diff --git a/Sources/Macros/Macros.swift b/Sources/Macros/Macros.swift
index 2e2f81f..c85a7d5 100644
--- a/Sources/Macros/Macros.swift
+++ b/Sources/Macros/Macros.swift
@@ -289,11 +289,95 @@ public struct MustOr: CodeItemMacro {
   }
 }
 
+public struct CliTestDeclarationMacro: PeerMacro, Sendable {
+
+  private static func doc_shrink(_ from: String) -> String {
+    return from.replacing(Regex(#/^.*\/\/\/[\s]+/#), with: "")
+  }
+
+  public static func expansion(
+    of node: AttributeSyntax,
+    providingPeersOf declaration: some DeclSyntaxProtocol,
+    in context: some MacroExpansionContext
+  ) throws -> [DeclSyntax] {
+
+    let cli_test_expected_output = node.leadingTrivia.filter({ $0.isComment }).map({
+      doc_shrink("\($0)")
+    }).joined(separator: "\\n")
+
+    let cli_test_driver_thunk_name = context.makeUniqueName(
+      declaration.cast(FunctionDeclSyntax.self).name.text + "_thunk_")
+    let cli_test_driver_thunk: DeclSyntax = """
+      @Sendable private func \(cli_test_driver_thunk_name)() async throws {
+        let expected = "\(raw: cli_test_expected_output)"
+
+        _ = unsafe try await Testing.__requiringUnsafe(
+          Testing.__requiringTry(
+            Testing.__requiringAwait(swiftCliTestRunner(\(declaration.cast(FunctionDeclSyntax.self).name), expected))))
+      }
+      """
+
+    let cli_test_driver_generator_name = context.makeUniqueName(
+      declaration.cast(FunctionDeclSyntax.self).name.text + "_generator_")
+    let cli_test_driver_generator: DeclSyntax = """
+        @Sendable private func \(cli_test_driver_generator_name)() async -> Testing.Test {
+          return .__function(
+            named: "xxxxxxxx()",
+            in: nil,
+            xcTestCompatibleSelector: Testing.__xcTestCompatibleSelector("xxxxxx:"),
+            traits: [],
+            sourceLocation: Testing.SourceLocation(
+              fileID: "Tests/CliTests/Cli.swift",
+              filePath: "/Users/hawkinsw/code/p4ce/Tests/p4rseTests/CliTests/Cli.swift", line: 359,
+              column: 2),
+            parameters: [],
+            testFunction: \(cli_test_driver_thunk_name)
+          )
+        }
+      """
+
+    let cli_test_driver_accessor_name = context.makeUniqueName(
+      declaration.cast(FunctionDeclSyntax.self).name.text + "_accessor_")
+    let cli_test_driver_accessor: DeclSyntax = """
+        private nonisolated let \(cli_test_driver_accessor_name): Accessor = { outValue, type, _, _ in
+
+          Testing.Test.__store(\(cli_test_driver_generator_name), into: outValue, asTypeAt: type)
+        }
+      """
+
+    let cli_test_driver_content_record_name = context.makeUniqueName(
+      declaration.cast(FunctionDeclSyntax.self).name.text + "_cr_")
+    let cli_test_driver_cr: DeclSyntax = """
+        private nonisolated let \(cli_test_driver_content_record_name): TestContentRecord = (
+          0x7465_7374, /* indicate a test */
+          0,
+          unsafe \(cli_test_driver_accessor_name),
+          0,
+          0
+        )
+      """
+
+    let cli_test_driver_content_container_name = context.makeUniqueName(
+      declaration.cast(FunctionDeclSyntax.self).name.text + "__🟡$_container_")
+    let cli_test_driver_container: DeclSyntax = """
+        struct \(cli_test_driver_content_container_name): Testing.__TestContentRecordContainer {
+          nonisolated static var __testContentRecord: TestContentRecord {
+            unsafe \(cli_test_driver_content_record_name)
+          }
+        }
+      """
+    return [
+      cli_test_driver_thunk, cli_test_driver_generator, cli_test_driver_accessor,
+      cli_test_driver_cr, cli_test_driver_container,
+    ]
+  }
+}
+
 @main
 struct P4Macros: CompilerPlugin {
   var providingMacros: [Macro.Type] = [
     RequireResult.self, RequireErrorResult.self, UseOkResult.self, UseErrorResult.self,
     RequireNodeType.self, SkipUnlessNodeType.self, SkipUnlessNodesTypes.self, RequireNodesType.self,
-    MustOr.self,
+    MustOr.self, CliTestDeclarationMacro.self,
   ]
 }
diff --git a/Tests/Support.swift b/Tests/Support.swift
new file mode 100644
index 0000000..a2f827c
--- /dev/null
+++ b/Tests/Support.swift
@@ -0,0 +1,104 @@
+// p4rse, Copyright 2026, Will Hawkins
+//
+// This file is part of p4rse.
