diff --git a/.github/workflow-scripts/assert-spm-deinit.sh b/.github/workflow-scripts/assert-spm-deinit.sh
new file mode 100755
index 000000000000..b8bd6f5072a3
--- /dev/null
+++ b/.github/workflow-scripts/assert-spm-deinit.sh
@@ -0,0 +1,128 @@
+#!/bin/bash
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+#
+# This source code is licensed under the MIT license found in the
+# LICENSE file in the root directory of this source tree.
+
+# Checks that `npx react-native spm deinit` left an iOS app with only its
+# CocoaPods files. It checks only what deinit removes; `pod install` restores
+# the CocoaPods references, so they are not checked here.
+#
+# It assumes the app had no Swift packages before `spm add`: any
+# packageReferences or packageProductDependencies left after deinit fail the
+# check. Do not use it on an app that already has Swift packages.
+#
+# Usage:
+#   assert-spm-deinit.sh record <ios-dir>
+#     Run before `spm add`. Prints PODFILE_SHA256 and GITIGNORE_SHA256, and
+#     appends them to $GITHUB_ENV when it is set.
+#   assert-spm-deinit.sh check <ios-dir> <name.xcodeproj> <podfile-sha256> <gitignore-sha256|absent>
+#     Run after `spm deinit`, with the values that record printed.
+
+usage() {
+  echo "Usage: $0 record <ios-dir>"
+  echo "       $0 check <ios-dir> <name.xcodeproj> <podfile-sha256> <gitignore-sha256|absent>"
+  exit 2
+}
+
+sha256_or_absent() {
+  if [ -f "$1" ]; then
+    shasum -a 256 "$1" | cut -d ' ' -f 1
+  else
+    echo absent
+  fi
+}
+
+record() {
+  local ios_dir=$1
+  if [ ! -f "$ios_dir/Podfile" ]; then
+    echo "::error::$ios_dir/Podfile is missing. Record the checksums while the app is still on CocoaPods."
+    exit 1
+  fi
+
+  local values
+  values="PODFILE_SHA256=$(sha256_or_absent "$ios_dir/Podfile")
+GITIGNORE_SHA256=$(sha256_or_absent "$ios_dir/.gitignore")"
+  echo "$values"
+  if [ -n "$GITHUB_ENV" ]; then
+    echo "$values" >> "$GITHUB_ENV"
+  fi
+}
+
+check() {
+  local ios_dir=$1
+  local xcodeproj="$ios_dir/$2"
+  local expected_podfile_sha256=$3
+  local expected_gitignore_sha256=$4
+  local pbxproj="$xcodeproj/project.pbxproj"
+  local failed=0
+
+  fail() {
+    echo "::error::$1"
+    failed=1
+  }
+
+  local actual_podfile_sha256
+  actual_podfile_sha256=$(sha256_or_absent "$ios_dir/Podfile")
+  if [ "$actual_podfile_sha256" != "$expected_podfile_sha256" ]; then
+    fail "Podfile changed: sha256 is '$actual_podfile_sha256', expected '$expected_podfile_sha256'."
+  fi
+
+  local actual_gitignore_sha256
+  actual_gitignore_sha256=$(sha256_or_absent "$ios_dir/.gitignore")
+  if [ "$actual_gitignore_sha256" != "$expected_gitignore_sha256" ]; then
+    fail ".gitignore changed: sha256 is '$actual_gitignore_sha256', expected '$expected_gitignore_sha256'."
+  fi
+
+  if [ -e "$xcodeproj/.spm-injected.json" ]; then
+    fail "$xcodeproj/.spm-injected.json is still there."
+  fi
+
+  if [ -e "$ios_dir/ReactNativeSPM" ]; then
+    fail "$ios_dir/ReactNativeSPM is still there."
+  fi
+
+  if [ ! -f "$pbxproj" ]; then
+    fail "$pbxproj is missing."
+  else
+    local injected
+    for injected in \
+      XCLocalSwiftPackageReference \
+      XCSwiftPackageProductDependency \
+      packageReferences \
+      packageProductDependencies \
+      ReactNativeSPM \
+      "Sync SPM Autolinking" \
+      "Embed React Native Flavored Frameworks"; do
+      if grep -qF "$injected" "$pbxproj"; then
+        fail "$pbxproj still contains '$injected'."
