SPICE-0025: pkl run CLI framework (#1367)

docs/modules/ROOT/partials/component-attributes.adoc

@@ -68,6 +68,7 @@ endif::[]
 :uri-pkldoc-example: {uri-pkl-examples-tree}/pkldoc
 
 :uri-stdlib-baseModule: {uri-pkl-stdlib-docs}/base
+:uri-stdlib-CommandModule: {uri-pkl-stdlib-docs}/Command
 :uri-stdlib-analyzeModule: {uri-pkl-stdlib-docs}/analyze
 :uri-stdlib-jsonnetModule: {uri-pkl-stdlib-docs}/jsonnet
 :uri-stdlib-reflectModule: {uri-pkl-stdlib-docs}/reflect
@@ -150,6 +151,13 @@ endif::[]
 :uri-stdlib-Resource: {uri-stdlib-baseModule}/Resource
 :uri-stdlib-outputFiles: {uri-stdlib-baseModule}/ModuleOutput#files
 :uri-stdlib-FileOutput: {uri-stdlib-baseModule}/FileOutput
+:uri-stdlib-Annotation: {uri-stdlib-baseModule}/Annotation
+:uri-stdlib-ConvertProperty: {uri-stdlib-baseModule}/ConvertProperty
+:uri-stdlib-Command-Flag: {uri-stdlib-CommandModule}/Flag
+:uri-stdlib-Command-BooleanFlag: {uri-stdlib-CommandModule}/BooleanFlag
+:uri-stdlib-Command-CountedFlag: {uri-stdlib-CommandModule}/CountedFlag
+:uri-stdlib-Command-Argument: {uri-stdlib-CommandModule}/Argument
+:uri-stdlib-Command-Import: {uri-stdlib-CommandModule}/Import
 
 :uri-messagepack: https://msgpack.org/index.html
 :uri-messagepack-spec: https://github.com/msgpack/msgpack/blob/master/spec.md

docs/modules/language-reference/pages/index.adoc

@@ -3961,6 +3961,7 @@ emailList: List<EmailAddress> // <2>
 <1> equivalent to `email: String(contains("@"))` for type checking purposes
 <2> equivalent to `emailList: List<String(contains("@"))>` for type checking purposes
 
+[[nullable-types]]
 ==== Nullable Types
 
 Class types such as `Bird` (see above) do not admit `null` values.

docs/modules/pkl-cli/pages/index.adoc

@@ -489,7 +489,9 @@ If these are the only failures, the command exits with exit code 10.
 Otherwise, failures result in exit code 1.
 
 <modules>::
-The absolute or relative URIs of the modules to test. Relative URIs are resolved against the working directory.
+The absolute or relative URIs of the modules to test.
+The module must extend `pkl:test`.
+Relative URIs are resolved against the working directory.
 
 ==== Options
 
@@ -546,6 +548,23 @@ Use `--no-power-assertions` to disable this feature if you prefer simpler output
 
 This command also takes <<common-options, common options>>.
 
+[[command-run]]
+=== `pkl run`
+
+*Synopsis:* `pkl run [<options>] [<module>] [<command options>]`
+
+Evaluate a <<cli-tools,CLI command>> defined by `<module>`.
+
+<module>::
+The absolute or relative URIs of the command module to run.
+The module must extend `pkl:Command`.
+Relative URIs are resolved against the working directory.
+
+<command options>::
+Additional CLI options and arguments defined by `<module>`.
+
+This command also takes <<common-options, common options>>, but they must be specified before `<module>`. 
+
 [[command-repl]]
 === `pkl repl`
 
@@ -800,7 +819,7 @@ Write the path of files with formatting violations to stdout.
 [[common-options]]
 === Common options
 
-The <<command-eval>>, <<command-test>>, <<command-repl>>, <<command-project-resolve>>, <<command-project-package>>, <<command-download-package>>, and <<command-analyze-imports>> commands support the following common options:
+The <<command-eval>>, <<command-test>>, <<command-run>>, <<command-repl>>, <<command-project-resolve>>, <<command-project-package>>, <<command-download-package>>, and <<command-analyze-imports>> commands support the following common options:
 
 include::../../pkl-cli/partials/cli-common-options.adoc[]
 
@@ -901,6 +920,301 @@ If multiple module outputs are written to the same file, or to standard output,
 By default, module outputs are separated with `---`, as in a YAML stream.
 The separator can be customized using the `--module-output-separator` option.
 
+[[cli-tools]]
+== Implementing CLI Tools
+
+CLI tools can be implemented in Pkl by modules extending the `pkl:Command` module.
+With `pkl:Command`, you can define a script in Pkl that is executed by your shell, providing a better CLI experience.
+
+Regular evaluation requires use of xref:language-reference:index.adoc#resources[resources] like properties and evironment variables to provide parameters:
+[source,bash]
+----
+$ pkl eval script.pkl -p username=me -p password=password
+----
+
+Commands provide a native, familiar CLI experience:
+[source,bash]
+----
+$ pkl run script.pkl --username=admin --password=hunter2
+$ ./script.pkl --username=admin --password=hunter2
+----
+
+Pkl commands have a few properties that distinguish them from standard module evaluation:
+
+* Users provide input to commands using familiar command line idioms, providing a better experience than deriving inputs from xref:language-reference:index.adoc#resources[resources] like external properties or environment variables.
+* Commands can dynamically import modules when they are specified as command line options.
+* Commands may write to standard output (via `output.text` or `output.bytes`) and the filesystem (via `output.files`) in the same evaluation.
+* Command file output may write to any absolute path (not only relative to the `--multiple-file-output-path` option).
+** Relative output paths are written relative to the current working directory (or `--working-dir`, if specified).
+** Paths of output file are printed to the command's standard error.
+
+IMPORTANT: Users of `pkl run` must be aware of the security implications of this behavior.
+Using `pkl eval` prevents accidental overwrites by not allowing absolute paths, but `pkl run` does not offer this protection.
+Commands may write to any path the invoking user has permissions to modify.
+
+Commands are implemented as regular modules and declare their supported command line flags and positional arguments using a class with annotated properties.
+
+=== Defining Commands
+
+Commands are defined by creating a module that extends `pkl:Command`:
+
+[source,pkl%tested]
+.my-tool.pkl
+----
+/// This doc comment becomes part of the command's CLI help!
+/// Markdown formatting is **allowed!**
+extends "pkl:Command"
+
+options: Options // <1>
+
+class Options {
+  // Define CLI flags/arguments...
+}
+
+// Regular module code...
+----
+<1> Re-declaration of the `options` property's type.
+
+Like `pkl eval`, when a command completes without an evaluation error the process exits successfully (exit code 0).
+Commands can return a failure using `throw` (exit code 1), but otherwise may not control the exit code.
+
+Other than the differences listed above, commands behave like any other Pkl module.
+For example, there is no way to execute other programs or make arbitrary HTTP requests.
+If additional functionality is desired, xref:language-reference:index.adoc#external-readers[external readers] may be used to extends Pkl's capabilities.
+
+=== Command Options
+
+Each property of a command's options class becomes a command line option.
+Properties with the `local`, `hidden`, `fixed`, and/or `const` modifiers are not parsed as options
+A property's doc comment, if present, becomes the corresponding option's CLI help description.
+Doc comments are interpreted as Markdown text and formatted nicely when displayed to users.
+Properties must have xref:language-reference:index.adoc#type-annotations[type annotations] to determine how they are parsed.
+
+Properties may be xref:language-reference:index.adoc#annotations[annotated] to influence how they behave:
+
+* Properties annotated with link:{uri-stdlib-Command-Flag}[`@Flag`] become CLI flags named `--<property name>` that accept a value.
+* `Boolean` properties annotated with link:{uri-stdlib-Command-BooleanFlag}[`@BooleanFlag`] become CLI flags named `--<property name>` and `--no-<property name>` that result in `true` and `false` values, respectively.
+* `Int` (and type aliases of `Int`) properties annotated with link:{uri-stdlib-Command-CountedFlag}[`@CountedFlag`] become CLI flags named `--<property name>` that produce a value equal to the number of times they are present on the command line.
+* Properties annotated with link:{uri-stdlib-Command-Argument}[`@Argument`] become positional CLI arguments and are parsed in the order they appear in the class.
+* Properties with no annotation are treated the same as `@Flag` with no further customization.
+
+Flag options may set a `shortName` property to define a single-character abbreviation (`-<short name>`).
+Flag abbreviations may be combined (e.g. `-a -b -v -v -f some-value` is equivalent to `-abvvf some-value`).
+
+A `@Flag` or `@Argument` property's type annotation determines how it is converted from the raw string value:
+
+|===
+|Type |Behavior
+
+|`String`
+|Value is used verbatim.
+
+|`Char`
+|Value is used verbatim but must be exactly one character.
+
+|`Boolean`
+|True values: `true`, `t`, `1`, `yes`, `y`, `on`
+
+False values: `false`, `f`, `0`, `no`, `n`, `off`
+
+|`Number`
+|Value is parsed as an `Int` if possible, otherwise parsed as `Float`.
+
+|`Float`
+|Value is parsed as a `Float`.
+
+|`Int`
+|Value is parsed as a `Int`.
+
+|`Int8`, `Int16`, `Int32`, `UInt`, `UInt8`, `UInt16`, `UInt32`
+|Value is parsed as a `Int` and must be within the type's range.
+
+|xref:language-reference:index.adoc#union-types[Union] of xref:language-reference:index.adoc#string-literal-types[string literals]
+|Value is used verbatim but must match a member of the union.
+
+|`List<Element>`, `Listing<Element>`, `Set<Element>`
+|Each occurrence of the option becomes an element of the final value.
+
+`Element` values are parsed based on the above primitive types.
+
+|`Map<Key, Value>`, `Mapping<Key, Value>`
+|Each occurrence of the option becomes an entry of the final value.
+
+Values are split on the first `"="` character; the first part is parsed as `Key` and the second as `Value`, both based on the above primitive types.
+
+|`Pair<First, Second>`
+|Value is split on the first `"="` character; the first part is parsed as `First` and the second as `Second`, both based on the above primitive types.
+
+|===
+
+If a flag that accepts only a single value is provided multiple times, the last occurrence becomes the final value.
+
+Only a single positional argument accepting multiple values is permitted per command.
+
+A property with a xref:language-reference:index.adoc#nullable-types[nullable type] is optional and, if not specified on the command line, will have value `null`.
+Properties with default values are also optional.
+Type constraints are evaluated when the command is executed, so additional restrictions on option values are enforced at runtime.
+
+==== Custom Option Conversion and Aggregation
+
+A property may be annotated with any type if its `@Flag` or `@Argument` annotation sets the `convert` or `transformAll` properties.
+The `convert` property is a xref:language-reference:index.adoc#anonymous-functions[function] that overrides how _each_ raw option value is interpreted.
+The `transformAll` property is a function that overrides how _all_ parsed option values become the final property value.
+
+The `convert` function may return an link:{uri-stdlib-Command-Import}[`Import`] value that is replaced during option parsing with the actual value of the module specified by its `uri` property.
+If `glob` is `true`, the replacement value is a `Mapping`; its keys are the _absolute_ URIs of the matched modules and its values are the actual module values.
+When specifying glob import options on the command line, it is often necessary to quote the value to avoid it being interpreted by the shell.
+If the return value of `convert` is a `List`, `Set`, `Map`, or `Pair`, each contained value (elements and entry keys/values) that are `Import` values are also replaced.
+
+[IMPORTANT]
+====
+If an option has type `Mapping<String, «some module type»>` and should accept a single glob pattern value, the option's annotation must also set `multiple = false` to override the default behavior of `Mapping` options accepting multiple values.
+Example:
+[source%parsed,{pkl}]
+----
+@Flag {
+  convert = (it) -> new Import { uri = it; glob = true }
+  multiple = false
+}
+birds: Mapping<String, Bird>
+----
+
+If multiple glob patterns values should be accepted and merged, `transformAll` may be used to merge every glob-imported `Mapping`:
+[source%parsed,{pkl}]
+----
+@Flag {
+  convert = (it) -> new Import { uri = it; glob = true }
+  transformAll =
+    (values) -> values.fold(new Mapping {}, (result, element) ->
+      (result) { ...element }
+    )
+}
+birds: Mapping<String, Bird>
+----
+====
+
+=== Subcommands
+
+Like many other command line libraries, `pkl:Command` allows building commands into a hierarchy with a root command and subcommands:
+
+[source,pkl%tested]
+.my-tool.pkl
+----
+extends "pkl:Command"
+
+command {
+  subcommands {
+    import("subcommand1.pkl")
+    import("subcommand2.pkl")
+    for (_, subcommand in import*("./subcommands/*.pkl")) {
+      subcommand
+    }
+  }
+}
+----
+
+[source,pkl%tested]
+.subcommand1.pkl
+----
+extends "pkl:Command"
+
+import "my-tool.pkl"
+
+parent: `my-tool` // <1>
+
+// Regular module code...
+----
+<1> Optional; asserts that this is a subcommand of `my-tool` and simplifies accessing properties and options of the parent command
+
+Each element of `subcommands` must have a unique value for `command.name`.
+
+=== Testing Commands
+
+Command modules are normal Pkl modules, so they may be imported and used like any other module.
+This is particularly helpful when testing commands, as the command's `options` and `parent` properties can be populated by test code.
+
+Testing the above command and subcommand might look like this:
+
+[source,pkl%tested]
+----
+amends "pkl:test"
+
+import "my-tool.pkl"
+import "subcommand1.pkl"
+
+examples {
+  ["Test my-tool"] {
+    (`my-tool`) {
+      options {
+        // Set my-tool options here...
+      }
+    }.output.text
+  }
+  ["Test subcommand1"] {
+    (subcommand1) {
+      parent { // this amends `my-tool`
+        options {
+          // Set my-tool options here...
+        }
+      }
+      options {
+        // Set subcommand options here...
+      }
+    }.output.text
+  }
+}
+----
+
+[[commands-as-standalone-scripts]]
+=== Commands as standalone scripts
+
+On *nix platforms, Pkl commands can be configured to run as standalone tools that can be invoked without the `pkl run` command.
+To achieve this, the command file must be marked executable (i.e. `chmod +x my-tool.pkl`) and a link:https://en.wikipedia.org/wiki/Shebang_(Unix)[shebang comment] must be added on the first line of the file:
+
+[source,pkl%parsed]
+----
+#!/usr/bin/env -S pkl run
+----
+
+NOTE: The `-S` flag for `env` is required on Linux systems due to a limitation of shebang handling in the Linux kernel.
+While not required on other *nix platforms like macOS, but it should be included for compatibility.
+
+==== Shell Completion
+
+Like with Pkl's own CLI, <<command-shell-completion, shell completions>> can be generated for standalone scripts.
+
+[source,shell]
+----
+# Generate shell completion script for bash
+./my-tool.pkl shell-completion bash
+
+# Generate shell completion script for zsh
+./my-tool.pkl shell-completion zsh
+----
+
+==== Customizing Completion Candidates
+
+`@Flag` and `@Argument` annotations may specify the `completionCandidates` to improve generated shell completions.
+
+Valid values include:
+
+* A `Listing<String>` of literal string values to offer for completion.
+* The literal string `"path"`, which offers local file paths for completion.
+
+Options with a string literal union type implicitly offer the members of the union as completion candidates.
+
+=== Flag name ambiguities
+
+It is possible for commands to define flags with names or short names that collide with Pkl's own command line options.
+To avoid ambiguity in parsing these options, all flags for Pkl itself (e.g. `--root-dir`) must be placed before the root command module's URI.
+Command authors are encouraged to avoid overlapping with Pkl's built-in flags, but this may not always be feasible, especially for single-character abbreviated names.
+
+This imposes a limitation around <<commands-as-standalone-scripts,standalone commands>> that prevents users from customizing Pkl evaluator options when they are invoked.
+There are two recommended workarounds for this limitation:
+
+* Use a `PklProject` to define evaluator settings instead of doing so on the command line.
+* If the command line must be used, switch to invoking via `pkl run [<flags>] [<root command module>]`.
+
 [[repl]]
 == Working with the REPL
 

