diff --git a/docs/content/commands/npm-diff.md b/docs/content/commands/npm-diff.md
index 57d20727a7533..479cb63b11213 100644
--- a/docs/content/commands/npm-diff.md
+++ b/docs/content/commands/npm-diff.md
@@ -242,6 +242,38 @@ command, if no explicit tag is given.
 When used by the `npm diff` command, this is the tag used to fetch the
 tarball that will be compared with the local files by default.
 
+#### `workspace`
+
+* Default:
+* Type: String (can be set multiple times)
+
+Enable running a command in the context of the configured workspaces of the
+current project while filtering by running only the workspaces defined by
+this configuration option.
+
+Valid values for the `workspace` config are either:
+
+* Workspace names
+* Path to a workspace directory
+* Path to a parent workspace directory (will result to selecting all of the
+  nested workspaces)
+
+When set for the `npm init` command, this may be set to the folder of a
+workspace which does not yet exist, to create the folder and set it up as a
+brand new workspace within the project.
+
+This value is not exported to the environment for child processes.
+
+#### `workspaces`
+
+* Default: false
+* Type: Boolean
+
+Enable running a command in the context of **all** the configured
+workspaces.
+
+This value is not exported to the environment for child processes.
+
 <!-- AUTOGENERATED CONFIG DESCRIPTIONS END -->
 ## See Also
 
diff --git a/lib/diff.js b/lib/diff.js
index 58834ca9c2674..d315551d443a5 100644
--- a/lib/diff.js
+++ b/lib/diff.js
@@ -1,13 +1,14 @@
 const { resolve } = require('path')
 
 const semver = require('semver')
-const libdiff = require('libnpmdiff')
+const libnpmdiff = require('libnpmdiff')
 const npa = require('npm-package-arg')
 const Arborist = require('@npmcli/arborist')
 const npmlog = require('npmlog')
 const pacote = require('pacote')
 const pickManifest = require('npm-pick-manifest')
 
+const getWorkspaces = require('./workspaces/get-workspaces.js')
 const readPackageName = require('./utils/read-package-name.js')
 const BaseCommand = require('./base-command.js')
 
@@ -25,10 +26,6 @@ class Diff extends BaseCommand {
   static get usage () {
     return [
       '[...<paths>]',
-      '--diff=<pkg-name> [...<paths>]',
-      '--diff=<version-a> [--diff=<version-b>] [...<paths>]',
-      '--diff=<spec-a> [--diff=<spec-b>] [...<paths>]',
-      '[--diff-ignore-all-space] [--diff-name-only] [...<paths>] [...<paths>]',
     ]
   }
 
@@ -45,19 +42,19 @@ class Diff extends BaseCommand {
       'diff-text',
       'global',
       'tag',
+      'workspace',
+      'workspaces',
     ]
   }
 
-  get where () {
-    const globalTop = resolve(this.npm.globalDir, '..')
-    const global = this.npm.config.get('global')
-    return global ? globalTop : this.npm.prefix
-  }
-
   exec (args, cb) {
     this.diff(args).then(() => cb()).catch(cb)
   }
 
+  execWorkspaces (args, filters, cb) {
+    this.diffWorkspaces(args, filters).then(() => cb()).catch(cb)
+  }
+
   async diff (args) {
     const specs = this.npm.config.get('diff').filter(d => d)
     if (specs.length > 2) {
@@ -67,75 +64,86 @@ class Diff extends BaseCommand {
       )
     }
 
+    // diffWorkspaces may have set this already
+    if (!this.prefix)
+      this.prefix = this.npm.prefix
+
+    // this is the "top" directory, one up from node_modules
+    // in global mode we have to walk one up from globalDir because our
+    // node_modules is sometimes under ./lib, and in global mode we're only ever
+    // walking through node_modules (because we will have been given a package
+    // name already)
+    if (this.npm.config.get('global'))
+      this.top = resolve(this.npm.globalDir, '..')
+    else
+      this.top = this.prefix
+
     const [a, b] = await this.retrieveSpecs(specs)
     npmlog.info('diff', { src: a, dst: b })
 
