diff --git a/packages/next-codemod/bin/__testfixtures__/react-18-installed-mixed-router/README.md b/packages/next-codemod/bin/__testfixtures__/react-18-installed-mixed-router/README.md
new file mode 100644
index 0000000000000..c6f744b30286b
--- /dev/null
+++ b/packages/next-codemod/bin/__testfixtures__/react-18-installed-mixed-router/README.md
@@ -0,0 +1 @@
+Upgrades React without prompt
diff --git a/packages/next-codemod/bin/__testfixtures__/react-18-installed-mixed-router/app/layout.ts b/packages/next-codemod/bin/__testfixtures__/react-18-installed-mixed-router/app/layout.ts
new file mode 100644
index 0000000000000..e69de29bb2d1d
diff --git a/packages/next-codemod/bin/__testfixtures__/react-18-installed-mixed-router/package.json b/packages/next-codemod/bin/__testfixtures__/react-18-installed-mixed-router/package.json
new file mode 100644
index 0000000000000..f0e0da30ae636
--- /dev/null
+++ b/packages/next-codemod/bin/__testfixtures__/react-18-installed-mixed-router/package.json
@@ -0,0 +1,11 @@
+{
+  "name": "react-18-installed-mixed-router",
+  "scripts": {
+    "dev": "next dev"
+  },
+  "dependencies": {
+    "next": "14.3.0-canary.44",
+    "react": "18.2.0",
+    "react-dom": "18.2.0"
+  }
+}
diff --git a/packages/next-codemod/bin/__testfixtures__/react-18-installed-mixed-router/pages/index.ts b/packages/next-codemod/bin/__testfixtures__/react-18-installed-mixed-router/pages/index.ts
new file mode 100644
index 0000000000000..e69de29bb2d1d
diff --git a/packages/next-codemod/bin/__testfixtures__/react-18-installed-pure-app-router/README.md b/packages/next-codemod/bin/__testfixtures__/react-18-installed-pure-app-router/README.md
new file mode 100644
index 0000000000000..c6f744b30286b
--- /dev/null
+++ b/packages/next-codemod/bin/__testfixtures__/react-18-installed-pure-app-router/README.md
@@ -0,0 +1 @@
+Upgrades React without prompt
diff --git a/packages/next-codemod/bin/__testfixtures__/react-18-installed-pure-app-router/app/layout.ts b/packages/next-codemod/bin/__testfixtures__/react-18-installed-pure-app-router/app/layout.ts
new file mode 100644
index 0000000000000..e69de29bb2d1d
diff --git a/packages/next-codemod/bin/__testfixtures__/react-18-installed-pure-app-router/package.json b/packages/next-codemod/bin/__testfixtures__/react-18-installed-pure-app-router/package.json
new file mode 100644
index 0000000000000..28587f48b3f75
--- /dev/null
+++ b/packages/next-codemod/bin/__testfixtures__/react-18-installed-pure-app-router/package.json
@@ -0,0 +1,11 @@
+{
+  "name": "react-18-installed-pure-app-router",
+  "scripts": {
+    "dev": "next dev"
+  },
+  "dependencies": {
+    "next": "14.3.0-canary.44",
+    "react": "18.2.0",
+    "react-dom": "18.2.0"
+  }
+}
diff --git a/packages/next-codemod/bin/__testfixtures__/react-18-installed-pure-pages-router/README.md b/packages/next-codemod/bin/__testfixtures__/react-18-installed-pure-pages-router/README.md
new file mode 100644
index 0000000000000..c6f744b30286b
--- /dev/null
+++ b/packages/next-codemod/bin/__testfixtures__/react-18-installed-pure-pages-router/README.md
@@ -0,0 +1 @@
+Upgrades React without prompt
diff --git a/packages/next-codemod/bin/__testfixtures__/react-18-installed-pure-pages-router/package.json b/packages/next-codemod/bin/__testfixtures__/react-18-installed-pure-pages-router/package.json
new file mode 100644
index 0000000000000..7ca58bd1489bb
--- /dev/null
+++ b/packages/next-codemod/bin/__testfixtures__/react-18-installed-pure-pages-router/package.json
@@ -0,0 +1,11 @@
+{
+  "name": "react-18-installed-pure-pages-router",
+  "scripts": {
+    "dev": "next dev"
+  },
+  "dependencies": {
+    "next": "14.3.0-canary.44",
+    "react": "18.2.0",
+    "react-dom": "18.2.0"
+  }
+}
diff --git a/packages/next-codemod/bin/__testfixtures__/react-18-installed-pure-pages-router/pages/index.ts b/packages/next-codemod/bin/__testfixtures__/react-18-installed-pure-pages-router/pages/index.ts
new file mode 100644
index 0000000000000..e69de29bb2d1d
diff --git a/packages/next-codemod/bin/__testfixtures__/react-19-installed-mixed-router/README.md b/packages/next-codemod/bin/__testfixtures__/react-19-installed-mixed-router/README.md
new file mode 100644
index 0000000000000..c6f744b30286b
--- /dev/null
+++ b/packages/next-codemod/bin/__testfixtures__/react-19-installed-mixed-router/README.md
@@ -0,0 +1 @@
+Upgrades React without prompt
diff --git a/packages/next-codemod/bin/__testfixtures__/react-19-installed-mixed-router/app/layout.ts b/packages/next-codemod/bin/__testfixtures__/react-19-installed-mixed-router/app/layout.ts
new file mode 100644
index 0000000000000..e69de29bb2d1d
diff --git a/packages/next-codemod/bin/__testfixtures__/react-19-installed-mixed-router/package.json b/packages/next-codemod/bin/__testfixtures__/react-19-installed-mixed-router/package.json
