# Tutorial: Build a CLI Tool in 30 Minutes with the Harness

> This is a complete walkthrough from zero to working software.
> Example project: A markdown link checker that finds broken links in .md files.
> Follow along exactly — copy-paste commands, see what works.

---

## What You'll Build

**mdlinkcheck** — A CLI tool that:
- Scans markdown files for links
- Tests each HTTP/HTTPS link (returns 200?)
- Identifies broken links (404, timeout, etc.)
- Outputs a report with line numbers

**Tech stack:** TypeScript, Node.js, Commander.js for CLI parsing

**Time investment:**
- Interview & spec: 10 minutes
- Agent planning: 2 minutes
- Agent building: 15-20 minutes (4-6 iterations)
- Your review: 5 minutes

---

## Prerequisites

- Node.js 18+ installed
- Claude CLI (`npm install -g @anthropic-ai/claude-cli`) or OpenClaw access
- Agent harness templates (this repo)
- 30 minutes of your time

---

## Phase 0: Setup (2 minutes)

### 1. Create the project directory
```bash
mkdir ~/projects/mdlinkcheck
cd ~/projects/mdlinkcheck
git init
```

### 2. Copy templates
```bash
cp /path/to/agent-harness/AGENT-INSTRUCTIONS.md ./AGENT.md
cp /path/to/agent-harness/PROJECT-SPEC.md ./PROJECT-SPEC.md
cp /path/to/agent-harness/DECISIONS.md ./DECISIONS.md
cp /path/to/agent-harness/ralph-loop.sh ./ralph-loop.sh
chmod +x ralph-loop.sh
```

### 3. Initial commit
```bash
git add .
git commit -m "Initial setup: agent harness templates"
```

You now have an empty project with the harness scaffolding.

---

## Phase 1: The Interview (10 minutes)

Open a chat with your agent (Claude, Cleo, etc.) and run through the interview protocol from SPEC-CREATION-GUIDE.md.

### Your opening message:
```
I want to build a CLI tool using the agent harness. Let's create 
the spec together using the interview protocol.

Project: A markdown link checker that finds broken links.
```

### Expected Interview Flow

**Agent:** "What are we building, in one sentence?"

**You:**
```
A CLI tool that scans markdown files for HTTP/HTTPS links and 
reports which ones are broken (404, timeout, etc.).
```

**Agent:** "Who is this for?"

**You:**
```
Developers who maintain documentation. They run it before 
committing to catch broken links.
```

**Agent:** "Why now? What's the trigger?"

**You:**
```
I maintain several docs repos and keep finding broken links after 
they're published. I want to catch them earlier.
```

**Agent:** "What does 'done' look like?"

**You:**
```
I can run `mdlinkcheck docs/**/*.md` and get a report like:
docs/api.md:15 - BROKEN - https://example.com/missing (404)
docs/guide.md:42 - BROKEN - https://oldsite.com (timeout)

Found 2 broken links out of 47 total.

Exit code 0 if all links work, 1 if any are broken (for CI).
```

**Agent:** "What have you tried before? What didn't work?"

**You:**
```
Tried markdown-link-check (npm package) but it's slow and has 
too many dependencies. I want something simple and fast.
```

**Agent:** "Walk me through how you'd use this feature."

**You:**
```
1. Install: npm install -g mdlinkcheck
2. Run: mdlinkcheck README.md
3. See output: list of broken links or "All links OK!"
4. Check exit code: 0 = success, 1 = broken links found
```

**Agent:** "What could go wrong?"

**You:**
```
- No markdown files found (should show helpful message)
- Network is down (should timeout gracefully, not hang)
- Huge file with 1000+ links (should show progress)
- Link requires auth (should report as "unreachable" not "broken")
```

**Agent:** "What's the input? What's the output?"

**You:**
```
Input: File paths or glob patterns (e.g., docs/**/*.md)
Output: 
  - For each broken link: filename:lineNumber - STATUS - URL (error)
  - Summary: "Found X broken links out of Y total"
  - Exit code: 0 (all good) or 1 (broken links)
```

**Agent:** "How would you test this?"

**You:**
```
Create a test.md with known good and bad links:
- https://google.com (should work)
- https://thisurldoesnotexist123456.com (should fail)

Run the tool, verify it reports the broken one.
```

