diff --git a/.pnp.cjs b/.pnp.cjs
index 1a769d037466..0ca97ca7f16b 100755
--- a/.pnp.cjs
+++ b/.pnp.cjs
@@ -186,6 +186,10 @@ function $$SETUP_STATE(hydrateRuntimeState, basePath) {
         "name": "@yarnpkg/shell",
         "reference": "workspace:packages/yarnpkg-shell"
       },
+      {
+        "name": "@yarnpkg/tools",
+        "reference": "workspace:packages/yarnpkg-tools"
+      },
       {
         "name": "pkg-tests-core",
         "reference": "workspace:packages/acceptance-tests/pkg-tests-core"
@@ -242,6 +246,7 @@ function $$SETUP_STATE(hydrateRuntimeState, basePath) {
       ["@yarnpkg/pnpify", ["workspace:packages/yarnpkg-pnpify"]],
       ["@yarnpkg/sdks", ["workspace:packages/yarnpkg-sdks"]],
       ["@yarnpkg/shell", ["workspace:packages/yarnpkg-shell"]],
+      ["@yarnpkg/tools", ["workspace:packages/yarnpkg-tools"]],
       ["acceptance-tests-229a13", ["workspace:packages/acceptance-tests"]],
       ["pkg-tests-core", ["workspace:packages/acceptance-tests/pkg-tests-core"]],
       ["pkg-tests-fixtures", ["workspace:packages/acceptance-tests/pkg-tests-fixtures"]],
@@ -14885,6 +14890,15 @@ function $$SETUP_STATE(hydrateRuntimeState, basePath) {
           "linkType": "SOFT",
         }]
       ]],
+      ["@yarnpkg/tools", [
+        ["workspace:packages/yarnpkg-tools", {
+          "packageLocation": "./packages/yarnpkg-tools/",
+          "packageDependencies": [
+            ["@yarnpkg/tools", "workspace:packages/yarnpkg-tools"]
+          ],
+          "linkType": "SOFT",
+        }]
+      ]],
       ["@zkochan/cmd-shim", [
         ["npm:5.1.0", {
           "packageLocation": "./.yarn/cache/@zkochan-cmd-shim-npm-5.1.0-3bdda00327-7be69cfede.zip/node_modules/@zkochan/cmd-shim/",
diff --git a/packages/yarnpkg-tools/package.json b/packages/yarnpkg-tools/package.json
new file mode 100644
index 000000000000..01553bf5f86c
--- /dev/null
+++ b/packages/yarnpkg-tools/package.json
@@ -0,0 +1,27 @@
+{
+  "name": "@yarnpkg/tools",
+  "version": "1.0.0",
+  "license": "BSD-2-Clause",
+  "main": "./sources/index.ts",
+  "scripts": {
+    "postpack": "rm -rf lib",
+    "prepack": "run build:compile \"$(pwd)\"",
+    "release": "yarn npm publish",
+    "test": "run test:unit \"$(pwd)\""
+  },
+  "publishConfig": {
+    "main": "./lib/index.js",
+    "typings": "./lib/index.d.ts"
+  },
+  "files": [
+    "/lib/**/*"
+  ],
+  "repository": {
+    "type": "git",
+    "url": "ssh://git@github.com/yarnpkg/berry.git",
+    "directory": "packages/yarnpkg-tools"
+  },
+  "engines": {
+    "node": ">=12 <14 || 14.2 - 14.9 || >14.10.0"
+  }
+}
diff --git a/packages/yarnpkg-tools/sources/index.ts b/packages/yarnpkg-tools/sources/index.ts
new file mode 100644
index 000000000000..028706f20817
--- /dev/null
+++ b/packages/yarnpkg-tools/sources/index.ts
@@ -0,0 +1,293 @@
+import fs   from 'fs';
+import path from 'path';
+
+const MANIFEST_FILENAME = `package.json`;
+const NODE_MODULES_FILENAME = `node_modules`;
+const PNP_JS_FILENAME = `.pnp.js`;
+
+const nodeModulesRegExp = /[\\/]node_modules[\\/](@[^\\/]*[\\/])?([^@\\/][^\\/]*)$/;
+
+class AmbiguousPackageManagerChoiceError extends Error {
+  constructor(message: string, cwd: string) {
+    super(`${message} Please specify your package manager choice using the "packageManager" field in ${path.join(cwd, MANIFEST_FILENAME)}.`);
+    this.name = `AmbiguousPackageManagerChoiceError`;
+  }
+}
+
+export enum PackageManager {
+  YARN = `yarn`,
+  PNPM = `pnpm`,
+  NPM = `npm`,
+}
+
+export const Lockfile = {
+  [PackageManager.YARN]: `yarn.lock`,
+  [PackageManager.PNPM]: `pnpm-lock.yaml`,
+  [PackageManager.NPM]: `package-lock.json`,
+};
+
+type BasePackageManagerSpec<T extends PackageManager> = {
+  name: T;
+  reason: string;
+};
+
+type MakePackageManagerSpec<T extends PackageManager, O extends object = {}> = BasePackageManagerSpec<T> & O;
+
+export type PackageManagerSpec =
+  | MakePackageManagerSpec<PackageManager.YARN, {isClassic: boolean}>
+  | MakePackageManagerSpec<PackageManager.PNPM>
+  | MakePackageManagerSpec<PackageManager.NPM>;
+
+/**
+ * Tries to detect the package manager from the user-agent.
