Module index: tests/e2e/tests/mod.rs — declares all test modules and re-exports infrastructure
• Backend abstraction: ServerBackend trait with three implementations: CargoBackend (static binary), DockerBackend (container), NpmBackend (npm package). Every test must work with all backends.
• Backend selection: CS_MCP_BACKEND env var (static / docker / npm); create_backend() factory
• Key environment variables: CS_ACCESS_TOKEN (required), CS_MCP_EXECUTABLE (optional override), CS_MCP_BACKEND

Infrastructure modules (all in tests/e2e/)

Re-exports via tests/mod.rs

Test modules use use super::*; to get all infrastructure via the re-exports in tests/mod.rs:

pub use crate::file_utils::{create_git_repo, create_temp_dir};
pub use crate::fixtures::get_sample_files;
pub use crate::mcp_client::MCPClient;
pub use crate::response_parsers::{extract_code_health_score, extract_result_text};
pub use crate::server_backends::{
    base_env, create_backend, docker_config_dir, fake_server_bind_host,
    fake_server_url_host, is_docker, skip_if_docker, ServerBackend,
};
pub use crate::{find_or_build_executable, make_client, setup};
pub use serde_json::json;
pub use std::path::Path;
pub use std::time::Duration;

Step-by-Step

1. Create the test file

Create tests/e2e/tests/<feature>.rs with the standard boilerplate:

//! <Feature> integration tests.
//!
//! Tests that the MCP server correctly <what this validates>.
//!
//! Validates:
//! - <First thing>
//! - <Second thing>

use super::*;

Conventions:

• The module docstring (//!) should explain what the test validates and why.
• use super::*; imports all infrastructure via tests/mod.rs re-exports.
• Define constants for tool names, timeouts, expected terms, and error patterns at the top.

2. Add fixtures if needed

If your test requires code samples with specific Code Health characteristics that the existing fixtures do not cover, add them to fixtures.rs:

const NEW_SAMPLE_CODE: &str = r#""""
Module docstring.
"""

def example():
    pass
"#;

Then update get_sample_files() and optionally get_expected_scores() if the new sample should be part of the standard test repository.

Skip this step if the existing fixtures are sufficient.

3. Add helper functions to reduce duplication

Extract shared setup logic into helper functions within the test module:

const TOOL_NAME: &str = "tool_name";
const TIMEOUT: Duration = Duration::from_secs(60);

fn call_tool(client: &mut MCPClient, repo_dir: &Path) -> String {
    let test_file = repo_dir.join("src/utils/calculator.py");
    let response = client
        .call_tool(
            TOOL_NAME,
            json!({"file_path": test_file.to_string_lossy()}),
            TIMEOUT,
        )
        .expect("Tool call should succeed");
    extract_result_text(&response)
}

fn setup_and_call(command: &[String], env: &[(String, String)], repo_dir: &Path) -> String {
    let mut client = make_client(command, env, repo_dir);
    assert!(client.start(), "Server should start");
    client.initialize().expect("Initialize should succeed");
    call_tool(&mut client, repo_dir)
}

4. Implement individual test functions

Each test function is a pub fn (not #[test]) — the #[test] wrappers live in main.rs:

pub fn test_feature_basic_response() {
    let (command, env, repo_dir, _tmp) = setup();
    let result_text = setup_and_call(&command, &env, &repo_dir);

    assert!(
        !result_text.is_empty(),
        "Tool should return content"
    );
}

pub fn test_feature_contains_expected_data() {
    let (command, env, repo_dir, _tmp) = setup();
    let result_text = setup_and_call(&command, &env, &repo_dir);
    let lower = result_text.to_lowercase();

    let expected_terms = &["term1", "term2", "term3"];
    let found = expected_terms.iter().filter(|t| lower.contains(*t)).count();

    assert!(found >= 2, "Expected at least 2 terms, found {found}");
}

pub fn test_feature_no_errors() {
    let (command, env, repo_dir, _tmp) = setup();
    let result_text = setup_and_call(&command, &env, &repo_dir);
    let lower = result_text.to_lowercase();

    let error_patterns = &["traceback", "no such file", "os error 2"];
    for pattern in error_patterns {
        assert!(
            !lower.contains(pattern),
            "Response must not contain '{pattern}': {result_text}"
        );
    }
}

