diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
new file mode 100644
index 0000000..72917b1
--- /dev/null
+++ b/.github/workflows/ci.yml
@@ -0,0 +1,68 @@
+name: CI
+
+# Every pull request runs the unit tests, the builder checks and the server game tests
+# (`./gradlew check`), then the client GUI smoke test with and without JEI.
+on:
+  pull_request:
+  push:
+    branches: [master-2.0-1.21.1-neo]
+
+jobs:
+  check:
+    name: Unit tests, checks and game tests
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v7
+      - uses: actions/setup-java@v6
+        with:
+          distribution: temurin
+          java-version: 21
+      - uses: gradle/actions/setup-gradle@v6
+
+      - name: Check
+        run: ./gradlew check --stacktrace
+
+      - name: Upload test reports and the game test log
+        if: always()
+        uses: actions/upload-artifact@v7
+        with:
+          name: check-reports
+          path: |
+            build/reports/tests/
+            build/gametest/logs/
+          if-no-files-found: ignore
+
+  gui-smoke:
+    name: Client GUI smoke test
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v7
+      - uses: actions/setup-java@v6
+        with:
+          distribution: temurin
+          java-version: 21
+      - uses: gradle/actions/setup-gradle@v6
+
+      # No display: Xvfb and Mesa's software OpenGL render the client off screen.
+      - name: Install Xvfb and Mesa
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y xvfb libgl1-mesa-dri
+
+      - name: GUI smoke test with JEI
+        timeout-minutes: 20
+        run: xvfb-run -a ./gradlew runClient -I scripts/gui-smoke.init.gradle
+
+      - name: GUI smoke test without JEI
+        timeout-minutes: 20
+        run: xvfb-run -a ./gradlew runClient -PwithoutJei -I scripts/gui-smoke.init.gradle
+
+      - name: Upload the client logs
+        if: always()
+        uses: actions/upload-artifact@v7
+        with:
+          name: gui-smoke-logs
+          path: |
+            build/gui-smoke/logs/
+            build/gui-smoke-without-jei/logs/
+          if-no-files-found: ignore
diff --git a/build.gradle b/build.gradle
index 51abddf..21c5508 100644
--- a/build.gradle
+++ b/build.gradle
@@ -68,7 +68,7 @@ tasks.configureEach { task ->
             || task.name.startsWith('test')
             || task.name in ['compileTestJava', 'compileBuilderTestJava', 'compileRuntimeTestJava',
                     'writeMinecraftClasspathJunit', 'writeMinecraftClasspathClient', 'writeMinecraftClasspathServer',
-                    'runClient', 'runServer']) {
+                    'runClient', 'runServer', 'runGameTestServer']) {
         task.mustRunAfter(refreshDataResources)
     }
 }
diff --git a/docs/vanilla-extensions.md b/docs/vanilla-extensions.md
index 0c8319b..38f8c67 100644
--- a/docs/vanilla-extensions.md
+++ b/docs/vanilla-extensions.md
@@ -141,21 +141,18 @@ checks every planned item/block has its packaged resources, verifies no unwanted
 vanilla tool/armor duplicates, and round-trips the runtime references through
 JSON/NBT and MaterialBuilder.
 
-An opt-in, self-stopping server smoke test exercises real registration, block
-placement, ammunition stack recovery after entity save/load, fluid placement and
-bucket recovery, actual mining tags, and packaged armor texture references:
+Server game tests exercise real registration, block placement, ammunition stack
+recovery after entity save/load, fluid placement and bucket recovery, actual mining
+tags, and packaged armor texture references. They are part of `./gradlew check`, or
+run them alone:
 
 ```sh
-./gradlew runServer --offline --stacktrace -I scripts/runtime-smoke.init.gradle
-./gradlew build check --offline --stacktrace
+./gradlew runGameTestServer
 ```
 
-The smoke run uses `build/runtime-smoke/server`, loopback networking, and a random
-port. It stops itself and the Gradle task fails if the success marker is absent.
-Its entrypoint lives in `src/runtimeTest/java`, outside release sources; `./gradlew check`
-compiles it to prevent API drift. The second command restores regular
-main class output after the opt-in test run. The release JAR must contain no
-`zone/moddev/mc/lib/smoke` classes.
+Each check is its own game test in `MMDLibGameTests` (`src/runtimeTest/java`, outside
+release sources), and the task fails naming every test that failed. The release JAR
+must contain no `zone/moddev/mc/lib/smoke` classes.
 
 Verified on a dedicated development server: all 50 block items placed, all twelve
 arrow/bolt types preserved the correct pickup item and components across entity
diff --git a/gradle/verification.gradle b/gradle/verification.gradle
index 2e48d76..844d990 100644
--- a/gradle/verification.gradle
+++ b/gradle/verification.gradle
@@ -13,6 +13,32 @@ sourceSets {
 }
 tasks.named('check') { dependsOn tasks.named('compileRuntimeTestJava') }
 
+// Server game tests in src/runtimeTest (MMDLibGameTests): `./gradlew runGameTestServer`, part of
+// `check`. The run loads main and runtimeTest as one mod, in a fresh world each time, and exits
+// with the number of failed tests. Release jars never contain runtimeTest (verifyReleaseClasses).
+def gameTestDir = layout.buildDirectory.dir('gametest')
+runs {
+    gameTestServer {
+        workingDirectory gameTestDir
+        modSource project.sourceSets.runtimeTest
+        systemProperty 'neoforge.enabledGameTestNamespaces', 'mmdlib'
+    }
+}
+// NeoGradle creates the run task late, so it is found by name.
+def gameTestTask = tasks.matching { it.name == 'runGameTestServer' }
+gameTestTask.configureEach { it.doFirst { project.delete(gameTestDir); gameTestDir.get().asFile.mkdirs() } }
+tasks.named('check') { dependsOn gameTestTask }
+
+// Unit tests in src/test: JUnit 5, no running game. `./gradlew test` (part of `check`).
+dependencies {
+    testImplementation 'org.junit.jupiter:junit-jupiter:5.12.2'
+    testRuntimeOnly 'org.junit.platform:junit-platform-launcher'
+}
+tasks.named('test', Test) {
+    useJUnitPlatform()
+    testLogging { events 'failed'; exceptionFormat 'full' }
+}
+
 // Standalone checks in src/builderTest, run as plain Java programs: task name -> main class
 // (in zone.moddev.mc.lib.common.builders), plus any system properties the check reads.
 def resources = file('src/main/resources').absolutePath
@@ -32,7 +58,6 @@ def builderChecks = [
     verifyMethodEvents      : ['MethodEventsTest'],
     verifyDefinitionLoader  : ['DefinitionLoaderTest'],
     verifyScreens           : ['zone.moddev.mc.lib.gui.ScreenLoaderTest'],
-    verifyMachineCore       : ['zone.moddev.mc.lib.machine.MachineCoreTest'],
     verifyVanillaExamples   : ['VanillaExamplesTest', ['mmdlib.resources': resources,
                                                        'mmdlib.generated': file('src/generated/resources').absolutePath]],
 ]
@@ -52,8 +77,9 @@ def verifyReleaseClasses = tasks.register('verifyReleaseClasses') {
     dependsOn tasks.named('jar')
     doLast {
         new java.util.zip.ZipFile(tasks.named('jar').get().archiveFile.get().asFile).withCloseable { archive ->
-            def leaked = archive.entries().findAll { entry -> entry.name.startsWith('zone/moddev/mc/lib/smoke/') }
-            if (!leaked.isEmpty()) throw new GradleException('Runtime smoke-test classes must not be packaged: ' + leaked)
+            def testOnly = ['zone/moddev/mc/lib/smoke/', 'data/mmdlib_smoke/', 'data/mmdlib/structure/']
+            def leaked = archive.entries().findAll { entry -> testOnly.any { entry.name.startsWith(it) } }
+            if (!leaked.isEmpty()) throw new GradleException('Test classes and data must not be packaged: ' + leaked.collect { it.name })
         }
     }
 }
diff --git a/scripts/runtime-smoke.init.gradle b/scripts/runtime-smoke.init.gradle
deleted file mode 100644
index 5cb642f..0000000
--- a/scripts/runtime-smoke.init.gradle
+++ /dev/null
@@ -1,36 +0,0 @@
-// Run: ./gradlew runServer --offline -I scripts/runtime-smoke.init.gradle
-// Run a normal ./gradlew build afterward to restore regular main output.
-allprojects {
-    afterEvaluate {
-        if (extensions.findByName('runs') != null) {
-            sourceSets.main.java.srcDir(file('src/runtimeTest/java'))
-            def smokeDir = file('build/runtime-smoke/server')
-            smokeDir.mkdirs()
-            new File(smokeDir, 'server.properties').text = '''server-ip=127.0.0.1
-server-port=0
-online-mode=true
-level-name=runtime-smoke
-spawn-protection=0
-'''
-            // A real datapack in the smoke world, so reload listeners read files the way packs ship them.
