test-driven-development

name	test-driven-development
description	Use when implementing any feature or bugfix, before writing implementation code

Test-Driven Development (TDD)

Overview

Write the test first. Watch it fail. Write minimal code to pass.

Core principle: If you didn't watch the test fail, you don't know if it tests the right thing.

Violating the letter of the rules is violating the spirit of the rules.

When to Use

The authority for this is the TDD Policy table in CLAUDE.md. It is narrower than the generic "always" and it wins. Reproduced here:

Scenario	TDD Required?
New feature / service / parser	Yes
New shared library or cross-tool code (Radoub.Formats, Radoub.UI)	Yes
Bug fix (reproducible)	Yes — failing test reproducing the bug first
Bug fix (investigation needed)	No — debug first, add regression test after
UI layout / styling / AXAML only	No — manual verification
Config / documentation only	No

The No rows are real exemptions, not rationalizations — do not talk yourself back into TDD for an AXAML-only change. Everything else in this skill applies to the Yes rows.

Thinking "skip TDD just this once" on a Yes row? Stop. That's rationalization.

Note the split: "investigation needed" means you genuinely cannot reproduce it yet. Once you can reproduce it, it is a reproducible bug fix — write the failing test.

The Iron Law

NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST

Write code before the test? Delete it. Start over.

No exceptions:

• Don't keep it as "reference"
• Don't "adapt" it while writing tests
• Don't look at it
• Delete means delete

Implement fresh from tests. Period.

Red-Green-Refactor

digraph tdd_cycle {
    rankdir=LR;
    red [label="RED\nWrite failing test", shape=box, style=filled, fillcolor="#ffcccc"];
    verify_red [label="Verify fails\ncorrectly", shape=diamond];
    green [label="GREEN\nMinimal code", shape=box, style=filled, fillcolor="#ccffcc"];
    verify_green [label="Verify passes\nAll green", shape=diamond];
    refactor [label="REFACTOR\nClean up", shape=box, style=filled, fillcolor="#ccccff"];
    next [label="Next", shape=ellipse];

    red -> verify_red;
    verify_red -> green [label="yes"];
    verify_red -> red [label="wrong\nfailure"];
    green -> verify_green;
    verify_green -> refactor [label="yes"];
    verify_green -> green [label="no"];
    refactor -> verify_green [label="stay\ngreen"];
    verify_green -> next;
    next -> red;
}

RED - Write Failing Test

Write one minimal test showing what should happen.

```csharp [Fact] public void ReadLocalizedString_TruncatesOverlongEntry_InsteadOfThrowing() { var gff = GffTestData.WithLocalizedString(length: 70_000);

var result = GffReader.ReadLocalizedString(gff);

Assert.Equal(65_535, result.Length);

}

Clear name, tests real behavior, one thing
</Good>

<Bad>
```csharp
[Fact]
public void ReaderWorks()
{
    var mock = new Mock<IGffReader>();
    mock.Setup(r => r.ReadLocalizedString(It.IsAny<GffStruct>())).Returns("ok");

    var result = mock.Object.ReadLocalizedString(null);

    Assert.Equal("ok", result);
}

Vague name, tests the mock not the code

Requirements:

• One behavior
• Clear name
• Real code (no mocks unless unavoidable)

Verify RED - Watch It Fail

MANDATORY. Never skip.

dotnet test Radoub.Formats/Radoub.Formats.Tests --filter "FullyQualifiedName~ReadLocalizedString"

Run the targeted test project, not Radoub.sln — the full suite takes ~30 minutes. See verification-before-completion for how to capture output without discarding failures.

Confirm:

• Test fails (not errors)
• Failure message is expected
• Fails because feature missing (not typos or a missing using)

Test passes? You're testing existing behavior. Fix test.

Test errors? Fix error, re-run until it fails correctly.