pkl-cli/src/main/kotlin/org/pkl/cli/CliCommandRunner.kt

@@ -0,0 +1,263 @@
+/*
+ * Copyright © 2025-2026 Apple Inc. and the Pkl project authors. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.pkl.cli
+
+import com.github.ajalt.clikt.completion.CompletionCandidates
+import com.github.ajalt.clikt.completion.CompletionCommand
+import com.github.ajalt.clikt.core.*
+import com.github.ajalt.clikt.parameters.arguments.*
+import com.github.ajalt.clikt.parameters.options.*
+import com.github.ajalt.clikt.parameters.types.int
+import java.io.OutputStream
+import java.nio.file.Path
+import kotlin.io.path.createParentDirectories
+import kotlin.io.path.exists
+import kotlin.io.path.isDirectory
+import kotlin.io.path.writeBytes
+import org.pkl.commons.cli.CliBaseOptions
+import org.pkl.commons.cli.CliCommand
+import org.pkl.commons.cli.CliException
+import org.pkl.commons.cli.commands.installCommonOptions
+import org.pkl.commons.currentWorkingDir
+import org.pkl.core.Closeables
+import org.pkl.core.CommandSpec
+import org.pkl.core.EvaluatorBuilder
+import org.pkl.core.FileOutput
+import org.pkl.core.ModuleSource.uri
+import org.pkl.core.PklBugException
+import org.pkl.core.PklException
+import org.pkl.core.util.IoUtils
+
+class CliCommandRunner
+@JvmOverloads
+constructor(
+  private val options: CliBaseOptions,
+  private val args: List<String>,
+  private val outputStream: OutputStream = System.out,
+  private val errStream: OutputStream = System.err,
+) : CliCommand(options) {
+
+  private val normalizedSourceModule = options.normalizedSourceModules.first()
+
+  override fun doRun() {
+    val builder = evaluatorBuilder()
+    try {
+      evalCmd(builder)
+    } finally {
+      Closeables.closeQuietly(builder.moduleKeyFactories)
+      Closeables.closeQuietly(builder.resourceReaders)
+    }
+  }
+
+  private fun evalCmd(builder: EvaluatorBuilder) {
+    val evaluator = builder.build()
+    evaluator.use {
+      evaluator.evaluateCommand(uri(normalizedSourceModule)) { spec ->
+        try {
+          val root = SynthesizedRunCommand(spec, this, options.sourceModules.first().toString())
+          root.installCommonOptions(includeVersion = false)
+          root.subcommands(
+            CompletionCommand(
+              name = "shell-completion",
+              help = "Generate a completion script for the given shell",
+            )
+          )
+          root.parse(args)
+        } catch (e: PklException) {
+          throw e
+        } catch (e: Exception) {
+          throw e.message?.let { PklException(it, e) } ?: PklException(e)
+        }
+      }
+    }
+  }
+
+  /** Renders the comand's `output.bytes`, writing it to the standard output stream. */
+  fun writeOutput(outputBytes: ByteArray) {
+    if (outputBytes.isEmpty()) return
+    outputStream.write(outputBytes)
+    outputStream.flush()
+  }
+
+  /**
+   * Renders the command's `output.files`, writing each entry as a file.
+   *
+   * File paths are written to the standard error stream.
+   *
+   * Unlike CliEvaluator, command outputs write relative to --working-dir and may write files
+   * anywhere in the filesystem. This is intentionally less sandboxed than `pkl eval` and directly
+   * targets the capabilities of CLI tools written in general purpose languages. Pkl commands should
+   * therefore be treated as untrusted code the way that any other CLI tool would be.
+   */
+  fun writeMultipleFileOutput(outputFiles: Map<String, FileOutput>) {
+    if (outputFiles.isEmpty()) return
+
+    val writtenFiles = mutableMapOf<Path, String>()
+    val outputDir = options.normalizedWorkingDir
+    if (outputDir.exists() && !outputDir.isDirectory()) {
+      throw CliException("Output path `$outputDir` exists and is not a directory.")
+    }
+    for ((pathSpec, fileOutput) in outputFiles) {
+      checkPathSpec(pathSpec)
+      val resolvedPath = outputDir.resolve(pathSpec).normalize()
+      val realPath = if (resolvedPath.exists()) resolvedPath.toRealPath() else resolvedPath
+      val previousOutput = writtenFiles[realPath]
+      if (previousOutput != null) {
+        throw CliException(
+          "Output file conflict: `output.files` entries `\"${previousOutput}\"` and `\"$pathSpec\"` resolve to the same file path `$realPath`."
+        )
+      }
+      if (realPath.isDirectory()) {
+        throw CliException(
+          "Output file conflict: `output.files` entry `\"$pathSpec\"` resolves to file path `$realPath`, which is a directory."
+        )
+      }
+      writtenFiles[realPath] = pathSpec
+      realPath.createParentDirectories()
+      realPath.writeBytes(fileOutput.bytes)
+      val displayPath =
+        if (Path.of(pathSpec).isAbsolute) pathSpec
+        else IoUtils.relativize(resolvedPath, currentWorkingDir).toString()
+      errStream.writeText(displayPath + IoUtils.getLineSeparator())
+      errStream.flush()
+    }
+  }
+
+  class SynthesizedRunCommand(
+    private val spec: CommandSpec,
+    private val runner: CliCommandRunner,
+    name: String? = null,
+  ) : CliktCommand(name = name ?: spec.name) {
+    init {
+      spec.options.forEach { opt ->
+        when (opt) {
+          is CommandSpec.Flag ->
+            registerOption(
+              option(
+                  names = opt.names,
+                  help = opt.helpText ?: "",
+                  metavar = opt.metavar,
+                  hidden = opt.hidden,
+                  completionCandidates = opt.completionCandidates?.toClikt(),
+                )
+                .convert {
+                  try {
+                    opt.transformEach.apply(it, runner.options.normalizedWorkingDir.toUri())
+                  } catch (e: CommandSpec.Option.BadValue) {
+                    fail(e.message!!)
+                  } catch (_: CommandSpec.Option.MissingOption) {
+                    throw MissingOption(option)
+                  }
+                }
+                .transformAll(opt.defaultValue, opt.showAsRequired) {
+                  try {
+                    opt.transformAll.apply(it)
+                  } catch (e: CommandSpec.Option.BadValue) {
+                    fail(e.message!!)
+                  } catch (_: CommandSpec.Option.MissingOption) {
+                    throw MissingOption(option)
+                  }
+                }
+            )
+          is CommandSpec.BooleanFlag ->
+            registerOption(
+              if (opt.defaultValue != null)
+                option(names = opt.names, help = opt.helpText ?: "", hidden = opt.hidden)
+                  .flag("--no-${opt.name}", default = opt.defaultValue!!)
+              else
+                option(names = opt.names, help = opt.helpText ?: "", hidden = opt.hidden)
+                  .nullableFlag("--no-${opt.name}")
+            )
+          is CommandSpec.CountedFlag ->
+            registerOption(
+              option(names = opt.names, help = opt.helpText ?: "", hidden = opt.hidden)
+                .int()
+                .transformValues(0..0) { it.lastOrNull() ?: 1 }
+                .transformAll { it.sum().toLong() }
+            )
+          is CommandSpec.Argument ->
+            registerArgument(
+              argument(
+                  opt.name,
+                  opt.helpText ?: "",
+                  completionCandidates = opt.completionCandidates?.toClikt(),
+                )
+                .convert {
+                  try {
+                    opt.transformEach.apply(it, runner.options.normalizedWorkingDir.toUri())
+                  } catch (e: CommandSpec.Option.BadValue) {
+                    fail(e.message!!)
+                  } catch (_: CommandSpec.Option.MissingOption) {
+                    throw MissingArgument(argument)
+                  }
+                }
+                .transformAll(if (opt.repeated) -1 else 1, !opt.repeated) {
+                  try {
+                    opt.transformAll.apply(it)
+                  } catch (e: CommandSpec.Option.BadValue) {
+                    fail(e.message!!)
+                  } catch (_: CommandSpec.Option.MissingOption) {
+                    throw MissingArgument(argument)
+                  }
+                }
+            )
+        }
+      }
+      spec.subcommands.forEach { subcommands(SynthesizedRunCommand(it, runner)) }
+    }
+
+    override val invokeWithoutSubcommand = true
+
+    override val hiddenFromHelp: Boolean = spec.hidden
+
+    override fun help(context: Context): String = spec.helpText ?: ""
+
+    override fun run() {
+      if (currentContext.invokedSubcommand is CompletionCommand) return
+
+      val opts =
+        registeredOptions()
+          .mapNotNull {
+            val opt = it as? OptionWithValues<*, *, *> ?: return@mapNotNull null
+            return@mapNotNull if (it.names.contains("--help")) null
+            else it.names.first().trimStart('-') to opt.value
+          }
+          .toMap() +
+          registeredArguments()
+            .mapNotNull { it as? ArgumentDelegate<*> }
+            .associateBy({ it.name }, { it.value })
+
+      val state = spec.apply.apply(opts, currentContext.obj as CommandSpec.State?)
+      currentContext.obj = state
+
+      if (currentContext.invokedSubcommand != null) return
+      if (spec.subcommands.isNotEmpty() && spec.noOp) {
+        throw PrintHelpMessage(currentContext, true, 1)
+      }
+
+      val result = state.evaluate()
+      runner.writeOutput(result.outputBytes)
+      runner.writeMultipleFileOutput(result.outputFiles)
+    }
+  }
+}
+
+fun CommandSpec.CompletionCandidates.toClikt(): CompletionCandidates =
+  when (this) {
+    CommandSpec.CompletionCandidates.PATH -> CompletionCandidates.Path
+    is CommandSpec.CompletionCandidates.Fixed -> CompletionCandidates.Fixed(values)
+    else -> throw PklBugException.unreachableCode()
+  }

pkl-cli/src/main/kotlin/org/pkl/cli/CliEvaluator.kt

@@ -143,17 +143,6 @@ constructor(
     }
   }
 
-  private fun Evaluator.writeOutput(moduleSource: ModuleSource, writeTo: Path): Boolean {
-    if (options.expression == null) {
-      val bytes = evaluateOutputBytes(moduleSource)
-      writeTo.writeBytes(bytes)
-      return bytes.isNotEmpty()
-    }
-    val text = evaluateExpressionString(moduleSource, options.expression)
-    writeTo.writeString(text)
-    return text.isNotEmpty()
-  }
-
   private fun Evaluator.evalOutput(moduleSource: ModuleSource): ByteArray {
     if (options.expression == null) {
       return evaluateOutputBytes(moduleSource)
@@ -227,13 +216,6 @@ constructor(
     }
   }
 
