Bring the plugin API and the Gradle plugin from the legacy Tazel

Tazel Plugins stay, built with Gradle and published to Tazel Portal.
The api/ (com.progmasoft.tazel:tazel-api) and gradle-plugin/
(com.progmasoft.tazel.plugin) projects come from Progmasoft/tazel-legacy
with their tests, with these changes:

- tazelPlugin { } holds group, name and version. The descriptor
  META-INF/tazel-plugin.properties records them first, so the JAR says
  who the plugin is; before, ViGet recorded the publisher, name and
  version at push time. The group must be a reversed domain of at least
  two parts (Tazel Portal checks the namespace by the Sonatype rules),
  the name letters, digits and _ . -, and the version a Semantic
  Version. The version of the Gradle project is not the plugin's.
- The API version starts again at 1, so plugins built for the legacy
  Tazel (API 3) are not loaded.
- Documentation no longer speaks of ViGet, Tala targets or
  tazel.tala.run; the site is tazel.net.
- Dependency locking of the legacy build is not carried over, like in
  the rest of this build.

The root build includes both projects, so ./gradlew check runs the
Gradle TestKit tests of the plugin as well.

13 files changed, 879 insertions, 2 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 532172f..569451d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -14,3 +14,6 @@ Versions follow [Semantic Versioning](https://semver.org). The earlier Tazel, 0.
   `c`, `cpp`, `objc`, `objcpp` and `zig` targets of the four kinds, with `isPublic` per file. A setting that a
   language or kind does not have is a compile error of the project file. A target block runs for each triple and
   build mode; its labels are resolved and visibility is checked.
+- The plugin API, `com.progmasoft.tazel:tazel-api` (API version 1), and the Gradle plugin `com.progmasoft.tazel.plugin`,
+  taken from the legacy Tazel. `tazelPlugin { }` now holds `group`, `name` and `version`, which the plugin descriptor
+  records; the group must be a reversed domain and the version a Semantic Version.
diff --git a/api/build.gradle.kts b/api/build.gradle.kts
new file mode 100644
index 0000000..135708e
--- /dev/null
+++ b/api/build.gradle.kts
@@ -0,0 +1,93 @@
+/*
+ * SPDX-FileCopyrightText: 2026 Progmasoft <support@progmasoft.com>
+ * SPDX-License-Identifier: MPL-2.0 WITH AdditionRef-Progmasoft-Exception-2.0
+ */
+
+import org.jetbrains.kotlin.gradle.dsl.JvmTarget
+
+// The API that plugins of the Tazel project file compile against: com.progmasoft.api.tazel and its
+// packages, published as com.progmasoft.tazel:tazel-api to https://maven.progmasoft.com/tazel.
+// A plugin does not bundle it; Tazel provides it at run time.
+plugins {
+  kotlin("jvm")
+  `java-library`
+  `maven-publish`
+}
+
+group = "com.progmasoft.tazel"
+version = rootProject.version
+
+repositories {
+  mavenCentral()
+}
+
+kotlin {
+  jvmToolchain(25)
+  compilerOptions {
+    jvmTarget = JvmTarget.JVM_25
+    allWarningsAsErrors = true
+  }
+}
+
+java {
+  sourceCompatibility = JavaVersion.VERSION_25
+  targetCompatibility = JavaVersion.VERSION_25
+  withSourcesJar()
+}
+
+sourceSets {
+  main { kotlin.srcDir("sources/main/kotlin") }
+}
+
+tasks.withType<AbstractArchiveTask>().configureEach {
+  isPreserveFileTimestamps = false
+  isReproducibleFileOrder = true
+}
+
+tasks.jar {
+  manifest {
+    attributes("Implementation-Title" to "Tazel API", "Implementation-Version" to project.version)
+  }
+}
+
+publishing {
+  publications {
+    create<MavenPublication>("api") {
+      artifactId = "tazel-api"
+      from(components["java"])
+      pom {
+        name = "Tazel API"
+        description = "The API that plugins of the Tazel project file compile against."
+        url = "https://tazel.net/"
+        licenses {
+          license {
+            name = "MPL-2.0 WITH AdditionRef-Progmasoft-Exception-2.0"
+            url = "https://github.com/Progmasoft/tazel/blob/main/LICENSE.txt"
+          }
+        }
+        developers {
+          developer {
+            name = "Progmasoft"
+            email = "support@progmasoft.com"
+          }
+        }
+        scm {
+          url = "https://github.com/Progmasoft/tazel"
+          connection = "scm:git:https://github.com/Progmasoft/tazel.git"
+        }
+      }
+    }
+  }
+  repositories {
+    // A local repository, which is copied to https://maven.progmasoft.com/tazel.