-    const res = await libdiff([a, b], {
+    const res = await libnpmdiff([a, b], {
       ...this.npm.flatOptions,
       diffFiles: args,
-      where: this.where,
+      where: this.top,
     })
     return this.npm.output(res)
   }
 
-  async retrieveSpecs ([a, b]) {
-    // no arguments, defaults to comparing cwd
-    // to its latest published registry version
-    if (!a)
-      return this.defaultSpec()
-
-    // single argument, used to compare wanted versions of an
-    // installed dependency or to compare the cwd to a published version
-    if (!b)
-      return this.transformSingleSpec(a)
-
-    const specs = await this.convertVersionsToSpecs([a, b])
-    return this.findVersionsByPackageName(specs)
+  async diffWorkspaces (args, filters) {
+    const workspaces =
+      await getWorkspaces(filters, { path: this.npm.localPrefix })
+    for (const workspacePath of workspaces.values()) {
+      this.top = workspacePath
+      this.prefix = workspacePath
+      await this.diff(args)
+    }
   }
 
-  async defaultSpec () {
-    let noPackageJson
-    let pkgName
+  // get the package name from the packument at `path`
+  // throws if no packument is present OR if it does not have `name` attribute
+  async packageName (path) {
+    let name
     try {
-      pkgName = await readPackageName(this.npm.prefix)
+      // TODO this won't work as expected in global mode
+      name = await readPackageName(this.prefix)
     } catch (e) {
       npmlog.verbose('diff', 'could not read project dir package.json')
-      noPackageJson = true
     }
 
-    if (!pkgName || noPackageJson) {
-      throw new Error(
-        'Needs multiple arguments to compare or run from a project dir.\n\n' +
-        `Usage:\n${this.usage}`
-      )
-    }
+    if (!name)
+      throw this.usageError('Needs multiple arguments to compare or run from a project dir.\n')
 
-    return [
-      `${pkgName}@${this.npm.config.get('tag')}`,
-      `file:${this.npm.prefix}`,
-    ]
+    return name
   }
 
-  async transformSingleSpec (a) {
+  async retrieveSpecs ([a, b]) {
+    if (a && b) {
+      const specs = await this.convertVersionsToSpecs([a, b])
+      return this.findVersionsByPackageName(specs)
+    }
+
+    // no arguments, defaults to comparing cwd
+    // to its latest published registry version
+    if (!a) {
+      const pkgName = await this.packageName(this.prefix)
+      return [
+        `${pkgName}@${this.npm.config.get('tag')}`,
+        `file:${this.prefix}`,
+      ]
+    }
+
+    // single argument, used to compare wanted versions of an
+    // installed dependency or to compare the cwd to a published version
     let noPackageJson
     let pkgName
     try {
-      pkgName = await readPackageName(this.npm.prefix)
+      pkgName = await readPackageName(this.prefix)
     } catch (e) {
       npmlog.verbose('diff', 'could not read project dir package.json')
       noPackageJson = true
     }
-    const missingPackageJson = new Error(
-      'Needs multiple arguments to compare or run from a project dir.\n\n' +
-      `Usage:\n${this.usage}`
-    )
 
-    const specSelf = () => {
-      if (noPackageJson)
-        throw missingPackageJson
-
-      return `file:${this.npm.prefix}`
-    }
+    const missingPackageJson = this.usageError('Needs multiple arguments to compare or run from a project dir.\n')
 
     // using a valid semver range, that means it should just diff
     // the cwd against a published version to the registry using the
@@ -143,10 +151,9 @@ class Diff extends BaseCommand {
     if (semver.validRange(a)) {
       if (!pkgName)
         throw missingPackageJson
-
       return [
         `${pkgName}@${a}`,
-        specSelf(),
+        `file:${this.prefix}`,
       ]
     }
 
@@ -160,7 +167,7 @@ class Diff extends BaseCommand {
       try {
         const opts = {
           ...this.npm.flatOptions,
-          path: this.where,
+          path: this.top,
         }
         const arb = new Arborist(opts)
         actualTree = await arb.loadActual(opts)
@@ -172,9 +179,11 @@ class Diff extends BaseCommand {
       }
 
       if (!node || !node.name || !node.package || !node.package.version) {
+        if (noPackageJson)
+          throw missingPackageJson
         return [
           `${spec.name}@${spec.fetchSpec}`,
-          specSelf(),
+          `file:${this.prefix}`,
         ]
       }
 