-            def datapack = new File(smokeDir, 'runtime-smoke/datapacks/mmdlib_smoke')
-            project.delete(datapack)
-            project.copy {
-                from file('src/runtimeTest/datapack')
-                into datapack
-            }
-            runs.server.workingDirectory = smokeDir
-            // NeoGradle creates run tasks in a later evaluation callback.
-            tasks.configureEach { task ->
-                if (task.name == 'runServer') {
-                    task.doLast {
-                        def report = new File(smokeDir, 'logs/latest.log')
-                        if (!report.exists() || !report.text.contains('MMDLIB_RUNTIME_SMOKE_PASS') || report.text.contains('MMDLIB_RUNTIME_SMOKE_FAIL')) {
-                            throw new GradleException('MMDLib runtime smoke test failed; inspect ' + report)
-                        }
-                    }
-                }
-            }
-        }
-    }
-}
diff --git a/src/builderTest/java/zone/moddev/mc/lib/machine/MachineCoreTest.java b/src/builderTest/java/zone/moddev/mc/lib/machine/MachineCoreTest.java
deleted file mode 100644
index 3c35f44..0000000
--- a/src/builderTest/java/zone/moddev/mc/lib/machine/MachineCoreTest.java
+++ /dev/null
@@ -1,485 +0,0 @@
-package zone.moddev.mc.lib.machine;
-
-import java.util.ArrayList;
-import java.util.Collections;
-import java.util.HashMap;
-import java.util.List;
-import java.util.Map;
-import java.util.Optional;
-import java.util.OptionalDouble;
-import java.util.OptionalInt;
-import java.util.Set;
-import java.util.function.Function;
-
-import com.google.gson.JsonElement;
-import com.google.gson.JsonParser;
-import com.google.gson.JsonPrimitive;
-
-import com.mojang.serialization.JsonOps;
-
-import net.minecraft.resources.ResourceLocation;
-import net.minecraft.world.entity.ai.attributes.AttributeModifier;
-
-import zone.moddev.mc.lib.common.types.MaterialProperties;
-import zone.moddev.mc.lib.machine.expr.Expr;
-import zone.moddev.mc.lib.machine.ingredient.AmountMatch;
-import zone.moddev.mc.lib.machine.internals.InternalsDefinition;
-import zone.moddev.mc.lib.machine.internals.MachineParts;
-import zone.moddev.mc.lib.machine.internals.PartDefinition;
-import zone.moddev.mc.lib.machine.internals.PartSubject;
-import zone.moddev.mc.lib.machine.internals.SwapRule;
-import zone.moddev.mc.lib.machine.recipe.InputClaims;
-import zone.moddev.mc.lib.machine.stat.MachineStats;
-import zone.moddev.mc.lib.machine.stat.StatDefinition;
-import zone.moddev.mc.lib.machine.stat.StatModifier;
-
-/** Checks the machine module's core types, which need no running game. */
-public final class MachineCoreTest {
-  public static void main(String[] args) {
-    expressions();
-    stats();
-    internalsDefinition();
-    parts();
-    amountMatch();
-    inputClaims();
-    System.out.println(
-        "Machine expressions, stats, internals definitions, part files, amount matching and input"
-            + " claims passed");
-  }
-
-  private static void expressions() {
-    require(
-        near(Expr.parse("0.08 * ($material.hardness - 3)").eval(v("material.hardness", 6)), .24));
-    require(Expr.parse("-2 + 3 * 4").eval(none()) == 10);
-    require(Expr.parse("(1 + 2) / 4").eval(none()) == .75);
-    require(Expr.parse("10 - 4 - 3").eval(none()) == 3);
-    require(Expr.parse("2 * -(1 + 1)").eval(none()) == -4);
-    require(Expr.parse("$a.b").variables().equals(Set.of("a.b")));
-    require(Expr.parse("1 + 2").variables().isEmpty());
-    require(Double.isInfinite(Expr.parse("1 / $x").eval(v("x", 0))));
-    require(Expr.constant(3).eval(none()) == 3);
-    expectFailure(() -> Expr.parse("2 *"));
-    expectFailure(() -> Expr.parse("2 $"));
-    expectFailure(() -> Expr.parse("(1"));
-    expectFailure(() -> Expr.parse(""));
-    expectFailure(() -> Expr.parse("$missing").eval(none()));
-    expectFailure(() -> Expr.parse("$a..b"));
-    expectFailure(() -> Expr.parse("$a."));
-    expectFailure(() -> Expr.parse("$.a"));
-    require(
-        Expr.CODEC.parse(JsonOps.INSTANCE, new JsonPrimitive(12)).getOrThrow().eval(none()) == 12);
-    require(
-        Expr.CODEC.parse(JsonOps.INSTANCE, new JsonPrimitive("$x * 2")).getOrThrow().eval(v("x", 4))
-            == 8);
-    require(Expr.CODEC.parse(JsonOps.INSTANCE, new JsonPrimitive("2 *")).error().isPresent());
-    // Encoding round-trips, including numbers Double.toString would write in E notation.
-    var big = Expr.parse("10000000 * $x + 0.0001");
-    var encoded = Expr.CODEC.encodeStart(JsonOps.INSTANCE, big).getOrThrow();
-    require(
-        near(
-            Expr.CODEC.parse(JsonOps.INSTANCE, encoded).getOrThrow().eval(v("x", 2)),
-            20000000.0001));
-  }
-
-  private static void stats() {
-    var defs =
-        Map.of(
-            "speed", new StatDefinition(1, OptionalDouble.of(.25), OptionalDouble.of(8)),
-            "energy_per_tick",
-                new StatDefinition(40, OptionalDouble.of(1), OptionalDouble.empty()));
-    // The worked table in machine-ui.md, with example hardness iron 4 and invar 6.
-    var iron = List.of(base("speed", .08), base("energy_per_tick", .12));
-    var invar = List.of(base("speed", .24), base("energy_per_tick", .18));
-    var motor = List.of(total("speed", .15), value("energy_per_tick", 8));
-    var upgrade = List.of(total("speed", .5), total("energy_per_tick", .5));
-    var twoIron = MachineStats.compute(defs, concat(iron, iron));
-    require(near(twoIron.get("speed"), 1.16) && near(twoIron.get("energy_per_tick"), 49.6));
-    var twoInvar = MachineStats.compute(defs, concat(invar, invar));
-    require(near(twoInvar.get("speed"), 1.48) && near(twoInvar.get("energy_per_tick"), 54.4));
-    var withMotor = MachineStats.compute(defs, concat(invar, invar, motor));
-    require(near(withMotor.get("speed"), 1.702) && near(withMotor.get("energy_per_tick"), 65.28));
-    var all = concat(invar, invar, motor, upgrade);
-    var full = MachineStats.compute(defs, all);
-    require(near(full.get("speed"), 2.553) && near(full.get("energy_per_tick"), 97.92));
-    var shuffled = new ArrayList<>(all);
-    Collections.reverse(shuffled);
-    require(MachineStats.compute(defs, shuffled).values().equals(full.values()));
-    require(MachineStats.compute(defs, List.of()).get("speed") == 1);
-    require(MachineStats.compute(defs, List.of(total("speed", 10))).get("speed") == 8);
-    require(MachineStats.compute(defs, List.of(value("speed", -5))).get("speed") == .25);
-    var undeclared = MachineStats.compute(defs, List.of(value("max_temperature", 400)));
-    require(undeclared.values().size() == 2 && undeclared.get("speed") == 1);
-    expectFailure(() -> MachineStats.EMPTY.get("speed"));
-    expectFailure(() -> value("speed", Double.NaN));
-    var parsed = parseStat("{\"base\":1,\"min\":0.25,\"max\":8}");
-    require(parsed.equals(defs.get("speed")));
-    require(parseStat("{\"base\":900}").min().isEmpty());
-    expectFailure(() -> parseStat("{\"base\":1,\"min\":5,\"max\":2}"));
-    expectFailure(() -> parseStat("{\"min\":1}"));
-  }
-
-  /** The internals example from machine-ui.md, "Sockets and stats". */
-  static final String INTERNALS =
-      """
-      {
-        "open_with": "#c:tools/wrench",
-        "swap": "when_idle",
-        "sockets": {
-          "gears": { "count": 2, "accepts": "#c:gears" },
-          "motor": { "count": 1, "accepts": "#example:motors" },
-          "coil":  { "count": 1, "accepts": "#example:heating_coils", "optional": true }
-        },
-        "stats": {
-          "speed":           { "base": 1.0, "min": 0.25, "max": 8 },
-          "energy_per_tick": { "base": 40, "min": 1 },
-          "max_temperature": { "base": 900 }
-        }
-      }