+    maven {
+      name = "staging"
+      url =
+        rootProject.layout.buildDirectory
+          .dir("maven")
+          .get()
+          .asFile
+          .toURI()
+    }
+  }
+}
diff --git a/api/sources/main/kotlin/com/progmasoft/api/tazel/dsl/TazelPluginDsl.kt b/api/sources/main/kotlin/com/progmasoft/api/tazel/dsl/TazelPluginDsl.kt
new file mode 100644
index 0000000..0babe77
--- /dev/null
+++ b/api/sources/main/kotlin/com/progmasoft/api/tazel/dsl/TazelPluginDsl.kt
@@ -0,0 +1,14 @@
+/*
+ * SPDX-FileCopyrightText: 2026 Progmasoft <support@progmasoft.com>
+ * SPDX-License-Identifier: MPL-2.0 WITH AdditionRef-Progmasoft-Exception-2.0
+ */
+
+package com.progmasoft.api.tazel.dsl
+
+/**
+ * Marks the scopes of a plugin DSL, so that a nested block of the project file cannot reach the
+ * members of the block around it by mistake: inside `cc { c { } }`, `c` sees only its own members.
+ */
+@DslMarker
+@Target(AnnotationTarget.CLASS, AnnotationTarget.TYPEALIAS, AnnotationTarget.TYPE)
+annotation class TazelPluginDsl
diff --git a/api/sources/main/kotlin/com/progmasoft/api/tazel/dsl/plugin/BuildStep.kt b/api/sources/main/kotlin/com/progmasoft/api/tazel/dsl/plugin/BuildStep.kt
new file mode 100644
index 0000000..d295f94
--- /dev/null
+++ b/api/sources/main/kotlin/com/progmasoft/api/tazel/dsl/plugin/BuildStep.kt
@@ -0,0 +1,77 @@
+/*
+ * SPDX-FileCopyrightText: 2026 Progmasoft <support@progmasoft.com>
+ * SPDX-License-Identifier: MPL-2.0 WITH AdditionRef-Progmasoft-Exception-2.0
+ */
+
+package com.progmasoft.api.tazel.dsl.plugin
+
+import java.nio.file.Path
+
+/** Work a plugin adds to the build of a project; see [ExtensionScope.buildStep]. */
+fun interface BuildStep {
+  fun run(context: BuildContext)
+}
+
+/** What a [BuildStep] runs with. */
+interface BuildContext {
+  val projectRoot: Path
+
+  /** The project the step builds. */
+  val project: ProjectSummary
+
+  /** The absolute directory the build mode of the project writes artifacts to. */
+  val outputDirectory: Path
+
+  val buildMode: BuildMode
+  val host: Host
+
+  /**
+   * A directory of this step for intermediate files, created before the step runs: the same one in
+   * every build, below `.tazel/build` of the project.
+   */
+  val workDirectory: Path
+
+  /** Prints a status line, such as `Configuring cmake (Release)`. */
+  fun status(
+    verb: String,
+    message: String,
+  )
+
+  fun warn(message: String)
+
+  /**
+   * Runs a program with the variables of [environment] added to those of Tazel and returns its exit
+   * code. What it prints goes to the output of Tazel.
+   */
+  fun run(
+    command: List<String>,
+    directory: Path = projectRoot,
+    environment: Map<String, String> = emptyMap(),
+  ): Int
+
+  /**
+   * Runs a program like [run], but gathers what it prints, output and errors in the order written,
+   * instead of showing it: for a probe whose failure is an answer, or a program whose output the
+   * step reads.
+   */
+  fun capture(
+    command: List<String>,
+    directory: Path = projectRoot,
+    environment: Map<String, String> = emptyMap(),
+  ): CapturedRun
+
+  /** The program [name] found on `PATH`, with the executable extension of the host, or `null`. */
+  fun findProgram(name: String): Path?