-  private fun OutputStream.writeText(text: String) = write(text.toByteArray())
-
-  private fun OutputStream.writeLine(text: String) {
-    writeText(text)
-    writeText("\n")
-  }
-
   private fun toModuleSource(uri: URI, reader: InputStream) =
     if (uri == VmUtils.REPL_TEXT_URI) {
       ModuleSource.create(uri, reader.readAllBytes().toString(StandardCharsets.UTF_8))
@@ -241,14 +223,6 @@ constructor(
       ModuleSource.uri(uri)
     }
 
-  private fun checkPathSpec(pathSpec: String) {
-    val illegal = pathSpec.indexOfFirst { IoUtils.isReservedFilenameChar(it) && it != '/' }
-    if (illegal == -1) {
-      return
-    }
-    throw CliException("Path spec `$pathSpec` contains illegal character `${pathSpec[illegal]}`.")
-  }
-
   /**
    * Renders each module's `output.files`, writing each entry as a file into the specified output
    * directory.

pkl-cli/src/main/kotlin/org/pkl/cli/OutputUtils.kt

@@ -0,0 +1,35 @@
+/*
+ * Copyright © 2025-2026 Apple Inc. and the Pkl project authors. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.pkl.cli
+
+import java.io.OutputStream
+import org.pkl.commons.cli.CliException
+import org.pkl.core.util.IoUtils
+
+fun checkPathSpec(pathSpec: String) {
+  val illegal = pathSpec.indexOfFirst { IoUtils.isReservedFilenameChar(it) && it != '/' }
+  if (illegal == -1) {
+    return
+  }
+  throw CliException("Path spec `$pathSpec` contains illegal character `${pathSpec[illegal]}`.")
+}
+
+fun OutputStream.writeText(text: String) = write(text.toByteArray())
+
+fun OutputStream.writeLine(text: String) {
+  writeText(text)
+  writeText("\n")
+}

pkl-cli/src/main/kotlin/org/pkl/cli/commands/RootCommand.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -50,6 +50,7 @@ class RootCommand : NoOpCliktCommand(name = "pkl") {
       DownloadPackageCommand(),
       AnalyzeCommand(),
       FormatterCommand(),
+      RunCommand(),
       CompletionCommand(
         name = "shell-completion",
         help = "Generate a completion script for the given shell",

pkl-cli/src/main/kotlin/org/pkl/cli/commands/RunCommand.kt

@@ -0,0 +1,50 @@
+/*
+ * Copyright © 2025-2026 Apple Inc. and the Pkl project authors. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.pkl.cli.commands
+
+import com.github.ajalt.clikt.completion.CompletionCandidates
+import com.github.ajalt.clikt.core.context
+import com.github.ajalt.clikt.parameters.arguments.argument
+import com.github.ajalt.clikt.parameters.arguments.convert
+import com.github.ajalt.clikt.parameters.arguments.multiple
+import com.github.ajalt.clikt.parameters.groups.provideDelegate
+import java.net.URI
+import org.pkl.cli.CliCommandRunner
+import org.pkl.commons.cli.commands.BaseCommand
+import org.pkl.commons.cli.commands.BaseOptions
+import org.pkl.commons.cli.commands.ProjectOptions
+
+class RunCommand : BaseCommand(name = "run", helpLink = helpLink) {
+  override val helpString = "Run a Pkl pkl:Command CLI tool"
+  override val treatUnknownOptionsAsArgs = true
+
+  init {
+    context { allowInterspersedArgs = false }
+  }
+
+  val module: URI by
+    argument(name = "module", completionCandidates = CompletionCandidates.Path).convert {
+      BaseOptions.parseModuleName(it)
+    }
+
+  val args: List<String> by argument(name = "args").multiple()
+
+  private val projectOptions by ProjectOptions()
+
+  override fun run() {
+    CliCommandRunner(baseOptions.baseOptions(listOf(module), projectOptions), args).run()
+  }
+}

pkl-commons-cli/src/main/kotlin/org/pkl/commons/cli/CliBaseOptions.kt

@@ -29,7 +29,7 @@ import org.pkl.core.util.IoUtils
 /** Base options shared between CLI commands. */
 data class CliBaseOptions(
   /** The source modules to evaluate. Relative URIs are resolved against [workingDir]. */
-  private val sourceModules: List<URI> = listOf(),
+  val sourceModules: List<URI> = listOf(),
 
   /**
    * The URI patterns that determine which modules can be loaded and evaluated. Patterns are matched

pkl-commons-cli/src/main/kotlin/org/pkl/commons/cli/CliCommand.kt

@@ -15,6 +15,7 @@
  */
 package org.pkl.commons.cli
 
+import com.github.ajalt.clikt.core.CliktError
 import java.net.URI
 import java.nio.file.Files
 import java.nio.file.Path
@@ -46,6 +47,7 @@ abstract class CliCommand(protected val cliOptions: CliBaseOptions) {
       proxyAddress?.let(IoUtils::setSystemProxy)
       doRun()
     } catch (e: PklException) {
+      if (e.cause is CliktError) throw e.cause!!
       throw CliException(e.message!!)
     } catch (e: CliException) {
       throw e

pkl-commons-cli/src/main/kotlin/org/pkl/commons/cli/commands/extensions.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -27,14 +27,16 @@ import org.pkl.core.Release
 
 private val theme = Theme { styles["markdown.code.span"] = TextStyle(bold = true) }
 
-fun <T : BaseCliktCommand<T>> T.installCommonOptions() {
+fun <T : BaseCliktCommand<T>> T.installCommonOptions(includeVersion: Boolean = true) {
   installMordantMarkdown()
 
-  versionOption(
-    Release.current().versionInfo,
-    names = setOf("-v", "--version"),
-    message = { if (commandName == "pkl") it else it.replaceFirst("Pkl", commandName) },
-  )
+  if (includeVersion) {
+    versionOption(
+      Release.current().versionInfo,
+      names = setOf("-v", "--version"),
+      message = { if (commandName == "pkl") it else it.replaceFirst("Pkl", commandName) },
+    )
+  }
 
   context { terminal = Terminal(theme = theme) }
 }

pkl-core/src/main/java/org/pkl/core/CommandSpec.java

@@ -0,0 +1,158 @@
+/*
+ * Copyright © 2025-2026 Apple Inc. and the Pkl project authors. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.pkl.core;
+
+import java.net.URI;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.function.BiFunction;
+import java.util.function.Function;
+import org.pkl.core.util.Nullable;
+
+public record CommandSpec(
+    String name,
+    @Nullable String helpText,
+    boolean hidden,
+    boolean noOp,
+    Iterable<Option> options,
+    List<CommandSpec> subcommands,
+    ApplyFunction apply) {
+
+  public sealed interface Option {
+    String name();
+
+    String[] getNames();
+
+    class MissingOption extends RuntimeException {
+      public MissingOption() {}
+    }
+
+    class BadValue extends RuntimeException {
+      public BadValue(String message) {
+        super(message);
+      }
+
+      public static BadValue invalid(String value, String type) {
+        return new BadValue(String.format("%s is not a valid %s", value, type));
+      }
+
+      public static BadValue badKeyValue(String value) {
+        return new BadValue(String.format("%s is not a valid key=value pair", value));
+      }
+
+      public static BadValue invalidChoice(String value, List<String> choices) {
+        return new BadValue(
+            String.format(
+                "invalid choice: %s. (choose from %s)", value, String.join(", ", choices)));
+      }
+
+      public static BadValue invalidChoice(String value, String choice) {
+        return new BadValue(String.format("invalid choice: %s. (choose from %s)", value, choice));
+      }
+    }
+  }
+
+  public abstract static sealed class CompletionCandidates {
+    public static final CompletionCandidates PATH = new StaticCompletionCandidates();
+
+    public static final class Fixed extends CompletionCandidates {
+      private final Set<String> values;
+
+      public Fixed(Set<String> values) {
+        this.values = values;
+      }
+
+      public Set<String> getValues() {
+        return values;
+      }
+    }
+
+    private static final class StaticCompletionCandidates extends CompletionCandidates {}
+  }
+
+  public record Flag(
+      String name,
+      @Nullable String helpText,
+      boolean showAsRequired,
+      BiFunction<String, URI, Object> transformEach,
+      Function<List<Object>, Object> transformAll,
+      @Nullable CompletionCandidates completionCandidates,
+      @Nullable String shortName,
+      String metavar,
+      boolean hidden,
+      @Nullable String defaultValue)
+      implements Option {
+    @Override
+    public String[] getNames() {
+      return shortName == null
+          ? new String[] {"--" + name}
+          : new String[] {"--" + name, "-" + shortName};
+    }
+  }
+
+  public record BooleanFlag(
+      String name,
+      @Nullable String helpText,
+      @Nullable String shortName,
+      boolean hidden,
+      @Nullable Boolean defaultValue)
+      implements Option {
+    @Override
+    public String[] getNames() {
+      return shortName == null
+          ? new String[] {"--" + name}
+          : new String[] {"--" + name, "-" + shortName};
+    }
+  }
+
+  public record CountedFlag(
+      String name, @Nullable String helpText, @Nullable String shortName, boolean hidden)
+      implements Option {
+    @Override
+    public String[] getNames() {
+      return shortName == null
+          ? new String[] {"--" + name}
+          : new String[] {"--" + name, "-" + shortName};
+    }
+  }
+
+  public record Argument(
+      String name,
+      @Nullable String helpText,
+      BiFunction<String, URI, Object> transformEach,
+      Function<List<Object>, Object> transformAll,
+      @Nullable CompletionCandidates completionCandidates,
+      boolean repeated)
+      implements Option {
+    @Override
+    public String[] getNames() {
+      return new String[] {name};
+    }
+  }
+
+  public interface ApplyFunction {
+    State apply(Map<String, Object> options, @Nullable State parent);
+  }
+
+  public record State(Object contents, Function<Object, Result> reify) {
+    public Result evaluate() {
+      return reify.apply(contents);
+    }
+  }
+
+  public record Result(byte[] outputBytes, Map<String, FileOutput> outputFiles) {}
+}

pkl-core/src/main/java/org/pkl/core/Evaluator.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -16,6 +16,7 @@
 package org.pkl.core;
 
 import java.util.Map;
+import java.util.function.Consumer;
 import org.pkl.core.runtime.VmEvalException;
 
 /**
@@ -224,6 +225,20 @@ public interface Evaluator extends AutoCloseable {
    */
   TestResults evaluateTest(ModuleSource moduleSource, boolean overwrite);
 
+  /**
+   * Parses the command module into a spec that describes the CLI options and subcommands.
+   *
+   * <p>This requires that the target module be a {@code "pkl:Command"} instance.
+   *
+   * <p>Unlike other evaluator methods, the resulting {@link CommandSpec} must be handled in a
+   * closure. This is because specs must be applied to parsed CLI options to produce command state
+   * that is eventually evaluated, which must happen in the active context of an evaluator.
+   *
+   * @throws PklException if an error occurs during evaluation
+   * @throws IllegalStateException if this evaluator has already been closed
+   */
+  void evaluateCommand(ModuleSource moduleSource, Consumer<CommandSpec> run);
+
   /**
    * Releases all resources held by this evaluator. If an {@code evaluate} method is currently
    * executing, this method blocks until cancellation of that execution has completed.

