Skill: add-pattern

Guide the agent through every step needed to add a new conflict-resolution pattern from end to end, touching all four required locations in the right order.

Step 1 — Gather information before writing any code

Ask the following questions up front. Do not start editing files until you have clear answers.

1. Name — What is the snake_case name for this pattern? (e.g. comment_only_change, empty_block_insertion)
2. Trigger condition — In plain English, when does this pattern apply? Be precise: what must be true of oursLines, theirsLines, and optionally baseLines for the pattern to fire?
3. Resolution strategy — What should the merged output look like when this pattern fires? (accept ours, accept theirs, union, base + additions, etc.)
4. Confidence score — What typeClassification value (0–100) do you propose, and why? What dataRisk value (0–100, where 0 = safe)? Use the existing patterns as a reference:
   • same_change / one_side_change / delete_no_change → typeClassification 100, dataRisk 0
   • non_overlapping → typeClassification 90, dataRisk 20
   • whitespace_only / reorder_only / insertion_at_boundary → typeClassification 85–90, dataRisk 8–20
   • value_only_change → typeClassification 72–88, dataRisk 25
5. Priority — What numeric priority should this pattern have? Lower = evaluated earlier. Current assignments:
   • 10 same_change, 20 delete_no_change, 30 one_side_change
   • 40 non_overlapping, 50 whitespace_only, 55 reorder_only
   • 57 insertion_at_boundary, 60 value_only_change, 999 complex Pick a value that places it at the right point in the evaluation chain.
6. requires — Does the pattern need the base (diff3), work without it (diff2), or both? Use "diff3", "diff2", or "both".

Step 2 — Add the ConflictType to types.ts

packages/core/src/types.ts contains the ConflictType union. Add the new type name there before creating the pattern file — TypeScript will then flag every location that needs updating.

// packages/core/src/types.ts
export type ConflictType =
  | "same_change"
  | "one_side_change"
  // ... existing types ...
  | "my_new_pattern"   // ← add here, with a short comment
  | "complex";

Also add a readable summary string for it in the buildSummary() switch inside packages/core/src/classifier.ts:

case "my_new_pattern":
  return "One-line human-readable description of the resolution.";

Step 3 — Create the pattern file

Create packages/core/src/patterns/<name>.ts.

The file must export a default PatternPlugin object — not a plain function. Copy the structure below and fill in every field.

/**
 * Pattern `my_new_pattern`
 *
 * <Two-sentence description of what this pattern detects and why it is safe
 * to auto-resolve.>
 *
 * Priority: <N> (<before/after which existing pattern>)
 * Requires: <diff3 | diff2 | both>
 */

import type { ClassifyInput, ConfidenceScore, PatternPlugin } from "../types.js";
import { scopeImpact, makeScore } from "./utils.js";

const myNewPattern: PatternPlugin = {
  type: "my_new_pattern",
  priority: <N>,
  requires: "<diff3|diff2|both>",

  detect(h: ClassifyInput): boolean {
    // Return true when this pattern applies.
    // Be conservative — false positives are worse than false negatives.
    return false; // replace with real logic
  },

  confidence(h: ClassifyInput): ConfidenceScore {
    const totalLines = Math.max(h.oursLines.length, h.theirsLines.length);
    return makeScore(
      <typeClassification>,  // 0–100, certainty of the classification
      <dataRisk>,            // 0–100, 0 = no data loss risk
      scopeImpact(totalLines),
      ["<booster: why we are confident>"],
      ["<penalty: why we are cautious, or empty array>"],
    );
  },

  explanation(h: ClassifyInput): string {
    return "Human-readable explanation shown in the UI (explain mode).";
  },

  passReason(h: ClassifyInput): string {
    return "Shown in DecisionTrace when this pattern matched.";
  },

  failReason(h: ClassifyInput): string {
    return "Shown in DecisionTrace when this pattern was tested but did not match.";
  },
};

export default myNewPattern;

Critical: context-line detection

If your detect() function ever inspects raw diff lines for context vs. added/removed:

// CORRECT — empty strings do NOT become phantom context lines
const isContext = (line: string) => line.startsWith(' ');

// WRONG — empty string passes this test and produces phantom context lines
const isContext = (line: string) => !line.startsWith('\\');

Step 4 — Register the pattern in classifier.ts

packages/core/src/classifier.ts owns the pattern registry. Add an import and add the plugin to the PATTERNS array in priority order.