Conventions:

• Functions are pub fn, not #[test] — the #[test] attribute goes on the wrapper in main.rs.
• Always hold _tmp (TempDir) — dropping it deletes the temp directory. The variable must live until the test completes.
• Use setup() for the standard git-repo-based test setup.
• Use make_client() to create an MCPClient from command/env/cwd.
• Use Duration::from_secs(60) for tool call timeouts.
• Use extract_result_text() to parse JSON-RPC responses — never parse manually.
• Docker-only considerations: Use skip_if_docker() at the start of tests that cannot run in Docker (e.g., filesystem path tests). Use fake_server_bind_host()/fake_server_url_host() for tests that start HTTP servers.

5. Register the module in tests/mod.rs

Add the module declaration to tests/e2e/tests/mod.rs:

pub mod <feature>;

Keep modules in alphabetical order.

6. Add #[test] wrappers in main.rs

Add wrapper functions in tests/e2e/main.rs under a comment section header:

// --- Feature Name ---
#[test]
fn test_feature_basic_response() {
    tests::feature::test_feature_basic_response();
}

#[test]
fn test_feature_contains_expected_data() {
    tests::feature::test_feature_contains_expected_data();
}

#[test]
fn test_feature_no_errors() {
    tests::feature::test_feature_no_errors();
}

Conventions:

• The wrapper function name matches the test function name exactly.
• Each wrapper is a one-liner that delegates to the module function.
• Group wrappers under a // --- Feature Name --- comment.
• Place new sections in logical order relative to existing tests.
• For ignored tests (e.g., stress tests), add #[ignore] above fn.

Canonical Example: business_case.rs

The simplest existing test to reference is tests/e2e/tests/business_case.rs. It demonstrates:

• Three focused test functions with the standard setup() + setup_and_call() pattern
• Constants for tool name, timeout, expected terms, and error patterns
• Helper functions to eliminate duplication
• Clean assertion messages

Refer to business_case.rs as the minimal template for any new test module.

Verification

Run the e2e test suite to verify your new test is picked up and passes:

# Run all e2e tests (builds release binary first)
cargo test --test e2e

# Run a specific test by name
cargo test --test e2e test_feature_basic_response

# Run all tests matching a pattern
cargo test --test e2e test_feature

Prerequisites:

• Rust toolchain (cargo)
• CS_ACCESS_TOKEN environment variable set
• git in PATH

Validate Code Health and Fix Any Issues

After the test is written and passing, run the CodeScene Code Health tools to ensure the new test code meets quality standards. Do not consider the test complete until Code Health passes without regressions.

Recommended workflow:

1. Run code_health_review on each new/modified file.
2. If any code smells or regressions are reported, refactor and re-check.
3. Before committing, run pre_commit_code_health_safeguard.
4. Before opening a PR, run analyze_change_set against the target branch.

Target: Code Health 10.0. Scores of 9+ are not "good enough" — aim for optimal.

Checklist

Before considering the test complete:

• Test file created at tests/e2e/tests/<feature>.rs
• Module docstring (//!) explains what the test validates and why
• use super::*; imports infrastructure
• Constants defined for tool names, timeouts, expected terms
• Helper functions extract shared logic to avoid duplication
• Each test function is pub fn (not #[test])
• _tmp (TempDir) held alive for the duration of each test
• Module declared in tests/e2e/tests/mod.rs (alphabetical order)
• #[test] wrappers added in tests/e2e/main.rs under a section comment
• Fixtures added to fixtures.rs if new code samples needed
• cargo test --test e2e test_<feature> passes
• code_health_review passes on all new/modified files
• pre_commit_code_health_safeguard reports no regressions