pkl-core/src/main/java/org/pkl/core/EvaluatorImpl.java

@@ -20,11 +20,11 @@ import java.io.IOException;
 import java.nio.file.Path;
 import java.time.Duration;
 import java.util.Collection;
-import java.util.LinkedHashMap;
 import java.util.Map;
 import java.util.concurrent.Executors;
 import java.util.concurrent.ScheduledExecutorService;
 import java.util.concurrent.TimeUnit;
+import java.util.function.Consumer;
 import java.util.function.Function;
 import java.util.function.Supplier;
 import org.graalvm.polyglot.Context;
@@ -40,6 +40,7 @@ import org.pkl.core.packages.PackageResolver;
 import org.pkl.core.project.DeclaredDependencies;
 import org.pkl.core.resource.ResourceReader;
 import org.pkl.core.runtime.BaseModule;
+import org.pkl.core.runtime.CommandSpecParser;
 import org.pkl.core.runtime.Identifier;
 import org.pkl.core.runtime.ModuleResolver;
 import org.pkl.core.runtime.ResourceManager;
@@ -48,8 +49,6 @@ import org.pkl.core.runtime.VmContext;
 import org.pkl.core.runtime.VmException;
 import org.pkl.core.runtime.VmExceptionBuilder;
 import org.pkl.core.runtime.VmLanguage;
-import org.pkl.core.runtime.VmMapping;
-import org.pkl.core.runtime.VmNull;
 import org.pkl.core.runtime.VmPklBinaryEncoder;
 import org.pkl.core.runtime.VmStackOverflowException;
 import org.pkl.core.runtime.VmTyped;
@@ -148,7 +147,7 @@ public final class EvaluatorImpl implements Evaluator {
     return doEvaluate(
         moduleSource,
         (module) -> {
-          var output = readModuleOutput(module);
+          var output = VmUtils.readModuleOutput(module);
           return VmUtils.readTextProperty(output);
         });
   }
@@ -157,7 +156,7 @@ public final class EvaluatorImpl implements Evaluator {
     return doEvaluate(
         moduleSource,
         (module) -> {
-          var output = readModuleOutput(module);
+          var output = VmUtils.readModuleOutput(module);
           var vmBytes = VmUtils.readBytesProperty(output);
           return vmBytes.export();
         });
@@ -168,7 +167,7 @@ public final class EvaluatorImpl implements Evaluator {
     return doEvaluate(
         moduleSource,
         (module) -> {
-          var output = readModuleOutput(module);
+          var output = VmUtils.readModuleOutput(module);
           var value = VmUtils.readMember(output, Identifier.VALUE);
           if (value instanceof VmValue vmValue) {
             vmValue.force(false);
@@ -183,20 +182,9 @@ public final class EvaluatorImpl implements Evaluator {
     return doEvaluate(
         moduleSource,
         (module) -> {
-          var output = readModuleOutput(module);
-          var filesOrNull = VmUtils.readMember(output, Identifier.FILES);
-          if (filesOrNull instanceof VmNull) {
-            return Map.of();
-          }
-          var files = (VmMapping) filesOrNull;
-          var result = new LinkedHashMap<String, FileOutput>();
-          files.forceAndIterateMemberValues(
-              (key, member, value) -> {
-                assert member.isEntry();
-                result.put((String) key, new FileOutputImpl(this, (VmTyped) value));
-                return true;
-              });
-          return result;
+          var output = VmUtils.readModuleOutput(module);
+          return VmUtils.readFilesProperty(
+              output, (fileOutput) -> new FileOutputImpl(this, fileOutput));
         });
   }
 
@@ -239,10 +227,10 @@ public final class EvaluatorImpl implements Evaluator {
           var expressionResult =
               switch (expression) {
                 case "module" -> module;
-                case "output.text" -> VmUtils.readTextProperty(readModuleOutput(module));
+                case "output.text" -> VmUtils.readTextProperty(VmUtils.readModuleOutput(module));
                 case "output.value" ->
-                    VmUtils.readMember(readModuleOutput(module), Identifier.VALUE);
-                case "output.bytes" -> VmUtils.readBytesProperty(readModuleOutput(module));
+                    VmUtils.readMember(VmUtils.readModuleOutput(module), Identifier.VALUE);
+                case "output.bytes" -> VmUtils.readBytesProperty(VmUtils.readModuleOutput(module));
                 default ->
                     VmUtils.evaluateExpression(module, expression, securityManager, moduleResolver);
               };
@@ -289,12 +277,27 @@ public final class EvaluatorImpl implements Evaluator {
         });
   }
 
+  @Override
+  public void evaluateCommand(ModuleSource moduleSource, Consumer<CommandSpec> run) {
+    doEvaluate(
+        moduleSource,
+        (module) -> {
+          var commandRunner =
+              new CommandSpecParser(
+                  moduleResolver,
+                  securityManager,
+                  (fileOutput) -> new FileOutputImpl(this, fileOutput));
+          run.accept(commandRunner.parse(module));
+          return null;
+        });
+  }
+
   @Override
   public <T> T evaluateOutputValueAs(ModuleSource moduleSource, PClassInfo<T> classInfo) {
     return doEvaluate(
         moduleSource,
         (module) -> {
-          var output = readModuleOutput(module);
+          var output = VmUtils.readModuleOutput(module);
           var value = VmUtils.readMember(output, Identifier.VALUE);
           var valueClassInfo = VmUtils.getClass(value).getPClassInfo();
           if (valueClassInfo.equals(classInfo)) {
@@ -367,6 +370,9 @@ public final class EvaluatorImpl implements Evaluator {
     } catch (VmException e) {
       handleTimeout(timeoutTask);
       throw e.toPklException(frameTransformer, color);
+    } catch (PklException e) {
+      // evaluateCommand can throw PklException from the CLI layer, pass them through
+      throw e;
     } catch (Exception e) {
       throw new PklBugException(e);
     } catch (ExceptionInInitializerError e) {
@@ -420,32 +426,6 @@ public final class EvaluatorImpl implements Evaluator {
             "evaluationTimedOut", (timeout.getSeconds() + timeout.getNano() / 1_000_000_000d)));
   }
 
-  private VmTyped readModuleOutput(VmTyped module) {
-    var value = VmUtils.readMember(module, Identifier.OUTPUT);
-    if (value instanceof VmTyped typedOutput
-        && typedOutput.getVmClass().getPClassInfo() == PClassInfo.ModuleOutput) {
-      return typedOutput;
-    }
-
-    var moduleUri = module.getModuleInfo().getModuleKey().getUri();
-    var builder =
-        new VmExceptionBuilder()
-            .evalError(
-                "invalidModuleOutput",
-                "output",
-                PClassInfo.ModuleOutput.getDisplayName(),
-                VmUtils.getClass(value).getPClassInfo().getDisplayName(),
-                moduleUri);
-    var outputMember = module.getMember(Identifier.OUTPUT);
-    assert outputMember != null;
-    var uriOfValueMember = outputMember.getSourceSection().getSource().getURI();
-    // If `output` was explicitly re-assigned, show that in the stack trace.
-    if (!uriOfValueMember.equals(PClassInfo.pklBaseUri)) {
-      builder.withSourceSection(outputMember.getBodySection()).withMemberName("output");
-    }
-    throw builder.build();
-  }
-
   private VmException moduleOutputValueTypeMismatch(
       VmTyped module, PClassInfo<?> expectedClassInfo, Object value, VmTyped output) {
     var moduleUri = module.getModuleInfo().getModuleKey().getUri();

pkl-core/src/main/java/org/pkl/core/ast/VmModifier.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -146,6 +146,10 @@ public final class VmModifier {
     return (modifiers & (LOCAL | EXTERNAL | ABSTRACT)) != 0;
   }
 
+  public static boolean isLocalOrExternalOrAbstractOrFixedOrConst(int modifiers) {
+    return (modifiers & (LOCAL | EXTERNAL | ABSTRACT | FIXED | CONST)) != 0;
+  }
+
   public static boolean isConstOrFixed(int modifiers) {
     return (modifiers & (CONST | FIXED)) != 0;
   }

pkl-core/src/main/java/org/pkl/core/ast/expression/literal/AmendModuleNode.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -45,6 +45,10 @@ public abstract class AmendModuleNode extends SpecializedObjectLiteralNode {
     this.moduleInfo = moduleInfo;
   }
 
+  public ModuleInfo getModuleInfo() {
+    return moduleInfo;
+  }
+
   @Override
   @TruffleBoundary
   protected AmendModuleNode copy(ExpressionNode newParentNode) {

pkl-core/src/main/java/org/pkl/core/ast/member/ClassMember.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -16,6 +16,7 @@
 package org.pkl.core.ast.member;
 
 import com.oracle.truffle.api.source.SourceSection;
+import java.util.ArrayList;
 import java.util.List;
 import org.pkl.core.runtime.Identifier;
 import org.pkl.core.runtime.VmClass;
@@ -53,6 +54,33 @@ public abstract class ClassMember extends Member {
     return annotations;
   }
 
+  public List<VmTyped> getAllAnnotations(boolean ascending) {
+    var annotations = new ArrayList<VmTyped>();
+
+    if (ascending) {
+      for (var clazz = getDeclaringClass(); clazz != null; clazz = clazz.getSuperclass()) {
+        var p = clazz.getDeclaredProperty(getName());
+        if (p != null) {
+          annotations.addAll(p.getAnnotations());
+        }
+      }
+    } else {
+      doGetAllAnnotationsDescending(getDeclaringClass(), annotations);
+    }
+
+    return annotations;
+  }
+
+  private void doGetAllAnnotationsDescending(VmClass clazz, List<VmTyped> annotations) {
+    if (clazz.getSuperclass() != null) {
+      doGetAllAnnotationsDescending(clazz.getSuperclass(), annotations);
+    }
+    var p = clazz.getDeclaredProperty(getName());
+    if (p != null) {
+      annotations.addAll(p.getAnnotations());
+    }
+  }
+
   /** Returns the prototype of the class that declares this member. */
   public final VmTyped getOwner() {
     return owner;

pkl-core/src/main/java/org/pkl/core/ast/member/ClassProperty.java

@@ -16,7 +16,6 @@
 package org.pkl.core.ast.member;
 
 import com.oracle.truffle.api.source.SourceSection;
-import java.util.ArrayList;
 import java.util.List;
 import org.pkl.core.Member.SourceLocation;
 import org.pkl.core.PClass;
@@ -54,33 +53,6 @@ public final class ClassProperty extends ClassMember {
     this.initializer = initializer;
   }
 
-  public List<VmTyped> getAllAnnotations(boolean ascending) {
-    var annotations = new ArrayList<VmTyped>();
-
-    if (ascending) {
-      for (var clazz = getDeclaringClass(); clazz != null; clazz = clazz.getSuperclass()) {
-        var p = clazz.getDeclaredProperty(getName());
-        if (p != null) {
-          annotations.addAll(p.getAnnotations());
-        }
-      }
-    } else {
-      doGetAllAnnotationsDescending(getDeclaringClass(), annotations);
-    }
-
-    return annotations;
-  }
-
-  private void doGetAllAnnotationsDescending(VmClass clazz, List<VmTyped> annotations) {
-    if (clazz.getSuperclass() != null) {
-      doGetAllAnnotationsDescending(clazz.getSuperclass(), annotations);
-    }
-    var p = clazz.getDeclaredProperty(getName());
-    if (p != null) {
-      annotations.addAll(p.getAnnotations());
-    }
-  }
-
   public VmSet getAllModifierMirrors() {
     var mods = 0;
     for (var clazz = getDeclaringClass(); clazz != null; clazz = clazz.getSuperclass()) {

pkl-core/src/main/java/org/pkl/core/ast/type/TypeNode.java

@@ -57,6 +57,10 @@ import org.pkl.core.util.Nullable;
 
 public abstract class TypeNode extends PklNode {
 
+  public interface ClassTypeNode {
+    VmClass getVmClass();
+  }
+
   protected TypeNode(SourceSection sourceSection) {
     super(sourceSection);
   }
@@ -402,7 +406,8 @@ public abstract class TypeNode extends PklNode {
   }
 
   /** The `module` type for a final module. */
-  public static final class FinalModuleTypeNode extends ObjectSlotTypeNode {
+  public static final class FinalModuleTypeNode extends ObjectSlotTypeNode
+      implements ClassTypeNode {
     private final VmClass moduleClass;
 
     public FinalModuleTypeNode(SourceSection sourceSection, VmClass moduleClass) {
@@ -456,7 +461,8 @@ public abstract class TypeNode extends PklNode {
   }
 
   /** The `module` type for an open module. */
-  public static final class NonFinalModuleTypeNode extends ObjectSlotTypeNode {
+  public static final class NonFinalModuleTypeNode extends ObjectSlotTypeNode
+      implements ClassTypeNode {
     private final VmClass moduleClass; // only used by getVmClass()
     @Child private ExpressionNode getModuleNode;
 
@@ -641,7 +647,7 @@ public abstract class TypeNode extends PklNode {
    * String/Boolean/Int/Float and their supertypes, only `VmValue`s can possibly pass its type
    * check.
    */
-  public static final class FinalClassTypeNode extends ObjectSlotTypeNode {
+  public static final class FinalClassTypeNode extends ObjectSlotTypeNode implements ClassTypeNode {
     private final VmClass clazz;
 
     public FinalClassTypeNode(SourceSection sourceSection, VmClass clazz) {
@@ -697,7 +703,8 @@ public abstract class TypeNode extends PklNode {
    * String/Boolean/Int/Float and their supertypes, only {@link VmValue}s can possibly pass its type
    * check.
    */
-  public abstract static class NonFinalClassTypeNode extends ObjectSlotTypeNode {
+  public abstract static class NonFinalClassTypeNode extends ObjectSlotTypeNode
+      implements ClassTypeNode {
     protected final VmClass clazz;
 
     public NonFinalClassTypeNode(SourceSection sourceSection, VmClass clazz) {
@@ -1088,6 +1095,14 @@ public abstract class TypeNode extends PklNode {
 
       return unionDefault;
     }
+
+    public Set<String> getStringLiterals() {
+      return stringLiterals;
+    }
+
+    public @Nullable String getUnionDefault() {
+      return unionDefault;
+    }
   }
 
   public static final class CollectionTypeNode extends ObjectSlotTypeNode {
@@ -1421,6 +1436,10 @@ public abstract class TypeNode extends PklNode {
       return BaseModule.getMapClass();
     }
 
