diff --git a/packages/rules-core/index.d.ts b/packages/rules-core/index.d.ts
new file mode 100644
index 000000000..e910bb060
--- /dev/null
+++ b/packages/rules-core/index.d.ts
@@ -0,0 +1 @@
+export * from "./src/index";
diff --git a/packages/rules-core/package.json b/packages/rules-core/package.json
new file mode 100644
index 000000000..ca4f75f3b
--- /dev/null
+++ b/packages/rules-core/package.json
@@ -0,0 +1,21 @@
+{
+  "name": "@oh-my-opencode/rules-core",
+  "version": "0.1.0",
+  "type": "module",
+  "private": true,
+  "description": "Pure TypeScript rule discovery, matching, and nested AGENTS.md context utilities for oh-my-opencode.",
+  "exports": {
+    ".": {
+      "types": "./index.d.ts",
+      "import": "./src/index.ts"
+    }
+  },
+  "types": "./index.d.ts",
+  "scripts": {
+    "typecheck": "tsgo --noEmit -p tsconfig.json",
+    "test": "bun test src/*.test.ts"
+  },
+  "dependencies": {
+    "picomatch": "^4.0.4"
+  }
+}
diff --git a/packages/rules-core/src/agents-md.ts b/packages/rules-core/src/agents-md.ts
new file mode 100644
index 000000000..5620f66db
--- /dev/null
+++ b/packages/rules-core/src/agents-md.ts
@@ -0,0 +1,50 @@
+import { existsSync, statSync } from "node:fs";
+import { dirname, isAbsolute, join, relative, resolve } from "node:path";
+import { AGENTS_FILENAME } from "./constants";
+import type { AgentsMdCache } from "./types";
+
+export interface FindAgentsMdUpInput {
+  readonly startDir: string;
+  readonly rootDir: string;
+  readonly skipRoot?: boolean;
+  readonly cache?: AgentsMdCache;
+}
+
+export async function findAgentsMdUp(input: FindAgentsMdUpInput): Promise<string[]> {
+  const startDir = resolve(input.startDir);
+  const rootDir = resolve(input.rootDir);
+  const skipRoot = input.skipRoot ?? true;
+  const cacheKey = [startDir, rootDir, skipRoot ? "1" : "0"].join("\0");
+  const cached = input.cache?.get(cacheKey);
+  if (cached) return [...cached];
+  const found: string[] = [];
+  let current = startDir;
+  while (true) {
+    const isRootDir = current === rootDir;
+    if (!(skipRoot && isRootDir)) {
+      const agentsPath = join(current, AGENTS_FILENAME);
+      if (isFile(agentsPath)) found.push(agentsPath);
+    }
+    if (isRootDir) break;
+    const parent = dirname(current);
+    if (parent === current || !isSameOrChildPath(parent, rootDir)) break;
+    current = parent;
+  }
+  const result = found.reverse();
+  input.cache?.set(cacheKey, result);
+  return result;
+}
+
+function isFile(path: string): boolean {
+  if (!existsSync(path)) return false;
+  try {
+    return statSync(path).isFile();
+  } catch {
+    return false;
+  }
+}
+
+function isSameOrChildPath(childPath: string, parentPath: string): boolean {
+  const relativePath = relative(parentPath, childPath);
+  return relativePath === "" || (!relativePath.startsWith("..") && !isAbsolute(relativePath));
+}
diff --git a/packages/rules-core/src/cache.ts b/packages/rules-core/src/cache.ts
new file mode 100644
index 000000000..2ac85ec79
--- /dev/null
+++ b/packages/rules-core/src/cache.ts
@@ -0,0 +1,26 @@
+import type { AgentsMdCache, DirectoryScanEntry, RuleFileCandidate, RuleScanCache, RuleScanCacheStats } from "./types";
+
+export function createRuleScanCache(): RuleScanCache {
+  const candidateCache = new Map<string, readonly RuleFileCandidate[]>();
+  const directoryCache = new Map<string, readonly DirectoryScanEntry[]>();
+  return {
+    get: (key) => candidateCache.get(key),
+    set: (key, value) => candidateCache.set(key, value),
+    getDirScan: (dir) => directoryCache.get(dir),
+    setDirScan: (dir, entries) => directoryCache.set(dir, entries),
+    stats: (): RuleScanCacheStats => ({ candidateEntries: candidateCache.size, directoryEntries: directoryCache.size }),
+    clear: () => {
+      candidateCache.clear();
+      directoryCache.clear();
+    },
+  };
+}
+
+export function createAgentsMdCache(): AgentsMdCache {
+  const cache = new Map<string, readonly string[]>();
+  return {
+    get: (key) => cache.get(key),
+    set: (key, value) => cache.set(key, value),
+    clear: () => cache.clear(),
+  };
+}
diff --git a/packages/rules-core/src/constants.ts b/packages/rules-core/src/constants.ts
new file mode 100644
index 000000000..aafdb0088
--- /dev/null
+++ b/packages/rules-core/src/constants.ts
@@ -0,0 +1,33 @@
+import type { RuleSource } from "./types";
+
+export const PROJECT_MARKERS = [".git", "pyproject.toml", "package.json", "Cargo.toml", "go.mod", ".venv"] as const;