+
+  /** Stops the build with [message]. */
+  fun fail(message: String): Nothing
+}
+
+/** The exit code of a program [BuildContext.capture] ran, and what it printed. */
+data class CapturedRun(
+  val exitCode: Int,
+  val output: String,
+)
+
+/** Thrown by [BuildContext.fail]; Tazel reports its message and stops the build. */
+class BuildStepException(message: String) : RuntimeException(message)
diff --git a/api/sources/main/kotlin/com/progmasoft/api/tazel/dsl/plugin/Host.kt b/api/sources/main/kotlin/com/progmasoft/api/tazel/dsl/plugin/Host.kt
new file mode 100644
index 0000000..1e138a8
--- /dev/null
+++ b/api/sources/main/kotlin/com/progmasoft/api/tazel/dsl/plugin/Host.kt
@@ -0,0 +1,40 @@
+/*
+ * SPDX-FileCopyrightText: 2026 Progmasoft <support@progmasoft.com>
+ * SPDX-License-Identifier: MPL-2.0 WITH AdditionRef-Progmasoft-Exception-2.0
+ */
+
+package com.progmasoft.api.tazel.dsl.plugin
+
+enum class OperatingSystem {
+  LINUX,
+  MACOS,
+  WINDOWS,
+  FREEBSD,
+  OPENBSD,
+  NETBSD,
+  UNKNOWN,
+}
+
+enum class Architecture {
+  X86_64,
+  AARCH64,
+  ARMV7H,
+  RISCV64,
+  UNKNOWN,
+}
+
+/** The machine Tazel runs on. */
+data class Host(
+  val os: OperatingSystem,
+  val architecture: Architecture,
+) {
+  /** The extension the host gives an executable file: `.exe` on Windows, none elsewhere. */
+  val executableExtension: String
+    get() = if (os == OperatingSystem.WINDOWS) ".exe" else ""
+}
+
+/** The build mode of the project. */
+enum class BuildMode {
+  DEBUG,
+  RELEASE,
+}
diff --git a/api/sources/main/kotlin/com/progmasoft/api/tazel/dsl/plugin/PluginExtensions.kt b/api/sources/main/kotlin/com/progmasoft/api/tazel/dsl/plugin/PluginExtensions.kt
new file mode 100644
index 0000000..f8ae8d1
--- /dev/null
+++ b/api/sources/main/kotlin/com/progmasoft/api/tazel/dsl/plugin/PluginExtensions.kt
@@ -0,0 +1,56 @@
+/*
+ * SPDX-FileCopyrightText: 2026 Progmasoft <support@progmasoft.com>
+ * SPDX-License-Identifier: MPL-2.0 WITH AdditionRef-Progmasoft-Exception-2.0
+ */
+
+package com.progmasoft.api.tazel.dsl.plugin
+
+import java.nio.file.Path
+import java.util.ServiceLoader
+
+/**
+ * The extensions of the project whose file is running.
+ *
+ * The DSL function of a plugin calls [require] with an extension its plugin registered and adds the
+ * work of its block through the [ExtensionScope] it receives:
+ * ```
+ * fun cmake(block: CMakeScope.() -> Unit) {
+ *   val settings = CMakeScope().apply(block)
+ *   PluginExtensions.require("cmake").buildStep("configure") { context -> ... }
+ * }
+ * ```
+ */
+object PluginExtensions {
+  private val host: ExtensionHost by lazy {
+    ServiceLoader.load(ExtensionHost::class.java, ExtensionHost::class.java.classLoader)
+      .firstOrNull()
+      ?: throw IllegalStateException("plugin extensions are available only inside Tazel")
+  }
+
+  /**
+   * The scope of [extension] in the project whose file is running. It fails when no plugin of the
+   * project registered [extension], or when the project file did not declare that plugin.
+   */
+  fun require(extension: String): ExtensionScope = host.require(extension)
+}
+
+/** What the block of an extension can add to its project. */
+interface ExtensionScope {
+  val projectRoot: Path
+  val host: Host
+
+  /**
+   * Adds [step] to the build of the project. `tazel build` and `tazel run` run the steps of a
+   * project in the order they were added, before its targets compile; a failed step stops the
+   * build. [name] names the step in the output and is one identifier, unique within the plugin.