@@ -220,14 +229,10 @@ class Diff extends BaseCommand {
     } else if (spec.type === 'directory') {
       return [
         `file:${spec.fetchSpec}`,
-        specSelf(),
+        `file:${this.prefix}`,
       ]
-    } else {
-      throw new Error(
-        'Spec type not supported.\n\n' +
-        `Usage:\n${this.usage}`
-      )
-    }
+    } else
+      throw this.usageError(`Spec type ${spec.type} not supported.\n`)
   }
 
   async convertVersionsToSpecs ([a, b]) {
@@ -238,17 +243,14 @@ class Diff extends BaseCommand {
     if (semverA && semverB) {
       let pkgName
       try {
-        pkgName = await readPackageName(this.npm.prefix)
+        pkgName = await readPackageName(this.prefix)
       } catch (e) {
         npmlog.verbose('diff', 'could not read project dir package.json')
       }
 
-      if (!pkgName) {
-        throw new Error(
-          'Needs to be run from a project dir in order to diff two versions.\n\n' +
-          `Usage:\n${this.usage}`
-        )
-      }
+      if (!pkgName)
+        throw this.usageError('Needs to be run from a project dir in order to diff two versions.\n')
+
       return [`${pkgName}@${a}`, `${pkgName}@${b}`]
     }
 
@@ -269,7 +271,7 @@ class Diff extends BaseCommand {
     try {
       const opts = {
         ...this.npm.flatOptions,
-        path: this.where,
+        path: this.top,
       }
       const arb = new Arborist(opts)
       actualTree = await arb.loadActual(opts)
diff --git a/lib/utils/config/definition.js b/lib/utils/config/definition.js
index 5ec2ba1ab1d58..507be6a644042 100644
--- a/lib/utils/config/definition.js
+++ b/lib/utils/config/definition.js
@@ -49,8 +49,12 @@ class Definition {
     if (!this.typeDescription)
       this.typeDescription = describeType(this.type)
     // hint is only used for non-boolean values
-    if (!this.hint)
-      this.hint = `<${this.key}>`
+    if (!this.hint) {
+      if (this.type === Number)
+        this.hint = '<number>'
+      else
+        this.hint = `<${this.key}>`
+    }
     if (!this.usage)
       this.usage = describeUsage(this)
   }
diff --git a/lib/utils/config/definitions.js b/lib/utils/config/definitions.js
index ea9665b5431f5..ce7702aaa4f79 100644
--- a/lib/utils/config/definitions.js
+++ b/lib/utils/config/definitions.js
@@ -506,6 +506,7 @@ define('dev', {
 
 define('diff', {
   default: [],
+  hint: '<pkg-name|spec|version>',
   type: [String, Array],
   description: `
     Define arguments to compare in \`npm diff\`.
@@ -545,6 +546,7 @@ define('diff-no-prefix', {
 
 define('diff-dst-prefix', {
   default: 'b/',
+  hint: '<path>',
   type: String,
   description: `
     Destination prefix to be used in \`npm diff\` output.
@@ -554,6 +556,7 @@ define('diff-dst-prefix', {
 
 define('diff-src-prefix', {
   default: 'a/',
+  hint: '<path>',
   type: String,
   description: `
     Source prefix to be used in \`npm diff\` output.
diff --git a/tap-snapshots/test/lib/load-all-commands.js.test.cjs b/tap-snapshots/test/lib/load-all-commands.js.test.cjs
index d40be42868184..097123d46a3cc 100644
--- a/tap-snapshots/test/lib/load-all-commands.js.test.cjs
+++ b/tap-snapshots/test/lib/load-all-commands.js.test.cjs
@@ -199,16 +199,14 @@ The registry diff command
 
 Usage:
 npm diff [...<paths>]
-npm diff --diff=<pkg-name> [...<paths>]
-npm diff --diff=<version-a> [--diff=<version-b>] [...<paths>]
-npm diff --diff=<spec-a> [--diff=<spec-b>] [...<paths>]
-npm diff [--diff-ignore-all-space] [--diff-name-only] [...<paths>] [...<paths>]
 