+
+export const PROJECT_RULE_SUBDIRS = [
+  [".omo", "rules"],
+  [".sisyphus", "rules"],
+  [".claude", "rules"],
+  [".cursor", "rules"],
+  [".github", "instructions"],
+] as const;
+
+export const PROJECT_RULE_FILES = [".github/copilot-instructions.md"] as const;
+export const OPENCODE_USER_RULE_DIRS = [".omo/rules", ".sisyphus/rules", ".opencode/rules"] as const;
+export const USER_RULE_DIR = ".claude/rules";
+export const RULE_EXTENSIONS = [".md", ".mdc"] as const;
+export const GITHUB_INSTRUCTIONS_PATTERN = /\.instructions\.md$/;
+export const AGENTS_FILENAME = "AGENTS.md";
+export const GLOBAL_DISTANCE = 9999;
+export const EXCLUDED_DIRS = new Set(["node_modules", ".git", "dist", "build", ".turbo", ".next", "coverage"]);
+
+export const SOURCE_PRIORITY: ReadonlyMap<RuleSource, number> = new Map([
+  [".omo/rules", 0],
+  [".sisyphus/rules", 1],
+  [".claude/rules", 2],
+  [".cursor/rules", 3],
+  [".github/instructions", 4],
+  [".github/copilot-instructions.md", 5],
+  ["~/.omo/rules", 100],
+  ["~/.sisyphus/rules", 101],
+  ["~/.opencode/rules", 102],
+  ["~/.claude/rules", 103],
+]);
diff --git a/packages/rules-core/src/distance.ts b/packages/rules-core/src/distance.ts
new file mode 100644
index 000000000..3e553ea19
--- /dev/null
+++ b/packages/rules-core/src/distance.ts
@@ -0,0 +1,25 @@
+import { dirname, relative } from "node:path";
+import { GLOBAL_DISTANCE } from "./constants";
+
+export function calculateDistance(rulePath: string, currentFile: string, projectRoot: string | null): number {
+  if (!projectRoot) return GLOBAL_DISTANCE;
+  try {
+    const ruleRelative = relative(projectRoot, dirname(rulePath));
+    const currentRelative = relative(projectRoot, dirname(currentFile));
+    if (ruleRelative.startsWith("..") || currentRelative.startsWith("..")) return GLOBAL_DISTANCE;
+    const ruleParts = toParts(ruleRelative);
+    const currentParts = toParts(currentRelative);
+    let shared = 0;
+    for (let index = 0; index < Math.min(ruleParts.length, currentParts.length); index += 1) {
+      if (ruleParts[index] !== currentParts[index]) break;
+      shared += 1;
+    }
+    return currentParts.length - shared;
+  } catch {
+    return GLOBAL_DISTANCE;
+  }
+}
+
+function toParts(path: string): string[] {
+  return path.split(/[/\\]/).filter(Boolean);
+}
diff --git a/packages/rules-core/src/finder.ts b/packages/rules-core/src/finder.ts
new file mode 100644
index 000000000..ebf6e7ab5
--- /dev/null
+++ b/packages/rules-core/src/finder.ts
@@ -0,0 +1,140 @@
+import { existsSync, statSync } from "node:fs";
+import { homedir } from "node:os";
+import { dirname, isAbsolute, join, relative, resolve } from "node:path";
+import { GLOBAL_DISTANCE, OPENCODE_USER_RULE_DIRS, PROJECT_RULE_FILES, PROJECT_RULE_SUBDIRS, USER_RULE_DIR } from "./constants";
+import { sortCandidates } from "./ordering";
+import { findRuleFilesRecursive, safeRealpathSync } from "./scanner";
+import type { DirectoryScanEntry, FindRuleFilesOptions, RuleFileCandidate, RuleScanCache, RuleSource } from "./types";
+
+export function findRuleFiles(
+  projectRoot: string | null,
+  homeDir: string,
+  currentFile: string,
+  options?: FindRuleFilesOptions,
+  cache?: RuleScanCache,
+): RuleFileCandidate[] {
+  const startDir = dirname(resolve(currentFile));
+  const skipClaudeUserRules = options?.skipClaudeUserRules ?? false;
+  const cacheKey = [projectRoot ?? "", startDir, skipClaudeUserRules ? "1" : "0"].join("\0");
+  const cached = cache?.get(cacheKey);
+  if (cached) return [...cached];
+  const candidates: RuleFileCandidate[] = [];
+  const seenRealPaths = new Set<string>();
+  if (projectRoot) {
+    addProjectRuleCandidates(projectRoot, startDir, candidates, seenRealPaths, cache);
+    addProjectSingleFileCandidates(projectRoot, candidates, seenRealPaths);
+  }
+  addUserRuleCandidates(homeDir || homedir(), skipClaudeUserRules, candidates, seenRealPaths, cache);
+  const sorted = sortCandidates(candidates);
+  cache?.set(cacheKey, sorted);
+  return sorted;
+}
+
+function addProjectRuleCandidates(
+  projectRoot: string,
+  startDir: string,
+  candidates: RuleFileCandidate[],
+  seenRealPaths: Set<string>,
+  cache: RuleScanCache | undefined,
+): void {
+  let currentDir = startDir;
+  let distance = 0;
+  while (true) {
+    for (const [parent, subdir] of PROJECT_RULE_SUBDIRS) {
+      const source = `${parent}/${subdir}` as RuleSource;