+    public TypeNode getKeyTypeNode() {
+      return keyTypeNode;
+    }
+
     public TypeNode getValueTypeNode() {
       return valueTypeNode;
     }
@@ -2131,6 +2150,14 @@ public abstract class TypeNode extends PklNode {
     protected boolean isParametric() {
       return true;
     }
+
+    public TypeNode getFirstTypeNode() {
+      return firstTypeNode;
+    }
+
+    public TypeNode getSecondTypeNode() {
+      return secondTypeNode;
+    }
   }
 
   public static class VarArgsTypeNode extends ObjectSlotTypeNode {
@@ -2313,6 +2340,10 @@ public abstract class TypeNode extends PklNode {
     protected final boolean acceptTypeNode(boolean visitTypeArguments, TypeNodeConsumer consumer) {
       return consumer.accept(this);
     }
+
+    public long getMask() {
+      return mask;
+    }
   }
 
   public static final class UIntTypeAliasTypeNode extends IntMaskSlotTypeNode {
@@ -2505,6 +2536,10 @@ public abstract class TypeNode extends PklNode {
       aliasedTypeNode = typeAlias.instantiate(typeArgumentNodes);
     }
 
+    public TypeNode getAliasedTypeNode() {
+      return aliasedTypeNode;
+    }
+
     @Override
     public FrameSlotKind getFrameSlotKind() {
       return aliasedTypeNode.getFrameSlotKind();
@@ -2670,6 +2705,10 @@ public abstract class TypeNode extends PklNode {
       this.constraintNodes = constraintNodes;
     }
 
+    public TypeNode getChildTypeNode() {
+      return childNode;
+    }
+
     @Override
     public FrameSlotKind getFrameSlotKind() {
       return childNode.getFrameSlotKind();

pkl-core/src/main/java/org/pkl/core/runtime/BaseModule.java

@@ -227,10 +227,26 @@ public final class BaseModule extends StdLibModule {
     return MixinTypeAlias.instance;
   }
 
+  public static VmTypeAlias getUIntTypeAlias() {
+    return UIntTypeAlias.instance;
+  }
+
   public static VmTypeAlias getUInt8TypeAlias() {
     return UInt8TypeAlias.instance;
   }
 
+  public static VmTypeAlias getUInt16TypeAlias() {
+    return UInt16TypeAlias.instance;
+  }
+
+  public static VmTypeAlias getUInt32TypeAlias() {
+    return UInt32TypeAlias.instance;
+  }
+
+  public static VmTypeAlias getCharTypeAlias() {
+    return CharTypeAlias.instance;
+  }
+
   private static final class AnyClass {
     static final VmClass instance = loadClass("Any");
   }
@@ -403,10 +419,26 @@ public final class BaseModule extends StdLibModule {
     static final VmTypeAlias instance = loadTypeAlias("Int32");
   }
 
+  private static final class UIntTypeAlias {
+    static final VmTypeAlias instance = loadTypeAlias("UInt");
+  }
+
   private static final class UInt8TypeAlias {
     static final VmTypeAlias instance = loadTypeAlias("UInt8");
   }
 
+  private static final class UInt16TypeAlias {
+    static final VmTypeAlias instance = loadTypeAlias("UInt16");
+  }
+
+  private static final class UInt32TypeAlias {
+    static final VmTypeAlias instance = loadTypeAlias("UInt32");
+  }
+
+  private static final class CharTypeAlias {
+    static final VmTypeAlias instance = loadTypeAlias("Char");
+  }
+
   private static final class MixinTypeAlias {
     static final VmTypeAlias instance = loadTypeAlias("Mixin");
   }

pkl-core/src/main/java/org/pkl/core/runtime/CommandModule.java

@@ -0,0 +1,95 @@
+/*
+ * Copyright © 2025-2026 Apple Inc. and the Pkl project authors. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.pkl.core.runtime;
+
+import com.oracle.truffle.api.CompilerDirectives.TruffleBoundary;
+import java.net.URI;
+
+public final class CommandModule extends StdLibModule {
+  static final VmTyped instance = VmUtils.createEmptyModule();
+
+  static {
+    loadModule(URI.create("pkl:Command"), instance);
+  }
+
+  private CommandModule() {}
+
+  public static VmTyped getModule() {
+    return instance;
+  }
+
+  public static VmClass getCommandInfoClass() {
+    return CommandInfoClass.instance;
+  }
+
+  public static VmClass getBaseFlagClass() {
+    return BaseFlagClass.instance;
+  }
+
+  public static VmClass getFlagClass() {
+    return FlagClass.instance;
+  }
+
+  public static VmClass getBooleanFlagClass() {
+    return BooleanFlagClass.instance;
+  }
+
+  public static VmClass getCountedFlagClass() {
+    return CountedFlagClass.instance;
+  }
+
+  public static VmClass getArgumentClass() {
+    return ArgumentClass.instance;
+  }
+
+  public static VmClass getImportClass() {
+    return ImportClass.instance;
+  }
+
+  private static final class CommandInfoClass {
+    static final VmClass instance = loadClass("CommandInfo");
+  }
+
+  private static final class BaseFlagClass {
+    static final VmClass instance = loadClass("BaseFlag");
+  }
+
+  private static final class FlagClass {
+    static final VmClass instance = loadClass("Flag");
+  }
+
+  private static final class BooleanFlagClass {
+    static final VmClass instance = loadClass("BooleanFlag");
+  }
+
+  private static final class CountedFlagClass {
+    static final VmClass instance = loadClass("CountedFlag");
+  }
+
+  private static final class ArgumentClass {
+    static final VmClass instance = loadClass("Argument");
+  }
+
+  private static final class ImportClass {
+    static final VmClass instance = loadClass("Import");
+  }
+
+  @TruffleBoundary
+  private static VmClass loadClass(String className) {
+    var theModule = getModule();
+    return (VmClass) VmUtils.readMember(theModule, Identifier.get(className));
+  }
+}

pkl-core/src/main/java/org/pkl/core/runtime/Identifier.java

@@ -147,6 +147,22 @@ public final class Identifier implements Comparable<Identifier> {
   // members of pkl.yaml
   public static final Identifier MAX_COLLECTION_ALIASES = get("maxCollectionAliases");
 
+  // members of pkl.Command
+  public static final Identifier OPTIONS = get("options");
+  public static final Identifier PARENT = get("parent");
+  public static final Identifier COMMAND = get("command");
+  public static final Identifier DESCRIPTION = get("description");
+  public static final Identifier HIDE = get("hide");
+  public static final Identifier NOOP = get("noOp");
+  public static final Identifier SUBCOMMANDS = get("subcommands");
+  public static final Identifier SHORT_NAME = get("shortName");
+  public static final Identifier METAVAR = get("metavar");
+  public static final Identifier MULTIPLE = get("multiple");
+  public static final Identifier CONVERT = get("convert");
+  public static final Identifier TRANSFORM_ALL = get("transformAll");
+  public static final Identifier GLOB = get("glob");
+  public static final Identifier COMPLETION_CANDIDATES = get("completionCandidates");
+
   // common in lambdas etc
   public static final Identifier IT = get("it");
 

pkl-core/src/main/java/org/pkl/core/runtime/ModuleCache.java

@@ -92,6 +92,8 @@ public final class ModuleCache {
           return BaseModule.getModule();
         case "Benchmark":
           return BenchmarkModule.getModule();
+        case "Command":
+          return CommandModule.getModule();
         case "jsonnet":
           return JsonnetModule.getModule();
         case "math":

pkl-core/src/main/java/org/pkl/core/runtime/TestRunner.java

@@ -69,7 +69,7 @@ public final class TestRunner {
     var resultsBuilder = new TestResults.Builder(info.getModuleName(), getDisplayUri(info));
 
     try {
-      checkAmendsPklTest(testModule);
+      VmUtils.checkAmends(testModule, TestModule.getModule().getVmClass());
     } catch (VmException v) {
       var error =
           new TestResults.Error(v.getMessage(), v.toPklException(stackFrameTransformer, useColor));
@@ -83,17 +83,6 @@ public final class TestRunner {
     return resultsBuilder.build();
   }
 
-  private void checkAmendsPklTest(VmTyped value) {
-    var testModuleClass = TestModule.getModule().getVmClass();
-    var moduleClass = value.getVmClass();
-    while (moduleClass != testModuleClass) {
-      moduleClass = moduleClass.getSuperclass();
-      if (moduleClass == null) {
-        throw new VmExceptionBuilder().typeMismatch(value, testModuleClass).build();
-      }
-    }
-  }
-
   private TestSectionResults runFacts(VmTyped testModule) {
     var facts = VmUtils.readMember(testModule, Identifier.FACTS);
     if (facts instanceof VmNull) return new TestSectionResults(TestSectionName.FACTS, List.of());

pkl-core/src/main/java/org/pkl/core/runtime/VmMap.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.

pkl-core/src/main/java/org/pkl/core/runtime/VmUtils.java

@@ -30,12 +30,14 @@ import java.net.URI;
 import java.text.DecimalFormat;
 import java.text.DecimalFormatSymbols;
 import java.util.*;
+import java.util.function.Function;
 import java.util.regex.Pattern;
 import java.util.regex.PatternSyntaxException;
 import java.util.stream.Collectors;
 import org.graalvm.polyglot.Context;
 import org.graalvm.polyglot.Engine;
 import org.organicdesign.fp.collections.ImMap;
+import org.pkl.core.FileOutput;
 import org.pkl.core.PClassInfo;
 import org.pkl.core.PObject;
 import org.pkl.core.SecurityManager;
@@ -179,6 +181,49 @@ public final class VmUtils {
     return (VmBytes) VmUtils.readMember(receiver, Identifier.BYTES);
   }
 
+  public static VmTyped readModuleOutput(VmTyped module) {
+    var value = VmUtils.readMember(module, Identifier.OUTPUT);
+    if (value instanceof VmTyped typedOutput
+        && typedOutput.getVmClass().getPClassInfo() == PClassInfo.ModuleOutput) {
+      return typedOutput;
+    }
+
+    var moduleUri = module.getModuleInfo().getModuleKey().getUri();
+    var builder =
+        new VmExceptionBuilder()
+            .evalError(
+                "invalidModuleOutput",
+                "output",
+                PClassInfo.ModuleOutput.getDisplayName(),
+                VmUtils.getClass(value).getPClassInfo().getDisplayName(),
+                moduleUri);
+    var outputMember = module.getMember(Identifier.OUTPUT);
+    assert outputMember != null;
+    var uriOfValueMember = outputMember.getSourceSection().getSource().getURI();
+    // If `output` was explicitly re-assigned, show that in the stack trace.
+    if (!uriOfValueMember.equals(PClassInfo.pklBaseUri)) {
+      builder.withSourceSection(outputMember.getBodySection()).withMemberName("output");
+    }
+    throw builder.build();
+  }
+
+  public static Map<String, FileOutput> readFilesProperty(
+      VmObjectLike receiver, Function<VmTyped, FileOutput> fileOutputFactory) {
+    var filesOrNull = VmUtils.readMember(receiver, Identifier.FILES);
+    if (filesOrNull instanceof VmNull) {
+      return Map.of();
+    }
+    var files = (VmMapping) filesOrNull;
+    var result = new LinkedHashMap<String, FileOutput>();
+    files.forceAndIterateMemberValues(
+        (key, member, value) -> {
+          assert member.isEntry();
+          result.put((String) key, fileOutputFactory.apply((VmTyped) value));
+          return true;
+        });
+    return result;
+  }
+
   @TruffleBoundary
   public static Object readMember(VmObjectLike receiver, Object memberKey) {
     var result = readMemberOrNull(receiver, memberKey);
@@ -855,7 +900,7 @@ public final class VmUtils {
       String expression,
       SecurityManager securityManager,
       ModuleResolver moduleResolver) {
-    var syntheticModule = ModuleKeys.synthetic(URI.create(REPL_TEXT), expression);
+    var syntheticModule = ModuleKeys.synthetic(REPL_TEXT_URI, expression);
     ResolvedModuleKey resolvedModule;
     try {
       resolvedModule = syntheticModule.resolve(securityManager);
@@ -870,7 +915,7 @@ public final class VmUtils {
             source.createSection(0, source.getLength()),
             VmUtils.unavailableSourceSection(),
             null,
-            "repl:text",
+            REPL_TEXT,
             syntheticModule,
             resolvedModule,
             false);
@@ -914,4 +959,20 @@ public final class VmUtils {
   public static String concat(String str1, String str2) {
     return str1 + str2;
   }
+
+  /** Check that a value is a VmTyped and that it inherits from the given class */
+  public static VmTyped checkAmends(Object value, VmClass clazz) {
+    if (!(value instanceof VmTyped typed)) {
+      throw new VmExceptionBuilder().typeMismatch(value, clazz).build();
+    }
+    return checkAmends(typed, clazz);
+  }
+
+  /** Check that a typed value inherits from the given class */
+  public static VmTyped checkAmends(VmTyped value, VmClass clazz) {
+    if (!value.getVmClass().isSubclassOf(clazz)) {
+      throw new VmExceptionBuilder().typeMismatch(value.getVmClass(), clazz).build();
+    }
+    return value;
+  }
 }

