diff --git a/dsh/sync/sync-from-upstream.mjs b/dsh/sync/sync-from-upstream.mjs
index b4cdd8b..517a87e 100644
--- a/dsh/sync/sync-from-upstream.mjs
+++ b/dsh/sync/sync-from-upstream.mjs
@@ -45,7 +45,7 @@
  */
 import { rmSync } from 'node:fs'
 import { cp, mkdir, readFile, readdir, rename, rm, stat, writeFile } from 'node:fs/promises'
-import { dirname, join, relative, resolve, sep } from 'node:path'
+import { dirname, isAbsolute, join, relative, resolve, sep } from 'node:path'
 import { fileURLToPath } from 'node:url'
 
 const scriptDir = dirname(fileURLToPath(import.meta.url))
@@ -322,6 +322,12 @@ function parseArgs(argv) {
   }
 }
 
+/** True when `parent` is `child` or one of its ancestors (`/a` → `/a/b`). */
+function isAncestorOrSame(parent, child) {
+  const rel = relative(parent, child)
+  return rel === '' || (!rel.startsWith('..') && !isAbsolute(rel))
+}
+
 /** @returns {Promise<string[]>} repo-relative file paths under `root`. */
 async function walk(root, current = root) {
   const out = []
@@ -368,6 +374,13 @@ async function main() {
   const upstreamRoot = args.upstream
   const upstreamSkills = join(upstreamRoot, 'skills')
 
+  // The destination must never overlap the source: swapping it in would
+  // delete the checkout we read from, and `--out <checkout>/skills` would
+  // delete the very tree being walked. Refuse before any write or delete.
+  if (isAncestorOrSame(args.out, upstreamRoot) || args.out === upstreamSkills) {
+    fail(`--out collides with the upstream checkout (${args.out})`)
+  }
+
   const upstreamStats = await stat(upstreamSkills).catch(() => null)
   if (!upstreamStats?.isDirectory()) {
     fail(`no skills/ directory in ${upstreamRoot}`)
diff --git a/dsh/tests/sync.test.mjs b/dsh/tests/sync.test.mjs
index bd75ec0..60a593b 100644
--- a/dsh/tests/sync.test.mjs
+++ b/dsh/tests/sync.test.mjs
@@ -107,6 +107,93 @@ test('sync fails loud on a broken anchor of another transform', () => {
   }
 })
 
+test('sync refuses --out that collides with the upstream checkout', () => {
+  const work = tempDir('dsh-sync-collision-')
+  try {
+    const upstream = fixtureCopy(work)
+    write(join(upstream, 'sentinel.txt'), 'source stays\n')
+
+    // `--out` equal to the checkout, an ancestor of it, and the skills/ tree
+    // itself — each would delete the source the sync is reading.
+    for (const out of [upstream, work, join(upstream, 'skills')]) {
+      const result = runSync([upstream, '--out', out, '--ref', 'v7.0.1'])
+
+      assert.equal(result.status, 1, `--out ${out} must be refused`)
+      assert.match(result.stderr, /--out collides with the upstream checkout/)
+    }
+
+    assert.equal(readFileSync(join(upstream, 'sentinel.txt'), 'utf8'), 'source stays\n')
+    assert.equal(existsSync(join(upstream, '.claude-plugin', 'plugin.json')), true)
+    assert.equal(readdirSync(join(upstream, 'skills')).length, 15)
+    assert.deepEqual(
+      readdirSync(work).filter((name) => name.includes('.tmp-')),
+      [],
+      'a refused sync must not create its staging tree',
+    )
+  } finally {
+    cleanup(work)
+  }
+})
+
+test('sync fails on a transform whose source file upstream dropped', () => {
+  const work = tempDir('dsh-sync-missing-transform-file-')
+  try {
+    const upstream = fixtureCopy(work)
+    rmSync(join(upstream, 'skills', 'brainstorming', 'visual-companion.md'))
+
+    const result = runSync([upstream, '--out', join(work, 'out'), '--ref', 'v7.0.1'])
+
+    assert.equal(result.status, 1)
+    assert.match(result.stderr, /never matched/)
+    assert.match(result.stderr, /DSH platform note for the visual companion/)
+    assert.equal(existsSync(join(work, 'out')), false)
+  } finally {
+    cleanup(work)
+  }
+})
+
+test('sync fails when an anchor matches twice', () => {
+  const work = tempDir('dsh-sync-duplicate-anchor-')
+  try {
+    const upstream = fixtureCopy(work)
+    const skillPath = join(upstream, 'skills', 'executing-plans', 'SKILL.md')
+    const anchor = "Run this skill's `scripts/task-start PLAN_FILE N`."
+    const original = readFileSync(skillPath, 'utf8')
+    assert.ok(original.includes(anchor), 'fixture must carry the task-start anchor verbatim')
+    write(skillPath, original.replace(anchor, `${anchor}\n\n${anchor}`))
+
+    const result = runSync([upstream, '--out', join(work, 'out'), '--ref', 'v7.0.1'])
+
+    assert.equal(result.status, 1)
+    assert.match(result.stderr, /matched 2 times/)
+    assert.match(result.stderr, /invoke task-start through bash/)
+  } finally {
+    cleanup(work)
+  }
+})
+
+test('sync warns, but succeeds, when the optional transform anchor is gone', () => {
+  const work = tempDir('dsh-sync-optional-anchor-')
+  try {
+    const upstream = fixtureCopy(work)
+    const skillPath = join(upstream, 'skills', 'subagent-driven-development', 'SKILL.md')
+    const anchor = 'You: "User level (~/.config/superpowers/hooks/)"'
+    const original = readFileSync(skillPath, 'utf8')
+    assert.ok(original.includes(anchor), 'fixture must carry the optional anchor verbatim')
+    write(skillPath, original.replace(anchor, 'You: "Install the hook at the project level"'))
+
+    const out = join(work, 'out')
+    const result = runSync([upstream, '--out', out, '--ref', 'v7.0.1'])
+
+    assert.equal(result.status, 0, `stderr: ${result.stderr}`)
+    assert.match(result.stderr, /\[warn\] optional transform/)
+    assert.match(result.stdout, /transforms applied: 15/)
+    assert.equal(existsSync(join(out, '.sync-meta.json')), true)
+  } finally {
+    cleanup(work)
+  }
+})
+
 test('sync fails on a changed skill set', () => {
   const work = tempDir('dsh-sync-skill-set-')
   try {