+//
+// This file is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program. If not, see <https://www.gnu.org/licenses/>.
+
+import Foundation
+import Testing
+
+typealias Accessor =
+  @convention(c) (
+    _ outValue: UnsafeMutableRawPointer,
+    _ type: UnsafeRawPointer,
+    _ hint: UnsafeRawPointer?,
+    _ reserved: UInt
+  ) -> CBool
+
+typealias TestContentRecord = (
+  kind: UInt32,
+  reserved1: UInt32,
+  accessor: Accessor?,
+  context: UInt,
+  reserved2: UInt
+)
+
+func findPathEnv() -> [URL]? {
+  for (key, value) in ProcessInfo().environment {
+    if key == "PATH" {
+      return value.split(separator: ":").map { URL(filePath: "\($0)") }
+    }
+  }
+
+  return .none
+}
+
+func findInPath(_ what: String) -> URL? {
+  guard let bin_paths = findPathEnv() else {
+    return .none
+  }
+
+  let fm = FileManager()
+  for bin_path in bin_paths {
+    let sought = bin_path.appending(path: what)
+    if fm.fileExists(atPath: sought.path) {
+      return sought
+    }
+  }
+
+  return .none
+}
+
+func swiftPath() -> URL? {
+  return findInPath("swift")
+}
+
+func swiftRun(withArgs args: [String]) throws -> String? {
+  let path = swiftPath()!
+  let child = Process()
+  let so = Pipe()
+  let se = Pipe()
+
+  child.standardOutput = so
+  child.executableURL = path
+  child.arguments =
+    [
+      "run",
+      "--ignore-lock" /* --ignore-lock needs to be early because it is a "location option". */,
+      "--skip-build",
+    ] + args
+
+  try! child.run()
+
+  let output =
+    switch child.standardOutput {
+    case let so as Pipe:
+      String(data: try! so.fileHandleForReading.readToEnd()!, encoding: String.Encoding.ascii)
+    default: Optional<String>.none
+    }
+  return output
+}
+
+func swiftCliTestRunner(_ arg_gen: () -> [String], _ expected: String) async throws {
+  let args = arg_gen()
+
+  let run_output = try! swiftRun(withArgs: args)
+
+  #expect(run_output == expected)
+}
+
+
+
+@attached(peer) public macro CliTest() =
+  #externalMacro(module: "Macros", type: "CliTestDeclarationMacro")
\ No newline at end of file
diff --git a/Tests/p4rseTests/CliTests/Cli.swift b/Tests/p4rseTests/CliTests/Cli.swift
new file mode 100644
index 0000000..3ac0b95
--- /dev/null
+++ b/Tests/p4rseTests/CliTests/Cli.swift
@@ -0,0 +1,26 @@
+// p4rse, Copyright 2026, Will Hawkins
+//
+// This file is part of p4rse.
+//
+// This file is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program. If not, see <https://www.gnu.org/licenses/>.
+
+import Foundation
+import Testing
+
+/// Success
+/// 
+@CliTest()
+func simple_cli_test() -> [String] {
+    return ["p4ce", "--plain", "compile", "simple.p4", "-I", "TestData/Sources/"]
+}
\ No newline at end of file