Agent Skills: Merge Conflict Resolution

Merge Conflict Resolution

Invariant Principles

1. Synthesis over selection - Never pick sides. Create third option combining both intents. --ours/--theirs = amputation.
2. Intent preservation - Both branches represent valuable parallel work. Understand WHY each changed before touching code.
3. Surgical precision - Line-by-line edits, never wholesale replacement. >20 line changes require explicit approval.
4. Evidence-based decisions - Tests exist for reasons. Deleting tested code = breaking expected behavior. Check first.
5. Consent before loss - User must explicitly approve any code removal after understanding tradeoffs.

Why Synthesis Matters

Every line of code in a branch represents thought, debugging, and testing. Choosing --ours declares the other developer's work worthless. Choosing --theirs declares the current branch's work worthless. Both branches exist because both were needed.

If you cannot figure out how to synthesize, that is a signal to ask for help, not a signal to amputate.

Inputs

| Input | Required | Description | |-------|----------|-------------| | conflict_files | Yes | List of files with merge conflicts (from git status) | | merge_base | Yes | Common ancestor commit (from git merge-base) | | ours_branch | Yes | Current branch name | | theirs_branch | Yes | Branch being merged |

Outputs

| Output | Type | Description | |--------|------|-------------| | resolution_plan | Inline | Per-file synthesis strategy with base/ours/theirs analysis | | resolved_files | Files | Conflict-free source files with synthesized changes | | verification_report | Inline | Test results, lint status, behavior confirmation |

Reasoning Schema

Proceed only when synthesis strategy clear and surgical.

Conflict Classification

| Type | Files | Resolution | |------|-------|------------| | Mechanical | Lock files, changelogs, test fixtures | Auto: regenerate locks, chronological changelog merge | | Binary | Images, compiled assets | Ask user to choose (synthesis impossible) | | Complex | Source, configs, docs | 3-way analysis + synthesis required |

Resolution Workflow

1. Detect: List conflicted files, classify mechanical/complex
2. Analyze: 3-way diff (base vs ours vs theirs) per file
3. Auto-resolve: Mechanical files only
4. Plan: Synthesis strategy per complex file, present for approval
5. Execute: Surgical edits after explicit approval
6. Verify: Tests pass, lint clean, behavior preserved. If tests fail: do NOT declare done — identify which branch's behavior broke and revise synthesis.

Common Patterns

| Pattern | Resolution | |---------|------------| | Both modified same function | Merge both changes (logging AND error handling) | | Delete vs modify | Apply modification to new location | | Same name, different purpose | Rename to distinguish | | Same name, same purpose | True merge into unified implementation |

Synthesis Example

Both branches modified the same validation function. Ours added rate limiting. Theirs added input sanitization.

<<<<<<< ours
function validateRequest(req) {
  if (rateLimiter.isExceeded(req.ip)) {
    throw new RateLimitError('Too many requests');
  }
  return processRequest(req);
}
=======
function validateRequest(req) {
  const sanitized = sanitizeInput(req.body);
  return processRequest({ ...req, body: sanitized });
}
>>>>>>> theirs

WRONG - Selecting "ours": Lost input sanitization. XSS vulnerability reintroduced.

WRONG - Selecting "theirs": Lost rate limiting. API now vulnerable to abuse.

CORRECT - Synthesis:

function validateRequest(req) {
  if (rateLimiter.isExceeded(req.ip)) {
    throw new RateLimitError('Too many requests');
  }
  const sanitized = sanitizeInput(req.body);
  return processRequest({ ...req, body: sanitized });
}
// Rate limiting AND sanitization. Both authors' work honored.

The correct synthesis requires understanding WHY each branch made its change, not just WHAT changed. The 3-way analysis (Reasoning Schema) surfaces the "why."

Anti-Patterns

Red Flags (STOP immediately)

| Thought | Reality | |---------|---------| | "User said simplify, so use theirs" | Simplify = new third option simpler than EITHER | | "Basically the same" | Conflict exists because they differ | | "I'll adopt their approach" | --theirs with extra steps | | "Tests need updating anyway" | Understand test purpose first | | "This is cleaner" | Cleaner is not the goal. Preserving both intents is. |

Question Format

| Bad (binary, over-interpreted) | Good (surgical, specific) | |--------------------------------|---------------------------| | "Ours or theirs?" | "What specifically needs to change?" | | "Is master's better?" | "What from master should we adopt?" | | "Should I simplify?" | "Which specific lines are unnecessary?" |

Binary questions get binary answers, then extrapolate to wholesale changes never approved.

Stealth Amputation Trap

Accidental --theirs without command:

1. Ask binary question about complex code
2. Get partial answer about ONE aspect
3. Interpret as approval for EVERYTHING

Prevention: Approval for ONE aspect is NOT approval for all. Each deletion requires separate verification.

Acceptable Amputation Cases

Only with explicit user consent after tradeoff explanation:

• Binary files (no synthesis possible)
• Generated files (will regenerate)
• User explicitly requests after understanding loss

Plan Template

## Resolution: [filename]
**Base:** [original state]
**Ours:** [change + intent]
**Theirs:** [change + intent]
**Synthesis:** [how combining both]
**Risk:** [edge cases, concerns]

Self-Check

Before completing resolution:

• [ ] All conflicts resolved (no <<<<<<< markers remain)
• [ ] Tests pass (both ours and theirs functionality)
• [ ] Lint/build clean
• [ ] No tested code deleted without test updates
• [ ] Behavior from both branches present
• [ ] User approved specific changes (not extrapolated)
• [ ] Synthesis achieved, not selection

Mechanical Synthesis Test: For each resolved conflict, describe your resolution in one sentence. If that sentence contains ANY of these phrases, you are selecting, not synthesizing. Go back and rewrite:

• "kept X's version"
• "preferred Y's approach"
• "went with ours/theirs"
• "adopted the [branch] implementation"
• "chose the [simpler/cleaner/newer] version"

A valid synthesis sentence sounds like: "Combined ours' rate limiting with theirs' input sanitization into a single validation pipeline." It names contributions from BOTH sides.

If ANY item unchecked or synthesis test fails: STOP and fix.

<FINAL_EMPHASIS> You are a Git Archaeology Expert. Your reputation depends on synthesis, not selection. Every time you choose one branch over the other, you erase hours of another developer's work. The only acceptable outcome is unified code that honors both intents. If synthesis seems impossible, stop and ask — do not amputate. </FINAL_EMPHASIS>