pkl-core/src/main/java/org/pkl/core/stdlib/math/MathNodes.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -51,14 +51,14 @@ public final class MathNodes {
   public abstract static class minInt8 extends ExternalPropertyNode {
     @Specialization
     protected long eval(VmTyped self) {
-      return -128;
+      return Byte.MIN_VALUE;
     }
   }
 
   public abstract static class maxInt8 extends ExternalPropertyNode {
     @Specialization
     protected long eval(VmTyped self) {
-      return 127;
+      return Byte.MAX_VALUE;
     }
   }
 

pkl-core/src/main/java/org/pkl/core/util/GlobResolver.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -471,13 +471,15 @@ public final class GlobResolver {
    * Resolves a glob expression.
    *
    * <p>Each pair is the expanded form of the glob pattern, paired with its resolved absolute URI.
+   *
+   * <p>globPattern must be absolute if enclosing module details are null.
    */
   @TruffleBoundary
   public static Map<String, ResolvedGlobElement> resolveGlob(
       SecurityManager securityManager,
       ReaderBase reader,
-      ModuleKey enclosingModuleKey,
-      URI enclosingUri,
+      @Nullable ModuleKey enclosingModuleKey,
+      @Nullable URI enclosingUri,
       String globPattern)
       throws IOException,
           SecurityManagerException,
@@ -486,6 +488,10 @@ public final class GlobResolver {
 
     var result = new LinkedHashMap<String, ResolvedGlobElement>();
     var hasAbsoluteGlob = globPattern.matches("\\w+:.*");
+    if ((enclosingModuleKey == null || enclosingUri == null) && !hasAbsoluteGlob) {
+      throw new PklBugException(
+          "GlobResolver.resolveGlob() callers must check that the glob pattern is absolute if calling with null enclosing module info");
+    }
 
     if (reader.hasHierarchicalUris()) {
       var splitPattern = splitGlobPatternIntoBaseAndWildcards(reader, globPattern, hasAbsoluteGlob);
@@ -493,7 +499,10 @@ public final class GlobResolver {
       var globParts = splitPattern.second;
       // short-circuit for glob pattern with no wildcards (can only match 0 or 1 element)
       if (globParts.length == 0) {
-        var resolvedUri = IoUtils.resolve(reader, enclosingUri, globPattern);
+        var resolvedUri =
+            enclosingUri == null
+                ? URI.create(globPattern)
+                : IoUtils.resolve(reader, enclosingUri, globPattern);
         if (reader.hasElement(securityManager, resolvedUri)) {
           result.put(globPattern, new ResolvedGlobElement(globPattern, resolvedUri, true));
         }
@@ -501,7 +510,10 @@ public final class GlobResolver {
       }
       URI baseUri;
       try {
-        baseUri = IoUtils.resolve(securityManager, enclosingModuleKey, URI.create(basePath));
+        baseUri =
+            enclosingModuleKey == null
+                ? URI.create(basePath)
+                : IoUtils.resolve(securityManager, enclosingModuleKey, URI.create(basePath));
       } catch (URISyntaxException e) {
         // assertion: this is only thrown if the pattern starts with a triple-dot import.
         // the language will throw an error if glob imports is combined with triple-dots.

pkl-core/src/main/resources/org/pkl/core/errorMessages.properties

@@ -1076,3 +1076,54 @@ invalidStringBase64=\
 
 characterCodingException=\
 Invalid bytes for charset "{0}".
+
+commandSubcommandConflict=\
+Command `{0}` has subcommands with conflicting name "{1}".\n\
+Elements of `command.subcommands` must have unique `name` values.
+
+commandMustNotAssignOrAmendProperty=\
+Commands must not assign or amend property `{0}`.
+
+commandOptionNoTypeAnnotation=\
+No type annotation found for `{0}` property.
+
+commandOptionsTypeNotClass=\
+Type annotation `{0}` on `options` property in `pkl:Command` subclass must be a class type.
+
+commandOptionsTypeAbstractClass=\
+Command options class `{0}` may not be abstract.
+
+commandOptionBothFlagAndArgument=\
+Found both `@Flag` and `@Argument` annotations for options property `{0}`.\n\
+\n\
+Only one option type may be specified.
+
+commandOptionTypeNullableWithDefaultValue=\
+Unexpected option property `{0}` with nullable type and default value.\n\
+\n\
+Options with default values must not be nullable.
+
+commandOptionUnsupportedType=\
+Command option property `{0}` has unsupported {1}type `{2}`.
+
+commandOptionUnexpectedDefaultValue=\
+Unexpected default value for `@{1}` property `{0}`.\n\
+\n\
+{1}s may not specify a default value.
+
+commandArgumentUnexpectedNonRepeatedNullableType=\
+Unexpected nullable type for non-collection `@Argument` property `{0}`.\n\
+\n\
+Arguments may not be nullable.
+
+commandArgumentsMultipleRepeated=\
+More than one repeated option annotated with `@Argument` found: `{0}` and `{1}`.\n\
+\n\
+Only one repeated argument is permitted per command.
+
+commandFlagHelpCollision=\
+Flag option `{0}` may not have name "help" or short name "h".
+
+commandFlagInvalidType=\
+Option `{0}` with annotation `@{1}` has invalid type `{2}`.\n\
+Expected type: `{3}`

pkl-core/src/test/files/LanguageSnippetTests/output/errors/cannotFindStdLibModule.err

@@ -9,6 +9,7 @@ Available standard library modules:
 pkl:analyze
 pkl:base
 pkl:Benchmark
+pkl:Command
 pkl:DocPackageInfo
 pkl:DocsiteInfo
 pkl:EvaluatorSettings