+      const ruleDir = join(currentDir, parent, subdir);
+      for (const entry of scanDirectoryWithCache(ruleDir, cache)) {
+        if (seenRealPaths.has(entry.realPath)) continue;
+        seenRealPaths.add(entry.realPath);
+        candidates.push({
+          path: entry.path,
+          realPath: entry.realPath,
+          source,
+          isGlobal: false,
+          distance,
+          relativePath: normalizePath(relative(projectRoot, entry.path)),
+        });
+      }
+    }
+    if (currentDir === projectRoot) break;
+    const parentDir = dirname(currentDir);
+    if (parentDir === currentDir || !isSameOrChildPath(parentDir, projectRoot)) break;
+    currentDir = parentDir;
+    distance += 1;
+  }
+}
+
+function addProjectSingleFileCandidates(
+  projectRoot: string,
+  candidates: RuleFileCandidate[],
+  seenRealPaths: Set<string>,
+): void {
+  for (const ruleFile of PROJECT_RULE_FILES) {
+    const filePath = join(projectRoot, ruleFile);
+    const realPath = validFileRealPath(filePath);
+    if (realPath === null || seenRealPaths.has(realPath)) continue;
+    seenRealPaths.add(realPath);
+    candidates.push({
+      path: filePath,
+      realPath,
+      source: ruleFile as RuleSource,
+      isGlobal: false,
+      distance: 0,
+      isSingleFile: true,
+      relativePath: normalizePath(ruleFile),
+    });
+  }
+}
+
+function addUserRuleCandidates(
+  homeDir: string,
+  skipClaudeUserRules: boolean,
+  candidates: RuleFileCandidate[],
+  seenRealPaths: Set<string>,
+  cache: RuleScanCache | undefined,
+): void {
+  const userRuleDirs: Array<readonly [string, RuleSource]> = OPENCODE_USER_RULE_DIRS.map((dir) => [join(homeDir, dir), `~/${dir}` as RuleSource]);
+  if (!skipClaudeUserRules) userRuleDirs.push([join(homeDir, USER_RULE_DIR), "~/.claude/rules"]);
+  for (const [userRuleDir, source] of userRuleDirs) {
+    for (const entry of scanDirectoryWithCache(userRuleDir, cache)) {
+      if (seenRealPaths.has(entry.realPath)) continue;
+      seenRealPaths.add(entry.realPath);
+      candidates.push({
+        path: entry.path,
+        realPath: entry.realPath,
+        source,
+        isGlobal: true,
+        distance: GLOBAL_DISTANCE,
+        relativePath: normalizePath(relative(homeDir, entry.path)),
+      });
+    }
+  }
+}
+
+function scanDirectoryWithCache(dir: string, cache: RuleScanCache | undefined): readonly DirectoryScanEntry[] {
+  const cached = cache?.getDirScan(dir);
+  if (cached) return cached;
+  const entries: DirectoryScanEntry[] = [];
+  findRuleFilesRecursive(dir, entries);
+  cache?.setDirScan(dir, entries);
+  return entries;
+}
+
+function validFileRealPath(filePath: string): string | null {
+  if (!existsSync(filePath)) return null;
+  try {
+    if (!statSync(filePath).isFile()) return null;
+    return safeRealpathSync(filePath);
+  } catch {
+    return null;
+  }
+}
+
+function isSameOrChildPath(childPath: string, parentPath: string): boolean {
+  const relativePath = relative(parentPath, childPath);
+  return relativePath === "" || (!relativePath.startsWith("..") && !isAbsolute(relativePath));
+}
+
+function normalizePath(path: string): string {
+  return path.replaceAll("\\", "/");
+}
diff --git a/packages/rules-core/src/index.test.ts b/packages/rules-core/src/index.test.ts
new file mode 100644
index 000000000..bfb069291
--- /dev/null
+++ b/packages/rules-core/src/index.test.ts
@@ -0,0 +1,133 @@
+import { mkdirSync, rmSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { afterEach, describe, expect, it } from "bun:test";
+
+import {
+  clearProjectRootCache,
+  createAgentsMdCache,
+  createRuleScanCache,
+  findAgentsMdUp,
+  findProjectRoot,
+  findRuleFiles,
+  parseRuleFrontmatter,
+  shouldApplyRule,
+} from "./index";
+
+let testRoot: string | null = null;
+
+function createTestRoot(name: string): string {
+  testRoot = join(tmpdir(), `${name}-${Date.now()}-${Math.random()}`);
+  mkdirSync(testRoot, { recursive: true });
+  return testRoot;
+}
+
+afterEach(() => {
+  if (testRoot) {
+    rmSync(testRoot, { recursive: true, force: true });
+    testRoot = null;
+  }
+  clearProjectRootCache();
+});
+
+describe("rules-core", () => {
+  it("#given mixed rule sources #when finding rule files #then returns deterministic source-priority order", () => {
+    // given
+    const root = createTestRoot("rules-core-order");
+    mkdirSync(join(root, ".git"));
+    mkdirSync(join(root, ".omo", "rules"), { recursive: true });