+ */
+function detectPackageManagerFromUserAgent(env: NodeJS.ProcessEnv): PackageManagerSpec | null {
+  const userAgent = env.npm_config_user_agent;
+  if (typeof userAgent === `undefined`)
+    return null;
+
+  const fields = userAgent.split(` `);
+
+  for (const field of fields) {
+    const [name, value] = field.split(`/`);
+
+    if (name === PackageManager.YARN) {
+      return {
+        name: PackageManager.YARN,
+        reason: `Found "${field}" in npm_config_user_agent`,
+        isClassic: value.startsWith(`0`) || value.startsWith(`1`),
+      };
+    }
+
+    for (const pm of [PackageManager.PNPM, PackageManager.NPM] as const) {
+      if (name === pm) {
+        return {
+          name,
+          reason: `Found "${field}" in npm_config_user_agent`,
+        };
+      }
+    }
+  }
+
+  return null;
+}
+
+/**
+ * Tries to detect the package manager by going up the directory tree and searching for install artifacts.
+ *
+ * Yarn modern can't hit this codepath because it always generates a lockfile.
+ */
+function detectPackageManagerFromInstallArtifacts(initialCwd: string): PackageManagerSpec | null {
+  let nextCwd = initialCwd;
+  let currCwd = ``;
+
+  const packageManagerSpecs: Array<PackageManagerSpec> = [];
+
+  while (nextCwd !== currCwd && packageManagerSpecs.length === 0) {
+    currCwd = nextCwd;
+    nextCwd = path.dirname(currCwd);
+
+    if (nodeModulesRegExp.test(currCwd))
+      continue;
+
+    const nodeModulesFolder = path.join(currCwd, NODE_MODULES_FILENAME);
+    if (fs.existsSync(nodeModulesFolder)) {
+      // Only generated by Yarn classic when operating in NM mode
+      if (fs.existsSync(path.join(nodeModulesFolder, `.yarn-integrity`))) {
+        packageManagerSpecs.push({
+          name: PackageManager.YARN,
+          reason: `Found .yarn-integrity in ${nodeModulesFolder}`,
+          isClassic: true,
+        });
+      }
+
+      // Only generated by PNPM when operating in any mode
+      if (fs.existsSync(path.join(nodeModulesFolder, `.modules.yaml`))) {
+        packageManagerSpecs.push({
+          name: PackageManager.PNPM,
+          reason: `Found .modules.yaml in ${nodeModulesFolder}`,
+        });
+      }
+
+      // Only generated by npm@7
+      if (fs.existsSync(path.join(nodeModulesFolder, `.package-lock.json`))) {
+        packageManagerSpecs.push({
+          name: PackageManager.NPM,
+          reason: `Found .package-lock.json in ${nodeModulesFolder}`,
+        });
+      }
+    }
+
+    // - Yarn modern can't hit this codepath because it always has a lockfile
+    // - PNPM can't hit this codepath because it always generates node_modules/.modules.yaml
+    // - Because of this, it's safe to assume it's Yarn classic operating in PnP mode
+    if (fs.existsSync(path.join(currCwd, PNP_JS_FILENAME)) && !fs.existsSync(path.join(nodeModulesFolder, `.modules.yaml`))) {
+      packageManagerSpecs.push({
+        name: PackageManager.YARN,
+        reason: `Found .pnp.js in ${currCwd}`,
+        isClassic: true,
+      });
+    }
+  }
+
+
+  if (packageManagerSpecs.length === 0)
+    return null;
+
+  if (packageManagerSpecs.length > 1)
+    throw new AmbiguousPackageManagerChoiceError(`Multiple install artifacts corresponding to the following package managers found: ${packageManagerSpecs.map(({name}) => JSON.stringify(name)).join(`, `)}`, currCwd);
+
+  return packageManagerSpecs[0];
+}
+
+/**
+ * Tries to detect the package manager by going up the directory tree and searching for lockfiles.
+ */
+function detectPackageManagerFromLockfiles(initialCwd: string): PackageManagerSpec | null {
+  let nextCwd = initialCwd;
+  let currCwd = ``;
+
+  const packageManagerSpecs: Array<PackageManagerSpec> = [];
+
+  while (nextCwd !== currCwd && packageManagerSpecs.length === 0) {
+    currCwd = nextCwd;
+    nextCwd = path.dirname(currCwd);
+
+    if (nodeModulesRegExp.test(currCwd))
+      continue;
+
+    if (fs.existsSync(path.join(currCwd, Lockfile[PackageManager.YARN]))) {
+      const lockfile = fs.readFileSync(path.join(currCwd, Lockfile[PackageManager.YARN]), `utf8`);
+      // Modern lockfiles always have a "__metadata" key.