+   */
+  fun buildStep(
+    name: String,
+    step: BuildStep,
+  )
+}
+
+/** Provided by Tazel, which [PluginExtensions] finds as a service. Plugins do not implement it. */
+interface ExtensionHost {
+  fun require(extension: String): ExtensionScope
+}
diff --git a/api/sources/main/kotlin/com/progmasoft/api/tazel/dsl/plugin/ProjectPlugin.kt b/api/sources/main/kotlin/com/progmasoft/api/tazel/dsl/plugin/ProjectPlugin.kt
new file mode 100644
index 0000000..9c50fe9
--- /dev/null
+++ b/api/sources/main/kotlin/com/progmasoft/api/tazel/dsl/plugin/ProjectPlugin.kt
@@ -0,0 +1,62 @@
+/*
+ * SPDX-FileCopyrightText: 2026 Progmasoft <support@progmasoft.com>
+ * SPDX-License-Identifier: MPL-2.0 WITH AdditionRef-Progmasoft-Exception-2.0
+ */
+
+package com.progmasoft.api.tazel.dsl.plugin
+
+import java.nio.file.Path
+
+/** The version of this API; a plugin built for another one is not loaded. */
+const val PROJECT_PLUGIN_API_VERSION = 1
+
+/**
+ * A plugin of the Tazel project file.
+ *
+ * Who the plugin is, its group, name and version, is in the descriptor of its JAR,
+ * `META-INF/tazel-plugin.properties`, which the Gradle plugin writes from `tazelPlugin { }`
+ * together with the API version, the imports it adds and what Tazel Portal shows about it. The
+ * service entry of the JAR, `META-INF/services/com.progmasoft.api.tazel.dsl.plugin.ProjectPlugin`,
+ * names the class, which needs a public constructor without parameters. Tazel creates it for every
+ * module that declares the plugin and calls [apply] before the project files run. The project files
+ * then use the DSL the plugin defines; see [PluginExtensions].
+ *
+ * A plugin runs with the same JVM permissions as Tazel. The integrity and the compatibility of its
+ * JAR are checked before it is activated; its code is not sandboxed.
+ */
+fun interface ProjectPlugin {
+  fun apply(context: ProjectPluginContext)
+}
+
+/** What a plugin can do while it is activated for one project. */
+interface ProjectPluginContext {
+  val projectRoot: Path
+
+  /** The machine Tazel runs on. */
+  val host: Host
+
+  /**
+   * Claims [name] as an extension of this plugin: a block of the project file that the DSL of the
+   * plugin defines, such as `cmake`. Two plugins of one project cannot claim the same name.
+   */
+  fun registerExtension(name: String)
+
+  /** Records [value] under [key] in the plan of the project; each key is written once. */
+  fun contribute(
+    key: String,
+    value: String,
+  )
+
+  /** Runs [action] once the project file has run and the plan of the project is finished. */
+  fun onFinalize(action: (ProjectSummary) -> Unit)
+}
+
+/** The project as a plugin sees it once its project file has run. */
+interface ProjectSummary {
+  val name: String?
+  val version: String?
+  val buildMode: BuildMode
+
+  /** The directory the build mode writes artifacts to, relative to the project root. */
+  val outputDirectory: String
+}
diff --git a/build.gradle.kts b/build.gradle.kts
index 8d3d62b..c2b69ae 100644
--- a/build.gradle.kts
+++ b/build.gradle.kts
@@ -63,13 +63,13 @@ sourceSets {
 
 spotless {
   kotlin {
-    target("sources/**/*.kt")
+    target("sources/**/*.kt", "api/sources/**/*.kt", "gradle-plugin/sources/**/*.kt")
     ktfmt().googleStyle()
     trimTrailingWhitespace()
     endWithNewline()
   }
   kotlinGradle {
-    target("*.gradle.kts")
+    target("*.gradle.kts", "api/*.gradle.kts", "gradle-plugin/*.gradle.kts")
     ktlint()
     trimTrailingWhitespace()
     endWithNewline()
diff --git a/gradle-plugin/build.gradle.kts b/gradle-plugin/build.gradle.kts
new file mode 100644
index 0000000..0aec44d
--- /dev/null
+++ b/gradle-plugin/build.gradle.kts
@@ -0,0 +1,90 @@
+/*
+ * SPDX-FileCopyrightText: 2026 Progmasoft <support@progmasoft.com>
+ * SPDX-License-Identifier: MPL-2.0 WITH AdditionRef-Progmasoft-Exception-2.0
+ */
+
+import org.jetbrains.kotlin.gradle.dsl.JvmTarget
+import org.jetbrains.kotlin.gradle.dsl.KotlinVersion
+
+// The Gradle plugin com.progmasoft.tazel.plugin, which builds plugins of the Tazel project file.
+// It runs inside Gradle, with the Kotlin standard library Gradle ships: it does not bundle one,
+// compiles for the Kotlin of Gradle 9.0 and runs on its JVM.