**Agent:** "Is this a must-have or nice-to-have?"

**You:**
```
Must-haves:
- Parse markdown, extract links
- Test HTTP/HTTPS links
- Report broken ones with filename and line number
- Proper exit codes for CI

Nice-to-haves (defer to v2):
- Check relative links (./other-doc.md)
- Parallel checking for speed
- Retry on timeout
- Config file for ignored URLs
```

---

## Phase 2: Spec Assembly (Agent does this)

**You:**
```
Great! Now fill out PROJECT-SPEC.md based on our interview. 
Use the template in the repo. Be specific about:
- Tech stack: TypeScript, Node.js, Commander.js
- Build command: npm run build (compile TS to dist/)
- Test command: npm test (use Node.js test runner)
- Data model: not needed (stateless tool)
- One requirement per feature with acceptance criteria
```

**Agent generates spec (2-3 minutes):**

The agent reads PROJECT-SPEC.md template and fills it out:

```markdown
# Project Specification: mdlinkcheck

## 1. Project Overview

### What are we building?
A command-line tool that scans markdown files for HTTP/HTTPS links 
and reports broken ones (404, timeout, connection errors).

### Why does it matter?
Developers maintaining documentation need to catch broken links 
before publishing. Existing tools are slow or over-complicated.

### Success criteria
- [ ] Scan .md files and extract all HTTP/HTTPS links
- [ ] Test each link (HEAD request, follow redirects)
- [ ] Report broken links with filename:line number
- [ ] Exit code 0 (all OK) or 1 (broken links found)
- [ ] Works on CLI: `mdlinkcheck docs/**/*.md`

## 2. Technical Foundation

### Tech stack
- Language: TypeScript
- Runtime: Node.js 18+
- CLI framework: Commander.js
- HTTP client: Native fetch (Node 18+)
- Build: tsc (TypeScript compiler)
- Test: Node.js test runner (node:test)

### Project structure
mdlinkcheck/
├── src/
│   ├── cli.ts          # Commander.js entry point
│   ├── parser.ts       # Extract links from markdown
│   ├── checker.ts      # Test links via HTTP
│   └── reporter.ts     # Format output
├── tests/
│   ├── parser.test.ts
│   ├── checker.test.ts
│   └── fixtures/       # Test markdown files
├── dist/               # Compiled JS (gitignored)
├── package.json
├── tsconfig.json
└── PROJECT-SPEC.md

### Build & test commands
npm install
npm run build    # tsc
npm test         # node --test tests/**/*.test.ts
npm run lint     # tsc --noEmit (type check only)

## 3. Requirements

### FR-001: Extract Links from Markdown
**Description:** Parse markdown files and extract all HTTP/HTTPS URLs.
**Acceptance criteria:**
- [ ] Find links in standard format: [text](https://url)
- [ ] Find links in reference format: [text][ref] ... [ref]: https://url
- [ ] Find bare URLs: https://example.com
- [ ] Return array of { url, line } objects
- [ ] Skip relative links (./file.md, /path/file.md)

### FR-002: Test HTTP/HTTPS Links
**Description:** Send HTTP request to each URL and determine status.
**Acceptance criteria:**
- [ ] Use HEAD request (faster than GET)
- [ ] Follow redirects (up to 3)
- [ ] Timeout after 10 seconds
- [ ] Return status: OK | BROKEN | TIMEOUT | UNREACHABLE
- [ ] Handle network errors gracefully (don't crash)

### FR-003: CLI Interface
**Description:** Command-line tool that accepts file paths or globs.
**Acceptance criteria:**
- [ ] Command: `mdlinkcheck <files...>`
- [ ] Expand glob patterns (docs/**/*.md)
- [ ] Show usage help with --help
- [ ] Show version with --version
- [ ] Exit code 0 if all links OK, 1 if any broken

### FR-004: Reporting
**Description:** Output broken links in readable format.
**Acceptance criteria:**
- [ ] Format: `filename:line - STATUS - URL (error message)`
- [ ] Summary: "Found X broken links out of Y total"
- [ ] Show "All links OK!" if none broken
- [ ] Color output: red for broken, green for OK (optional)

## 4. Anti-patterns
- Don't use regex for markdown parsing — use a markdown parser
- Don't use GET requests — HEAD is faster and sufficient
- Don't hang on timeout — set explicit timeout
- Don't crash on network errors — catch and report

## 5. Phasing

### Phase 1: Core (must-have)
- [ ] Project setup (package.json, tsconfig, build)
- [ ] Link extraction from markdown
- [ ] HTTP link checking
- [ ] CLI interface
- [ ] Basic reporting

### Phase 2: Polish (nice-to-have, defer)
- [ ] Parallel checking
- [ ] Progress bar
- [ ] Config file for ignored URLs
- [ ] Check relative links
```