pkl-core/src/test/kotlin/org/pkl/core/runtime/CommandSpecParserTest.kt

@@ -0,0 +1,791 @@
+/*
+ * Copyright © 2025-2026 Apple Inc. and the Pkl project authors. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.pkl.core.runtime
+
+import java.net.URI
+import java.nio.file.Path
+import kotlin.io.path.createParentDirectories
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.jupiter.api.Test
+import org.junit.jupiter.api.assertDoesNotThrow
+import org.junit.jupiter.api.assertThrows
+import org.junit.jupiter.api.io.TempDir
+import org.pkl.commons.writeString
+import org.pkl.core.CommandSpec
+import org.pkl.core.Evaluator
+import org.pkl.core.ModuleSource.uri
+import org.pkl.core.PklException
+
+class CommandSpecParserTest {
+  companion object {
+    private val renderOptions =
+      """
+      extends "pkl:Command"
+      import "pkl:Command"
+      
+      options: Options
+      
+      output {
+        value = options
+      }
+      
+    """
+        .trimIndent()
+
+    private val evaluator = Evaluator.preconfigured()
+  }
+
+  @TempDir private lateinit var tempDir: Path
+
+  private fun writePklFile(fileName: String, contents: String): URI {
+    tempDir.resolve(fileName).createParentDirectories()
+    return tempDir.resolve(fileName).writeString(contents).toUri()
+  }
+
+  private fun parse(moduleUri: URI): CommandSpec {
+    var spec: CommandSpec? = null
+    evaluator.evaluateCommand(uri(moduleUri)) { spec = it }
+    return spec!!
+  }
+
+  @Test
+  fun `command module does not amend pkl_Command`() {
+    val moduleUri = writePklFile("cmd.pkl", "")
+
+    val exc = assertThrows<PklException> { parse(moduleUri) }
+    assertThat(exc.message).contains("Expected value of type `pkl.Command`, but got type")
+  }
+
+  @Test
+  fun `options property assigned`() {
+    val moduleUri =
+      writePklFile(
+        "cmd.pkl",
+        """
+          extends "pkl:Command"
+          options = new {}
+        """
+          .trimIndent(),
+      )
+
+    val exc = assertThrows<PklException> { parse(moduleUri) }
+    assertThat(exc.message).contains("options = ")
+    assertThat(exc.message).contains("Commands must not assign or amend property `options`.")
+  }
+
+  @Test
+  fun `options property amended`() {
+    val moduleUri =
+      writePklFile(
+        "cmd.pkl",
+        """
+          extends "pkl:Command"
+          options {}
+        """
+          .trimIndent(),
+      )
+
+    val exc = assertThrows<PklException> { parse(moduleUri) }
+    assertThat(exc.message).contains("options {")
+    assertThat(exc.message).contains("Commands must not assign or amend property `options`.")
+  }
+
+  @Test
+  fun `parent property assigned`() {
+    val moduleUri =
+      writePklFile(
+        "cmd.pkl",
+        """
+          extends "pkl:Command"
+          parent = new {}
+        """
+          .trimIndent(),
+      )
+
+    val exc = assertThrows<PklException> { parse(moduleUri) }
+    assertThat(exc.message).contains("parent = ")
+    assertThat(exc.message).contains("Commands must not assign or amend property `parent`.")
+  }
+
+  @Test
+  fun `parent property amended`() {
+    val moduleUri =
+      writePklFile(
+        "cmd.pkl",
+        """
+          extends "pkl:Command"
+          parent {}
+        """
+          .trimIndent(),
+      )
+
+    val exc = assertThrows<PklException> { parse(moduleUri) }
+    assertThat(exc.message).contains("parent {")
+    assertThat(exc.message).contains("Commands must not assign or amend property `parent`.")
+  }
+
+  @Test
+  fun `options type annotation does not reference class`() {
+    val moduleUri =
+      writePklFile(
+        "cmd.pkl",
+        """
+          extends "pkl:Command"
+          options: "nope" | "try again"
+        """
+          .trimIndent(),
+      )
+
+    val exc = assertThrows<PklException> { parse(moduleUri) }
+    assertThat(exc.message).contains("options: \"nope\" | \"try again\"")
+    assertThat(exc.message)
+      .contains(
+        "Type annotation `\"nope\" | \"try again\"` on `options` property in `pkl:Command` subclass must be a class type."
+      )
+  }
+
+  @Test
+  fun `options class is abstract`() {
+    val moduleUri =
+      writePklFile(
+        "cmd.pkl",
+        """
+          extends "pkl:Command"
+          options: Options
+          abstract class Options {}
+        """
+          .trimIndent(),
+      )
+
+    val exc = assertThrows<PklException> { parse(moduleUri) }
+    assertThat(exc.message).contains("abstract class Options {")
+    assertThat(exc.message).contains("Command options class `cmd#Options` may not be abstract.")
+  }
+
+  @Test
+  fun `command property value does not amend CommandInfo`() {
+    val moduleUri =
+      writePklFile(
+        "cmd.pkl",
+        """
+          extends "pkl:Command"
+          command = new Foo {}
+          class Foo
+        """
+          .trimIndent(),
+      )
+
+    val exc = assertThrows<PklException> { parse(moduleUri) }
+    assertThat(exc.message).contains("command = new Foo {}")
+    assertThat(exc.message)
+      .contains("Expected value of type `pkl.Command#CommandInfo`, but got type `cmd#Foo`.")
+  }
+
+  @Test
+  fun `first annotation of the same type wins`() {
+    val moduleUri =
+      writePklFile(
+        "cmd.pkl",
+        renderOptions +
+          """
+          open class BaseOptions {
+            /// foo in BaseOptions
+            @Flag { shortName = "a" }
+            foo: String
+            
+            /// bar in BaseOptions
+            @Flag { shortName = "b" }
+            bar: String
+          }
+          class Options extends BaseOptions {
+            /// bar in Options
+            @Flag { shortName = "x" }
+            bar: String
+            
+            /// baz in Options
+            @Flag { shortName = "y" }
+            @CountedFlag { shortName = "z" }
+            baz: Int
+          }
+        """
+            .trimIndent(),
+      )
+
+    val spec = parse(moduleUri)
+
+    // assert class overrides its superclass
+    assertThat(spec.options.toList()[0]).isInstanceOf(CommandSpec.Flag::class.java)
+    (spec.options.toList()[0] as CommandSpec.Flag).apply {
+      assertThat(this.name).isEqualTo("bar")
+      assertThat(this.shortName).isEqualTo("x")
+      assertThat(this.helpText).isEqualTo("bar in Options")
+    }
+
+    // assert first flag annotation wins
+    assertThat(spec.options.toList()[1]).isInstanceOf(CommandSpec.Flag::class.java)
+    (spec.options.toList()[1] as CommandSpec.Flag).apply {
+      assertThat(this.name).isEqualTo("baz")
+      assertThat(this.shortName).isEqualTo("y")
+      assertThat(this.helpText).isEqualTo("baz in Options")
+    }
+
+    // assert superclass options are inherited
+    assertThat(spec.options.toList()[2]).isInstanceOf(CommandSpec.Flag::class.java)
+    (spec.options.toList()[2] as CommandSpec.Flag).apply {
+      assertThat(this.name).isEqualTo("foo")
+      assertThat(this.shortName).isEqualTo("a")
+      assertThat(this.helpText).isEqualTo("foo in BaseOptions")
+    }
+  }
+
+  @Test
+  fun `@Flag and @Argument on the same option`() {
+    val moduleUri =
+      writePklFile(
+        "cmd.pkl",
+        renderOptions +
+          """
+      class Options {
+        @Flag
+        @Argument
+        foo: String
+      }
+    """
+            .trimIndent(),
+      )
+
+    val exc = assertThrows<PklException> { parse(moduleUri) }
+    assertThat(exc.message).contains("foo: String")
+    assertThat(exc.message)
+      .contains("Found both `@Flag` and `@Argument` annotations for options property `foo`.")
+  }
+
+  @Test
+  fun `option with no type annotation`() {
+    val moduleUri =
+      writePklFile(
+        "cmd.pkl",
+        renderOptions +
+          """
+      class Options {
+        foo = "bar"
+      }
+    """
+            .trimIndent(),
+      )
+
+    val exc = assertThrows<PklException> { parse(moduleUri) }
+    assertThat(exc.message).contains("foo = \"bar\"")
+    assertThat(exc.message).contains("No type annotation found for `foo` property.")
+  }
+
+  @Test
+  fun `nullable option with default not allowed`() {
+    val moduleUri =
+      writePklFile(
+        "cmd.pkl",
+        renderOptions +
+          """
+      class Options {
+        foo: String? = "bar"
+      }
+    """
+            .trimIndent(),
+      )
+
+    val exc = assertThrows<PklException> { parse(moduleUri) }
+    assertThat(exc.message).contains("foo: String? = \"bar\"")
+    assertThat(exc.message)
+      .contains("Unexpected option property `foo` with nullable type and default value")
+  }
+
+  @Test
+  fun `option with union type containing non-string-literals`() {
+    val moduleUri =
+      writePklFile(
+        "cmd.pkl",
+        renderOptions +
+          """
+      class Options {
+        foo: "oops" | String
+      }
+    """
+            .trimIndent(),
+      )
+
+    val exc = assertThrows<PklException> { parse(moduleUri) }
+    assertThat(exc.message).contains("foo: \"oops\" | String")
+    assertThat(exc.message)
+      .contains("Command option property `foo` has unsupported type `\"oops\" | String`.")
+  }
+
+  @Test
+  fun `argument with default not allowed`() {
+    val moduleUri =
+      writePklFile(
+        "cmd.pkl",
+        renderOptions +
+          """
+      class Options {
+        @Argument
+        foo: String = "bar"
+      }
+    """
+            .trimIndent(),
+      )
+
+    val exc = assertThrows<PklException> { parse(moduleUri) }
+    assertThat(exc.message).contains("foo: String = \"bar\"")
+    assertThat(exc.message).contains("Unexpected default value for `@Argument` property `foo`.")
+  }
+
+  @Test
+  fun `nullable non-collection argument not allowed`() {
+    val moduleUri =
+      writePklFile(
+        "cmd.pkl",
+        renderOptions +
+          """
+      class Options {
+        @Argument
+        foo: String?
+      }
+    """
+            .trimIndent(),
+      )
+
+    val exc = assertThrows<PklException> { parse(moduleUri) }
+    assertThat(exc.message).contains("foo: String?")
+    assertThat(exc.message)
+      .contains("Unexpected nullable type for non-collection `@Argument` property `foo`.")
+  }
+
+  @Test
+  fun `non-constant default values result in an optional flag with no default`() {
+    val moduleUri =
+      writePklFile(
+        "cmd.pkl",
+        renderOptions +
+          """
+          class Options {
+            foo: String = "hi"
+            bar: String = foo
+            baz: Map<String, String> = Map()
+            qux: Map<String, String> = baz
+            quux: Int = 5
+          }
+        """
+            .trimIndent(),
+      )
+
+    val spec = parse(moduleUri)
+
+    assertThat(spec.options.toList()[0]).isInstanceOf(CommandSpec.Flag::class.java)
+    (spec.options.toList()[0] as CommandSpec.Flag).apply {
+      assertThat(this.name).isEqualTo("foo")
+      assertThat(this.defaultValue).isEqualTo("hi")
+    }
+
+    assertThat(spec.options.toList()[1]).isInstanceOf(CommandSpec.Flag::class.java)
+    (spec.options.toList()[1] as CommandSpec.Flag).apply {
+      assertThat(this.name).isEqualTo("bar")
+      assertThat(this.defaultValue).isNull()
+    }
+
+    assertThat(spec.options.toList()[2]).isInstanceOf(CommandSpec.Flag::class.java)
+    (spec.options.toList()[2] as CommandSpec.Flag).apply {
+      assertThat(this.name).isEqualTo("baz")
+      assertThat(this.defaultValue).isNull()
+    }
+
+    assertThat(spec.options.toList()[3]).isInstanceOf(CommandSpec.Flag::class.java)
+    (spec.options.toList()[3] as CommandSpec.Flag).apply {
+      assertThat(this.name).isEqualTo("qux")
+      assertThat(this.defaultValue).isNull()
+    }
+
+    assertThat(spec.options.toList()[4]).isInstanceOf(CommandSpec.Flag::class.java)
+    (spec.options.toList()[4] as CommandSpec.Flag).apply {
+      assertThat(this.name).isEqualTo("quux")
+      assertThat(this.defaultValue).isEqualTo("5")
+    }
+  }
+
+  @Test
+  fun `flag with collision on --help`() {
+    val moduleUri =
+      writePklFile(
+        "cmd.pkl",
+        renderOptions +
+          """
+      class Options {
+        help: Boolean
+      }
+    """
+            .trimIndent(),
+      )
+
+    val exc = assertThrows<PklException> { parse(moduleUri) }
+    assertThat(exc.message).contains("help: Boolean")
+    assertThat(exc.message)
+      .contains("Flag option `help` may not have name \"help\" or short name \"h\".")
+  }
+
+  @Test
+  fun `flag with collision on -h`() {
+    val moduleUri =
+      writePklFile(
+        "cmd.pkl",
+        renderOptions +
+          """
+      class Options {
+        @Flag { shortName = "h" }
+        showHelp: Boolean
+      }
+    """
+            .trimIndent(),
+      )
+
+    val exc = assertThrows<PklException> { parse(moduleUri) }
+    assertThat(exc.message).contains("showHelp: Boolean")
+    assertThat(exc.message)
+      .contains("Flag option `showHelp` may not have name \"help\" or short name \"h\".")
+  }
+
+  @Test
+  fun `multiple arguments with collection types not allowed`() {
+    val moduleUri =
+      writePklFile(
+        "cmd.pkl",
+        renderOptions +
+          """
+      class Options {
+        @Argument
+        list: List<String>
+        @Argument
+        set: Set<String>
+      }
+    """
+            .trimIndent(),
+      )
+
+    val exc = assertThrows<PklException> { parse(moduleUri) }
+    assertThat(exc.message).contains("class Options {")
+    assertThat(exc.message)
+      .contains("More than one repeated option annotated with `@Argument` found: `list` and `set`.")
+    assertThat(exc.message).contains("Only one repeated argument is permitted per command.")
+  }
+
+  @Test
+  fun `collection option with collection element type`() {
+    val moduleUri =
+      writePklFile(
+        "cmd.pkl",
+        renderOptions +
+          """
+      class Options {
+        foo: List<List<"a" | "b">>
+      }
+    """
+            .trimIndent(),
+      )
+
+    val exc = assertThrows<PklException> { parse(moduleUri) }
+    assertThat(exc.message).contains("foo: List<List<\"a\" | \"b\">>")
+    assertThat(exc.message)
+      .contains("Command option property `foo` has unsupported element type `List<\"a\" | \"b\">`.")
+  }
+
+  @Test
+  fun `collection option with map element type`() {
+    val moduleUri =
+      writePklFile(
+        "cmd.pkl",
+        renderOptions +
+          """
+      class Options {
+        foo: List<Map<String, "a" | "b">>
+      }
+    """
+            .trimIndent(),
+      )
+
+    val exc = assertThrows<PklException> { parse(moduleUri) }
+    assertThat(exc.message).contains("foo: List<Map<String, \"a\" | \"b\">>")
+    assertThat(exc.message)
+      .contains(
+        "Command option property `foo` has unsupported element type `Map<String, \"a\" | \"b\">`."
+      )
+  }
+
+  @Test
+  fun `map option with collection value type`() {
+    val moduleUri =
+      writePklFile(
+        "cmd.pkl",
+        renderOptions +
+          """
+      class Options {
+        foo: Map<String, List<"a" | "b">>
+      }
+    """
+            .trimIndent(),
+      )
+
+    val exc = assertThrows<PklException> { parse(moduleUri) }
+    assertThat(exc.message).contains("foo: Map<String, List<\"a\" | \"b\">>")
+    assertThat(exc.message)
+      .contains("Command option property `foo` has unsupported value type `List<\"a\" | \"b\">`.")
+  }
+
+  @Test
+  fun `map option with map value type`() {
+    val moduleUri =
+      writePklFile(
+        "cmd.pkl",
+        renderOptions +
+          """
+      class Options {
+        foo: Map<String, Map<String, "a" | "b">>
+      }
+    """
+            .trimIndent(),
+      )
+
+    val exc = assertThrows<PklException> { parse(moduleUri) }
+    assertThat(exc.message).contains("foo: Map<String, Map<String, \"a\" | \"b\">>")
+    assertThat(exc.message)
+      .contains(
+        "Command option property `foo` has unsupported value type `Map<String, \"a\" | \"b\">`."
+      )
+  }
+
+  @Test
+  fun `map option with collection key type`() {
+    val moduleUri =
+      writePklFile(
+        "cmd.pkl",
+        renderOptions +
+          """
+      class Options {
+        foo: Map<Map<String, "a" | "b">, String>
+      }
+    """
+            .trimIndent(),
+      )
+
+    val exc = assertThrows<PklException> { parse(moduleUri) }
+    assertThat(exc.message).contains("foo: Map<Map<String, \"a\" | \"b\">, String>")
+    assertThat(exc.message)
+      .contains(
+        "Command option property `foo` has unsupported key type `Map<String, \"a\" | \"b\">`."
+      )
+  }
+
+  @Test
+  fun `map option with map key type`() {
+    val moduleUri =
+      writePklFile(
+        "cmd.pkl",
+        renderOptions +
+          """
+      class Options {
+        foo: Map<Map<String, "a" | "b">, String>
+      }
+    """
+            .trimIndent(),
+      )
+
+    val exc = assertThrows<PklException> { parse(moduleUri) }
+    assertThat(exc.message).contains("foo: Map<Map<String, \"a\" | \"b\">, String>")
+    assertThat(exc.message)
+      .contains(
+        "Command option property `foo` has unsupported key type `Map<String, \"a\" | \"b\">`."
+      )
+  }
+
+  @Test
+  fun `map option with map key type allowed with convert`() {
+    val moduleUri =
+      writePklFile(
+        "cmd.pkl",
+        renderOptions +
+          """
+      class Options {
+        @Flag { convert = (it) -> Pair("foo", "a") }
+        foo: Map<Map<String, "a" | "b">, String>
+      }
+    """
+            .trimIndent(),
+      )
+
+    assertDoesNotThrow { parse(moduleUri) }
+  }
+
+  @Test
+  fun `unsupported option type`() {
+    val moduleUri =
+      writePklFile(
+        "cmd.pkl",
+        renderOptions +
+          """
+      class Options {
+        foo: Foo
+      }
+      class Foo
+    """
+            .trimIndent(),
+      )
+
+    val exc = assertThrows<PklException> { parse(moduleUri) }
+    assertThat(exc.message).contains("foo: Foo")
+    assertThat(exc.message).contains("Command option property `foo` has unsupported type `Foo`.")
+  }
+
+  @Test
+  fun `options constraints in all positions are erased`() {
+    val moduleUri =
+      writePklFile(
+        "cmd.pkl",
+        renderOptions +
+          """
+      class Options {
+        a: String(true)
+        b: String?(true)
+        c: String(true)?
+        d: List<String(true)>
+        e: List<String(true)>(true)
+        f: List<String(true)>(true)?(true)
+        g: (Map<String(true), String(true)>(true)?(true))(true)
+      }
+    """
+            .trimIndent(),
+      )
+
+    parse(moduleUri)
+  }
+
+  @Test
+  fun `conflicting subcommand names`() {
+    val moduleUri =
+      writePklFile(
+        "cmd.pkl",
+        """
+      extends "pkl:Command"
+      import "pkl:Command"
+      command {
+        subcommands {
+          new Sub { command { name = "foo" } }
+          new Sub { command { name = "foo" } }
+        }
+      }
+      
+      class Sub extends Command
+    """
+          .trimIndent(),
+      )
+
+    val exc = assertThrows<PklException> { parse(moduleUri) }
+    assertThat(exc.message).contains("Command `cmd` has subcommands with conflicting name \"foo\".")
+  }
+
+  @Test
+  fun `list or set option with no type arguments`() {
+    for (type in listOf("List", "Set")) {
+      val moduleUri =
+        writePklFile(
+          "cmd_$type.pkl",
+          renderOptions +
+            """
+      class Options {
+        foo: $type
+      }
+    """
+              .trimIndent(),
+        )
+
+      val exc = assertThrows<PklException> { parse(moduleUri) }
+      assertThat(exc.message).contains("foo: $type")
+      assertThat(exc.message)
+        .contains("Command option property `foo` has unsupported type `$type`.")
+      assertThat(exc.message).contains("$type options must provide one type argument.")
+    }
+  }
+
+  @Test
+  fun `map option with no type arguments`() {
+    val moduleUri =
+      writePklFile(
+        "cmd.pkl",
+        renderOptions +
+          """
+      class Options {
+        foo: Map
+      }
+    """
+            .trimIndent(),
+      )
+
+    val exc = assertThrows<PklException> { parse(moduleUri) }
+    assertThat(exc.message).contains("foo: Map")
+    assertThat(exc.message).contains("Command option property `foo` has unsupported type `Map`.")
+    assertThat(exc.message).contains("Map options must provide two type arguments.")
+  }
+
+  @Test
+  fun `boolean flag with incorrect type`() {
+    val moduleUri =
+      writePklFile(
+        "cmd.pkl",
+        renderOptions +
+          """
+      class Options {
+        @BooleanFlag
+        foo: String
+      }
+    """
+            .trimIndent(),
+      )
+
+    val exc = assertThrows<PklException> { parse(moduleUri) }
+    assertThat(exc.message).contains("foo: String")
+    assertThat(exc.message)
+      .contains("Option `foo` with annotation `@BooleanFlag` has invalid type `String`.")
+    assertThat(exc.message).contains("Expected type: `Boolean`")
+  }
+
+  @Test
+  fun `counted flag with incorrect type`() {
+    val moduleUri =
+      writePklFile(
+        "cmd.pkl",
+        renderOptions +
+          """
+      class Options {
+        @CountedFlag
+        foo: String
+      }
+    """
+            .trimIndent(),
+      )
+
+    val exc = assertThrows<PklException> { parse(moduleUri) }
+    assertThat(exc.message).contains("foo: String")
+    assertThat(exc.message)
+      .contains("Option `foo` with annotation `@CountedFlag` has invalid type `String`.")
+    assertThat(exc.message).contains("Expected type: `Int`")
+  }
+}

