diff --git a/publish/src/core/change-detection.ts b/publish/src/core/change-detection.ts
new file mode 100644
index 0000000..3a8208b
--- /dev/null
+++ b/publish/src/core/change-detection.ts
@@ -0,0 +1,67 @@
+export type GitRunner = (args: string[]) => Promise<string>
+
+function escapeRegex(s: string): string {
+  return s.replace(/[.*+?^${}()|[\]\\]/g, '\\$&')
+}
+
+export function filterPostDirs(lines: string[], contentRoot: string): string[] {
+  const root = contentRoot.replace(/\/$/, '')
+  const prefix = root + '/'
+  const indexRe = new RegExp(`^${escapeRegex(prefix)}([^/]+)/index\\.md$`)
+  const assetRe = new RegExp(`^${escapeRegex(prefix)}([^/]+)/`)
+  const drafts = prefix + '_'
+  const dirs = new Set<string>()
+  for (const line of lines) {
+    const l = line.trim()
+    if (!l) continue
+    if (l.startsWith(drafts)) continue
+    const indexMatch = l.match(indexRe)
+    if (indexMatch) {
+      dirs.add(`${prefix}${indexMatch[1]}`)
+      continue
+    }
+    const assetMatch = l.match(assetRe)
+    if (assetMatch && !l.endsWith('.md')) {
+      dirs.add(`${prefix}${assetMatch[1]}`)
+    }
+  }
+  return [...dirs].sort()
+}
+
+const defaultRunner: GitRunner = async (args) => {
+  const proc = new Deno.Command('git', { args, stdout: 'piped', stderr: 'piped' })
+  const out = await proc.output()
+  if (out.code !== 0) {
+    throw new Error(`git ${args.join(' ')} failed: ${new TextDecoder().decode(out.stderr)}`)
+  }
+  return new TextDecoder().decode(out.stdout)
+}
+
+export interface DiffArgs {
+  from: string
+  to: string
+  contentRoot: string
+  runner?: GitRunner
+}
+
+export async function changedPostDirs(args: DiffArgs): Promise<string[]> {
+  const runner = args.runner ?? defaultRunner
+  const stdout = await runner(['diff', '--name-only', `${args.from}..${args.to}`])
+  return filterPostDirs(stdout.split('\n'), args.contentRoot)
+}
+
+export async function allPostDirs(contentRoot: string): Promise<string[]> {
+  const result: string[] = []
+  for await (const entry of Deno.readDir(contentRoot)) {
+    if (entry.isDirectory && !entry.name.startsWith('_')) {
+      const indexPath = `${contentRoot}/${entry.name}/index.md`
+      try {
+        const stat = await Deno.stat(indexPath)
+        if (stat.isFile) result.push(`${contentRoot}/${entry.name}`)
+      } catch {
+        // skip folders without index.md
+      }
+    }
+  }
+  return result.sort()
+}
diff --git a/publish/tests/change-detection_test.ts b/publish/tests/change-detection_test.ts
new file mode 100644
index 0000000..8da17cd
--- /dev/null
+++ b/publish/tests/change-detection_test.ts
@@ -0,0 +1,55 @@
+import { assertEquals } from '@std/assert'
+import {
+  changedPostDirs,
+  filterPostDirs,
+  type GitRunner,
+} from '../src/core/change-detection.ts'
+
+Deno.test('filterPostDirs: extrahiert post-ordner aus dateipfaden (content/posts)', () => {
+  const lines = [
+    'content/posts/a/index.md',
+    'content/posts/b/image.png',
+    'content/posts/c/other.md',
+    'README.md',
+    'app/src/lib/x.ts',
+  ]
+  assertEquals(
+    filterPostDirs(lines, 'content/posts').sort(),
+    ['content/posts/a', 'content/posts/b'],
+  )
+})
+
+Deno.test('filterPostDirs: respektiert alternativen root (blog/)', () => {
+  const lines = [
+    'blog/x/index.md',
+    'blog/y/pic.png',
+    'content/posts/z/index.md',
+    'README.md',
+  ]
+  assertEquals(filterPostDirs(lines, 'blog').sort(), ['blog/x', 'blog/y'])
+})
+
+Deno.test('filterPostDirs: ignoriert _drafts und non-index.md', () => {
+  const lines = [
+    'content/posts/a/index.md',
+    'content/posts/a/extra.md',
+    'content/posts/_drafts/x/index.md',
+  ]
+  assertEquals(filterPostDirs(lines, 'content/posts'), ['content/posts/a'])
+})
+
+Deno.test('changedPostDirs: nutzt git diff --name-only A..B', async () => {
+  const runner: GitRunner = (args) => {
+    assertEquals(args[0], 'diff')
+    assertEquals(args[1], '--name-only')
+    assertEquals(args[2], 'HEAD~1..HEAD')
+    return Promise.resolve('content/posts/x/index.md\nREADME.md\n')
+  }
+  const dirs = await changedPostDirs({
+    from: 'HEAD~1',
+    to: 'HEAD',
+    contentRoot: 'content/posts',
+    runner,
+  })
+  assertEquals(dirs, ['content/posts/x'])
+})