 Options:
-[--diff <diff> [--diff <diff> ...]] [--diff-name-only]
-[--diff-unified <diff-unified>] [--diff-ignore-all-space] [--diff-no-prefix]
-[--diff-src-prefix <diff-src-prefix>] [--diff-dst-prefix <diff-dst-prefix>]
+[--diff <pkg-name|spec|version> [--diff <pkg-name|spec|version> ...]]
+[--diff-name-only] [--diff-unified <number>] [--diff-ignore-all-space]
+[--diff-no-prefix] [--diff-src-prefix <path>] [--diff-dst-prefix <path>]
 [--diff-text] [-g|--global] [--tag <tag>]
+[-w|--workspace <workspace-name> [-w|--workspace <workspace-name> ...]]
+[-ws|--workspaces]
 
 Run "npm help diff" for more info
 `
diff --git a/tap-snapshots/test/lib/utils/npm-usage.js.test.cjs b/tap-snapshots/test/lib/utils/npm-usage.js.test.cjs
index 7fdcf0c5d2dba..54f6c3d2feb2a 100644
--- a/tap-snapshots/test/lib/utils/npm-usage.js.test.cjs
+++ b/tap-snapshots/test/lib/utils/npm-usage.js.test.cjs
@@ -336,16 +336,14 @@ All commands:
                     
                     Usage:
                     npm diff [...<paths>]
-                    npm diff --diff=<pkg-name> [...<paths>]
-                    npm diff --diff=<version-a> [--diff=<version-b>] [...<paths>]
-                    npm diff --diff=<spec-a> [--diff=<spec-b>] [...<paths>]
-                    npm diff [--diff-ignore-all-space] [--diff-name-only] [...<paths>] [...<paths>]
                     
                     Options:
-                    [--diff <diff> [--diff <diff> ...]] [--diff-name-only]
-                    [--diff-unified <diff-unified>] [--diff-ignore-all-space] [--diff-no-prefix]
-                    [--diff-src-prefix <diff-src-prefix>] [--diff-dst-prefix <diff-dst-prefix>]
+                    [--diff <pkg-name|spec|version> [--diff <pkg-name|spec|version> ...]]
+                    [--diff-name-only] [--diff-unified <number>] [--diff-ignore-all-space]
+                    [--diff-no-prefix] [--diff-src-prefix <path>] [--diff-dst-prefix <path>]
                     [--diff-text] [-g|--global] [--tag <tag>]
+                    [-w|--workspace <workspace-name> [-w|--workspace <workspace-name> ...]]
+                    [-ws|--workspaces]
                     
                     Run "npm help diff" for more info
 
diff --git a/test/lib/diff.js b/test/lib/diff.js
index 7a52ea5ee0ae1..993dfa4d60718 100644
--- a/test/lib/diff.js
+++ b/test/lib/diff.js
@@ -1,10 +1,9 @@
-const { resolve } = require('path')
+const { resolve, join } = require('path')
 const t = require('tap')
 const mockNpm = require('../fixtures/mock-npm')
 
 const noop = () => null
 let libnpmdiff = noop
-let rpn = () => 'foo'
 
 const config = {
   global: false,
@@ -21,9 +20,11 @@ const flatOptions = {
   diffText: false,
   savePrefix: '^',
 }
+const fooPath = t.testdir({
+  'package.json': JSON.stringify({ name: 'foo', version: '1.0.0' }),
+})
 const npm = mockNpm({
-  globalDir: __dirname,
-  prefix: '.',
+  prefix: fooPath,
   config,
   flatOptions,
   output: noop,
@@ -33,7 +34,6 @@ const mocks = {
   npmlog: { info: noop, verbose: noop },
   libnpmdiff: (...args) => libnpmdiff(...args),
   'npm-registry-fetch': async () => ({}),
-  '../../lib/utils/read-package-name.js': async (prefix) => rpn(prefix),
   '../../lib/utils/usage.js': () => 'usage instructions',
 }
 