**You review the spec:**
```
Looks good! One clarification: use the `marked` library for parsing
markdown (don't write a regex-based parser). Update FR-001.
```

**Agent updates spec**, commits:
```bash
git add PROJECT-SPEC.md
git commit -m "Add project specification for mdlinkcheck"
```

---

## Phase 3: Planning (Agent does this)

**You:**
```
Read PROJECT-SPEC.md. Create IMPLEMENTATION_PLAN.md by decomposing 
the project into discrete tasks ordered by dependency. Output 
<promise>PLANNED</promise> when done.
```

**Agent thinks and writes plan:**

```markdown
# Implementation Plan: mdlinkcheck

## Phase 1: Foundation
- [ ] Project scaffolding (package.json, tsconfig, src/, tests/)
- [ ] CLI entry point with Commander.js (--help, --version)
- [ ] Markdown link extraction (using marked lexer)
- [ ] HTTP link checker (fetch with timeout)
- [ ] Reporter (format output)
- [ ] Wire together: CLI → parser → checker → reporter
- [ ] Integration test with fixtures

## Phase 2: Polish (Deferred)
- [ ] Parallel checking
- [ ] Progress indicator
```

**Agent outputs:**
```
<promise>PLANNED</promise>

Plan created with 7 tasks. Review IMPLEMENTATION_PLAN.md.
```

**You review:**
```
Good! Let's start building.
```

---

## Phase 4: Build Iterations (Agent does this, you watch)

### Method A: Using ralph-loop.sh
```bash
./ralph-loop.sh --agent claude --max 10
```

### Method B: Using OpenClaw sessions_spawn
```
You: "Run the first agent harness iteration for mdlinkcheck"
Cleo: *spawns sub-agent* → reads AGENT.md → picks first task → builds
```

### Method C: Manual (for learning)
```
You: "Read AGENT.md in /path/to/mdlinkcheck. Follow the core loop:
      orient, pick the first unchecked task in IMPLEMENTATION_PLAN.md,
      implement it, verify (npm run build && npm test), commit, exit."
```

---

### Iteration 1: Project scaffolding

**Agent output:**
```
Reading PROJECT-SPEC.md and IMPLEMENTATION_PLAN.md...
First task: Project scaffolding

Creating package.json...
Creating tsconfig.json...
Creating src/, tests/, tests/fixtures/...
Installing dependencies: commander, marked, @types/node...
Running build: ✅ compiles successfully
Running tests: ✅ (no tests yet, but runner works)

Committing...
[main a1b2c3d] chore: project scaffolding with TypeScript and Commander
Task complete. Exiting for fresh context.
```

**Git log:**
```
a1b2c3d chore: project scaffolding with TypeScript and Commander
```

**IMPLEMENTATION_PLAN.md updated:**
```markdown
- [x] Project scaffolding (package.json, tsconfig, src/, tests/)
- [ ] CLI entry point with Commander.js (--help, --version)
...
```

---

### Iteration 2: CLI entry point

**Agent output:**
```
Reading IMPLEMENTATION_PLAN.md...
Next task: CLI entry point with Commander.js

Creating src/cli.ts...
Wiring up Commander: program.name('mdlinkcheck').description('...')
Adding --help and --version...
Adding bin entry to package.json...
Testing: npm link (local install)
Running: mdlinkcheck --help ✅
Running: mdlinkcheck --version ✅

Committing...
[main b2c3d4e] feat: CLI entry point with Commander.js
```

**You can now run:**
```bash
npm link
mdlinkcheck --help
# mdlinkcheck - Find broken links in markdown files
# Usage: mdlinkcheck <files...>
```

---

### Iteration 3: Markdown link extraction

**Agent output:**
```
Next task: Markdown link extraction

Creating src/parser.ts...
Using marked.lexer to parse markdown into tokens...
Extracting links from link and image tokens...
Tracking line numbers...
Writing tests with fixtures/test.md...

Test results:
✅ Extracts standard markdown links
✅ Extracts reference-style links
✅ Skips relative links
✅ Returns line numbers

Committing...
[main c3d4e5f] feat: markdown link extraction with line numbers
```

**Files created:**
- `src/parser.ts`
- `tests/parser.test.ts`
- `tests/fixtures/test.md` (sample markdown with various link formats)