+    mkdirSync(join(root, ".sisyphus", "rules"), { recursive: true });
+    mkdirSync(join(root, ".claude", "rules"), { recursive: true });
+    mkdirSync(join(root, ".cursor", "rules"), { recursive: true });
+    mkdirSync(join(root, ".github", "instructions"), { recursive: true });
+    mkdirSync(join(root, "src"), { recursive: true });
+    writeFileSync(join(root, ".github", "copilot-instructions.md"), "copilot");
+    writeFileSync(join(root, ".omo", "rules", "omo.md"), "omo");
+    writeFileSync(join(root, ".sisyphus", "rules", "sisyphus.md"), "sisyphus");
+    writeFileSync(join(root, ".claude", "rules", "claude.md"), "claude");
+    writeFileSync(join(root, ".cursor", "rules", "cursor.md"), "cursor");
+    writeFileSync(join(root, ".github", "instructions", "github.instructions.md"), "github");
+
+    // when
+    const found = findRuleFiles(root, root, join(root, "src", "index.ts"));
+
+    // then
+    expect(found.map((rule) => rule.relativePath)).toEqual([
+      ".github/copilot-instructions.md",
+      ".omo/rules/omo.md",
+      ".sisyphus/rules/sisyphus.md",
+      ".claude/rules/claude.md",
+      ".cursor/rules/cursor.md",
+      ".github/instructions/github.instructions.md",
+    ]);
+  });
+
+  it("#given frontmatter aliases and negative glob #when matching #then honors applyTo paths and exclusions", () => {
+    // given
+    const { metadata } = parseRuleFrontmatter(`---\npaths: ["src/**/*.ts"]\napplyTo:\n  - "!src/**/*.test.ts"\n---\nRule\n`);
+
+    // when
+    const sourceMatch = shouldApplyRule(metadata, "/repo/src/index.ts", "/repo");
+    const testMatch = shouldApplyRule(metadata, "/repo/src/index.test.ts", "/repo");
+
+    // then
+    expect(sourceMatch).toEqual({ applies: true, reason: "glob: src/**/*.ts" });
+    expect(testMatch).toEqual({ applies: false });
+  });
+
+  it("#given nested AGENTS.md files #when walking up with root skip #then returns parent-to-child non-root files", async () => {
+    // given
+    const root = createTestRoot("rules-core-agents");
+    mkdirSync(join(root, ".git"));
+    mkdirSync(join(root, "packages", "app", "src"), { recursive: true });
+    writeFileSync(join(root, "AGENTS.md"), "root");
+    writeFileSync(join(root, "packages", "AGENTS.md"), "packages");
+    writeFileSync(join(root, "packages", "app", "AGENTS.md"), "app");
+
+    // when
+    const found = await findAgentsMdUp({
+      startDir: join(root, "packages", "app", "src"),
+      rootDir: root,
+      cache: createAgentsMdCache(),
+    });
+
+    // then
+    expect(found).toEqual([
+      join(root, "packages", "AGENTS.md"),
+      join(root, "packages", "app", "AGENTS.md"),
+    ]);
+  });
+
+  it("#given repeated same-directory targets #when using scan caches #then reuses cached candidates", () => {
+    // given
+    const root = createTestRoot("rules-core-cache");
+    mkdirSync(join(root, ".git"));
+    mkdirSync(join(root, ".omo", "rules"), { recursive: true });
+    mkdirSync(join(root, "src"), { recursive: true });
+    writeFileSync(join(root, ".omo", "rules", "typescript.md"), "typescript");
+    const cache = createRuleScanCache();
+
+    // when
+    const first = findRuleFiles(root, root, join(root, "src", "a.ts"), undefined, cache);
+    const second = findRuleFiles(root, root, join(root, "src", "b.ts"), undefined, cache);
+
+    // then
+    expect(first).toEqual(second);
+    expect(cache.stats()).toEqual({ candidateEntries: 1, directoryEntries: 11 });
+  });
+
+  it("#given nested project markers #when finding project root #then memoizes ancestor lookups", () => {
+    // given
+    const root = createTestRoot("rules-core-project-root");
+    mkdirSync(join(root, ".git"));
+    mkdirSync(join(root, "a", "b", "c"), { recursive: true });
+
+    // when
+    const first = findProjectRoot(join(root, "a", "b", "c", "file.ts"));
+    const second = findProjectRoot(join(root, "a", "b", "other.ts"));
+
+    // then
+    expect(first).toBe(root);
+    expect(second).toBe(root);
+  });
+});
diff --git a/packages/rules-core/src/index.ts b/packages/rules-core/src/index.ts
new file mode 100644
index 000000000..e4f3c247e
--- /dev/null
+++ b/packages/rules-core/src/index.ts
@@ -0,0 +1,20 @@
+export { createAgentsMdCache, createRuleScanCache } from "./cache";
+export { findAgentsMdUp, type FindAgentsMdUpInput } from "./agents-md";
+export { findRuleFiles } from "./finder";
+export { parseRuleFrontmatter } from "./parser";