@@ -49,10 +49,11 @@ t.afterEach(() => {
   flatOptions.diffDstPrefix = ''
   flatOptions.diffText = false
   flatOptions.savePrefix = '^'
-  npm.globalDir = __dirname
-  npm.prefix = '..'
+  npm.globalDir = fooPath
+  npm.prefix = fooPath
   libnpmdiff = noop
-  rpn = () => 'foo'
+  diff.prefix = undefined
+  diff.top = undefined
 })
 
 const Diff = t.mock('../../lib/diff.js', mocks)
@@ -62,23 +63,23 @@ t.test('no args', t => {
   t.test('in a project dir', t => {
     t.plan(3)
 
-    const path = t.testdir({})
     libnpmdiff = async ([a, b], opts) => {
       t.equal(a, 'foo@latest', 'should have default spec comparison')
-      t.equal(b, `file:${path}`, 'should compare to cwd')
+      t.equal(b, `file:${fooPath}`, 'should compare to cwd')
       t.match(opts, npm.flatOptions, 'should forward flat options')
     }
 
-    npm.prefix = path
+    npm.prefix = fooPath
     diff.exec([], err => {
       if (err)
         throw err
+      t.end()
     })
   })
 
   t.test('no args, missing package.json name in cwd', t => {
-    rpn = () => undefined
-
+    const path = t.testdir({})
+    npm.prefix = path
     diff.exec([], err => {
       t.match(
         err,
@@ -89,10 +90,11 @@ t.test('no args', t => {
     })
   })
 
-  t.test('no args, missing package.json in cwd', t => {
-    rpn = () => {
-      throw new Error('ERR')
-    }
+  t.test('no args, bad package.json in cwd', t => {
+    const path = t.testdir({
+      'package.json': '{invalid"json',
+    })
+    npm.prefix = path
 
     diff.exec([], err => {
       t.match(
@@ -109,21 +111,16 @@ t.test('no args', t => {
 
 t.test('single arg', t => {
   t.test('spec using cwd package name', t => {
-    t.plan(4)
+    t.plan(3)
 
-    rpn = (prefix) => {
-      t.equal(prefix, path, 'read-package-name gets proper prefix')
-      return 'foo'
-    }
-    const path = t.testdir({})
     libnpmdiff = async ([a, b], opts) => {
       t.equal(a, 'foo@1.0.0', 'should forward single spec')
-      t.equal(b, `file:${path}`, 'should compare to cwd')
+      t.equal(b, `file:${fooPath}`, 'should compare to cwd')
       t.match(opts, npm.flatOptions, 'should forward flat options')
     }
 
     config.diff = ['foo@1.0.0']
-    npm.prefix = path
+    npm.prefix = fooPath
     diff.exec([], err => {
       if (err)
         throw err
@@ -133,9 +130,6 @@ t.test('single arg', t => {
 
   t.test('unknown spec, no package.json', t => {
     const path = t.testdir({})
-    rpn = () => {
-      throw new Error('ERR')
-    }
 
     config.diff = ['foo@1.0.0']
     npm.prefix = path
@@ -152,15 +146,13 @@ t.test('single arg', t => {
   t.test('spec using semver range', t => {
     t.plan(3)
 
-    const path = t.testdir({})
     libnpmdiff = async ([a, b], opts) => {
       t.equal(a, 'foo@~1.0.0', 'should forward single spec')
-      t.equal(b, `file:${path}`, 'should compare to cwd')
+      t.equal(b, `file:${fooPath}`, 'should compare to cwd')
       t.match(opts, npm.flatOptions, 'should forward flat options')
     }
 
     config.diff = ['foo@~1.0.0']
-    npm.prefix = path
     diff.exec([], err => {
       if (err)
         throw err
@@ -170,15 +162,13 @@ t.test('single arg', t => {
   t.test('version', t => {
     t.plan(3)
 
-    const path = t.testdir({})
     libnpmdiff = async ([a, b], opts) => {
       t.equal(a, 'foo@2.1.4', 'should convert to expected first spec')
-      t.equal(b, `file:${path}`, 'should compare to cwd')
+      t.equal(b, `file:${fooPath}`, 'should compare to cwd')
       t.match(opts, npm.flatOptions, 'should forward flat options')
     }
 
     config.diff = ['2.1.4']
-    npm.prefix = path
     diff.exec([], err => {
       if (err)
         throw err
@@ -186,10 +176,8 @@ t.test('single arg', t => {
   })
 
   t.test('version, no package.json', t => {
-    rpn = () => {
-      throw new Error('ERR')
-    }
-
+    const path = t.testdir({})
+    npm.prefix = path
     config.diff = ['2.1.4']
     diff.exec([], err => {
       t.match(
@@ -204,10 +192,9 @@ t.test('single arg', t => {
   t.test('version, filtering by files', t => {
     t.plan(3)
 