+plugins {
+  kotlin("jvm")
+  `java-gradle-plugin`
+  id("com.gradle.plugin-publish") version "2.0.0"
+}
+
+group = "com.progmasoft.tazel"
+version = rootProject.version
+
+repositories {
+  mavenCentral()
+}
+
+dependencies {
+  compileOnly(kotlin("stdlib"))
+  testImplementation(kotlin("test"))
+}
+
+kotlin {
+  jvmToolchain(25)
+  compilerOptions {
+    jvmTarget = JvmTarget.JVM_17
+    apiVersion = KotlinVersion.KOTLIN_2_2
+    languageVersion = KotlinVersion.KOTLIN_2_2
+  }
+}
+
+java {
+  sourceCompatibility = JavaVersion.VERSION_17
+  targetCompatibility = JavaVersion.VERSION_17
+}
+
+sourceSets {
+  main { kotlin.srcDir("sources/main/kotlin") }
+  test { kotlin.srcDir("sources/test/kotlin") }
+}
+
+// The version of the plugin is the version of Tazel; tazelApi() asks for the API of that version.
+val versionResource =
+  tasks.register("versionResource") {
+    val directory = layout.buildDirectory.dir("generated/version")
+    val pluginVersion = project.version.toString()
+    inputs.property("version", pluginVersion)
+    outputs.dir(directory)
+    doLast {
+      directory.get().asFile.resolve("tazel-gradle-plugin.properties").apply {
+        parentFile.mkdirs()
+        writeText("version=$pluginVersion\n")
+      }
+    }
+  }
+sourceSets.main { resources.srcDir(versionResource) }
+
+tasks.withType<AbstractArchiveTask>().configureEach {
+  isPreserveFileTimestamps = false
+  isReproducibleFileOrder = true
+}
+
+tasks.test {
+  useJUnitPlatform()
+}
+
+gradlePlugin {
+  website = "https://tazel.net/"
+  vcsUrl = "https://github.com/Progmasoft/tazel"
+  plugins {
+    create("tazelPlugin") {
+      id = "com.progmasoft.tazel.plugin"
+      implementationClass = "com.progmasoft.tazel.gradle.TazelPluginPlugin"
+      displayName = "Tazel plugin"
+      description =
+        "Builds a plugin of the Tazel project file: adds the Tazel API from " +
+        "https://maven.progmasoft.com/tazel/ as tazelApi(), and writes the descriptor and the " +
+        "service entry of the plugin from the tazelPlugin { } block."
+      // The Portal refuses the tags "gradle" and "plugin".
+      tags = listOf("tazel", "build", "kotlin-dsl")
+    }
+  }
+}
diff --git a/gradle-plugin/sources/main/kotlin/com/progmasoft/tazel/gradle/TazelPluginDescriptorTask.kt b/gradle-plugin/sources/main/kotlin/com/progmasoft/tazel/gradle/TazelPluginDescriptorTask.kt
new file mode 100644
index 0000000..78ac736
--- /dev/null
+++ b/gradle-plugin/sources/main/kotlin/com/progmasoft/tazel/gradle/TazelPluginDescriptorTask.kt
@@ -0,0 +1,139 @@
+/*
+ * SPDX-FileCopyrightText: 2026 Progmasoft <support@progmasoft.com>
+ * SPDX-License-Identifier: MPL-2.0 WITH AdditionRef-Progmasoft-Exception-2.0
+ */
+
+package com.progmasoft.tazel.gradle
+
+import java.io.StringWriter
+import java.util.Properties
+import org.gradle.api.DefaultTask
+import org.gradle.api.GradleException
+import org.gradle.api.file.DirectoryProperty
+import org.gradle.api.provider.ListProperty
+import org.gradle.api.provider.Property
+import org.gradle.api.tasks.CacheableTask
+import org.gradle.api.tasks.Input
+import org.gradle.api.tasks.Optional
+import org.gradle.api.tasks.OutputDirectory
+import org.gradle.api.tasks.TaskAction
+
+/**
+ * Writes `META-INF/tazel-plugin.properties` and the `ProjectPlugin` service entry of a Tazel
+ * plugin, which Tazel reads before it loads the plugin and when it pushes it to Tazel Portal. The
+ * descriptor says who the plugin is: its group, name and version.