-      """;
-
-  private static void internalsDefinition() {
-    var internals = parseInternals(INTERNALS);
-    require(internals.swap() == SwapRule.WHEN_IDLE);
-    require(List.copyOf(internals.sockets().keySet()).equals(List.of("coil", "gears", "motor")));
-    require(internals.slotCount() == 4);
-    require(internals.slots("coil").equals(List.of(0)));
-    require(internals.slots("gears").equals(List.of(1, 2)));
-    require(internals.slots("motor").equals(List.of(3)));
-    require(
-        internals.sockets().get("coil").optional() && !internals.sockets().get("gears").optional());
-    require(
-        internals
-            .openWith()
-            .orElseThrow()
-            .value()
-            .left()
-            .orElseThrow()
-            .location()
-            .equals(ResourceLocation.parse("c:tools/wrench")));
-    require(internals.stats().get("max_temperature").base() == 900);
-    var plain =
-        parseInternals("{\"sockets\":{\"motor\":{\"accepts\":\"example:motor\"}},\"stats\":{}}");
-    require(plain.swap() == SwapRule.WHEN_IDLE && plain.openWith().isEmpty());
-    require(plain.sockets().get("motor").count() == 1);
-    require(
-        plain
-            .sockets()
-            .get("motor")
-            .accepts()
-            .orElseThrow()
-            .value()
-            .right()
-            .orElseThrow()
-            .equals(ResourceLocation.parse("example:motor")));
-    require(SwapRule.WHEN_IDLE.allows(false) && !SwapRule.WHEN_IDLE.allows(true));
-    require(SwapRule.ALWAYS.allows(true) && !SwapRule.NEVER.allows(false));
-    expectFailure(() -> parseInternals(INTERNALS.replace("\"count\": 2", "\"count\": 0")));
-    expectFailure(() -> parseInternals(INTERNALS.replace("when_idle", "sometimes")));
-    expectFailure(() -> parseInternals(INTERNALS.replace("#c:gears", "#")));
-    expectFailure(() -> parseInternals(INTERNALS.replace("#c:gears", "Not An Id")));
-    expectFailure(() -> internals.slots("pump"));
-    require(internals.socketAt(0).equals("coil") && internals.socketAt(3).equals("motor"));
-    expectFailure(() -> internals.socketAt(-1));
-    expectFailure(() -> internals.socketAt(4));
-  }
-
-  /** The gear template from machine-ui.md, "Part files". */
-  static final String GEAR_TEMPLATE =
-      """
-{
-  "part": "mmdlib:gear",
-  "socket": "gears",
-  "modifiers": [
-    { "stat": "speed",           "op": "add_multiplied_base", "amount": "0.08 * ($material.hardness - 3)" },
-    { "stat": "energy_per_tick", "op": "add_multiplied_base", "amount": "0.03 * $material.hardness" }
-  ],
-  "durability": "500 * $material.hardness"
-}
-""";
-
-  private static void parts() {
-    var subjects =
-        Map.of(
-            id("test:iron_gear"), gear("test:iron_gear", 4, Map.of()),
-            id("test:invar_gear"), gear("test:invar_gear", 6, Map.of("grip", 2.0)),
-            id("test:soft_gear"), gear("test:soft_gear", 0, Map.of()),
-            id("test:diamond_coated_gear"), gear("test:diamond_coated_gear", 10, Map.of()),
-            id("test:plain_rod"),
-                new PartSubject(
-                    id("test:plain_rod"), Optional.of(id("mmdlib:rod")), Optional.empty()),
-            id("test:plain_pipe"),
-                new PartSubject(id("test:plain_pipe"), Optional.empty(), Optional.empty()));
-    var files = new HashMap<ResourceLocation, JsonElement>();
-    files.put(id("test:gear"), JsonParser.parseString(GEAR_TEMPLATE));
-    files.put(
-        id("test:diamond_coated_gear"),
-        JsonParser.parseString(
-            """
-            {
-              "item": "test:diamond_coated_gear",
-              "socket": "gears",
-              "modifiers": [
-                { "stat": "speed", "op": "add_multiplied_total", "amount": 0.4 },
-                { "stat": "energy_per_tick", "op": "add_value", "amount": 12 }
-              ]
-            }
-            """));
-    // A second template for the same part: the lower resource ID (test:gear) wins.
-    files.put(
-        id("test:zz_gear"),
-        JsonParser.parseString(GEAR_TEMPLATE.replace("\"gears\"", "\"other\"")));
-    // An unknown variable: logged and skipped, the rest still load.
-    files.put(
-        id("test:broken"),
-        JsonParser.parseString(
-            GEAR_TEMPLATE
-                .replace("mmdlib:gear", "mmdlib:plate")
-                .replace("$material.hardness - 3", "$material.colour")));
-    files.put(
-        id("test:pipe"),
-        JsonParser.parseString(
-            """
-{"item": "test:plain_pipe", "socket": "pipes",
- "modifiers": [{"stat": "speed", "op": "add_value", "amount": "$material.special.grip + 1"}]}
-"""));
-    files.put(
-        id("test:grip"),
-        JsonParser.parseString(
-            """
-            {"item": "test:invar_gear", "socket": "gears",
-             "modifiers": [{"stat": "grip", "op": "add_value",
-                            "amount": "$material.special.grip + $material.special.none"}]}
-            """));
-    var parts = MachineParts.parse(files, item -> Optional.ofNullable(subjects.get(item)));
-    var iron = parts.resolve(id("test:iron_gear")).orElseThrow();
-    require(iron.socket().equals("gears"));
-    require(near(iron.modifiers().get(0).amount(), .08));
-    require(near(iron.modifiers().get(1).amount(), .12));
-    require(iron.modifiers().get(0).op() == AttributeModifier.Operation.ADD_MULTIPLIED_BASE);
-    require(iron.durability().equals(OptionalInt.of(2000)));
-    // An item file beats the template; missing special keys count as 0.
-    var invar = parts.resolve(id("test:invar_gear")).orElseThrow();
-    require(invar.modifiers().size() == 1 && invar.modifiers().get(0).amount() == 2);
-    require(invar.durability().isEmpty());
-    var coated = parts.resolve(id("test:diamond_coated_gear")).orElseThrow();
-    require(coated.modifiers().get(0).op() == AttributeModifier.Operation.ADD_MULTIPLIED_TOTAL);
-    require(coated.modifiers().get(0).amount() == .4 && coated.durability().isEmpty());
-    // Hardness 0: the template's durability is 0, so this gear is not a part.
-    require(parts.resolve(id("test:soft_gear")).isEmpty());
-    // An item without a material can't use $material.
-    require(parts.resolve(id("test:plain_pipe")).isEmpty());
-    require(parts.resolve(id("test:plain_rod")).isEmpty());
-    require(parts.resolve(id("test:unknown")).isEmpty());
-    require(parts.resolve(id("test:iron_gear")).orElseThrow() == iron); // cached
-    // A non-finite amount (10 / hardness 0) makes the item not a part.
-    var rods =
-        MachineParts.parse(
-            Map.of(
-                id("test:rod"),
-                JsonParser.parseString(
-                    """
-{"part": "mmdlib:rod", "socket": "shafts",
- "modifiers": [{"stat": "speed", "op": "add_value", "amount": "10 / $material.hardness"}]}
-""")),
-            item ->
-                Optional.of(
-                    new PartSubject(
-                        item,
-                        Optional.of(id("mmdlib:rod")),
-                        gear(item.toString(), item.getPath().equals("zero_rod") ? 0 : 5, Map.of())
-                            .material())));
-    require(rods.resolve(id("test:zero_rod")).isEmpty());
-    require(rods.resolve(id("test:steel_rod")).orElseThrow().modifiers().get(0).amount() == 2);
-    expectFailure(() -> parsePart("{\"socket\":\"gears\",\"modifiers\":[]}"));
-    expectFailure(
-        () ->
-            parsePart(
-                "{\"part\":\"mmdlib:gear\",\"item\":\"test:x\",\"socket\":\"gears\",\"modifiers\":[]}"));
-    expectFailure(() -> parsePart(GEAR_TEMPLATE.replace("$material.hardness - 3", "$speed")));
-    // A misspelt key is an error, not a part that quietly never wears.