-    const path = t.testdir({})
     libnpmdiff = async ([a, b], opts) => {
       t.equal(a, 'foo@2.1.4', 'should use expected spec')
-      t.equal(b, `file:${path}`, 'should compare to cwd')
+      t.equal(b, `file:${fooPath}`, 'should compare to cwd')
       t.match(opts, {
         ...npm.flatOptions,
         diffFiles: [
@@ -218,7 +205,6 @@ t.test('single arg', t => {
     }
 
     config.diff = ['2.1.4']
-    npm.prefix = path
     diff.exec(['./foo.js', './bar.js'], err => {
       if (err)
         throw err
@@ -277,9 +263,6 @@ t.test('single arg', t => {
 
   t.test('unknown package name, no package.json', t => {
     const path = t.testdir({})
-    rpn = () => {
-      throw new Error('ERR')
-    }
 
     config.diff = ['bar']
     npm.prefix = path
@@ -531,8 +514,9 @@ t.test('single arg', t => {
   t.test('unknown package name', t => {
     t.plan(2)
 
-    const path = t.testdir({})
-    rpn = async () => undefined
+    const path = t.testdir({
+      'package.json': JSON.stringify({ version: '1.0.0' }),
+    })
     libnpmdiff = async ([a, b], opts) => {
       t.equal(a, 'bar@latest', 'should target latest tag of name')
       t.equal(b, `file:${path}`, 'should compare to cwd')
@@ -550,15 +534,12 @@ t.test('single arg', t => {
   t.test('use project name in project dir', t => {
     t.plan(2)
 
-    const path = t.testdir({})
-    rpn = async () => 'my-project'
     libnpmdiff = async ([a, b], opts) => {
-      t.equal(a, 'my-project@latest', 'should target latest tag of name')
-      t.equal(b, `file:${path}`, 'should compare to cwd')
+      t.equal(a, 'foo@latest', 'should target latest tag of name')
+      t.equal(b, `file:${fooPath}`, 'should compare to cwd')
     }
 
-    config.diff = ['my-project']
-    npm.prefix = path
+    config.diff = ['foo']
     diff.exec([], err => {
       if (err)
         throw err
@@ -568,15 +549,12 @@ t.test('single arg', t => {
   t.test('dir spec type', t => {
     t.plan(2)
 
-    const path = t.testdir({})
-    rpn = async () => 'my-project'
     libnpmdiff = async ([a, b], opts) => {
       t.equal(a, 'file:/path/to/other-dir', 'should target dir')
-      t.equal(b, `file:${path}`, 'should compare to cwd')
+      t.equal(b, `file:${fooPath}`, 'should compare to cwd')
     }
 
     config.diff = ['/path/to/other-dir']
-    npm.prefix = path
     diff.exec([], err => {
       if (err)
         throw err
@@ -584,14 +562,11 @@ t.test('single arg', t => {
   })
 
   t.test('unsupported spec type', t => {
-    rpn = async () => 'my-project'
-
     config.diff = ['git+https://github.com/user/foo']
-
     diff.exec([], err => {
       t.match(
         err,
-        /Spec type not supported./,
+        /Spec type git not supported./,
         'should throw spec type not supported error.'
       )
       t.end()
@@ -638,7 +613,6 @@ t.test('first arg is a qualified spec', t => {
       }),
     })
 
-    rpn = async () => 'my-project'
     libnpmdiff = async ([a, b], opts) => {
       t.equal(a, 'bar@2.0.0', 'should set expected first spec')
       t.equal(b, `bar@file:${resolve(path, 'node_modules/bar')}`, 'should target local node_modules pkg')
@@ -707,7 +681,6 @@ t.test('first arg is a known dependency name', t => {
       }),
     })
 