+ */
+@CacheableTask
+abstract class TazelPluginDescriptorTask : DefaultTask() {
+  // A required field is optional to Gradle, so the task reports it as a tazelPlugin setting.
+  @get:Input @get:Optional abstract val pluginGroup: Property<String>
+
+  @get:Input @get:Optional abstract val pluginName: Property<String>
+
+  @get:Input @get:Optional abstract val pluginVersion: Property<String>
+
+  @get:Input @get:Optional abstract val pluginClass: Property<String>
+
+  @get:Input abstract val imports: ListProperty<String>
+
+  // `description` is a property of every task already.
+  @get:Input @get:Optional abstract val description_: Property<String>
+
+  @get:Input abstract val authors: ListProperty<String>
+
+  @get:Input @get:Optional abstract val license: Property<String>
+
+  @get:Input @get:Optional abstract val homepage: Property<String>
+
+  @get:Input @get:Optional abstract val repository: Property<String>
+
+  @get:Input abstract val apiVersion: Property<Int>
+
+  @get:OutputDirectory abstract val outputDirectory: DirectoryProperty
+
+  @TaskAction
+  fun write() {
+    val group = required(pluginGroup, "group")
+    if (!group.matches(GROUP)) {
+      throw GradleException(
+        "tazelPlugin.group is '$group'; a group is a reversed domain such as com.example, " +
+          "of at least two parts of letters, digits, - and _"
+      )
+    }
+    val name = required(pluginName, "name")
+    if (!name.matches(NAME)) {
+      throw GradleException(
+        "tazelPlugin.name is '$name'; a name is letters, digits and _ . -, " +
+          "starting with a letter or digit"
+      )
+    }
+    val version = required(pluginVersion, "version")
+    if (!version.matches(SEMVER)) {
+      throw GradleException(
+        "tazelPlugin.version is '$version', which is not a Semantic Version such as 0.1.0"
+      )
+    }
+    val pluginClass = required(pluginClass, "pluginClass")
+    val imports = imports.get()
+    imports.forEach { import ->
+      if (!import.matches(IMPORT)) {
+        throw GradleException("tazelPlugin.imports has '$import', which is not a dotted name")
+      }
+    }
+
+    val entries =
+      linkedMapOf(
+        "group" to group,
+        "name" to name,
+        "version" to version,
+        "apiVersion" to apiVersion.get().toString(),
+      )
+    if (imports.isNotEmpty()) entries["imports"] = imports.joinToString(",")
+    description_.orNull?.let { entries["description"] = it }
+    authors.get().forEachIndexed { index, author -> entries["author.$index"] = author }
+    license.orNull?.let { entries["license"] = it }
+    homepage.orNull?.let { entries["homepage"] = it }
+    repository.orNull?.let { entries["repository"] = it }
+
+    val metaInf = outputDirectory.get().asFile.resolve("META-INF")
+    metaInf.resolve("services").mkdirs()
+    metaInf.resolve("tazel-plugin.properties").writeText(properties(entries))
+    metaInf
+      .resolve("services/com.progmasoft.api.tazel.dsl.plugin.ProjectPlugin")
+      .writeText("$pluginClass\n")
+  }
+
+  private fun required(
+    property: Property<String>,
+    name: String,
+  ): String =
+    property.orNull?.trim()?.takeIf(String::isNotEmpty)
+      ?: throw GradleException("tazelPlugin.$name is required")
+
+  /**
+   * The entries in the format of [Properties], escaped by it, without the date line it writes, so
+   * the same settings always give the same bytes.
+   */
+  private fun properties(entries: Map<String, String>): String {
+    val text = StringWriter()
+    Properties()
+      .apply { entries.forEach { (key, value) -> setProperty(key, value) } }
+      .store(text, null)
+    val lines = text.toString().lines().filter { it.isNotEmpty() && !it.startsWith("#") }
+    val order = entries.keys.toList()
+    return lines
+      .sortedBy { line -> order.indexOf(line.substringBefore('=').replace("\\", "")) }
+      .joinToString("\n", postfix = "\n")
+  }
+
+  private companion object {
+    val GROUP = Regex("[A-Za-z0-9][A-Za-z0-9_-]*(?:\\.[A-Za-z0-9][A-Za-z0-9_-]*)+")
+    val NAME = Regex("[A-Za-z0-9][A-Za-z0-9_.-]*")
+    val SEMVER =
+      Regex(
+        """(0|[1-9]\d*)\.(0|[1-9]\d*)\.(0|[1-9]\d*)""" +
+          """(-[0-9A-Za-z-]+(\.[0-9A-Za-z-]+)*)?(\+[0-9A-Za-z-]+(\.[0-9A-Za-z-]+)*)?"""