new file mode 100644
index 0000000000000..a3ee83b34a39b
--- /dev/null
+++ b/packages/next-codemod/bin/__testfixtures__/react-19-installed-mixed-router/package.json
@@ -0,0 +1,11 @@
+{
+  "name": "react-19-installed-mixed-router",
+  "scripts": {
+    "dev": "next dev"
+  },
+  "dependencies": {
+    "next": "14.3.0-canary.45",
+    "react": "19.0.0-beta-4508873393-20240430",
+    "react-dom": "19.0.0-beta-4508873393-20240430"
+  }
+}
diff --git a/packages/next-codemod/bin/__testfixtures__/react-19-installed-mixed-router/pages/index.ts b/packages/next-codemod/bin/__testfixtures__/react-19-installed-mixed-router/pages/index.ts
new file mode 100644
index 0000000000000..e69de29bb2d1d
diff --git a/packages/next-codemod/bin/__testfixtures__/react-19-installed-pure-app-router/README.md b/packages/next-codemod/bin/__testfixtures__/react-19-installed-pure-app-router/README.md
new file mode 100644
index 0000000000000..c6f744b30286b
--- /dev/null
+++ b/packages/next-codemod/bin/__testfixtures__/react-19-installed-pure-app-router/README.md
@@ -0,0 +1 @@
+Upgrades React without prompt
diff --git a/packages/next-codemod/bin/__testfixtures__/react-19-installed-pure-app-router/app/layout.ts b/packages/next-codemod/bin/__testfixtures__/react-19-installed-pure-app-router/app/layout.ts
new file mode 100644
index 0000000000000..e69de29bb2d1d
diff --git a/packages/next-codemod/bin/__testfixtures__/react-19-installed-pure-app-router/package.json b/packages/next-codemod/bin/__testfixtures__/react-19-installed-pure-app-router/package.json
new file mode 100644
index 0000000000000..9031ec28ac412
--- /dev/null
+++ b/packages/next-codemod/bin/__testfixtures__/react-19-installed-pure-app-router/package.json
@@ -0,0 +1,11 @@
+{
+  "name": "react-19-installed-pure-app-router",
+  "scripts": {
+    "dev": "next dev"
+  },
+  "dependencies": {
+    "next": "14.3.0-canary.45",
+    "react": "19.0.0-beta-4508873393-20240430",
+    "react-dom": "19.0.0-beta-4508873393-20240430"
+  }
+}
diff --git a/packages/next-codemod/bin/__testfixtures__/react-19-installed-pure-pages-router/README.md b/packages/next-codemod/bin/__testfixtures__/react-19-installed-pure-pages-router/README.md
new file mode 100644
index 0000000000000..c6f744b30286b
--- /dev/null
+++ b/packages/next-codemod/bin/__testfixtures__/react-19-installed-pure-pages-router/README.md
@@ -0,0 +1 @@
+Upgrades React without prompt
diff --git a/packages/next-codemod/bin/__testfixtures__/react-19-installed-pure-pages-router/package.json b/packages/next-codemod/bin/__testfixtures__/react-19-installed-pure-pages-router/package.json
new file mode 100644
index 0000000000000..881881779fbd0
--- /dev/null
+++ b/packages/next-codemod/bin/__testfixtures__/react-19-installed-pure-pages-router/package.json
@@ -0,0 +1,11 @@
+{
+  "name": "react-19-installed-pure-pages-router",
+  "scripts": {
+    "dev": "next dev"
+  },
+  "dependencies": {
+    "next": "14.3.0-canary.45",
+    "react": "19.0.0-beta-4508873393-20240430",
+    "react-dom": "19.0.0-beta-4508873393-20240430"
+  }
+}
diff --git a/packages/next-codemod/bin/__testfixtures__/react-19-installed-pure-pages-router/pages/index.ts b/packages/next-codemod/bin/__testfixtures__/react-19-installed-pure-pages-router/pages/index.ts
new file mode 100644
index 0000000000000..e69de29bb2d1d
diff --git a/packages/next-codemod/package.json b/packages/next-codemod/package.json
index 4965e5fee684f..77724903e6254 100644
--- a/packages/next-codemod/package.json
+++ b/packages/next-codemod/package.json
@@ -30,7 +30,8 @@
     "build": "pnpm tsc -d -p tsconfig.json",
     "prepublishOnly": "cd ../../ && turbo run build",
     "dev": "pnpm tsc -d -w -p tsconfig.json",
-    "test": "jest"
+    "test": "jest",
+    "test:upgrade-fixture": "./scripts/test-upgrade-fixture.sh"
   },
   "bin": "./bin/next-codemod.js",
   "devDependencies": {
diff --git a/packages/next-codemod/scripts/test-upgrade-fixture.sh b/packages/next-codemod/scripts/test-upgrade-fixture.sh
new file mode 100755
index 0000000000000..1a86df2c23823
--- /dev/null
+++ b/packages/next-codemod/scripts/test-upgrade-fixture.sh
@@ -0,0 +1,18 @@
+#!/usr/bin/env bash
+# Simple test runner for upgrade fixtures.
+# For manual verification only (check the README.md) in the fixtures
+# for the expected behavior.
+# Usage:
+# - cwd must be ~/packages/next-codemod
+# - `pnpm test:upgrade-fixture <fixture-name> <...next-codemod-args>`
+
+NEXT_CODEMOD_BIN=$(pwd)/bin/next-codemod.js
+cd "$1" || exit 1
+# We're only interested in the changes the upgrade command does.
+git add -A .
+rm -rf node_modules
+pnpm install --ignore-workspace --no-lockfile
+node "$NEXT_CODEMOD_BIN" upgrade "${@:2}"
+git --no-pager diff .
+git restore .
+git reset HEAD -- .