-    rpn = async () => 'my-project'
     libnpmdiff = async ([a, b], opts) => {
       t.equal(a, `bar@file:${resolve(path, 'node_modules/bar')}`, 'should target local node_modules pkg')
       t.equal(b, 'bar@2.0.0', 'should set expected second spec')
@@ -747,7 +720,6 @@ t.test('first arg is a known dependency name', t => {
       }),
     })
 
-    rpn = async () => 'my-project'
     libnpmdiff = async ([a, b], opts) => {
       t.equal(a, `bar@file:${resolve(path, 'node_modules/bar')}`, 'should target local node_modules pkg')
       t.equal(b, `bar-fork@file:${resolve(path, 'node_modules/bar-fork')}`, 'should target fork local node_modules pkg')
@@ -781,7 +753,6 @@ t.test('first arg is a known dependency name', t => {
       }),
     })
 
-    rpn = async () => 'my-project'
     libnpmdiff = async ([a, b], opts) => {
       t.equal(a, `bar@file:${resolve(path, 'node_modules/bar')}`, 'should target local node_modules pkg')
       t.equal(b, 'bar@2.0.0', 'should use package name from first arg')
@@ -815,7 +786,6 @@ t.test('first arg is a known dependency name', t => {
       }),
     })
 
-    rpn = async () => 'my-project'
     libnpmdiff = async ([a, b], opts) => {
       t.equal(a, `bar@file:${resolve(path, 'node_modules/bar')}`, 'should target local node_modules pkg')
       t.equal(b, 'bar-fork@latest', 'should set expected second spec')
@@ -869,7 +839,6 @@ t.test('first arg is a valid semver range', t => {
       }),
     })
 
-    rpn = async () => 'my-project'
     libnpmdiff = async ([a, b], opts) => {
       t.equal(a, 'bar@1.0.0', 'should use name from second arg')
       t.equal(b, `bar@file:${resolve(path, 'node_modules/bar')}`, 'should set expected second spec from nm')
@@ -886,10 +855,9 @@ t.test('first arg is a valid semver range', t => {
   t.test('second arg is ALSO a semver version', t => {
     t.plan(2)
 
-    rpn = async () => 'my-project'
     libnpmdiff = async ([a, b], opts) => {
-      t.equal(a, 'my-project@1.0.0', 'should use name from project dir')
-      t.equal(b, 'my-project@2.0.0', 'should use name from project dir')
+      t.equal(a, 'foo@1.0.0', 'should use name from project dir')
+      t.equal(b, 'foo@2.0.0', 'should use name from project dir')
     }
 
     config.diff = ['1.0.0', '2.0.0']
@@ -901,10 +869,6 @@ t.test('first arg is a valid semver range', t => {
 
   t.test('second arg is ALSO a semver version BUT cwd not a project dir', t => {
     const path = t.testdir({})
-    rpn = () => {
-      throw new Error('ERR')
-    }
-
     config.diff = ['1.0.0', '2.0.0']
     npm.prefix = path
     diff.exec([], err => {
@@ -920,7 +884,6 @@ t.test('first arg is a valid semver range', t => {
   t.test('second arg is an unknown dependency name', t => {
     t.plan(2)
 
-    rpn = async () => 'my-project'
     libnpmdiff = async ([a, b], opts) => {
       t.equal(a, 'bar@1.0.0', 'should use name from second arg')
       t.equal(b, 'bar@latest', 'should compare against latest tag')
@@ -944,7 +907,6 @@ t.test('first arg is a valid semver range', t => {
 
     const Diff = t.mock('../../lib/diff.js', {
       ...mocks,
-      '../../lib/utils/read-package-name.js': async () => 'my-project',
       '@npmcli/arborist': class {
         constructor () {
           throw new Error('ERR')
@@ -977,7 +939,7 @@ t.test('first arg is an unknown dependency name', t => {
       t.equal(a, 'bar@latest', 'should set expected first spec')
       t.equal(b, 'bar@2.0.0', 'should set expected second spec')
       t.match(opts, npm.flatOptions, 'should forward flat options')
-      t.match(opts, { where: '.' }, 'should forward pacote options')
+      t.match(opts, { where: fooPath }, 'should forward pacote options')
     }
 
     config.diff = ['bar', 'bar@2.0.0']
@@ -1007,7 +969,6 @@ t.test('first arg is an unknown dependency name', t => {
       }),
     })
 