+      )
+    val IMPORT = Regex("[A-Za-z_][A-Za-z0-9_]*(?:\\.[A-Za-z_][A-Za-z0-9_]*)*(?:\\.\\*)?")
+  }
+}
diff --git a/gradle-plugin/sources/main/kotlin/com/progmasoft/tazel/gradle/TazelPluginPlugin.kt b/gradle-plugin/sources/main/kotlin/com/progmasoft/tazel/gradle/TazelPluginPlugin.kt
new file mode 100644
index 0000000..e86ceaa
--- /dev/null
+++ b/gradle-plugin/sources/main/kotlin/com/progmasoft/tazel/gradle/TazelPluginPlugin.kt
@@ -0,0 +1,123 @@
+/*
+ * SPDX-FileCopyrightText: 2026 Progmasoft <support@progmasoft.com>
+ * SPDX-License-Identifier: MPL-2.0 WITH AdditionRef-Progmasoft-Exception-2.0
+ */
+
+package com.progmasoft.tazel.gradle
+
+import java.net.URI
+import java.util.Properties
+import javax.inject.Inject
+import org.gradle.api.Plugin
+import org.gradle.api.Project
+import org.gradle.api.plugins.JavaPluginExtension
+import org.gradle.api.provider.ListProperty
+import org.gradle.api.provider.Property
+
+/**
+ * `id("com.progmasoft.tazel.plugin")`: builds a plugin of the Tazel project file.
+ *
+ * It adds the repository of the Tazel API, https://maven.progmasoft.com/tazel, the `tazelApi()`
+ * dependency and the `tazelPlugin { }` block, from which it writes the descriptor and the service
+ * entry of the plugin into its JAR. The version of this Gradle plugin is the version of Tazel, and
+ * `tazelApi()` takes the API of that version.
+ */
+class TazelPluginPlugin : Plugin<Project> {
+  override fun apply(project: Project) {
+    project.pluginManager.apply("java-library")
+    project.repositories.maven { repository ->
+      repository.name = "ProgmasoftTazel"
+      repository.url = URI.create(REPOSITORY)
+      // Only the API comes from it; everything else keeps its own repositories.
+      repository.content { it.includeGroup(GROUP) }
+    }
+    val settings = project.extensions.create("tazelPlugin", TazelPluginExtension::class.java)
+    project.extensions.create("tazelApi", TazelApiDependency::class.java, project)
+
+    val descriptor =
+      project.tasks.register("tazelPluginDescriptor", TazelPluginDescriptorTask::class.java) { task
+        ->
+        task.group = "build"
+        task.description = "Writes the descriptor and the service entry of the Tazel plugin."
+        task.pluginGroup.set(settings.group)
+        task.pluginName.set(settings.name)
+        task.pluginVersion.set(settings.version)
+        task.pluginClass.set(settings.pluginClass)
+        task.imports.set(settings.imports)
+        task.description_.set(settings.description)
+        task.authors.set(settings.authors)
+        task.license.set(settings.license)
+        task.homepage.set(settings.homepage)
+        task.repository.set(settings.repository)
+        task.apiVersion.set(API_VERSION)
+        task.outputDirectory.set(project.layout.buildDirectory.dir("generated/tazel-plugin"))
+      }
+    project.extensions.getByType(JavaPluginExtension::class.java).sourceSets.named("main") {
+      it.resources.srcDir(descriptor)
+    }
+  }
+
+  companion object {
+    const val REPOSITORY = "https://maven.progmasoft.com/tazel"
+    const val GROUP = "com.progmasoft.tazel"
+
+    /** The plugin API of the Tazel this Gradle plugin belongs to. */
+    const val API_VERSION = 1
+
+    /** The version of this Gradle plugin, which is the version of Tazel and of its API. */
+    val VERSION: String by lazy {
+      val properties = Properties()
+      TazelPluginPlugin::class.java.getResourceAsStream("/tazel-gradle-plugin.properties").use {
+        stream ->
+        requireNotNull(stream) { "the Tazel Gradle plugin has no version resource" }
+        properties.load(stream)
+      }
+      properties.getProperty("version")
+    }
+  }
+}
+
+/**
+ * The `tazelPlugin { }` block: who the plugin is, its class, and what Tazel Portal shows about it.