+      if (lockfile.match(/^__metadata:$/m)) {
+        packageManagerSpecs.push({
+          name: PackageManager.YARN,
+          isClassic: false,
+          reason: `Found ${Lockfile[PackageManager.YARN]} containing "__metadata:" key in ${currCwd}`,
+        });
+      } else {
+        packageManagerSpecs.push({
+          name: PackageManager.YARN,
+          isClassic: true,
+          reason: `Found ${Lockfile[PackageManager.YARN]} not containing "__metadata:" key in ${currCwd}`,
+        });
+      }
+    }
+
+    for (const pm of [PackageManager.PNPM, PackageManager.NPM] as const) {
+      if (fs.existsSync(path.join(currCwd, Lockfile[pm]))) {
+        packageManagerSpecs.push({
+          name: pm,
+          reason: `Found ${Lockfile[pm]} in ${currCwd}`,
+        });
+      }
+    }
+  }
+
+  if (packageManagerSpecs.length === 0)
+    return null;
+
+  if (packageManagerSpecs.length > 1)
+    throw new AmbiguousPackageManagerChoiceError(`Multiple lockfiles found: ${packageManagerSpecs.map(({name}) => JSON.stringify(Lockfile[name])).join(`, `)}`, currCwd);
+
+  return packageManagerSpecs[0];
+}
+
+/**
+ * Tries to detect the package manager from the closest manifest going up the directory tree by reading the "packageManager" field.
+ */
+function detectPackageManagerFromManifest(initialCwd: string): PackageManagerSpec | null {
+  let nextCwd = initialCwd;
+  let currCwd = ``;
+
+  let selection: {
+    data: any;
+    manifestPath: string;
+  } | null = null;
+
+  while (nextCwd !== currCwd && (!selection || !selection.data.packageManager)) {
+    currCwd = nextCwd;
+    nextCwd = path.dirname(currCwd);
+
+    if (nodeModulesRegExp.test(currCwd))
+      continue;
+
+    const manifestPath = path.join(currCwd, MANIFEST_FILENAME);
+    if (!fs.existsSync(manifestPath))
+      continue;
+
+    const content = fs.readFileSync(manifestPath, `utf8`);
+
+    let data;
+    try {
+      data = JSON.parse(content);
+    } catch {}
+
+    if (typeof data !== `object` || data === null)
+      throw new Error(`Invalid package.json in ${path.relative(initialCwd, manifestPath)}`);
+
+    selection = {data, manifestPath};
+  }
+
+  if (selection === null)
+    return null;
+
+  const rawPmSpec = selection.data.packageManager;
+  if (typeof rawPmSpec !== `string`)
+    return null;
+
+  const [name, value] = rawPmSpec.split(`@`);
+  if (name === PackageManager.YARN) {
+    return {
+      name: PackageManager.YARN,
+      reason: `Found "${rawPmSpec}" in the "packageManager" field of ${selection.manifestPath}`,
+      isClassic: value.startsWith(`0`) || value.startsWith(`1`),
+    };
+  }
+
+  for (const pm of [PackageManager.PNPM, PackageManager.NPM] as const) {
+    if (name === pm) {
+      return {
+        name,
+        reason: `Found "${rawPmSpec}" in the "packageManager" field of ${selection.manifestPath}`,
+      };
+    }
+  }
+
+  return null;
+}
+
+/**
+ * Tries to detect the package manager from the current project.
+ *
+ * _**Important:**_ You likely want to use `detectPackageManager` instead.
+ *
+ * @internal
+ */
+export function detectPackageManagerFromProject(cwd: string): PackageManagerSpec | null {
+  const manifestPm = detectPackageManagerFromManifest(cwd);
+  if (manifestPm !== null)
+    return manifestPm;
+
+  const lockfilePm = detectPackageManagerFromLockfiles(cwd);
+  if (lockfilePm !== null)
+    return lockfilePm;
+
+  const installArtifactPm = detectPackageManagerFromInstallArtifacts(cwd);
+  if (installArtifactPm !== null)
+    return installArtifactPm;
+
+  return null;
+}
+
+export function detectPackageManager(cwd: string, env: NodeJS.ProcessEnv = process.env): PackageManagerSpec | null {
+  const projectPm = detectPackageManagerFromProject(cwd);
+  if (projectPm !== null)
+    return projectPm;
+
+  const userAgentPm = detectPackageManagerFromUserAgent(env);
+  if (userAgentPm !== null)
+    return userAgentPm;
+
+  return null;
+}
diff --git a/yarn.lock b/yarn.lock
index 84821b3e5010..2ce7806c6f30 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -6121,6 +6121,12 @@ __metadata:
   languageName: unknown
   linkType: soft
 
+"@yarnpkg/tools@workspace:packages/yarnpkg-tools":
+  version: 0.0.0-use.local
+  resolution: "@yarnpkg/tools@workspace:packages/yarnpkg-tools"
+  languageName: unknown
+  linkType: soft
+
 "@zkochan/cmd-shim@npm:^5.1.0":
   version: 5.1.0
   resolution: "@zkochan/cmd-shim@npm:5.1.0"