-    rpn = async () => 'my-project'
     libnpmdiff = async ([a, b], opts) => {
       t.equal(a, 'bar-fork@latest', 'should use latest tag')
       t.equal(b, `bar@file:${resolve(path, 'node_modules/bar')}`, 'should target local node_modules pkg')
@@ -1055,9 +1016,6 @@ t.test('first arg is an unknown dependency name', t => {
     t.plan(2)
 
     const path = t.testdir({})
-    rpn = () => {
-      throw new Error('ERR')
-    }
     libnpmdiff = async ([a, b], opts) => {
       t.equal(a, 'bar@latest', 'should use latest tag')
       t.equal(b, 'bar-fork@latest', 'should use latest tag')
@@ -1120,11 +1078,9 @@ t.test('various options', t => {
   t.test('set files no diff args', t => {
     t.plan(3)
 
-    const path = t.testdir({})
-    rpn = async () => 'my-project'
     libnpmdiff = async ([a, b], opts) => {
-      t.equal(a, 'my-project@latest', 'should have default spec')
-      t.equal(b, `file:${path}`, 'should compare to cwd')
+      t.equal(a, 'foo@latest', 'should have default spec')
+      t.equal(b, `file:${fooPath}`, 'should compare to cwd')
       t.match(opts, {
         ...npm.flatOptions,
         diffFiles: [
@@ -1134,7 +1090,6 @@ t.test('various options', t => {
       }, 'should forward all remaining items as filenames')
     }
 
-    npm.prefix = path
     diff.exec(['./foo.js', './bar.js'], err => {
       if (err)
         throw err
@@ -1183,3 +1138,80 @@ t.test('too many args', t => {
     t.end()
   })
 })
+
+t.test('workspaces', t => {
+  const path = t.testdir({
+    'package.json': JSON.stringify({
+      name: 'workspaces-test',
+      version: '1.2.3-test',
+      workspaces: ['workspace-a', 'workspace-b', 'workspace-c'],
+    }),
+    'workspace-a': {
+      'package.json': JSON.stringify({
+        name: 'workspace-a',
+        version: '1.2.3-a',
+      }),
+    },
+    'workspace-b': {
+      'package.json': JSON.stringify({
+        name: 'workspace-b',
+        version: '1.2.3-b',
+      }),
+    },
+    'workspace-c': JSON.stringify({
+      'package.json': {
+        name: 'workspace-n',
+        version: '1.2.3-n',
+      },
+    }),
+  })
+
+  t.test('all workspaces', t => {
+    const diffCalls = []
+    libnpmdiff = async ([a, b]) => {
+      diffCalls.push([a, b])
+    }
+    npm.prefix = path
+    npm.localPrefix = path
+    diff.execWorkspaces([], [], (err) => {
+      if (err)
+        throw err
+      t.same(diffCalls, [
+        ['workspace-a@latest', join(`file:${path}`, 'workspace-a')],
+        ['workspace-b@latest', join(`file:${path}`, 'workspace-b')],
+      ], 'should call libnpmdiff with workspaces params')
+      t.end()
+    })
+  })
+
+  t.test('one workspace', t => {
+    const diffCalls = []
+    libnpmdiff = async ([a, b]) => {
+      diffCalls.push([a, b])
+    }
+    npm.prefix = path
+    npm.localPrefix = path
+    diff.execWorkspaces([], ['workspace-a'], (err) => {
+      if (err)
+        throw err
+      t.same(diffCalls, [
+        ['workspace-a@latest', join(`file:${path}`, 'workspace-a')],
+      ], 'should call libnpmdiff with workspaces params')
+      t.end()
+    })
+  })
+
+  t.test('invalid workspace', t => {
+    libnpmdiff = () => {
+      t.fail('should not call libnpmdiff')
+    }
+    npm.prefix = path
+    npm.localPrefix = path
+    diff.execWorkspaces([], ['workspace-x'], (err) => {
+      t.match(err, /No workspaces found/)
+      t.match(err, /workspace-x/)
+      t.end()
+    })
+  })
+  t.end()
+})