+ * `tazel portal plugins push plugin.jar` publishes the JAR under its group, name and version.
+ */
+abstract class TazelPluginExtension {
+  /**
+   * The namespace of the plugin, such as `com.example`; Tazel Portal checks it by the Sonatype
+   * rules.
+   */
+  abstract val group: Property<String>
+
+  /** The name of the plugin within its group, such as `cmake`. */
+  abstract val name: Property<String>
+
+  /** A Semantic Version, such as `0.1.0`. */
+  abstract val version: Property<String>
+
+  /** The class that implements `ProjectPlugin`. */
+  abstract val pluginClass: Property<String>
+
+  /** Packages or classes the project file imports by default, such as `com.example.cmake.*`. */
+  abstract val imports: ListProperty<String>
+
+  abstract val description: Property<String>
+  abstract val authors: ListProperty<String>
+
+  /** An SPDX license expression, such as `MIT`. */
+  abstract val license: Property<String>
+
+  abstract val homepage: Property<String>
+  abstract val repository: Property<String>
+}
+
+/**
+ * `tazelApi()` in the `dependencies` block: the API of Tazel, which the plugin compiles against and
+ * its tests use. It is not bundled; Tazel provides it at run time.
+ */
+abstract class TazelApiDependency @Inject constructor(private val project: Project) {
+  operator fun invoke() {
+    val coordinate = "${TazelPluginPlugin.GROUP}:tazel-api:${TazelPluginPlugin.VERSION}"
+    project.dependencies.add("compileOnly", coordinate)
+    project.dependencies.add("testImplementation", coordinate)
+  }
+}
diff --git a/gradle-plugin/sources/test/kotlin/com/progmasoft/tazel/gradle/TazelPluginPluginTest.kt b/gradle-plugin/sources/test/kotlin/com/progmasoft/tazel/gradle/TazelPluginPluginTest.kt
new file mode 100644
index 0000000..205f1c6
--- /dev/null
+++ b/gradle-plugin/sources/test/kotlin/com/progmasoft/tazel/gradle/TazelPluginPluginTest.kt
@@ -0,0 +1,172 @@
+/*
+ * SPDX-FileCopyrightText: 2026 Progmasoft <support@progmasoft.com>
+ * SPDX-License-Identifier: MPL-2.0 WITH AdditionRef-Progmasoft-Exception-2.0
+ */
+
+package com.progmasoft.tazel.gradle
+
+import java.io.File
+import java.nio.file.Files
+import kotlin.test.AfterTest
+import kotlin.test.BeforeTest
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertTrue
+import org.gradle.testkit.runner.GradleRunner
+
+/** Runs the plugin in a project of its own, as a plugin author would. */
+class TazelPluginPluginTest {
+  private lateinit var project: File
+
+  @BeforeTest
+  fun create() {
+    project = Files.createTempDirectory("tazel-gradle-plugin-").toFile()
+    project.resolve("settings.gradle.kts").writeText("rootProject.name = \"native\"\n")
+  }
+
+  @AfterTest
+  fun delete() {
+    project.deleteRecursively()
+  }
+
+  private fun build(
+    tazelPlugin: String,
+    vararg arguments: String,
+  ) =
+    GradleRunner.create()
+      .withProjectDir(project)
+      .withPluginClasspath()
+      .also {
+        project
+          .resolve("build.gradle.kts")
+          .writeText(
+            """
+            plugins { id("com.progmasoft.tazel.plugin") }
+
+            // The project version is not the plugin version: tazelPlugin { version } is.
+            version = "1.2.0.3.4-build5"
+
+            tazelPlugin {
+            $tazelPlugin
+            }
+
+            dependencies { tazelApi() }
+
+            tasks.register("printApi") {
+              val compileOnly = configurations.compileOnly.get().dependencies.map { "${'$'}{it.group}:${'$'}{it.name}:${'$'}{it.version}" }
+              val tests = configurations.testImplementation.get().dependencies.map { "${'$'}{it.group}:${'$'}{it.name}:${'$'}{it.version}" }
+              doLast { println("compileOnly=${'$'}compileOnly testImplementation=${'$'}tests") }
+            }
+            """
+              .trimIndent()
+          )
+      }
+      .withArguments(*arguments, "--stacktrace")
+
+  private val identity =
+    """
+    group = "com.example.plugins"
+    name = "native"
+