-    expectFailure(() -> parsePart(GEAR_TEMPLATE.replace("\"durability\"", "\"durabilty\"")));
-    expectFailure(() -> parsePart(GEAR_TEMPLATE.replace("\"stat\"", "\"note\": 1, \"stat\"")));
-    checkParts();
-    int generation = MachineParts.current().generation();
-    MachineParts.install(parts);
-    require(MachineParts.current() == parts && parts.generation() > generation);
-    MachineParts.install(MachineParts.NONE);
-  }
-
-  /** Files that decode but can't do anything are reported by check, which also resolves early. */
-  private static void checkParts() {
-    var files = new HashMap<ResourceLocation, JsonElement>();
-    files.put(id("test:gear"), JsonParser.parseString(GEAR_TEMPLATE));
-    files.put(
-        id("test:typo_part"),
-        JsonParser.parseString(GEAR_TEMPLATE.replace("mmdlib:gear", "mmdlib:gaer")));
-    files.put(
-        id("test:typo_item"),
-        JsonParser.parseString(
-            "{\"item\": \"test:iron_geer\", \"socket\": \"gears\", \"modifiers\": []}"));
-    files.put(
-        id("test:special"),
-        JsonParser.parseString(
-            """
-            {"item": "test:invar_gear", "socket": "gears",
-             "modifiers": [{"stat": "grip", "op": "add_value",
-                            "amount": "$material.special.grip + $material.special.gripp"}]}
-            """));
-    var subjects =
-        Map.of(
-            id("test:iron_gear"), gear("test:iron_gear", 4, Map.of()),
-            id("test:invar_gear"), gear("test:invar_gear", 6, Map.of("grip", 2.0)),
-            id("test:soft_gear"), gear("test:soft_gear", 0, Map.of()));
-    var lookups = new int[1];
-    var parts =
-        MachineParts.parse(
-            files,
-            item -> {
-              lookups[0]++;
-              return Optional.ofNullable(subjects.get(item));
-            });
-    var problems = parts.check(subjects.keySet());
-    require(problems.size() == 3);
-    require(mentions(problems, "test:typo_part", "mmdlib:gaer"));
-    require(mentions(problems, "test:typo_item", "test:iron_geer"));
-    require(mentions(problems, "test:special", "$material.special.gripp"));
-    // Every item was resolved, so template errors are already logged and nothing looks up again.
-    int after = lookups[0];
-    require(parts.resolve(id("test:iron_gear")).isPresent());
-    require(parts.resolve(id("test:soft_gear")).isEmpty());
-    require(lookups[0] == after);
-  }
-
-  private static boolean mentions(List<String> problems, String... words) {
-    return problems.stream()
-        .anyMatch(problem -> List.of(words).stream().allMatch(problem::contains));
-  }
-
-  private static void amountMatch() {
-    require(AmountMatch.EXACT.test(5, 5) && !AmountMatch.EXACT.test(5, 6));
-    require(AmountMatch.AT_LEAST.test(5, 6) && AmountMatch.AT_LEAST.test(5, 5));
-    require(!AmountMatch.AT_LEAST.test(5, 4));
-    require(AmountMatch.ANY_AMOUNT.test(5, 0));
-    require(AmountMatch.AT_LEAST.test(3_000_000_000L, 3_000_000_001L));
-    require(!AmountMatch.AT_LEAST.test(3_000_000_000L, 2_999_999_999L));
-  }
-
-  private static void inputClaims() {
-    // A broad part listed first moves off the only entry a later part accepts.
-    var two = new InputClaims(new long[] {1, 1});
-    require(two.claim(1, any -> true) == 0);
-    require(two.claim(1, entry -> entry == 0) == 0);
-    require(two.claimed(0) == 1 && two.claimed(1) == 1);
-    // Listing them the other way round gives the same answer.
-    var swapped = new InputClaims(new long[] {1, 1});
-    require(swapped.claim(1, entry -> entry == 0) == 0);
-    require(swapped.claim(1, any -> true) == 0);
-    require(swapped.claimed(0) == 1 && swapped.claimed(1) == 1);
-    // A chain: the third part moves the second, which moves the first.
-    var chain = new InputClaims(new long[] {1, 1, 1});
-    require(chain.claim(1, entry -> entry != 1) == 0);
-    require(chain.claim(1, entry -> entry != 2) == 0);
-    require(chain.claim(1, entry -> entry == 0) == 0);
-    require(chain.claimed(0) == 1 && chain.claimed(1) == 1 && chain.claimed(2) == 1);
-    // Part of a stack moves, and the rest stays.
-    var stacks = new InputClaims(new long[] {3, 3});
-    require(stacks.claim(4, any -> true) == 0);
-    require(stacks.claim(2, entry -> entry == 0) == 0);
-    require(stacks.claimed(0) == 3 && stacks.claimed(1) == 3);
-    // Nothing counts twice, and a claim that can't be met reports how much it's short.
-    var shared = new InputClaims(new long[] {3});
-    require(shared.claim(2, any -> true) == 0);
-    require(shared.claim(2, any -> true) == 1);
-    require(shared.claimed(0) == 3);
-    // Two parts that only accept one entry can't both be met by moving.
-    var stuck = new InputClaims(new long[] {1, 1});
-    require(stuck.claim(1, entry -> entry == 0) == 0);
-    require(stuck.claim(1, entry -> entry == 0) == 1);
-    require(stuck.claimed(0) == 1 && stuck.claimed(1) == 0);
-    // Empty entries aren't asked about, so an ingredient never tests an empty stack.
-    var empty = new InputClaims(new long[] {0, 2});
-    require(
-        empty.claim(
-                1,
-                entry -> {
-                  require(entry == 1);
-                  return true;
-                })
-            == 0);
-    // Energy amounts don't fit an int.
-    var energy = new InputClaims(new long[] {3_000_000_000L});
-    require(energy.claim(2_999_999_999L, any -> true) == 0);
-    require(energy.claim(2, any -> true) == 1);
-  }
-
-  private static PartSubject gear(String item, double hardness, Map<String, Double> special) {
-    return new PartSubject(
-        id(item),
-        Optional.of(id("mmdlib:gear")),
-        Optional.of(new MaterialProperties("metal", -1, hardness, 1, 0, 0, Set.of(), special)));
-  }
-
-  private static PartDefinition parsePart(String json) {
-    return PartDefinition.CODEC.parse(JsonOps.INSTANCE, JsonParser.parseString(json)).getOrThrow();
-  }
-
-  private static ResourceLocation id(String value) {
-    return ResourceLocation.parse(value);
-  }
-
-  static InternalsDefinition parseInternals(String json) {
-    return InternalsDefinition.CODEC
-        .parse(JsonOps.INSTANCE, JsonParser.parseString(json))
-        .getOrThrow();
-  }
-
-  private static StatDefinition parseStat(String json) {
-    return StatDefinition.CODEC.parse(JsonOps.INSTANCE, JsonParser.parseString(json)).getOrThrow();
-  }
-
-  private static StatModifier value(String stat, double amount) {
-    return new StatModifier(stat, AttributeModifier.Operation.ADD_VALUE, amount);
-  }
-
-  private static StatModifier base(String stat, double amount) {
-    return new StatModifier(stat, AttributeModifier.Operation.ADD_MULTIPLIED_BASE, amount);
-  }
-
-  private static StatModifier total(String stat, double amount) {
-    return new StatModifier(stat, AttributeModifier.Operation.ADD_MULTIPLIED_TOTAL, amount);
-  }
-
-  @SafeVarargs
-  private static <T> List<T> concat(List<T>... lists) {
-    var all = new ArrayList<T>();
-    for (var list : lists) all.addAll(list);
-    return all;
-  }
-
-  private static Function<String, OptionalDouble> v(String name, double value) {
-    var values = Map.of(name, value);
-    return key ->
-        values.containsKey(key) ? OptionalDouble.of(values.get(key)) : OptionalDouble.empty();
-  }
-
-  private static Function<String, OptionalDouble> none() {
-    return name -> OptionalDouble.empty();
-  }
-
-  static boolean near(double a, double b) {
-    return Math.abs(a - b) < 1e-9;
-  }
-
-  static void require(boolean condition) {
-    if (!condition) throw new AssertionError("Machine core assertion failed");
-  }
-
-  static void expectFailure(Runnable action) {
-    try {
-      action.run();
-    } catch (IllegalArgumentException | IllegalStateException ex) {
-      return;
-    }
-    throw new AssertionError("Invalid machine input accepted");
-  }
-}
diff --git a/src/runtimeTest/datapack/pack.mcmeta b/src/runtimeTest/datapack/pack.mcmeta
deleted file mode 100644
index c98bb4b..0000000
--- a/src/runtimeTest/datapack/pack.mcmeta
+++ /dev/null
@@ -1,6 +0,0 @@
-{
-  "pack": {
-    "pack_format": 48,
-    "description": "MMDLib runtime smoke test data"
-  }
-}
diff --git a/src/runtimeTest/java/zone/moddev/mc/lib/smoke/ContentSmoke.java b/src/runtimeTest/java/zone/moddev/mc/lib/smoke/ContentSmoke.java
new file mode 100644
index 0000000..95de728
--- /dev/null
+++ b/src/runtimeTest/java/zone/moddev/mc/lib/smoke/ContentSmoke.java
@@ -0,0 +1,211 @@
+package zone.moddev.mc.lib.smoke;
+
+import net.minecraft.core.BlockPos;
+import net.minecraft.core.Direction;
+import net.minecraft.core.component.DataComponents;
+import net.minecraft.core.registries.BuiltInRegistries;
+import net.minecraft.nbt.CompoundTag;
+import net.minecraft.network.chat.Component;
+import net.minecraft.resources.ResourceLocation;
+import net.minecraft.server.level.ServerLevel;
+import net.minecraft.world.InteractionHand;
+import net.minecraft.world.entity.Entity;
+import net.minecraft.world.entity.item.ItemEntity;
+import net.minecraft.world.entity.projectile.AbstractArrow;
+import net.minecraft.world.item.*;
+import net.minecraft.world.item.context.UseOnContext;
+import net.minecraft.world.level.block.*;
+import net.minecraft.world.phys.AABB;
+import net.minecraft.world.phys.BlockHitResult;
+import net.minecraft.world.phys.Vec3;
+
+import net.neoforged.neoforge.common.CommonHooks;
+import net.neoforged.neoforge.common.util.FakePlayerFactory;
+
+import zone.moddev.mc.lib.MMDLib;
+import zone.moddev.mc.lib.common.recipes.CrushingRecipe;
+import zone.moddev.mc.lib.common.runtime.*;
+
+/**
+ * Checks MMDLib's own material content on a running server: registration, block placement,
+ * ammunition recovery, buckets, crushing, mining tiers and armor textures.