---

### Iteration 4: HTTP link checker

**Agent output:**
```
Next task: HTTP link checker

Creating src/checker.ts...
Using fetch with HEAD method...
Following redirects (up to 3)...
10-second timeout via AbortController...
Error handling for network failures...

Test results:
✅ Returns OK for 200 status
✅ Returns BROKEN for 404
✅ Returns TIMEOUT for hung requests
✅ Returns UNREACHABLE for network errors

Committing...
[main d4e5f6g] feat: HTTP link checker with timeout and error handling
```

---

### Iteration 5: Reporter

**Agent output:**
```
Next task: Reporter

Creating src/reporter.ts...
Format: filename:line - STATUS - URL (error)...
Summary line with counts...
Color output using ANSI codes (red for broken, green for OK)...

Test results:
✅ Formats broken link correctly
✅ Shows summary
✅ Handles no broken links case

Committing...
[main e5f6g7h] feat: reporter with colored output and summary
```

---

### Iteration 6: Wire together

**Agent output:**
```
Next task: Wire together CLI → parser → checker → reporter

Updating src/cli.ts...
For each file argument:
  1. Read file
  2. Extract links (parser)
  3. Check each link (checker)
  4. Collect broken links
Report results (reporter)
Set exit code (0 or 1)

Build: ✅
Test: ✅

Manual test with real URLs:
Created tests/fixtures/real-links.md:
- https://google.com
- https://thisurldoesnotexist123456.com

Running: mdlinkcheck tests/fixtures/real-links.md
Output:
  tests/fixtures/real-links.md:2 - BROKEN - https://thisurldoesnotexist123456.com (404)
  Found 1 broken link out of 2 total.

Exit code: 1 ✅

Committing...
[main f6g7h8i] feat: wire CLI components together, end-to-end working
```

---

### Iteration 7: Integration test

**Agent output:**
```
Next task: Integration test with fixtures

Creating tests/integration.test.ts...
Test fixture with known good and bad links...
Run CLI programmatically...
Assert output contains expected broken links...
Assert exit code is 1...

Test results:
✅ Detects broken links in fixture
✅ Reports correct filename and line number
✅ Exit code is 1 when links are broken
✅ Exit code is 0 when all links are OK

All tasks complete!
<promise>DONE</promise>

Committing...
[main g7h8i9j] test: add integration tests with fixtures
```

---

## Phase 5: Review (You do this)

### 1. Check the git log
```bash
git log --oneline
```

Output:
```
g7h8i9j test: add integration tests with fixtures
f6g7h8i feat: wire CLI components together, end-to-end working
e5f6g7h feat: reporter with colored output and summary
d4e5f6g feat: HTTP link checker with timeout and error handling
c3d4e5f feat: markdown link extraction with line numbers
b2c3d4e feat: CLI entry point with Commander.js
a1b2c3d chore: project scaffolding with TypeScript and Commander
```

✅ **7 iterations, 7 commits** — clean history

### 2. Run the build and tests
```bash
npm run build
npm test
```

Output:
```
> build
tsc

> test
✅ parser extracts links correctly (4 tests)
✅ checker tests URLs correctly (4 tests)
✅ reporter formats output (3 tests)
✅ integration end-to-end (2 tests)

13 tests passed
```

✅ **All tests pass**

### 3. Try it for real
```bash
npm link
mdlinkcheck README.md
```

Output:
```
All links OK! ✅
Found 0 broken links out of 5 total.
```

**Try with a broken link:**
```bash
echo "[broken](https://thisurldoesnotexist123456.com)" > test.md
mdlinkcheck test.md
```

Output:
```
test.md:1 - BROKEN - https://thisurldoesnotexist123456.com (404)
Found 1 broken link out of 1 total.
```

✅ **Works as specified**

### 4. Review the code quality

**Check src/parser.ts:**
```typescript
import { marked } from 'marked';

interface LinkInfo {
  url: string;
  line: number;
}

export function extractLinks(markdown: string): LinkInfo[] {
  const tokens = marked.lexer(markdown);
  const links: LinkInfo[] = [];
  
  function walkTokens(token: any, lineOffset = 0) {
    if (token.type === 'link' || token.type === 'image') {
      if (token.href.startsWith('http://') || token.href.startsWith('https://')) {
        links.push({
          url: token.href,
          line: lineOffset + (token.line || 1)
        });
      }
    }
    if (token.tokens) {
      token.tokens.forEach((t: any) => walkTokens(t, lineOffset));
    }
  }
  
  tokens.forEach(token => walkTokens(token));
  return links;
}
```