// 1. Add the import near the other pattern imports
import myNewPattern from "./patterns/my-new-pattern.js";

// 2. Add to PATTERNS array — the array order is cosmetic (sort is by .priority),
//    but keep it in priority order for readability
const PATTERNS: PatternPlugin[] = [
  sameChange,           // priority 10
  // ...
  myNewPattern,         // priority <N>  ← add here
  // ...
  complex,              // priority 999 — MUST stay last
];

complex must always remain the last entry (priority 999, detect() always returns true) — it is the fallback for every unrecognized conflict.

Step 5 — Add a resolution case in resolver/assemble.ts

packages/core/src/resolver/assemble.ts contains the assembleResolution() function, which switches on hunk.type to produce merged lines. Every new ConflictType needs a case here, otherwise the pattern will be classified correctly but its resolution will never be assembled.

case "my_new_pattern": {
  // Produce the merged lines according to the resolution strategy.
  const merged = /* ... */;
  return {
    lines: merged,
    reason: "Short one-line reason logged in verbose mode.",
  };
}

Place the new case near the patterns of similar priority. Return { lines: null, reason: "..." } if under certain option flags the pattern should refuse to auto-resolve (see whitespace_only for an example).

Step 6 — Write the tests

Create packages/core/src/__tests__/patterns/<name>.test.ts.

Minimum coverage: 10 test cases.

import { describe, it, expect } from "vitest";
import { resolve } from "../../resolver.js";

describe("<name> — should resolve", () => {
  // At least 5 cases where the pattern fires and produces the expected output.
  // Include at least one diff3 case (with ||||||| base section) and one diff2.
  // Include at least one case with empty strings.
  it("resolves <scenario>", () => {
    const input = [
      "<<<<<<< ours",
      /* ours lines */
      "||||||| base",
      /* base lines */
      "=======",
      /* theirs lines */
      ">>>>>>> theirs",
    ].join("\n");
    const result = resolve(input, "src/file.ts");
    expect(result.hunks[0].type).toBe("<name>");
    expect(result.stats.autoResolved).toBe(1);
    expect(result.mergedContent).toBe(/* expected output */);
  });
});

describe("<name> — should NOT resolve", () => {
  // At least 5 cases where the pattern must NOT fire (returns null / classified
  // as a different type). These guard against false positives.
  // Include at least one edge case (single line, all-empty lines, etc.).
  it("does not resolve when <counter-scenario>", () => {
    const input = [
      "<<<<<<< ours",
      /* ours lines that look similar but must not match */
      "=======",
      /* theirs lines */
      ">>>>>>> theirs",
    ].join("\n");
    const result = resolve(input, "src/file.ts");
    expect(result.hunks[0].type).not.toBe("<name>");
  });
});

Step 7 — Check Rust parity

Open apps/desktop/src-tauri/examples/parity_probe.rs and check whether the new pattern corresponds to a conflict type that the Rust side also classifies.

The parity probe binary is compiled and run by pnpm test:parity (from apps/desktop/). If the Rust classifier uses a matching concept, add the equivalent classification logic there and update the parity fixtures.

If the new pattern is purely a TypeScript-side heuristic with no Rust equivalent, add a comment in parity_probe.rs documenting that this type is TS-only.

Step 8 — Validation

Run both suites before marking the task done:

# TypeScript unit tests
cd packages/core && pnpm test

# Rust ↔ TypeScript parity tests
cd apps/desktop && pnpm test:parity

Both must pass with zero failures. If the parity suite fails, the Rust implementation is out of sync — fix it before merging.