+ */
+final class ContentSmoke {
+  private ContentSmoke() {}
+
+  /** The materials, the standalone detector plate and the item count. */
+  static void registered() {
+    if (MMDLib.loadedMaterials().size() != 15) throw new AssertionError("Missing materials");
+    var detector = BuiltInRegistries.BLOCK.get(ResourceLocation.parse("mmdlib:human_detector"));
+    if (MMDLib.standaloneBlocks().size() != 1
+        || !(detector instanceof StandardBlocks.DetectorPressurePlate))
+      throw new AssertionError("Standalone player-only plate missing: " + detector);
+    if (registrar().items().size() != 234)
+      throw new AssertionError("Wrong runtime item count: " + registrar().items().size());
+  }
+
+  /** Every block item places its block. */
+  static void blocksPlace(ServerLevel level) {
+    var player = FakePlayerFactory.getMinecraft(level);
+    int placed = 0;
+    for (var entry : registrar().blocks().entrySet()) {
+      var itemHolder = registrar().items().get(entry.getKey());
+      if (itemHolder == null || !(itemHolder.get() instanceof BlockItem item)) continue;
+      var pos = new BlockPos((placed % 8) * 4, level.getMaxBuildHeight() - 12, (placed / 8) * 4);
+      level.getChunkAt(pos);
+      level.setBlock(pos, Blocks.STONE.defaultBlockState(), 3);
+      level.setBlock(pos.above(), Blocks.AIR.defaultBlockState(), 3);
+      level.setBlock(pos.above(2), Blocks.AIR.defaultBlockState(), 3);
+      player.setPos(pos.getX() + .5, pos.getY() + 1, pos.getZ() + 2.5);
+      player.setItemInHand(InteractionHand.MAIN_HAND, new ItemStack(item));
+      var hit = new BlockHitResult(Vec3.atCenterOf(pos).add(0, .5, 0), Direction.UP, pos, false);
+      var result = item.useOn(new UseOnContext(player, InteractionHand.MAIN_HAND, hit));
+      if (!result.consumesAction()
+          || level.getBlockState(pos.above()).getBlock() != entry.getValue().get())
+        throw new AssertionError("Placement failed: " + entry.getKey() + " result " + result);
+      placed++;
+    }
+    if (placed != 59) throw new AssertionError("Wrong placed block count: " + placed);
+  }
+
+  /** Arrows and bolts keep their pickup item and its components across save and load. */
+  static void ammunitionRecovers(ServerLevel level) {
+    var player = FakePlayerFactory.getMinecraft(level);
+    int projectiles = 0;
+    for (var entry : registrar().projectiles().entrySet()) {
+      var ammo = (MaterialAmmoItem) registrar().items().get(entry.getKey()).get();
+      var stack = new ItemStack(ammo, 12);
+      stack.set(DataComponents.CUSTOM_NAME, Component.literal("Recovery smoke test"));
+      var projectile = ammo.createArrow(level, stack, player, null);
+      if (projectile.pickup != AbstractArrow.Pickup.ALLOWED
+          || projectile.getType() != entry.getValue().get())
+        throw new AssertionError("Wrong projectile type/pickup");
+      checkStack(stack.copyWithCount(1), projectile.getPickupItemStackOrigin());
+      if (stack.getCount() != 12)
+        throw new AssertionError("Ammunition consumed during construction");
+      var tag = projectile.saveWithoutId(new CompoundTag());
+      var restored = entry.getValue().get().create(level);
+      restored.load(tag);
+      checkStack(stack.copyWithCount(1), restored.getPickupItemStackOrigin());
+      projectiles++;
+    }
+    if (projectiles != 14) throw new AssertionError("Wrong projectile count: " + projectiles);
+  }
+
+  /** Every bucket places its fluid and picks it back up. */
+  static void bucketsPlaceAndRecover(ServerLevel level) {
+    var player = FakePlayerFactory.getMinecraft(level);
+    int fluids = 0;
+    for (var family : registrar().fluidFamilies()) {
+      var pos =
+          new BlockPos((fluids % 8) * 4, level.getMaxBuildHeight() - 12, 48 + (fluids / 8) * 4);
+      level.getChunkAt(pos);
+      level.setBlock(pos.below(), Blocks.STONE.defaultBlockState(), 3);
+      level.setBlock(pos, Blocks.AIR.defaultBlockState(), 3);
+      var bucket = family.bucket.get();
+      var stack = new ItemStack(bucket);
+      if (!bucket.emptyContents(player, level, pos, null, stack))
+        throw new AssertionError("Bucket placement failed: " + family.definition.name());
+      var state = level.getBlockState(pos);
+      if (level.getFluidState(pos).getType() != family.source.get())
+        throw new AssertionError("Wrong fluid placed");
+      var recovered = ((BucketPickup) state.getBlock()).pickupBlock(player, level, pos, state);
+      if (recovered.getItem() != bucket) throw new AssertionError("Wrong fluid bucket recovered");
+      fluids++;
+    }
+    if (fluids != 15) throw new AssertionError("Wrong bucket count: " + fluids);
+  }
+
+  /** A real dropped-item crushing recipe, not just recipe decoding. */
+  static void crackhammerCrushesDrops(ServerLevel level, BlockPos floor) {
+    var player = FakePlayerFactory.getMinecraft(level);
+    level.getChunkAt(floor);
+    level.setBlock(floor, Blocks.STONE.defaultBlockState(), 3);
+    level
+        .getEntitiesOfClass(ItemEntity.class, new AABB(floor.above()).inflate(2))
+        .forEach(Entity::discard);
+    var input =
+        new ItemEntity(
+            level,
+            floor.getX() + .5,
+            floor.getY() + 1.1,
+            floor.getZ() + .5,
+            new ItemStack(Items.IRON_INGOT, 2));
+    level.addFreshEntity(input);
+    var hammer = new ItemStack(item("mmdlib:iron_crackhammer"));
+    player.getAbilities().instabuild = false;
+    player.setShiftKeyDown(false);
+    player.setItemInHand(InteractionHand.MAIN_HAND, hammer);
+    // Aim horizontally at a floating drop: there is no clicked block
+    // to accidentally exercise only the old useOn path.
+    player.setPos(input.getX(), input.getY() + .1 - player.getEyeHeight(), input.getZ() - 2);
+    player.setYRot(0);
+    player.setXRot(0);
+    // Crushing is a data-bound tool-right-click handler, so fire the real NeoForge event the
+    // bridge listens to.
+    CommonHooks.onItemRightClick(player, InteractionHand.MAIN_HAND);
+    var dust = item("mmdlib:iron_dust");
+    var outputs =
+        level.getEntitiesOfClass(
+            ItemEntity.class,
+            new AABB(floor.above()),
+            entity -> entity.getItem().getItem() == dust);
+    // The default server config crushes up to 8 per click, 1 wear each: both ingots go.