+export { shouldApplyRule, createContentHash, isDuplicateByContentHash, isDuplicateByRealPath, resetMatcherCache, getMatcherCacheStats } from "./matcher";
+export { findProjectRoot, clearProjectRootCache } from "./project-root";
+export { calculateDistance } from "./distance";
+export { findRuleFilesRecursive, safeRealpathSync } from "./scanner";
+export type {
+  AgentsMdCache,
+  DirectoryScanEntry,
+  FindRuleFilesOptions,
+  MatchResult,
+  RuleFileCandidate,
+  RuleFrontmatterResult,
+  RuleMetadata,
+  RuleScanCache,
+  RuleScanCacheStats,
+  RuleSource,
+} from "./types";
diff --git a/packages/rules-core/src/matcher.ts b/packages/rules-core/src/matcher.ts
new file mode 100644
index 000000000..a2d64eb29
--- /dev/null
+++ b/packages/rules-core/src/matcher.ts
@@ -0,0 +1,77 @@
+import { createHash } from "node:crypto";
+import { basename, relative } from "node:path";
+import picomatch from "picomatch";
+import type { MatchResult, RuleMetadata } from "./types";
+
+const matcherCache = new Map<string, (path: string) => boolean>();
+const MAX_MATCHER_CACHE_ENTRIES = 256;
+const PICOMATCH_OPTIONS = { dot: true, bash: true } as const;
+
+export function resetMatcherCache(): void {
+  matcherCache.clear();
+}
+
+export function getMatcherCacheStats(): { readonly entries: number } {
+  return { entries: matcherCache.size };
+}
+
+export function shouldApplyRule(metadata: RuleMetadata, currentFilePath: string, projectRoot: string | null): MatchResult {
+  if (metadata.alwaysApply === true) return { applies: true, reason: "alwaysApply" };
+  const patterns = normalizeGlobs(metadata);
+  if (patterns.length === 0) return { applies: false };
+  const pathBases = [
+    toPosix(projectRoot ? relative(projectRoot, currentFilePath) : currentFilePath),
+    toPosix(basename(currentFilePath)),
+  ];
+  const negativeMatchers = patterns.filter((pattern) => pattern.startsWith("!")).map((pattern) => matcherFor(pattern.slice(1)));
+  for (const pattern of patterns) {
+    if (pattern.startsWith("!")) continue;
+    const isMatch = matcherFor(pattern);
+    if (!pathBases.some((pathBase) => isMatch(pathBase))) continue;
+    if (pathBases.some((pathBase) => negativeMatchers.some((isExcluded) => isExcluded(pathBase)))) return { applies: false };
+    return { applies: true, reason: `glob: ${pattern}` };
+  }
+  return { applies: false };
+}
+
+export function createContentHash(content: string): string {
+  return createHash("sha256").update(content).digest("hex").slice(0, 16);
+}
+
+export function isDuplicateByRealPath(realPath: string, cache: ReadonlySet<string>): boolean {
+  return cache.has(realPath);
+}
+
+export function isDuplicateByContentHash(hash: string, cache: ReadonlySet<string>): boolean {
+  return cache.has(hash);
+}
+
+function normalizeGlobs(metadata: RuleMetadata): string[] {
+  const patterns = [...normalizePatternList(metadata.globs), ...normalizePatternList(metadata.paths), ...normalizePatternList(metadata.applyTo)];
+  return [...new Set(patterns.map(toPosix))];
+}
+
+function normalizePatternList(patterns: string | readonly string[] | undefined): string[] {
+  if (patterns === undefined) return [];
+  return typeof patterns === "string" ? [patterns] : [...patterns];
+}
+
+function matcherFor(pattern: string): (path: string) => boolean {
+  const cached = matcherCache.get(pattern);
+  if (cached) {
+    matcherCache.delete(pattern);
+    matcherCache.set(pattern, cached);
+    return cached;
+  }
+  const matcher = picomatch(pattern, PICOMATCH_OPTIONS);
+  if (matcherCache.size >= MAX_MATCHER_CACHE_ENTRIES) {
+    const oldest = matcherCache.keys().next().value;
+    if (oldest !== undefined) matcherCache.delete(oldest);
+  }
+  matcherCache.set(pattern, matcher);
+  return matcher;
+}
+
+function toPosix(path: string): string {
+  return path.replaceAll("\\", "/");
+}
diff --git a/packages/rules-core/src/ordering.ts b/packages/rules-core/src/ordering.ts
new file mode 100644
index 000000000..9ef97ccf7
--- /dev/null
+++ b/packages/rules-core/src/ordering.ts
@@ -0,0 +1,26 @@
+import { SOURCE_PRIORITY } from "./constants";
+import type { RuleFileCandidate } from "./types";
+
+export function sortCandidates<T extends RuleFileCandidate>(candidates: readonly T[]): T[] {
+  return candidates
+    .map((candidate, index) => ({ candidate, index }))
+    .sort((left, right) => compareCandidates(left.candidate, right.candidate) || left.index - right.index)
+    .map(({ candidate }) => candidate);
+}