✅ **Clean, readable, uses marked instead of regex**

**Check src/checker.ts:**
```typescript
export async function checkLink(url: string): Promise<LinkStatus> {
  const controller = new AbortController();
  const timeout = setTimeout(() => controller.abort(), 10000);
  
  try {
    const response = await fetch(url, {
      method: 'HEAD',
      signal: controller.signal,
      redirect: 'follow'
    });
    
    clearTimeout(timeout);
    return response.ok ? 'OK' : 'BROKEN';
  } catch (error) {
    clearTimeout(timeout);
    if (error.name === 'AbortError') return 'TIMEOUT';
    return 'UNREACHABLE';
  }
}
```

✅ **Proper timeout handling, follows redirects, good error handling**

### 5. Review against spec

| Requirement | Status |
|-------------|--------|
| FR-001: Extract links from markdown | ✅ Implemented with marked |
| FR-002: Test HTTP/HTTPS links | ✅ HEAD requests with timeout |
| FR-003: CLI interface | ✅ Commander.js with --help |
| FR-004: Reporting | ✅ Colored output with summary |
| Exit codes | ✅ 0 for success, 1 for broken |
| Tests | ✅ 13 tests, all passing |

---

## Phase 6: Polish (Optional)

**You:**
```
This looks great! Let's add one enhancement: show a progress 
indicator when checking many links. Add this as a task to the plan.
```

**You update IMPLEMENTATION_PLAN.md:**
```markdown
## Phase 2: Polish
- [ ] Progress indicator (show "Checking... 5/47" while running)
```

**You:**
```
Run one more iteration to implement the progress indicator.
```

**Agent (iteration 8):**
```
Reading plan...
Next task: Progress indicator

Adding progress output to src/cli.ts...
Show "Checking link 5/47..." as each link is tested...
Clear line and show summary when done...

Build: ✅
Test: ✅
Manual test: Shows progress ✅

Committing...
[main h8i9j0k] feat: add progress indicator for link checking
<promise>DONE</promise>
```

**Now when you run it:**
```bash
mdlinkcheck docs/**/*.md
```

Output:
```
Checking link 1/47...
Checking link 2/47...
...
Checking link 47/47...

All links OK! ✅
Found 0 broken links out of 47 total.
```

---

## What You Just Did

In **30 minutes**, you:

1. ✅ Interviewed with an agent to create a clear spec
2. ✅ Let the agent plan the task decomposition
3. ✅ Ran 7 autonomous iterations (each: implement → test → commit)
4. ✅ Reviewed the output (5 minutes)
5. ✅ Added a polish enhancement (1 more iteration)
6. ✅ Shipped a working CLI tool with tests

**Your effort:**
- Interview: 10 min (you answered questions)
- Review: 5 min (you ran the code and checked quality)
- Total: 15 minutes of YOUR time

**Agent's effort:**
- Planning: 2 min
- Building: 18 min (8 iterations × ~2 min each)
- Total: 20 minutes of autonomous work

**You wrote zero lines of code.** You defined WHAT to build, and the agent figured out HOW.

---

## What to Try Next

### Package it for npm
```markdown
- [ ] Add README.md with usage examples
- [ ] Add LICENSE (MIT)
- [ ] Publish to npm: `npm publish`
```

### Add features
```markdown
- [ ] Check relative links (./other-doc.md)
- [ ] Config file to ignore certain URLs
- [ ] Parallel checking (Promise.all for speed)
- [ ] JSON output mode for CI integration
```

### Use it in CI
```yaml
# .github/workflows/docs.yml
name: Check Links
on: [push]
jobs:
  check:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v3
      - run: npm install -g mdlinkcheck
      - run: mdlinkcheck docs/**/*.md
```

---

## Key Lessons from This Tutorial

### 1. The spec is everything
The 10-minute interview created a spec that guided 8 flawless iterations. Bad spec = bad output.

### 2. Fresh context prevents drift
Each iteration started fresh. No context overflow, no confusion from stale reasoning.

### 3. Tests validate autonomy
Every iteration ran `npm test`. Failing tests forced the agent to fix before proceeding. No test theater.

### 4. Git history tells the story
One commit per task. Clean, reviewable, revertable.

### 5. You shift from writer to reviewer
Your job: define the goal, review the output, course-correct. The agent writes the code.

---

_Now go build something real. Interview, spec, run the loop, review. Repeat._