fix-bug

Fix bugs using Chicago School TDD, root cause analysis, and rich domain design.

Workflow

┌─────────────────────────────────────────────────────────────┐
│  1. REPRODUCE & UNDERSTAND                                   │
├─────────────────────────────────────────────────────────────┤
│  • Reproduce the bug                                         │
│  • Identify expected vs actual behavior                      │
│  • Locate the root cause in code                             │
└─────────────────────────────────────────────────────────────┘
                            │
                            ▼
┌─────────────────────────────────────────────────────────────┐
│  2. WRITE FAILING TEST (Red)                                 │
├─────────────────────────────────────────────────────────────┤
│  • Write test that exposes the bug                           │
│  • Test should FAIL before fix                               │
│  • Test should verify CORRECT behavior                       │
└─────────────────────────────────────────────────────────────┘
                            │
                            ▼
┌─────────────────────────────────────────────────────────────┐
│  3. FIX & VERIFY (Green)                                     │
├─────────────────────────────────────────────────────────────┤
│  • Implement minimal fix                                     │
│  • Test now PASSES                                           │
│  • All existing tests still pass                             │
└─────────────────────────────────────────────────────────────┘

Phase 1: Reproduce & Understand

Identify the Bug

1. Reproduce: Follow exact steps to trigger the bug
2. Expected: What SHOULD happen (user's mental model)
3. Actual: What IS happening (current behavior)
4. Root cause: WHY it's happening (code analysis)

Locate in Architecture

Reference: docs/ARCHITECTURE.md

| Layer | Location | What to look for | |-------|----------|------------------| | Domain | Sources/Domain/ | Incorrect business logic, missing invariants | | Infrastructure | Sources/Infrastructure/ | Parsing errors, CLI/API issues | | App | Sources/App/ | View state issues, binding problems |

Domain Invariants

Check if the bug violates domain invariants that should be maintained:

// Example: QuotaMonitor should maintain selection invariants
// - selectedProviderId should always point to an enabled provider
// - Domain should be self-validating (no external "ensure" calls needed)

Phase 2: Write Failing Test (Red)

Chicago School TDD

We follow Chicago School TDD (state-based testing):

• Test state changes and return values, not interactions
• Focus on the "what" (observable outcomes), not the "how" (method calls)
• Mocks stub dependencies to return data, not to verify calls
• No verify() calls - assert on resulting state instead

Test Pattern

Test the CORRECT behavior, not the bug:

@Suite
struct {Component}Tests {

    @Test func `{describes correct behavior}`() {
        // Given - setup that triggers the bug scenario
        let settings = makeSettingsRepository()
        let claude = ClaudeProvider(probe: MockUsageProbe(), settingsRepository: settings)
        claude.isEnabled = false  // Bug trigger condition

        // When - action that should work correctly
        let monitor = QuotaMonitor(providers: AIProviders(providers: [claude, codex]))

        // Then - assert EXPECTED behavior (will FAIL before fix)
        #expect(monitor.selectedProviderId == "codex")  // Not "claude"
    }
}

Test Location

| Bug Location | Test Location | |--------------|---------------| | Sources/Domain/Monitor/ | Tests/DomainTests/Monitor/ | | Sources/Domain/Provider/ | Tests/DomainTests/Provider/ | | Sources/Infrastructure/CLI/ | Tests/InfrastructureTests/CLI/ |

Run Test (Should FAIL)

swift test --filter "{TestSuiteName}"

Phase 3: Fix & Verify (Green)

Fix Guidelines

1. Minimal change: Fix only what's broken
2. Domain first: Prefer fixing in domain layer when possible
3. Maintain invariants: Domain should be self-validating
4. No over-engineering: Don't refactor unrelated code

Domain Design Principles

When fixing domain bugs, ensure:

// 1. Domain maintains its own invariants
public init(...) {
    // Validate on construction
    selectFirstEnabledIfNeeded()  // Called internally, not externally
}

// 2. Public API hides implementation details
public func setProviderEnabled(_ id: String, enabled: Bool) {
    provider.isEnabled = enabled
    if !enabled {
        selectFirstEnabledIfNeeded()  // Private - called automatically
    }
}

// 3. Private methods for internal invariant maintenance
private func selectFirstEnabledIfNeeded() { ... }

Verify Fix

# Run the specific test (should PASS now)
swift test --filter "{TestSuiteName}"

# Run ALL tests to ensure no regressions
swift test

Checklist

• [ ] Bug reproduced and understood
• [ ] Root cause identified in code
• [ ] Failing test written (exposes bug)
• [ ] Test FAILS before fix
• [ ] Minimal fix implemented
• [ ] Test PASSES after fix
• [ ] All existing tests still pass
• [ ] Domain invariants maintained (if applicable)
• [ ] CHANGELOG updated with fix description