+
+function compareCandidates(left: RuleFileCandidate, right: RuleFileCandidate): number {
+  return (
+    Number(left.isGlobal) - Number(right.isGlobal) ||
+    left.distance - right.distance ||
+    (SOURCE_PRIORITY.get(left.source) ?? Number.POSITIVE_INFINITY) -
+      (SOURCE_PRIORITY.get(right.source) ?? Number.POSITIVE_INFINITY) ||
+    compareString(left.relativePath, right.relativePath) ||
+    compareString(left.realPath, right.realPath)
+  );
+}
+
+function compareString(left: string, right: string): number {
+  if (left < right) return -1;
+  if (left > right) return 1;
+  return 0;
+}
diff --git a/packages/rules-core/src/parser.ts b/packages/rules-core/src/parser.ts
new file mode 100644
index 000000000..a75c9f03a
--- /dev/null
+++ b/packages/rules-core/src/parser.ts
@@ -0,0 +1,175 @@
+import type { RuleFrontmatterResult, RuleMetadata } from "./types";
+
+type GlobValue = string | readonly string[];
+
+type ParsedGlobValue = {
+  readonly value: GlobValue;
+  readonly consumed: number;
+};
+
+export function parseRuleFrontmatter(content: string): RuleFrontmatterResult {
+  const normalized = stripBom(content);
+  const openingLength = openingDelimiterLength(normalized);
+  if (openingLength === 0) return { metadata: {}, body: normalized };
+  const closing = findClosingDelimiter(normalized, openingLength);
+  if (!closing) return { metadata: {}, body: normalized };
+  try {
+    return { metadata: parseYaml(normalized.slice(openingLength, closing.start)), body: normalized.slice(closing.bodyStart) };
+  } catch {
+    return { metadata: {}, body: normalized };
+  }
+}
+
+function parseYaml(yaml: string): RuleMetadata {
+  const lines = yaml.replace(/\r\n/g, "\n").split("\n");
+  const metadata: { description?: string; alwaysApply?: boolean; globs?: string | string[] } = {};
+  let index = 0;
+  while (index < lines.length) {
+    const line = stripComment(lines[index] ?? "").trim();
+    if (!line) {
+      index += 1;
+      continue;
+    }
+    const colon = line.indexOf(":");
+    if (colon === -1) {
+      index += 1;
+      continue;
+    }
+    const key = line.slice(0, colon).trim();
+    const rawValue = line.slice(colon + 1).trim();
+    if (key === "description") metadata.description = parseString(rawValue);
+    else if (key === "alwaysApply") metadata.alwaysApply = rawValue === "true";
+    else if (key === "globs" || key === "paths" || key === "applyTo") {
+      const parsed = parseGlobValue(rawValue, lines, index);
+      metadata.globs = mergeGlobs(metadata.globs, parsed.value);
+      index += parsed.consumed;
+      continue;
+    }
+    index += 1;
+  }
+  return metadata;
+}
+
+function parseGlobValue(rawValue: string, lines: readonly string[], currentIndex: number): ParsedGlobValue {
+  if (rawValue.startsWith("[")) return { value: parseInlineArray(rawValue), consumed: 1 };
+  if (!rawValue) {
+    const parsed = parseMultilineArray(lines, currentIndex);
+    return parsed.values.length > 0 ? { value: parsed.values, consumed: parsed.consumed } : { value: "", consumed: 1 };
+  }
+  const value = parseString(rawValue);
+  if (value.includes(",")) return { value: value.split(",").map((item) => item.trim()).filter(Boolean), consumed: 1 };
+  return { value, consumed: 1 };
+}
+
+function parseMultilineArray(lines: readonly string[], currentIndex: number): { readonly values: readonly string[]; readonly consumed: number } {
+  const values: string[] = [];
+  let consumed = 1;
+  for (let index = currentIndex + 1; index < lines.length; index += 1) {
+    const line = stripComment(lines[index] ?? "");
+    if (line.trim().length === 0) {
+      consumed += 1;
+      continue;
+    }
+    const item = line.match(/^\s+-\s*(.*)$/);
+    if (!item) break;
+    const value = parseString(item[1] ?? "");
+    if (value) values.push(value);
+    consumed += 1;
+  }
+  return { values, consumed };
+}
+
+function parseInlineArray(value: string): string[] {
+  const closing = value.lastIndexOf("]");
+  if (closing === -1) return [];
+  return splitCommaSeparated(value.slice(1, closing)).map(parseString).filter(Boolean);
+}
+
+function mergeGlobs(existing: string | string[] | undefined, next: GlobValue): string | string[] {
+  if (Array.isArray(next) && next.length === 0) return existing ?? [];
+  if (!Array.isArray(next) && next.length === 0) return existing ?? "";
+  if (existing === undefined) {
+    if (typeof next === "string") return next;
+    return [...next];
