TDD with LLMs

Subhomoy Haldar

A worry-free guide to developing software with confidence using Claude Code and Test Driven Development.

Hint: Press Space to go forward. Shift + Space to go back.

In The Past

• Ship Fast → Skip tests
• Save time (short-term)

In The Past

• Code review = read + manual test
• Verify it works → Spot issues/smells

In The Past

• Requires 100% focus
• Merge conflicts, typos, bad assumptions
• Technical debt compounds silently

What’s The Solution Here?

• Guarantee correctness?
• Make reviews repeatable?

Enter:

Test-Driven Development

• Business logic → Code
• Executable
• Repeatable
• Never forgotten

Why Now?

• What changed?
• “Too much work”?
• “Ship first”?

Answer: Claude Code

• Natural language → Code
• Describe needs → Get output
• Massive time savings

Do We Still Need Tests With Claude?

• You tell me.

“The fastest code is the code which does not run.

The code easiest to maintain is the code that was never written.”

- Robert Galanakis

Untested Code is a Liability

• My code = unproven until tested
• LLM output = trust even less

Good Thing About Claude…

• Automates menial code generation
• Use time savings for tests & guardrails

Traditional TDD

1. Write tests [Hard]
2. Run → Compilation errors [Easy]
3. Stubs → Tests fail (Red) [Easy]
4. Implement → Tests pass (Green) [Medium/Hard]
5. Iterate

LLM Assisted TDD

1. Dictate business requirements in detail [Medium/Hard]

LLM Assisted TDD

1. Dictate business requirements in detail [Medium/Hard]
2. Claude writes tests [Easy]

LLM Assisted TDD

1. Dictate business requirements in detail [Medium/Hard]
2. Claude writes tests [Easy]
3. Review tests → Add missed edge cases [Medium]

LLM Assisted TDD

1. Dictate business requirements in detail [Medium/Hard]
2. Claude writes tests [Easy]
3. Review tests → Add missed edge cases [Medium]
4. Claude: Run → Stubs → Implement [Easy]

LLM Assisted TDD

1. Dictate business requirements in detail [Medium/Hard]
2. Claude writes tests [Easy]
3. Review tests → Add missed edge cases [Medium]
4. Claude: Run → Stubs → Implement [Easy]
5. Manual review [Medium/Hard]

LLM Assisted TDD

1. Dictate business requirements in detail [Medium/Hard]
2. Claude writes tests [Easy]
3. Review tests → Add missed edge cases [Medium]
4. Claude: Run → Stubs → Implement [Easy]
5. Manual review [Medium/Hard]
6. Refine requirements [Medium]

LLM Assisted TDD

1. Dictate business requirements in detail [Medium/Hard]
2. Claude writes tests [Easy]
3. Review tests → Add missed edge cases [Medium]
4. Claude: Run → Stubs → Implement [Easy]
5. Manual review [Medium/Hard]
6. Refine requirements [Medium]
7. Loop or done

Practical Example

Upgrading Chart.js

Practical Example

• Requirement: Upgrade Chart.js
• Success criteria: Charts keep working
• Regression tests → Nothing breaks
• Iterate until pass
• Ready to refactor

Practical Example

• Upgrade versions
• Re-run regression tests
• Manual testing (high confidence)

Practical Example

• Code review (auto + manual)
• Resolve comments → Merge
• Done

Discussions

• Doubts?
• Concerns?