stdlib/Command.pkl

@@ -0,0 +1,290 @@
+//===----------------------------------------------------------------------===//
+// Copyright © 2026 Apple Inc. and the Pkl project authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//===----------------------------------------------------------------------===//
+
+/// Defines inputs and outputs for CLI commands implemented in Pkl.
+///
+/// Modules extending `pkl:Command` may configure [ModuleOutput.text], [ModuleOutput.bytes], and/or
+/// [ModuleOutput.files] of [output] to influence the effect of the command.
+///
+/// Command modules should override [options] and provide their own class declaring options:
+/// ```
+/// extends "pkl:Command"
+///
+/// options: Options
+///
+/// class Options {
+///   // ...
+/// }
+/// ```
+@ModuleInfo { minPklVersion = "0.31.0" }
+open module pkl.Command
+
+import "pkl:Command"
+import "pkl:reflect"
+
+local commandClass = reflect.Class(getClass())
+
+/// Command configuration.
+hidden command: CommandInfo = new {
+  name = // choose the name of the module if the command is a module or the class otherwise
+    if (commandClass.name == "ModuleClass") reflect.Module(module).name else commandClass.name
+  description = commandClass.docComment // works for both classes and module classes
+}
+
+/// Command line options.
+///
+/// This property is set by the runtime during command execution.
+/// It must not be amended or overridden by modules or classes extending [Command].
+///
+/// Command modules should override this property and provide their own options type.
+/// The properties of the specified type declare the command line flags and arguments accepted by
+/// the command.
+///
+/// Example:
+/// ```
+/// extends "pkl:Command"
+///
+/// options: Options
+///
+/// class Options {
+///   /// Maximum number of tries to attempt operation before giving up.
+///   `max-tries`: UInt = 3
+///
+///   /// Duration after which operation will be timed out.
+///   @Flag { convert = module.convertDuration; metavar = "duration" }
+///   timeout: Duration = 30.s
+///
+///   /// Whether to use cache data locally.
+///   @BooleanFlag
+///   cache: Boolean = true
+///
+///   /// Log verbosity.
+///   @CountedFlag { shortName = "v" }
+///   verbose: Int
+///
+///   /// File paths to operate on.
+///   @Argument { completionCandidates = "paths" }
+///   path: Listing<String>
+/// }
+/// ```
+hidden options: Typed
+
+/// The value of the parent command with parsed options.
+///
+/// This property is set by the runtime during command execution.
+/// It must not be amended or overridden by modules or classes extending [Command].
+///
+/// The parent command is `null` for root commands.
+hidden parent: Command?
+
+/// The value of the root command with parsed options.
+///
+/// This property is set by the runtime during command execution.
+///
+/// The root command is `null` for root commands.
+hidden fixed root: Command? = parent?.root ?? parent
+
+/// Command configuration.
+class CommandInfo {
+  /// The name of the subcommand.
+  ///
+  /// Default value: the name of the module or class extending [Command].
+  name: String
+
+  /// A description of the command; shown in its CLI help.
+  ///
+  /// Default value: the doc comment of the module or class extending [Command].
+  description: String?
+
+  /// Hide this command from CLI help.
+  hide: Boolean = false
+
+  /// If this command is executed, return an error and print CLI usage.
+  ///
+  /// Only applicable to commands with [subcommands].
+  ///
+  /// This is enabled by default when this command has [subcommands].
+  /// Overriding it to `false` will allow this command to be executed directly.
+  noOp: Boolean(implies(!subcommands.isEmpty)) = !subcommands.isEmpty
+
+  /// Child commands.
+  ///
+  /// Must have unique [name] values.
+  // NB: not using isDistinctBy constraint because the command runtime can give better errors
+  subcommands: Listing<Command>
+}
+
+/// Annotates [options] properties to configure them as named CLI flags.
+// NB: this should be a sealed class once Pkl supports them
+abstract class BaseFlag extends Annotation {
+  /// Abbreviated flag name.
+  shortName: Char?
+
+  /// Hide this option from CLI help.
+  hide: Boolean = false
+}
+
+/// Annotates an [options] property to configure it as a named CLI flag that accepts a value.
+class Flag extends BaseFlag {
+  /// Text to use in place of the option value in CLI help.
+  ///
+  /// If not specified, the value is derived from the flag's type.
+  metavar: String?
+
+  /// Customize the behavior of parsing the raw option values.
+  ///
+  /// When the return value is an [Import] value or a [Pair] member, [List] or [Set] element
+  /// containing an [Import], the URI or glob URI specified by the value is imported and the value
+  /// is replaced with the value of the imported module(s).
+  ///
+  /// If no transform is provided, the raw flag value are parsed according to the option's type:
+  ///
+  /// | Type                       | Behavior |
+  /// | -------------------------- | -------- |
+  /// | [String]                   | Value is used verbatim |
+  /// | [Char]                     | Value is used verbatim; must be exactly one character |
+  /// | [Boolean]                  | True values: `true`, `t`, `1`, `yes`, `y`, `on`; False values: `false`, `f`, `0`, `no`, `n`, `off` |
+  /// | [Number]                   | Value is parsed as an [Int] if possible, otherwise parsed as [Float] |
+  /// | [Float]                    | Value is parsed as a [Float] |
+  /// | [Int]                      | Value is parsed as an [Int] |
+  /// | [Int8], [Int16], [Int32], [UInt], [UInt8], [UInt16], [UInt32] | Value is parsed as an [Int] and must be within the type's range |
+  /// | Union of [String] literals | Value is used verbatim; must match a member of the union |
+  /// | [Listing], [List], [Set]   | Element values are parsed based on the above primitive types |
+  /// | [Mapping], [Map], [Pair]   | Value is split into a [Pair] on the first `"="` and each substring is parsed based on the above primitive types |
+  /// | Other types                | An error is thrown; `convert` should be defined explicitly |
+  convert: ((String) -> Any)?
+
+  /// Specifies whether the flag may be specified more than once.
+  ///
+  /// If not specified, this is determined based on the option's type.
+  /// Options with type [Listing], [List], [Set], [Mapping], or [Map] are mulitple by default.
+  /// Overriding this behavior generally requires setting [convert] and/or [transformAll].
+  multiple: Boolean?
+
+  /// Customize the behavior of turning all parsed flag values into the final option value.
+  ///
+  /// If no value is provided, all flag values are transformed according to the option's type:
+  ///
+  /// | Type              | Behavior |
+  /// | ----------------- | -------- |
+  /// | [Mapping], [Map]  | Each value must be a [Pair], each pair becomes an entry in the result. |
+  /// | [Listing], [List] | Result is all option values in the order specified |
+  /// | [Set]             | Result is all unique option values |
+  /// | Other types       | Result is the last value specified option value |
+  transformAll: ((List<Any>) -> Any)?
+
+  /// Specify how this flag should be completed in generated shell completions.
+  ///
+  /// If set to `"paths"`, the completion candidates will be local file paths. 
+  /// If set to a [Listing], the completion candidates will be the specified literal strings.
+  ///
+  /// Options with a string literal union type use the members of the union as completion candidates
+  /// by default.
+  completionCandidates: "paths" | *Listing<String>(isDistinct)
+}
+
+/// Annotates [Boolean] [options] properties to configure them as named CLI flags that may be
+/// specified zero or one times.
+///
+/// Boolean flags produce a pair of flags in the form `--<name>`/`--no-<name>`.
+///
+/// Annotating a property with a type other than [Boolean] is an error.
+class BooleanFlag extends BaseFlag
+
+/// Annotates [Integer] [options] properties to configure them as named CLI flags that may be
+/// specified zero or more times.
+///
+/// Counted flags produce a value equal to the number of times a flag is specified.
+///
+/// Annotating a property with a type other than [Int] is an error.
+class CountedFlag extends BaseFlag
+
+/// Annotates an [options] property to configure it as a positional CLI argument.
+class Argument extends Annotation {
+  /// Customize the behavior of turning the raw option values string into the final value.
+  ///
+  /// When the return value is an [Import] value or a [Pair] member, [List] or [Set] element
+  /// containing an [Import], the URI or glob URI specified by the value is imported and the value
+  /// is replaced with the value of the imported module(s).
+  ///
+  /// If no value is provided, each option value is transformed using the same rules as
+  /// [Flag.convert].
+  convert: ((String) -> Any)?
+
+  /// Specifies whether the argument may be specified more than once.
+  ///
+  /// If not specified, this is determined based on the option's type.
+  /// Options with type [Listing], [List], [Set], [Mapping], or [Map] are multiple by default.
+  /// Overriding this behavior generally requires setting [convert] and/or [transformAll].
+  ///
+  /// Only one argument per command may be multiple.
+  multiple: Boolean?
+
+  /// Customize the behavior of turning all parsed flag values into the final option value.
+  ///
+  /// If no value is provided, all option values are transformed using the same rules as
+  /// [Flag.transformAll].
+  transformAll: ((List<Any>) -> Any)?
+
+  /// Specify how this flag should be completed in generated shell completions.
+  ///
+  /// If set to `"paths"`, the completion candidates will be local file paths. 
+  /// If set to a [Listing], the completion candidates will be the specified literal strings.
+  ///
+  /// Options with a string literal union type use the members of the union as completion candidates
+  /// by default.
+  completionCandidates: "paths" | *Listing<String>(isDistinct)
+}
+
+/// A value used in [Flag.convert] and [Argument.convert] to declare an option as a dynamic
+/// import.
+class Import {
+  /// The absolute URI of the module to import.
+  uri: String
+
+  /// Whether [uri] should be interpreted as a glob pattern.
+  ///
+  /// When `false`, the replacement value is the value of the specified module.
+  /// When `true`, the replacement value is a [Mapping] from [String] keys to matched module values. 
+  glob: Boolean = false
+}
+
+local const quantityRegex = Regex(#"([0-9]+(?:\.[0-9]+)?)\.?([A-Za-z]+)"#)
+
+local const function parseQuantity(value: String, typeName: String): Pair<Float, String> =
+  let (match = quantityRegex.matchEntire(value))
+    if (match == null)
+      throw("Unable to parse \(typeName) from string '\(value)'")
+    else
+      Pair(match.groups[1].value.toFloat(), match.groups[2].value.toLowerCase())
+
+/// A convert function for [Duration] values.
+///
+/// For use with [Flag.convert] and [Argument.convert].
+hidden const convertDuration: (String) -> Duration = (value: String) ->
+  let (quantity = parseQuantity(value, "Duration"))
+  let (_unit = quantity.second)
+  let (unit = if (_unit is DurationUnit) _unit else null)
+    quantity.first.toDuration(unit ?? throw("Unable to parse DurationUnit from '\(_unit)'"))
+
+/// A convert function for [DataSize] values.
+///
+/// For use with [Flag.convert] and [Argument.convert].
+hidden const convertDataSize: (String) -> DataSize = (value: String) ->
+  let (quantity = parseQuantity(value, "DataSize"))
+  let (_unit = quantity.second)
+  let (unit = if (_unit is DataSizeUnit) _unit else null)
+    quantity.first.toDataSize(unit ?? throw("Unable to parse DataSizeUnit from '\(_unit)'"))

stdlib/test.pkl

@@ -17,7 +17,7 @@
 /// A template for writing tests.
 ///
 /// To write tests, amend this module and define [facts] or [examples] (or both).
-/// To run tests, evaluate the amended module.
+/// To run tests, use the `pkl test` command to evaluate the amended module.
 @ModuleInfo { minPklVersion = "0.31.0" }
 open module pkl.test