+  }
+  const existingValues = Array.isArray(existing) ? existing : [existing];
+  const nextValues = typeof next === "string" ? [next] : [...next];
+  return [...existingValues, ...nextValues];
+}
+
+function splitCommaSeparated(value: string): string[] {
+  const values: string[] = [];
+  let current = "";
+  let quote: string | null = null;
+  let escaped = false;
+  for (const character of value) {
+    if (escaped) {
+      current += character;
+      escaped = false;
+      continue;
+    }
+    if (quote && character === "\\") {
+      escaped = true;
+      continue;
+    }
+    if (character === '"' || character === "'") {
+      if (!quote) quote = character;
+      else if (quote === character) quote = null;
+      current += character;
+      continue;
+    }
+    if (!quote && character === ",") {
+      values.push(current.trim());
+      current = "";
+      continue;
+    }
+    current += character;
+  }
+  values.push(current.trim());
+  return values;
+}
+
+function parseString(value: string): string {
+  const trimmed = value.trim();
+  if ((trimmed.startsWith('"') && trimmed.endsWith('"')) || (trimmed.startsWith("'") && trimmed.endsWith("'"))) {
+    return trimmed.slice(1, -1);
+  }
+  return trimmed;
+}
+
+function stripComment(line: string): string {
+  let quote: string | null = null;
+  for (let index = 0; index < line.length; index += 1) {
+    const character = line[index];
+    if (character === '"' || character === "'") {
+      if (!quote) quote = character;
+      else if (quote === character) quote = null;
+    }
+    if (!quote && character === "#") return line.slice(0, index);
+  }
+  return line;
+}
+
+function stripBom(content: string): string {
+  return content.startsWith("\uFEFF") ? content.slice(1) : content;
+}
+
+function openingDelimiterLength(content: string): number {
+  if (content.startsWith("---\r\n")) return 5;
+  if (content.startsWith("---\n")) return 4;
+  return 0;
+}
+
+function findClosingDelimiter(content: string, openingLength: number): { readonly start: number; readonly bodyStart: number } | null {
+  let lineStart = openingLength;
+  while (lineStart <= content.length) {
+    const nextNewline = content.indexOf("\n", lineStart);
+    const lineEnd = nextNewline === -1 ? content.length : nextNewline;
+    if (content.slice(lineStart, lineEnd).replace(/\r$/, "") === "---") {
+      return { start: lineStart, bodyStart: nextNewline === -1 ? content.length : nextNewline + 1 };
+    }
+    if (nextNewline === -1) break;
+    lineStart = nextNewline + 1;
+  }
+  return null;
+}
diff --git a/packages/rules-core/src/project-root.ts b/packages/rules-core/src/project-root.ts
new file mode 100644
index 000000000..4e1341a28
--- /dev/null
+++ b/packages/rules-core/src/project-root.ts
@@ -0,0 +1,55 @@
+import { existsSync, statSync } from "node:fs";
+import { dirname, join } from "node:path";
+import { PROJECT_MARKERS } from "./constants";
+
+const projectRootCache = new Map<string, string | null>();
+
+export function clearProjectRootCache(): void {
+  projectRootCache.clear();
+}
+
+export function findProjectRoot(startPath: string): string | null {
+  const cached = projectRootCache.get(startPath);
+  if (cached !== undefined) return cached;
+  const startDir = resolveStartDir(startPath);
+  const cachedStartDir = projectRootCache.get(startDir);
+  if (cachedStartDir !== undefined) {
+    projectRootCache.set(startPath, cachedStartDir);
+    return cachedStartDir;
+  }
+
+  const visited: string[] = [];
+  let current = startDir;
+  let resolved: string | null = null;
+  while (true) {
+    const cachedAncestor = projectRootCache.get(current);
+    if (cachedAncestor !== undefined) {
+      resolved = cachedAncestor;
+      break;
+    }
+    visited.push(current);
+    if (hasProjectMarker(current)) {
+      resolved = current;
+      break;
+    }
+    const parent = dirname(current);
+    if (parent === current) break;
+    current = parent;
+  }
+
+  for (const directory of visited) projectRootCache.set(directory, resolved);
+  projectRootCache.set(startPath, resolved);
+  return resolved;
+}
+
+function resolveStartDir(startPath: string): string {
+  try {
+    return statSync(startPath).isDirectory() ? startPath : dirname(startPath);
+  } catch {
+    return dirname(startPath);
+  }
+}
+
+function hasProjectMarker(directory: string): boolean {
+  return PROJECT_MARKERS.some((marker) => existsSync(join(directory, marker)));
