mdproof-implement-feature

name: mdproof-implement-feature description: >- Implement a feature from a spec file or description using TDD workflow. Use this skill whenever the user asks to: add a new CLI flag, implement a feature, build new functionality, create a new internal package, add an assertion type, extend the parser, or write Go code for mdproof. This skill enforces test-first development and proper package structure. If the request involves writing Go code and tests, use this skill — even if the user doesn't explicitly say "implement". argument-hint: "[spec-file-path | feature description]" targets: [claude, codex]

Implement a feature following TDD workflow. $ARGUMENTS is a spec file path or a plain-text feature description.

Scope: This skill writes Go code and tests. It does NOT update CHANGELOG (use changelog after).

Workflow

Step 1: Understand Requirements

If $ARGUMENTS is a file path:

1. Read the spec file
2. Extract acceptance criteria and edge cases
3. Identify affected packages

If $ARGUMENTS is a description:

1. Search existing code for related functionality
2. Identify the right package to extend
3. Confirm scope with user before proceeding

Step 2: Identify Affected Files

List all files that will be created or modified:

# Typical patterns
internal/<package>/<feature>.go       # Core logic
internal/<package>/<feature>_test.go  # Unit test
cmd/mdproof/main.go                   # CLI flag additions
mdproof.go                            # Facade re-exports (if new public types)

Display the file list and continue. If scope is unclear, ask the user.

Step 3: Write Failing Tests First (RED)

Write tests in the appropriate internal/ package:

func TestFeature_BasicCase(t *testing.T) {
    // Setup
    input := "test input"

    // Act
    result := SomeFunction(input)

    // Assert
    if result != expected {
        t.Errorf("got %v, want %v", result, expected)
    }
}

For executor/runner tests that need shell execution:

func TestMain(m *testing.M) {
    os.Setenv("MDPROOF_ALLOW_EXECUTE", "1")
    os.Exit(m.Run())
}

Verify tests fail:

go test ./internal/<package> -run TestFeature_BasicCase

Step 4: Implement (GREEN)

Write minimal code to make tests pass:

1. Follow existing patterns in internal/
2. Keep the dependency graph acyclic:

   core → (nothing)
   config → (nothing)
   parser → core
   assertion → core
   executor → core, assertion
   report → core
   runner → core, parser, executor, config, report
   sandbox → config
   

3. New shared types go in internal/core/types.go
4. If adding public API, update mdproof.go facade with type aliases

Verify tests pass:

go test ./internal/<package>

Step 5: Refactor and Verify

1. Clean up code while keeping tests green
2. Run full quality check:

   make check  # fmt-check + lint + test
   

3. Fix any formatting or lint issues

Step 6: Stage and Report

1. List all created/modified files
2. Confirm each acceptance criterion is met with test evidence

Package Reference

Key Design Patterns

Facade Pattern (mdproof.go)

Root package re-exports types via aliases for clean public API:

type Step = core.Step
type Report = core.Report

Functions wrap internal calls:

func Run(r io.Reader, name string, opts RunOptions) (Report, error) {
    return runner.Run(r, name, opts)
}

Container Safety

executor.IsContainerEnv() checks /.dockerenv or cgroup. Override with MDPROOF_ALLOW_EXECUTE=1. Tests must set this in TestMain.

Shell Session Persistence

Single bash process per runbook. Env vars persist via env file written after each step. This is core to mdproof's design — steps share state.

Rules

• Test-first — always write failing test before implementation
• Minimal code — only write what's needed to pass tests
• Follow patterns — match existing code style in each package
• No cycles — respect the dependency graph
• 3-strike rule — if a test fails 3 times after fixes, stop and report
• Spec ambiguity — ask the user rather than guessing