+    if (input.isAlive() || outputs.isEmpty() || hammer.getDamageValue() != 2)
+      throw new AssertionError(
+          "Crackhammer failed: input="
+              + input.getItem().getCount()
+              + ", outputs="
+              + outputs.size()
+              + ", wear="
+              + hammer.getDamageValue()
+              + ", recipes="
+              + level.getRecipeManager().getAllRecipesFor(CrushingRecipe.TYPE.get()).size());
+  }
+
+  /** Tag-based mining tiers and packaged armor textures. */
+  static void toolsAndArmor() throws Exception {
+    var emerald = new ItemStack(item("mmdlib:emerald_pickaxe"));
+    var quartz = new ItemStack(item("mmdlib:quartz_pickaxe"));
+    if (!emerald.isCorrectToolForDrops(Blocks.OBSIDIAN.defaultBlockState())
+        || quartz.isCorrectToolForDrops(Blocks.OBSIDIAN.defaultBlockState()))
+      throw new AssertionError("Incorrect tag-based mining tiers");
+    var helmet = new ItemStack(item("mmdlib:emerald_helmet"));
+    if (helmet.get(DataComponents.DYED_COLOR) != null)
+      throw new AssertionError("Precolored armor must not be tinted twice");
+    var texture =
+        ((ArmorItem) helmet.getItem()).getMaterial().value().layers().getFirst().texture(false);
+    if (!texture.getNamespace().equals("mmdlib"))
+      throw new AssertionError("Missing custom armor layer");
+    requireResource(texture, "Armor texture");
+    var horse = (AnimalArmorItem) item("mmdlib:emerald_horse_armor");
+    requireResource(horse.getTexture(), "Horse armor texture");
+  }
+
+  /** MMDLib's own registrar; the machine smoke registrar is skipped. */
+  private static RuntimeContentRegistrar registrar() {
+    var own =
+        RuntimeContentRegistrar.instances().stream()
+            .filter(registrar -> registrar != MachineSmoke.registrar())
+            .toList();
+    if (own.size() != 1) throw new AssertionError("Expected one MMDLib registrar, got " + own);
+    return own.getFirst();
+  }
+
+  private static Item item(String id) {
+    return registrar().items().get(ResourceLocation.parse(id)).get();
+  }
+
+  private static void requireResource(ResourceLocation texture, String what) throws Exception {
+    try (var stream =
+        ContentSmoke.class.getResourceAsStream(
+            "/assets/" + texture.getNamespace() + "/" + texture.getPath())) {
+      if (stream == null) throw new AssertionError(what + " not packaged: " + texture);
+    }
+  }
+
+  private static void checkStack(ItemStack expected, ItemStack actual) {
+    if (actual.getCount() != 1 || !ItemStack.isSameItemSameComponents(expected, actual))
+      throw new AssertionError("Wrong projectile recovery stack");
+  }
+}
diff --git a/src/runtimeTest/java/zone/moddev/mc/lib/smoke/MMDLibGameTests.java b/src/runtimeTest/java/zone/moddev/mc/lib/smoke/MMDLibGameTests.java
new file mode 100644
index 0000000..fb62048
--- /dev/null
+++ b/src/runtimeTest/java/zone/moddev/mc/lib/smoke/MMDLibGameTests.java
@@ -0,0 +1,94 @@
+package zone.moddev.mc.lib.smoke;
+
+import net.minecraft.core.BlockPos;
+import net.minecraft.gametest.framework.GameTest;
+import net.minecraft.gametest.framework.GameTestHelper;
+
+import net.neoforged.neoforge.gametest.GameTestHolder;
+import net.neoforged.neoforge.gametest.PrefixGameTestTemplate;
+
+/**
+ * MMDLib's server game tests, run by {@code ./gradlew runGameTestServer}. Each one runs a check in
+ * an empty 5×5×5 structure ({@code mmdlib:empty}) and passes when the check returns; a thrown error
+ * fails it with that error's message. The server exits with the number of failed tests.
+ */
+@GameTestHolder("mmdlib")
+@PrefixGameTestTemplate(false)
+public final class MMDLibGameTests {
+  private static final String EMPTY = "empty";
+
+  private MMDLibGameTests() {}
+
+  @GameTest(template = EMPTY)
+  public static void contentRegistered(GameTestHelper helper) {
+    ContentSmoke.registered();
+    helper.succeed();
+  }
+
+  @GameTest(template = EMPTY)
+  public static void blocksPlace(GameTestHelper helper) {
+    ContentSmoke.blocksPlace(helper.getLevel());
+    helper.succeed();
+  }
+
+  @GameTest(template = EMPTY)
+  public static void ammunitionRecovers(GameTestHelper helper) {
+    ContentSmoke.ammunitionRecovers(helper.getLevel());
+    helper.succeed();
+  }
+
+  @GameTest(template = EMPTY)
+  public static void bucketsPlaceAndRecover(GameTestHelper helper) {
+    ContentSmoke.bucketsPlaceAndRecover(helper.getLevel());
+    helper.succeed();
+  }
+
+  @GameTest(template = EMPTY)
+  public static void crackhammerCrushesDrops(GameTestHelper helper) {
+    ContentSmoke.crackhammerCrushesDrops(
+        helper.getLevel(), helper.absolutePos(new BlockPos(2, 1, 2)));
+    helper.succeed();
+  }
+
+  @GameTest(template = EMPTY)
+  public static void toolsAndArmor(GameTestHelper helper) throws Exception {
+    ContentSmoke.toolsAndArmor();
+    helper.succeed();
+  }
+
+  @GameTest(template = EMPTY)
+  public static void capabilities(GameTestHelper helper) {
+    CapabilitySmoke.check(helper.getLevel());
+    helper.succeed();
+  }
+
+  @GameTest(template = EMPTY)
+  public static void standaloneMachine(GameTestHelper helper) {
+    MachineSmoke.check(helper.getLevel());
+    helper.succeed();
+  }
+
+  @GameTest(template = EMPTY)
+  public static void machinePartsLoad(GameTestHelper helper) {
+    MachinePartsLoaderSmoke.check();
+    helper.succeed();
+  }
+
+  @GameTest(template = EMPTY)
+  public static void machineInternals(GameTestHelper helper) {
+    MachineInternalsSmoke.check(helper.getLevel());
+    helper.succeed();
+  }
+
+  @GameTest(template = EMPTY)
+  public static void machineInventory(GameTestHelper helper) {
+    MachineInventorySmoke.check(helper.getLevel());
+    helper.succeed();
+  }
+
+  @GameTest(template = EMPTY)
+  public static void machineRecipes(GameTestHelper helper) {
+    MachineRecipeSmoke.check(helper.getLevel());
+    helper.succeed();
+  }
+}
diff --git a/src/runtimeTest/java/zone/moddev/mc/lib/smoke/MachinePartsLoaderSmoke.java b/src/runtimeTest/java/zone/moddev/mc/lib/smoke/MachinePartsLoaderSmoke.java
index 06bc4b3..ff1d663 100644
--- a/src/runtimeTest/java/zone/moddev/mc/lib/smoke/MachinePartsLoaderSmoke.java
+++ b/src/runtimeTest/java/zone/moddev/mc/lib/smoke/MachinePartsLoaderSmoke.java
@@ -8,8 +8,8 @@
 import zone.moddev.mc.lib.machine.internals.MachineParts;
 
 /**
- * Checks that part files in a real datapack reach {@link MachineParts#current()} through the reload
- * listener. The smoke init script copies {@code src/runtimeTest/datapack} into the smoke world.
+ * Checks that part files shipped as resources ({@code src/runtimeTest/resources/data/mmdlib_smoke})
+ * reach {@link MachineParts#current()} through the reload listener.
