diff --git a/.github/workflows/pull_request.yml b/.github/workflows/pull_request.yml
index ea40b32..9edab19 100644
--- a/.github/workflows/pull_request.yml
+++ b/.github/workflows/pull_request.yml
@@ -43,4 +43,3 @@ jobs:
     with:
       api_breakage_check_enabled: false
       license_header_check_project_name: "Swift.org"
-      format_check_enabled: false
diff --git a/.github/workflows/scripts/cross-pr-checkout.swift b/.github/workflows/scripts/cross-pr-checkout.swift
new file mode 100644
index 0000000..7a4550a
--- /dev/null
+++ b/.github/workflows/scripts/cross-pr-checkout.swift
@@ -0,0 +1,247 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2024 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import Foundation
+
+#if canImport(FoundationNetworking)
+// FoundationNetworking is a separate module in swift-foundation but not swift-corelibs-foundation.
+import FoundationNetworking
+#endif
+
+#if canImport(WinSDK)
+import WinSDK
+#endif
+
+struct GenericError: Error, CustomStringConvertible {
+  var description: String
+
+  init(_ description: String) {
+    self.description = description
+  }
+}
+
+/// Escape the given command to be printed for log output.
+func escapeCommand(_ executable: URL, _ arguments: [String]) -> String {
+  return ([executable.path] + arguments).map {
+    if $0.contains(" ") {
+      return "'\($0)'"
+    }
+    return $0
+  }.joined(separator: " ")
+}
+
+/// Launch a subprocess with the given command and wait for it to finish
+func run(_ executable: URL, _ arguments: String..., workingDirectory: URL? = nil) throws {
+  print("Running \(escapeCommand(executable, arguments)) (working directory: \(workingDirectory?.path ?? "<nil>"))")
+  let process = Process()
+  process.executableURL = executable
+  process.arguments = arguments
+  if let workingDirectory {
+    process.currentDirectoryURL = workingDirectory
+  }
+
+  try process.run()
+  process.waitUntilExit()
+  guard process.terminationStatus == 0 else {
+    throw GenericError(
+      "\(escapeCommand(executable, arguments)) failed with non-zero exit code: \(process.terminationStatus)"
+    )
+  }
+}
+
+/// Find the executable with the given name in PATH.
+public func lookup(executable: String) throws -> URL {
+  #if os(Windows)
+  let pathSeparator: Character = ";"
+  let executable = executable + ".exe"
+  #else
+  let pathSeparator: Character = ":"
+  #endif
+  for pathVariable in ["PATH", "Path"] {
+    guard let pathString = ProcessInfo.processInfo.environment[pathVariable] else {
+      continue
+    }
+    for searchPath in pathString.split(separator: pathSeparator) {
+      let candidateUrl = URL(fileURLWithPath: String(searchPath)).appendingPathComponent(executable)
+      if FileManager.default.isExecutableFile(atPath: candidateUrl.path) {
+        return candidateUrl
+      }
+    }
+  }
+  throw GenericError("Did not find \(executable)")
+}
+
+func downloadData(from url: URL) async throws -> Data {
+  return try await withCheckedThrowingContinuation { continuation in
+    URLSession.shared.dataTask(with: url) { data, _, error in
+      if let error {
+        continuation.resume(throwing: error)
+        return
+      }
+      guard let data else {
+        continuation.resume(throwing: GenericError("Received no data for \(url)"))
+        return
+      }
+      continuation.resume(returning: data)
+    }
+    .resume()
+  }
+}
+
+/// The JSON fields of the `https://api.github.com/repos/<repository>/pulls/<prNumber>` endpoint that we care about.
+struct PRInfo: Codable {
+  struct Base: Codable {
+    /// The name of the PR's base branch.
+    let ref: String
+  }
+  /// The base branch of the PR
+  let base: Base
+
+  /// The PR's description.
+  let body: String?
+}
+
+/// - Parameters:
+///   - repository: The repository's name, eg. `swiftlang/swift-syntax`
+func getPRInfo(repository: String, prNumber: String) async throws -> PRInfo {
+  guard let prInfoUrl = URL(string: "https://api.github.com/repos/\(repository)/pulls/\(prNumber)") else {
+    throw GenericError("Failed to form URL for GitHub API")
+  }
+
+  do {
+    let data = try await downloadData(from: prInfoUrl)
+    return try JSONDecoder().decode(PRInfo.self, from: data)
+  } catch {
+    throw GenericError("Failed to load PR info from \(prInfoUrl): \(error)")
+  }
+}
+
+/// Information about a PR that should be tested with this PR.
