diff --git a/src/hooks/rules-injector/matcher.test.ts b/src/hooks/rules-injector/matcher.test.ts
new file mode 100644
index 000000000..009c19645
--- /dev/null
+++ b/src/hooks/rules-injector/matcher.test.ts
@@ -0,0 +1,70 @@
+/// <reference path="../../../bun-test.d.ts" />
+
+import { beforeEach, describe, expect, it } from "bun:test"
+import {
+  createContentHash,
+  getMatcherCacheStats,
+  isDuplicateByContentHash,
+  isDuplicateByRealPath,
+  resetMatcherCache,
+  shouldApplyRule,
+} from "./matcher"
+
+describe("shouldApplyRule", () => {
+  beforeEach(() => {
+    resetMatcherCache()
+  })
+
+  it("#given repeated glob metadata #when matching many files #then compiles each pattern once", () => {
+    // given
+    const metadata = { globs: ["src/**/*.ts", "test/**/*.ts"] }
+    const projectRoot = "/workspace/project"
+
+    // when
+    for (let index = 0; index < 20; index += 1) {
+      shouldApplyRule(metadata, `${projectRoot}/src/file-${index}.ts`, projectRoot)
+      shouldApplyRule(metadata, `${projectRoot}/test/file-${index}.ts`, projectRoot)
+    }
+
+    // then
+    expect(getMatcherCacheStats()).toEqual({ entries: 2 })
+  })
+
+  it("#given matching glob #when path is under project root #then returns matching reason", () => {
+    // given / when
+    const result = shouldApplyRule({ globs: "src/**/*.ts" }, "/workspace/project/src/index.ts", "/workspace/project")
+
+    // then
+    expect(result).toEqual({ applies: true, reason: "glob: src/**/*.ts" })
+  })
+
+  it("#given always apply metadata #when no globs exist #then applies without compiling matchers", () => {
+    // given / when
+    const result = shouldApplyRule({ alwaysApply: true }, "/workspace/project/src/index.ts", "/workspace/project")
+
+    // then
+    expect(result).toEqual({ applies: true, reason: "alwaysApply" })
+    expect(getMatcherCacheStats()).toEqual({ entries: 0 })
+  })
+})
+
+describe("rule duplicate helpers", () => {
+  it("#given real path cache #when path exists #then reports duplicate", () => {
+    // given
+    const cache = new Set(["/workspace/project/AGENTS.md"])
+
+    // when / then
+    expect(isDuplicateByRealPath("/workspace/project/AGENTS.md", cache)).toBe(true)
+    expect(isDuplicateByRealPath("/workspace/project/src/AGENTS.md", cache)).toBe(false)
+  })
+
+  it("#given content #when hashing #then duplicate helper uses truncated hash", () => {
+    // given
+    const hash = createContentHash("rule-content")
+    const cache = new Set([hash])
+
+    // when / then
+    expect(hash).toHaveLength(16)
+    expect(isDuplicateByContentHash(hash, cache)).toBe(true)
+  })
+})
diff --git a/src/hooks/rules-injector/matcher.ts b/src/hooks/rules-injector/matcher.ts
index 13f6d51c5..d68dae109 100644
--- a/src/hooks/rules-injector/matcher.ts
+++ b/src/hooks/rules-injector/matcher.ts
@@ -3,11 +3,37 @@ import { relative } from "node:path"
 import picomatch from "picomatch"
 import type { RuleMetadata } from "./types"
 
+type PathMatcher = (path: string) => boolean
+
 export interface MatchResult {
   applies: boolean
   reason?: string
 }
 
+export interface MatcherCacheStats {
+  entries: number
+}
+
+const PICOMATCH_OPTIONS = { dot: true, bash: true } as const
+const matcherCache = new Map<string, PathMatcher>()
+
+function matcherFor(pattern: string): PathMatcher {
+  const cached = matcherCache.get(pattern)
+  if (cached) return cached
+
+  const matcher = picomatch(pattern, PICOMATCH_OPTIONS)
+  matcherCache.set(pattern, matcher)
+  return matcher
+}
+
+export function resetMatcherCache(): void {
+  matcherCache.clear()
+}
+
+export function getMatcherCacheStats(): MatcherCacheStats {
+  return { entries: matcherCache.size }
+}
+
 /**
  * Check if a rule should apply to the current file based on metadata
  */
@@ -33,7 +59,7 @@ export function shouldApplyRule(
   const relativePath = projectRoot ? relative(projectRoot, currentFilePath) : currentFilePath
 
   for (const pattern of patterns) {
-    if (picomatch.isMatch(relativePath, pattern, { dot: true, bash: true })) {
+    if (matcherFor(pattern)(relativePath)) {
       return { applies: true, reason: `glob: ${pattern}` }
     }
   }