+}
diff --git a/packages/rules-core/src/scanner.ts b/packages/rules-core/src/scanner.ts
new file mode 100644
index 000000000..ff1fbce6c
--- /dev/null
+++ b/packages/rules-core/src/scanner.ts
@@ -0,0 +1,44 @@
+import { existsSync, readdirSync, realpathSync } from "node:fs";
+import { join } from "node:path";
+import { EXCLUDED_DIRS, GITHUB_INSTRUCTIONS_PATTERN, RULE_EXTENSIONS } from "./constants";
+import type { DirectoryScanEntry } from "./types";
+
+function isGitHubInstructionsDir(dir: string): boolean {
+  return dir.includes(".github/instructions") || dir.endsWith(".github/instructions");
+}
+
+function isRuleFile(fileName: string, dir: string): boolean {
+  if (isGitHubInstructionsDir(dir)) return GITHUB_INSTRUCTIONS_PATTERN.test(fileName);
+  return RULE_EXTENSIONS.some((extension) => fileName.endsWith(extension));
+}
+
+export function safeRealpathSync(filePath: string): string {
+  try {
+    return realpathSync.native(filePath);
+  } catch {
+    return filePath;
+  }
+}
+
+export function findRuleFilesRecursive(dir: string, results: DirectoryScanEntry[], visited = new Set<string>()): void {
+  if (!existsSync(dir)) return;
+  const realDir = safeRealpathSync(dir);
+  if (visited.has(realDir)) return;
+  visited.add(realDir);
+  let entries;
+  try {
+    entries = readdirSync(dir, { withFileTypes: true, encoding: "utf8" }).sort((left, right) => left.name.localeCompare(right.name));
+  } catch {
+    return;
+  }
+  for (const entry of entries) {
+    const fullPath = join(dir, entry.name);
+    if (entry.isDirectory()) {
+      if (!EXCLUDED_DIRS.has(entry.name)) findRuleFilesRecursive(fullPath, results, visited);
+      continue;
+    }
+    if (entry.isFile() && isRuleFile(entry.name, dir)) {
+      results.push({ path: fullPath, realPath: safeRealpathSync(fullPath), relativePath: entry.name });
+    }
+  }
+}
diff --git a/packages/rules-core/src/types.ts b/packages/rules-core/src/types.ts
new file mode 100644
index 000000000..4bea6d989
--- /dev/null
+++ b/packages/rules-core/src/types.ts
@@ -0,0 +1,69 @@
+export interface RuleMetadata {
+  readonly description?: string;
+  readonly globs?: string | readonly string[];
+  readonly paths?: string | readonly string[];
+  readonly applyTo?: string | readonly string[];
+  readonly alwaysApply?: boolean;
+}
+
+export interface RuleFrontmatterResult {
+  readonly metadata: RuleMetadata;
+  readonly body: string;
+}
+
+export interface RuleFileCandidate {
+  readonly path: string;
+  readonly realPath: string;
+  readonly isGlobal: boolean;
+  readonly distance: number;
+  readonly relativePath: string;
+  readonly source: RuleSource;
+  readonly isSingleFile?: boolean;
+}
+
+export type RuleSource =
+  | ".omo/rules"
+  | ".sisyphus/rules"
+  | ".claude/rules"
+  | ".cursor/rules"
+  | ".github/instructions"
+  | ".github/copilot-instructions.md"
+  | "~/.omo/rules"
+  | "~/.sisyphus/rules"
+  | "~/.opencode/rules"
+  | "~/.claude/rules";
+
+export interface MatchResult {
+  readonly applies: boolean;
+  readonly reason?: string;
+}
+
+export interface DirectoryScanEntry {
+  readonly path: string;
+  readonly realPath: string;
+  readonly relativePath: string;
+}
+
+export interface RuleScanCacheStats {
+  readonly candidateEntries: number;
+  readonly directoryEntries: number;
+}
+
+export interface RuleScanCache {
+  get(key: string): readonly RuleFileCandidate[] | undefined;
+  set(key: string, value: readonly RuleFileCandidate[]): void;
+  getDirScan(dir: string): readonly DirectoryScanEntry[] | undefined;
+  setDirScan(dir: string, entries: readonly DirectoryScanEntry[]): void;
+  stats(): RuleScanCacheStats;
+  clear(): void;
+}
+
+export interface FindRuleFilesOptions {
+  readonly skipClaudeUserRules?: boolean;
+}
+
+export interface AgentsMdCache {
+  get(key: string): readonly string[] | undefined;
+  set(key: string, value: readonly string[]): void;
+  clear(): void;
+}
diff --git a/packages/rules-core/tsconfig.json b/packages/rules-core/tsconfig.json
new file mode 100644
index 000000000..7a69d1222
--- /dev/null
+++ b/packages/rules-core/tsconfig.json
@@ -0,0 +1,14 @@
+{
+  "compilerOptions": {
+    "target": "ESNext",
+    "module": "ESNext",
+    "moduleResolution": "bundler",
+    "strict": true,
+    "esModuleInterop": true,
+    "skipLibCheck": true,
+    "forceConsistentCasingInFileNames": true,
+    "lib": ["ESNext"],
+    "types": ["bun-types"]
+  },
+  "include": ["src/**/*"]
+}