  */
 final class MachinePartsLoaderSmoke {
   private MachinePartsLoaderSmoke() {}
diff --git a/src/runtimeTest/java/zone/moddev/mc/lib/smoke/RuntimeSmoke.java b/src/runtimeTest/java/zone/moddev/mc/lib/smoke/RuntimeSmoke.java
deleted file mode 100644
index 22ec14b..0000000
--- a/src/runtimeTest/java/zone/moddev/mc/lib/smoke/RuntimeSmoke.java
+++ /dev/null
@@ -1,221 +0,0 @@
-package zone.moddev.mc.lib.smoke;
-
-import net.minecraft.core.BlockPos;
-import net.minecraft.core.Direction;
-import net.minecraft.core.component.DataComponents;
-import net.minecraft.nbt.CompoundTag;
-import net.minecraft.network.chat.Component;
-import net.minecraft.resources.ResourceLocation;
-import net.minecraft.world.InteractionHand;
-import net.minecraft.world.entity.projectile.AbstractArrow;
-import net.minecraft.world.item.*;
-import net.minecraft.world.item.context.UseOnContext;
-import net.minecraft.world.level.block.*;
-import net.minecraft.world.phys.BlockHitResult;
-import net.minecraft.world.phys.Vec3;
-
-import net.neoforged.bus.api.SubscribeEvent;
-import net.neoforged.fml.common.EventBusSubscriber;
-import net.neoforged.neoforge.common.util.FakePlayerFactory;
-import net.neoforged.neoforge.event.server.ServerStartedEvent;
-
-import zone.moddev.mc.lib.MMDLib;
-import zone.moddev.mc.lib.common.runtime.*;
-
-/** Injected into a dedicated development run only; excluded from the released JAR. */
-@EventBusSubscriber(modid = "mmdlib")
-public final class RuntimeSmoke {
-  @SubscribeEvent
-  public static void started(ServerStartedEvent event) {
-    if (Boolean.getBoolean("mmdlib.guiSmoke")) return;
-    var server = event.getServer();
-    try {
-      var level = server.overworld();
-      var player = FakePlayerFactory.getMinecraft(level);
-      int placed = 0, projectiles = 0, fluids = 0;
-      if (MMDLib.loadedMaterials().size() != 15) throw new AssertionError("Missing materials");
-      var detector =
-          net.minecraft.core.registries.BuiltInRegistries.BLOCK.get(
-              net.minecraft.resources.ResourceLocation.parse("mmdlib:human_detector"));
-      if (MMDLib.standaloneBlocks().size() != 1
-          || !(detector
-              instanceof zone.moddev.mc.lib.common.runtime.StandardBlocks.DetectorPressurePlate))
-        throw new AssertionError("Standalone player-only plate missing: " + detector);
-      for (var registrar : RuntimeContentRegistrar.instances()) {
-        if (registrar == MachineSmoke.registrar()) continue;
-        if (registrar.items().size() != 234)
-          throw new AssertionError("Wrong runtime item count: " + registrar.items().size());
-        for (var entry : registrar.blocks().entrySet()) {
-          var itemHolder = registrar.items().get(entry.getKey());
-          if (itemHolder == null || !(itemHolder.get() instanceof BlockItem item)) continue;
-          var pos =
-              new BlockPos((placed % 8) * 4, level.getMaxBuildHeight() - 12, (placed / 8) * 4);
-          level.getChunkAt(pos);
-          level.setBlock(pos, Blocks.STONE.defaultBlockState(), 3);
-          level.setBlock(pos.above(), Blocks.AIR.defaultBlockState(), 3);
-          level.setBlock(pos.above(2), Blocks.AIR.defaultBlockState(), 3);
-          player.setPos(pos.getX() + .5, pos.getY() + 1, pos.getZ() + 2.5);
-          player.setItemInHand(InteractionHand.MAIN_HAND, new ItemStack(item));
-          var hit =
-              new BlockHitResult(Vec3.atCenterOf(pos).add(0, .5, 0), Direction.UP, pos, false);
-          var result = item.useOn(new UseOnContext(player, InteractionHand.MAIN_HAND, hit));
-          if (!result.consumesAction()
-              || level.getBlockState(pos.above()).getBlock() != entry.getValue().get())
-            throw new AssertionError("Placement failed: " + entry.getKey() + " result " + result);
-          placed++;
-        }
-        for (var entry : registrar.projectiles().entrySet()) {
-          var ammo = (MaterialAmmoItem) registrar.items().get(entry.getKey()).get();
-          var stack = new ItemStack(ammo, 12);
-          stack.set(DataComponents.CUSTOM_NAME, Component.literal("Recovery smoke test"));
-          var projectile = ammo.createArrow(level, stack, player, null);
-          if (projectile.pickup != AbstractArrow.Pickup.ALLOWED
-              || projectile.getType() != entry.getValue().get())
-            throw new AssertionError("Wrong projectile type/pickup");
-          checkStack(stack.copyWithCount(1), projectile.getPickupItemStackOrigin());
-          if (stack.getCount() != 12)
-            throw new AssertionError("Ammunition consumed during construction");
-          var tag = projectile.saveWithoutId(new CompoundTag());
-          var restored = entry.getValue().get().create(level);
-          restored.load(tag);
-          checkStack(stack.copyWithCount(1), restored.getPickupItemStackOrigin());
-          projectiles++;
-        }
-        for (var family : registrar.fluidFamilies()) {
-          var pos =
-              new BlockPos((fluids % 8) * 4, level.getMaxBuildHeight() - 12, 48 + (fluids / 8) * 4);
-          level.getChunkAt(pos);
-          level.setBlock(pos.below(), Blocks.STONE.defaultBlockState(), 3);
-          level.setBlock(pos, Blocks.AIR.defaultBlockState(), 3);
-          var bucket = family.bucket.get();
-          var stack = new ItemStack(bucket);
-          if (!bucket.emptyContents(player, level, pos, null, stack))
-            throw new AssertionError("Bucket placement failed: " + family.definition.name());
-          var state = level.getBlockState(pos);
-          if (level.getFluidState(pos).getType() != family.source.get())
-            throw new AssertionError("Wrong fluid placed");
-          var recovered = ((BucketPickup) state.getBlock()).pickupBlock(player, level, pos, state);
-          if (recovered.getItem() != bucket)
-            throw new AssertionError("Wrong fluid bucket recovered");
-          fluids++;
-        }
-        // Exercise an actual dropped-item crushing recipe, not just recipe decoding.
-        var floor = level.getSharedSpawnPos().above(8);
-        level.getChunkAt(floor);
-        level.setBlock(floor, Blocks.STONE.defaultBlockState(), 3);
-        level
-            .getEntitiesOfClass(
-                net.minecraft.world.entity.item.ItemEntity.class,
-                new net.minecraft.world.phys.AABB(floor.above()).inflate(2))
-            .forEach(net.minecraft.world.entity.Entity::discard);
-        var input =
-            new net.minecraft.world.entity.item.ItemEntity(
-                level,
-                floor.getX() + .5,
-                floor.getY() + 1.1,
-                floor.getZ() + .5,
-                new ItemStack(Items.IRON_INGOT, 2));
-        level.addFreshEntity(input);
-        var hammer =
-            new ItemStack(
-                registrar.items().get(ResourceLocation.parse("mmdlib:iron_crackhammer")).get());
-        player.getAbilities().instabuild = false;
-        player.setShiftKeyDown(false);
-        player.setItemInHand(InteractionHand.MAIN_HAND, hammer);
-        // Aim horizontally at a floating drop: there is no clicked block
-        // to accidentally exercise only the old useOn path.
-        player.setPos(input.getX(), input.getY() + .1 - player.getEyeHeight(), input.getZ() - 2);
-        player.setYRot(0);
-        player.setXRot(0);
-        // Crushing is a data-bound tool-right-click handler, so fire the real NeoForge event the
-        // bridge listens to.
-        net.neoforged.neoforge.common.CommonHooks.onItemRightClick(
-            player, InteractionHand.MAIN_HAND);
-        var hammerResult = "event";
-        var dust = registrar.items().get(ResourceLocation.parse("mmdlib:iron_dust")).get();
-        var outputs =
-            level.getEntitiesOfClass(
-                net.minecraft.world.entity.item.ItemEntity.class,
-                new net.minecraft.world.phys.AABB(floor.above()),
-                entity -> entity.getItem().getItem() == dust);
-        // The default server config crushes up to 8 per click, 1 wear each: both ingots go.
-        if (input.isAlive() || outputs.isEmpty() || hammer.getDamageValue() != 2)
-          throw new AssertionError(
-              "Crackhammer failed: result="
-                  + hammerResult
-                  + ", input="
-                  + input.getItem().getCount()
-                  + ", outputs="
-                  + outputs.size()
-                  + ", wear="
-                  + hammer.getDamageValue()
-                  + ", recipes="
-                  + level
-                      .getRecipeManager()
-                      .getAllRecipesFor(zone.moddev.mc.lib.common.recipes.CrushingRecipe.TYPE.get())
-                      .size());
-        var emerald =
-            new ItemStack(
-                registrar.items().get(ResourceLocation.parse("mmdlib:emerald_pickaxe")).get());
-        var quartz =
-            new ItemStack(
-                registrar.items().get(ResourceLocation.parse("mmdlib:quartz_pickaxe")).get());
-        if (!emerald.isCorrectToolForDrops(Blocks.OBSIDIAN.defaultBlockState())
-            || quartz.isCorrectToolForDrops(Blocks.OBSIDIAN.defaultBlockState()))
-          throw new AssertionError("Incorrect tag-based mining tiers");
-        var helmet =
-            new ItemStack(
-                registrar.items().get(ResourceLocation.parse("mmdlib:emerald_helmet")).get());