+      fi
+    done
+  fi
+
+  local scheme
+  for scheme in "$xcodeproj"/xcshareddata/xcschemes/*.xcscheme; do
+    if [ -f "$scheme" ] && grep -qF 'title = "Sync SPM Autolinking"' "$scheme"; then
+      fail "$scheme still has the Sync SPM Autolinking pre-action."
+    fi
+  done
+
+  if [ $failed -ne 0 ]; then
+    exit 1
+  fi
+  echo "spm deinit removed every SwiftPM change."
+}
+
+case "$1" in
+  record)
+    [ $# -eq 2 ] || usage
+    record "$2"
+    ;;
+  check)
+    [ $# -eq 5 ] || usage
+    check "$2" "$3" "$4" "$5"
+    ;;
+  *)
+    usage
+    ;;
+esac
diff --git a/.github/workflows/test-ios-spm-newapp.yml b/.github/workflows/test-ios-spm-newapp.yml
index 5607e8b94804..b21f653f199a 100644
--- a/.github/workflows/test-ios-spm-newapp.yml
+++ b/.github/workflows/test-ios-spm-newapp.yml
@@ -46,6 +46,14 @@ jobs:
         uses: ./.github/actions/setup-spm-artifacts
         with:
           working-directory: ${{ env.APP_IOS_DIR }}
+      # The `spm deinit` round trip runs in the Debug cell only: it does not
+      # depend on the flavor.
+      - name: Record HelloWorld before SwiftPM
+        if: matrix.flavor == 'Debug'
+        shell: bash
+        run: |
+          ./.github/workflow-scripts/assert-spm-deinit.sh record "$APP_IOS_DIR"
+          cp -R "$APP_IOS_DIR" "$RUNNER_TEMP/ios-before-spm"
       # HelloWorld is a CocoaPods project in git. CI converts it to SwiftPM on every run.
       - name: Convert HelloWorld to SwiftPM
         shell: bash
@@ -100,3 +108,47 @@ jobs:
             echo "::error::Release build embeds a Debug React.framework ($COUNT getDebugProps symbols, expected none)"
             exit 1
           fi
+      - name: Take HelloWorld back to CocoaPods
+        if: matrix.flavor == 'Debug'
+        shell: bash
+        working-directory: ${{ env.APP_IOS_DIR }}
+        run: npx react-native spm deinit
+      # Informational only. Expected differences include the CocoaPods
+      # references and the .xcworkspace, which `pod install` restores, and the
+      # JavaScriptCore.framework reference and empty Pods group, which deinit
+      # does not restore on purpose.
+      - name: Show what changed since before SwiftPM
+        if: ${{ !cancelled() && matrix.flavor == 'Debug' }}
+        shell: bash
+        run: diff -r -x build "$RUNNER_TEMP/ios-before-spm" "$APP_IOS_DIR" || true
+      - name: Check that only the CocoaPods files are left
+        if: matrix.flavor == 'Debug'
+        shell: bash
+        run: ./.github/workflow-scripts/assert-spm-deinit.sh check "$APP_IOS_DIR" "$XCODE_PROJECT" "$PODFILE_SHA256" "$GITIGNORE_SHA256"
+      - name: Setup ruby
+        if: matrix.flavor == 'Debug'
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: 2.6.10
+      # The tarballs are the Debug ones that setup-spm-artifacts downloaded.
+      - name: Install the CocoaPods dependencies
+        if: matrix.flavor == 'Debug'
+        shell: bash
+        working-directory: private/helloworld
+        run: |
+          export RCT_USE_LOCAL_RN_DEP=/tmp/deps-debug/ReactNativeDependenciesDebug.xcframework.tar.gz
+          export RCT_TESTONLY_RNCORE_TARBALL_PATH=/tmp/rc-debug/ReactCoreDebug.xcframework.tar.gz
+          yarn bootstrap ios | cat
+      - name: Build ${{ env.XCODE_SCHEME }} with CocoaPods (Debug)
+        if: matrix.flavor == 'Debug'
+        shell: bash
+        working-directory: ${{ env.APP_IOS_DIR }}
+        run: |
+          xcodebuild \
+            -workspace "${XCODE_PROJECT%.xcodeproj}.xcworkspace" \
+            -scheme "$XCODE_SCHEME" \
+            -configuration Debug \
+            -sdk iphonesimulator \
+            -destination 'generic/platform=iOS Simulator' \
+            -derivedDataPath build/cocoapods-e2e-dd \
+            build