+struct CrossRepoPR {
+  /// The owner of the repository, eg. `swiftlang`
+  let repositoryOwner: String
+
+  /// The name of the repository, eg. `swift-syntax`
+  let repositoryName: String
+
+  /// The PR number that's referenced.
+  let prNumber: String
+}
+
+/// Retrieve all PRs that are referenced from PR `prNumber` in `repository`.
+/// `repository` is the owner and repo name joined by `/`, eg. `swiftlang/swift-syntax`.
+func getCrossRepoPrs(repository: String, prNumber: String) async throws -> [CrossRepoPR] {
+  var result: [CrossRepoPR] = []
+  let prInfo = try await getPRInfo(repository: repository, prNumber: prNumber)
+  for line in prInfo.body?.split(separator: "\n") ?? [] {
+    guard line.lowercased().starts(with: "linked pr:") else {
+      continue
+    }
+    // We can't use Swift's Regex here because this script needs to run on Windows with Swift 5.9, which doesn't support
+    // Swift Regex.
+    var remainder = line[...]
+    guard let ownerRange = remainder.firstRange(of: "swiftlang/") ?? remainder.firstRange(of: "apple/") else {
+      continue
+    }
+    let repositoryOwner = remainder[ownerRange].dropLast()
+    remainder = remainder[ownerRange.upperBound...]
+    let repositoryName = remainder.prefix { $0.isLetter || $0.isNumber || $0 == "-" || $0 == "_" }
+    if repositoryName.isEmpty {
+      continue
+    }
+    remainder = remainder.dropFirst(repositoryName.count)
+    if remainder.starts(with: "/pull/") {
+      remainder = remainder.dropFirst(6)
+    } else if remainder.starts(with: "#") {
+      remainder = remainder.dropFirst()
+    } else {
+      continue
+    }
+    let pullRequestNum = remainder.prefix { $0.isNumber }
+    if pullRequestNum.isEmpty {
+      continue
+    }
+    result.append(
+      CrossRepoPR(
+        repositoryOwner: String(repositoryOwner),
+        repositoryName: String(repositoryName),
+        prNumber: String(pullRequestNum)
+      )
+    )
+  }
+  return result
+}
+
+func main() async throws {
+  guard ProcessInfo.processInfo.arguments.count >= 3 else {
+    throw GenericError(
+      """
+      Expected two arguments:
+      - Repository name, eg. `swiftlang/swift-syntax
+      - PR number
+      """
+    )
+  }
+  let repository = ProcessInfo.processInfo.arguments[1]
+  let prNumber = ProcessInfo.processInfo.arguments[2]
+
+  let crossRepoPrs = try await getCrossRepoPrs(repository: repository, prNumber: prNumber)
+  if !crossRepoPrs.isEmpty {
+    print("Detected cross-repo PRs")
+    for crossRepoPr in crossRepoPrs {
+      print(" - \(crossRepoPr.repositoryOwner)/\(crossRepoPr.repositoryName)#\(crossRepoPr.prNumber)")
+    }
+  }
+
+  for crossRepoPr in crossRepoPrs {
+    let git = try lookup(executable: "git")
+    let swift = try lookup(executable: "swift")
+    let baseBranch = try await getPRInfo(
+      repository: "\(crossRepoPr.repositoryOwner)/\(crossRepoPr.repositoryName)",
+      prNumber: crossRepoPr.prNumber
+    ).base.ref
+
+    let workspaceDir = URL(fileURLWithPath: "..").resolvingSymlinksInPath()
+    let repoDir = workspaceDir.appendingPathComponent(crossRepoPr.repositoryName)
+    try run(
+      git,
+      "clone",
+      "https://github.com/\(crossRepoPr.repositoryOwner)/\(crossRepoPr.repositoryName).git",
+      "\(crossRepoPr.repositoryName)",
+      workingDirectory: workspaceDir
+    )
+    try run(git, "fetch", "origin", "pull/\(crossRepoPr.prNumber)/merge:pr_merge", workingDirectory: repoDir)
+    try run(git, "checkout", baseBranch, workingDirectory: repoDir)
+    try run(git, "reset", "--hard", "pr_merge", workingDirectory: repoDir)
+    try run(
+      swift,
+      "package",
+      "config",
+      "set-mirror",
+      "--package-url",
+      "https://github.com/\(crossRepoPr.repositoryOwner)/\(crossRepoPr.repositoryName).git",
+      "--mirror-url",
+      repoDir.path
+    )
+  }
+}
+
+do {
+  try await main()
+} catch {
+  print(error)
+  #if os(Windows)
+  _Exit(1)
+  #else
+  exit(1)
+  #endif
+}
diff --git a/.github/workflows/swift_package_test.yml b/.github/workflows/swift_package_test.yml
index b61a8f3..e218f19 100644
--- a/.github/workflows/swift_package_test.yml
+++ b/.github/workflows/swift_package_test.yml
@@ -64,6 +64,10 @@ on:
         type: boolean
         description: "Boolean to enable providing the GITHUB_TOKEN to downstream job."