GREEN - Minimal Code

Write simplest code to pass the test.

```csharp public static string ReadLocalizedString(GffStruct gff) { var raw = gff.GetString(Field.LocalizedString); return raw.Length > MaxLength ? raw[..MaxLength] : raw; } ``` Just enough to pass ```csharp public static string ReadLocalizedString( GffStruct gff, int? maxLength = null, TruncationMode mode = TruncationMode.Tail, Action? onTruncated = null, bool preserveWordBoundaries = false) { // YAGNI — none of this is required by the test } ``` Over-engineered

Don't add features, refactor other code, or "improve" beyond the test.

Verify GREEN - Watch It Pass

MANDATORY.

dotnet test Radoub.Formats/Radoub.Formats.Tests

Confirm:

• Test passes
• Other tests in the affected project still pass
• Output pristine (no new build warnings)

Changed a shared library? Radoub.Formats and Radoub.UI are consumed by every tool — also test the tools that use the code you touched.

Test fails? Fix code, not test.

Other tests fail? Fix now.

REFACTOR - Clean Up

After green only:

• Remove duplication
• Improve names
• Extract helpers

Keep tests green. Don't add behavior.

Repeat

Next failing test for next feature.

Good Tests

Quality	Good	Bad
Minimal	One thing. "and" in name? Split it.	test('validates email and domain and whitespace')
Clear	Name describes behavior	test('test1')
Shows intent	Demonstrates desired API	Obscures what code should do

Why Order Matters

"I'll write tests after to verify it works"

Tests written after code pass immediately. Passing immediately proves nothing:

• Might test wrong thing
• Might test implementation, not behavior
• Might miss edge cases you forgot
• You never saw it catch the bug

Test-first forces you to see the test fail, proving it actually tests something.

"I already manually tested all the edge cases"

Manual testing is ad-hoc. You think you tested everything but:

• No record of what you tested
• Can't re-run when code changes
• Easy to forget cases under pressure
• "It worked when I tried it" ≠ comprehensive

Automated tests are systematic. They run the same way every time.

"Deleting X hours of work is wasteful"

Sunk cost fallacy. The time is already gone. Your choice now:

• Delete and rewrite with TDD (X more hours, high confidence)
• Keep it and add tests after (30 min, low confidence, likely bugs)

The "waste" is keeping code you can't trust. Working code without real tests is technical debt.

"TDD is dogmatic, being pragmatic means adapting"

TDD IS pragmatic:

• Finds bugs before commit (faster than debugging after)
• Prevents regressions (tests catch breaks immediately)
• Documents behavior (tests show how to use code)
• Enables refactoring (change freely, tests catch breaks)

"Pragmatic" shortcuts = debugging in production = slower.

"Tests after achieve the same goals - it's spirit not ritual"

No. Tests-after answer "What does this do?" Tests-first answer "What should this do?"

Tests-after are biased by your implementation. You test what you built, not what's required. You verify remembered edge cases, not discovered ones.

Tests-first force edge case discovery before implementing. Tests-after verify you remembered everything (you didn't).

30 minutes of tests after ≠ TDD. You get coverage, lose proof tests work.

Common Rationalizations

Excuse	Reality
"Too simple to test"	Simple code breaks. Test takes 30 seconds.
"I'll test after"	Tests passing immediately prove nothing.
"Tests after achieve same goals"	Tests-after = "what does this do?" Tests-first = "what should this do?"
"Already manually tested"	Ad-hoc ≠ systematic. No record, can't re-run.
"Deleting X hours is wasteful"	Sunk cost fallacy. Keeping unverified code is technical debt.
"Keep as reference, write tests first"	You'll adapt it. That's testing after. Delete means delete.
"Need to explore first"	Fine. Throw away exploration, start with TDD.
"Test hard = design unclear"	Listen to test. Hard to test = hard to use.
"TDD will slow me down"	TDD faster than debugging. Pragmatic = test-first.
"Manual test faster"	Manual doesn't prove edge cases. You'll re-test every change.
"Existing code has no tests"	You're improving it. Add tests for existing code.

Red Flags - STOP and Start Over

• Code before test
• Test after implementation
• Test passes immediately
• Can't explain why test failed
• Tests added "later"
• Rationalizing "just this once"
• "I already manually tested it"
• "Tests after achieve the same purpose"
• "It's about spirit not ritual"
• "Keep as reference" or "adapt existing code"
• "Already spent X hours, deleting is wasteful"
• "TDD is dogmatic, I'm being pragmatic"
• "This is different because..."

All of these mean: Delete code. Start over with TDD.

Example: Bug Fix

Bug: A blueprint saves with a 20-character resref. The tool accepts it, but Aurora silently cannot load the file in-game.

RED

[Fact]
public void Validate_ResrefOver16Chars_ReturnsError()
{
    var result = ResrefValidator.Validate("my_very_long_blueprint");

    Assert.False(result.IsValid);
    Assert.Contains("16 characters", result.Error);
}

Verify RED

$ dotnet test Relique/Relique.Tests --filter "FullyQualifiedName~ResrefOver16Chars"
Failed!  - Failed: 1, Passed: 0
  Assert.False() Failure: Expected False, actual True

Failing for the right reason: the validator currently accepts it.

GREEN

public static ValidationResult Validate(string resref)
{
    if (string.IsNullOrWhiteSpace(resref))
        return ValidationResult.Fail("Resref is required");

    if (resref.Length > 16)
        return ValidationResult.Fail($"Resref must be 16 characters or fewer (got {resref.Length})");

    return ValidationResult.Ok();
}

Verify GREEN

$ dotnet test Relique/Relique.Tests
Passed!  - Failed: 0, Passed: 47

REFACTOR This constraint is Aurora-wide, not Relique-specific — if a second tool needs it, promote the validator to Radoub.Formats and add tests there.

Note what the test asserts: the behavior Aurora requires, not the implementation. It would still pass if the length check moved elsewhere.

Verification Checklist

Before marking work complete:

• Every new function/method has a test
• Watched each test fail before implementing
• Each test failed for expected reason (feature missing, not typo)
• Wrote minimal code to pass each test
• All tests pass
• Output pristine (no errors, warnings)
• Tests use real code (mocks only if unavoidable)
• Edge cases and errors covered

Can't check all boxes? You skipped TDD. Start over.

When Stuck

Problem	Solution
Don't know how to test	Write wished-for API. Write assertion first. Ask your human partner.
Test too complicated	Design too complicated. Simplify interface.
Must mock everything	Code too coupled. Use dependency injection.
Test setup huge	Extract helpers. Still complex? Simplify design.

Debugging Integration

Bug found? Write failing test reproducing it. Follow TDD cycle. Test proves fix and prevents regression.

Never fix bugs without a test.

Testing Anti-Patterns

When adding mocks or test utilities, read @testing-anti-patterns.md to avoid common pitfalls:

• Testing mock behavior instead of real behavior
• Adding test-only methods to production classes
• Mocking without understanding dependencies

Final Rule

Production code → test exists and failed first
Otherwise → not TDD

No exceptions without your human partner's permission.