         default: false
+      enable_cross_pr_testing:
+        type: boolean
+        description: "Whether PRs can be tested in combination with other PRs by mentioning them as `Linked PR: <link to PR>` in the PR description"
+        default: false
 
 jobs:
   linux-build:
@@ -88,6 +92,12 @@ jobs:
         if: ${{ inputs.needs_token }}
         run: |
             echo "GITHUB_TOKEN=${{ secrets.GITHUB_TOKEN }}" >> $GITHUB_ENV
+      - name: Check out related PRs
+        if: ${{ inputs.enable_cross_pr_testing && github.event_name == 'pull_request' }}
+        run: |
+          apt-get update && apt-get install -y curl
+          curl -s https://raw.githubusercontent.com/swiftlang/github-workflows/refs/heads/main/.github/workflows/scripts/cross-pr-checkout.swift > /tmp/cross-pr-checkout.swift
+          swift /tmp/cross-pr-checkout.swift "${{ github.repository }}" "${{ github.event.number }}"
       - name: Set environment variables
         if: ${{ inputs.linux_env_vars }}
         run: |
@@ -170,6 +180,18 @@ jobs:
           Invoke-Program swift --version
           Invoke-Program swift test --version
           Invoke-Program cd $Source
+          '@ >> $env:TEMP\test-script\run.ps1
+
+          if ("${{ inputs.enable_cross_pr_testing && github.event_name == 'pull_request' }}" -eq "true") {
+            echo @'
+          Invoke-WebRequest https://raw.githubusercontent.com/swiftlang/github-workflows/refs/heads/main/.github/workflows/scripts/cross-pr-checkout.swift -OutFile $env:TEMP\cross-pr-checkout.swift
+          # Running in script mode fails on Windows (https://github.com/swiftlang/swift/issues/77263), compile and run the script.
+          Invoke-Program swiftc -sdk $env:SDKROOT $env:TEMP\cross-pr-checkout.swift -o $env:TEMP\cross-pr-checkout.exe
+          Invoke-Program $env:TEMP\cross-pr-checkout.exe "${{ github.repository }}" "${{ github.event.number }}"
+          '@ >> $env:TEMP\test-script\run.ps1
+          }
+
+          echo @'
           ${{ inputs.windows_pre_build_command }}
           Invoke-Program ${{ inputs.windows_build_command }} ${{ (contains(matrix.swift_version, 'nightly') && inputs.swift_nightly_flags) || inputs.swift_flags }}
           '@ >> $env:TEMP\test-script\run.ps1
diff --git a/.swift-format b/.swift-format
new file mode 100644
index 0000000..41a022f
--- /dev/null
+++ b/.swift-format
@@ -0,0 +1,18 @@
+{
+    "version": 1,
+    "lineLength": 120,
+    "indentation": {
+        "spaces": 2
+    },
+    "lineBreakBeforeEachArgument": true,
+    "indentConditionalCompilationBlocks": false,
+    "prioritizeKeepingFunctionOutputTogether": true,
+    "rules": {
+        "AlwaysUseLowerCamelCase": false,
+        "AmbiguousTrailingClosureOverload": false,
+        "NoBlockComments": false,
+        "OrderedImports": true,
+        "UseLetInEveryBoundCaseVariable": false,
+        "UseSynthesizedInitializer": false
+    }
+}
diff --git a/README.md b/README.md
index 7d929a7..73705bc 100644
--- a/README.md
+++ b/README.md
@@ -75,6 +75,25 @@ pre_build_command: "which example || (apt update -q && apt install -yq example"
 
 macOS platform support will be available soon.
 
+#### Cross-PR testing
+
+To support testing of PRs together with PRs for one of the package’s dependencies, set add the following to your PR job.
+
+```yaml
+with:
+  enable_cross_pr_testing: true
+```
+
+To reference a linked PR, add `Linked PR: <link to PR>` to the PR description, eg.
+
+```
+Linked PR: https://github.com/swiftlang/swift-syntax/pull/2859
+// or alternatively
+Linked PR: swiftlang/swift-syntax#2859
+```
+
+Enabling cross-PR testing will add about 10s to PR testing time.
+
 ## Running workflows locally
 
 You can run the Github Actions workflows locally using