-        if (helmet.get(DataComponents.DYED_COLOR) != null)
-          throw new AssertionError("Precolored armor must not be tinted twice");
-        var texture =
-            ((ArmorItem) helmet.getItem()).getMaterial().value().layers().getFirst().texture(false);
-        if (!texture.getNamespace().equals("mmdlib"))
-          throw new AssertionError("Missing custom armor layer");
-        try (var stream =
-            RuntimeSmoke.class.getResourceAsStream(
-                "/assets/" + texture.getNamespace() + "/" + texture.getPath())) {
-          if (stream == null) throw new AssertionError("Armor texture not packaged: " + texture);
-        }
-        var horse =
-            (AnimalArmorItem)
-                registrar.items().get(ResourceLocation.parse("mmdlib:emerald_horse_armor")).get();
-        try (var stream =
-            RuntimeSmoke.class.getResourceAsStream(
-                "/assets/"
-                    + horse.getTexture().getNamespace()
-                    + "/"
-                    + horse.getTexture().getPath())) {
-          if (stream == null)
-            throw new AssertionError("Horse armor texture not packaged: " + horse.getTexture());
-        }
-      }
-      CapabilitySmoke.check(level);
-      MachineSmoke.check(level);
-      MachinePartsLoaderSmoke.check();
-      MachineInternalsSmoke.check(level);
-      MachineInventorySmoke.check(level);
-      MachineRecipeSmoke.check(level);
-      if (projectiles != 14 || fluids != 15 || placed != 59)
-        throw new AssertionError(
-            "Wrong smoke counts: " + placed + "/" + projectiles + "/" + fluids);
-      MMDLib.LOGGER.info(
-          "MMDLIB_RUNTIME_SMOKE_PASS: {} blocks placed; {} arrow/bolt component-preserving save"
-              + " round trips; {} buckets placed/recovered; tag-based tiers, authored armor"
-              + " textures, dropped-item crushing, capability wrappers and a standalone ticking"
-              + " machine and item verified",
-          placed,
-          projectiles,
-          fluids);
-    } catch (Throwable ex) {
-      MMDLib.LOGGER.error("MMDLIB_RUNTIME_SMOKE_FAIL", ex);
-    } finally {
-      server.halt(false);
-    }
-  }
-
-  private static void checkStack(ItemStack expected, ItemStack actual) {
-    if (actual.getCount() != 1 || !ItemStack.isSameItemSameComponents(expected, actual))
-      throw new AssertionError("Wrong projectile recovery stack");
-  }
-}
diff --git a/src/runtimeTest/resources/data/mmdlib/structure/empty.nbt b/src/runtimeTest/resources/data/mmdlib/structure/empty.nbt
new file mode 100644
index 0000000..0a14490
Binary files /dev/null and b/src/runtimeTest/resources/data/mmdlib/structure/empty.nbt differ
diff --git a/src/runtimeTest/datapack/data/mmdlib_smoke/mmdlib/machine_part/broken.json b/src/runtimeTest/resources/data/mmdlib_smoke/mmdlib/machine_part/broken.json
similarity index 100%
rename from src/runtimeTest/datapack/data/mmdlib_smoke/mmdlib/machine_part/broken.json
rename to src/runtimeTest/resources/data/mmdlib_smoke/mmdlib/machine_part/broken.json
diff --git a/src/runtimeTest/datapack/data/mmdlib_smoke/mmdlib/machine_part/gear.json b/src/runtimeTest/resources/data/mmdlib_smoke/mmdlib/machine_part/gear.json
similarity index 100%
rename from src/runtimeTest/datapack/data/mmdlib_smoke/mmdlib/machine_part/gear.json
rename to src/runtimeTest/resources/data/mmdlib_smoke/mmdlib/machine_part/gear.json
diff --git a/src/runtimeTest/datapack/data/mmdlib_smoke/mmdlib/machine_part/unknown_item.json b/src/runtimeTest/resources/data/mmdlib_smoke/mmdlib/machine_part/unknown_item.json
similarity index 100%
rename from src/runtimeTest/datapack/data/mmdlib_smoke/mmdlib/machine_part/unknown_item.json
rename to src/runtimeTest/resources/data/mmdlib_smoke/mmdlib/machine_part/unknown_item.json
diff --git a/src/test/java/zone/moddev/mc/lib/machine/MachineAssertions.java b/src/test/java/zone/moddev/mc/lib/machine/MachineAssertions.java
new file mode 100644
index 0000000..2485c06
--- /dev/null
+++ b/src/test/java/zone/moddev/mc/lib/machine/MachineAssertions.java
@@ -0,0 +1,47 @@
+package zone.moddev.mc.lib.machine;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.fail;
+
+import java.util.Map;
+import java.util.OptionalDouble;
+import java.util.function.Function;
+
+import org.junit.jupiter.api.function.Executable;
+
+/** Shared assertions and inputs for the machine module's unit tests. */
+public final class MachineAssertions {
+  private MachineAssertions() {}
+
+  /** Asserts {@code a} and {@code b} agree to 1e-9. */
+  public static void assertNear(double expected, double actual) {
+    assertEquals(expected, actual, 1e-9);
+  }
+
+  /**
+   * Asserts {@code action} refuses its input the way machine code does: with an {@link
+   * IllegalArgumentException}, or the {@link IllegalStateException} of a failed {@code getOrThrow}.
+   */
+  public static void assertRejected(Executable action) {
+    try {
+      action.execute();
+    } catch (IllegalArgumentException | IllegalStateException expected) {
+      return;
+    } catch (Throwable other) {
+      fail("Expected the input to be refused, got " + other, other);
+    }
+    fail("Expected the input to be refused, but it was accepted");
+  }
+
+  /** Expression variables with one value. */
+  public static Function<String, OptionalDouble> vars(String name, double value) {
+    var values = Map.of(name, value);
+    return key ->
+        values.containsKey(key) ? OptionalDouble.of(values.get(key)) : OptionalDouble.empty();
+  }
+
+  /** Expression variables with no values. */
+  public static Function<String, OptionalDouble> noVars() {
+    return name -> OptionalDouble.empty();
+  }
+}
diff --git a/src/test/java/zone/moddev/mc/lib/machine/expr/ExprTest.java b/src/test/java/zone/moddev/mc/lib/machine/expr/ExprTest.java
new file mode 100644
index 0000000..24ea349
--- /dev/null
+++ b/src/test/java/zone/moddev/mc/lib/machine/expr/ExprTest.java
@@ -0,0 +1,86 @@
+package zone.moddev.mc.lib.machine.expr;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+import static zone.moddev.mc.lib.machine.MachineAssertions.assertNear;
+import static zone.moddev.mc.lib.machine.MachineAssertions.assertRejected;
+import static zone.moddev.mc.lib.machine.MachineAssertions.noVars;
+import static zone.moddev.mc.lib.machine.MachineAssertions.vars;
+
+import java.util.Set;
+
+import com.google.gson.JsonPrimitive;
+
+import com.mojang.serialization.JsonOps;
+
+import org.junit.jupiter.api.Test;
+
+class ExprTest {
+  @Test
+  void evaluatesMaterialVariables() {
+    assertNear(
+        .24, Expr.parse("0.08 * ($material.hardness - 3)").eval(vars("material.hardness", 6)));
+  }
+
+  @Test
+  void followsPrecedenceAndAssociativity() {
+    assertEquals(10, Expr.parse("-2 + 3 * 4").eval(noVars()));
+    assertEquals(.75, Expr.parse("(1 + 2) / 4").eval(noVars()));
+    assertEquals(3, Expr.parse("10 - 4 - 3").eval(noVars()));
+    assertEquals(-4, Expr.parse("2 * -(1 + 1)").eval(noVars()));
+    assertEquals(3, Expr.constant(3).eval(noVars()));
+  }
+
+  @Test
+  void listsItsVariables() {
+    assertEquals(Set.of("a.b"), Expr.parse("$a.b").variables());
+    assertTrue(Expr.parse("1 + 2").variables().isEmpty());
+  }
+
+  @Test
+  void divisionByZeroIsInfiniteNotAnError() {
+    assertTrue(Double.isInfinite(Expr.parse("1 / $x").eval(vars("x", 0))));
+  }
+
+  @Test
+  void refusesMalformedText() {
+    assertRejected(() -> Expr.parse("2 *"));
+    assertRejected(() -> Expr.parse("2 $"));
+    assertRejected(() -> Expr.parse("(1"));
+    assertRejected(() -> Expr.parse(""));
+  }
+
+  @Test
+  void refusesMalformedVariableNames() {
+    assertRejected(() -> Expr.parse("$a..b"));
+    assertRejected(() -> Expr.parse("$a."));
+    assertRejected(() -> Expr.parse("$.a"));
+  }
+
+  @Test
+  void refusesAMissingVariable() {
+    assertRejected(() -> Expr.parse("$missing").eval(noVars()));
+  }
+
+  @Test
+  void decodesNumbersAndStrings() {
+    assertEquals(
+        12, Expr.CODEC.parse(JsonOps.INSTANCE, new JsonPrimitive(12)).getOrThrow().eval(noVars()));
+    assertEquals(
+        8,
+        Expr.CODEC
+            .parse(JsonOps.INSTANCE, new JsonPrimitive("$x * 2"))
+            .getOrThrow()
+            .eval(vars("x", 4)));
+    assertTrue(Expr.CODEC.parse(JsonOps.INSTANCE, new JsonPrimitive("2 *")).error().isPresent());
+  }
+
+  // Double.toString would write these in E notation, which the parser doesn't read.
+  @Test
+  void roundTripsLargeAndSmallNumbers() {
+    var encoded =
+        Expr.CODEC.encodeStart(JsonOps.INSTANCE, Expr.parse("10000000 * $x + 0.0001")).getOrThrow();
+    assertNear(
+        20000000.0001, Expr.CODEC.parse(JsonOps.INSTANCE, encoded).getOrThrow().eval(vars("x", 2)));
+  }
+}
diff --git a/src/test/java/zone/moddev/mc/lib/machine/ingredient/AmountMatchTest.java b/src/test/java/zone/moddev/mc/lib/machine/ingredient/AmountMatchTest.java
new file mode 100644
index 0000000..050bda8
--- /dev/null
+++ b/src/test/java/zone/moddev/mc/lib/machine/ingredient/AmountMatchTest.java
@@ -0,0 +1,32 @@
+package zone.moddev.mc.lib.machine.ingredient;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import org.junit.jupiter.api.Test;
+
+class AmountMatchTest {
+  @Test
+  void exactNeedsTheSameAmount() {
+    assertTrue(AmountMatch.EXACT.test(5, 5));
+    assertFalse(AmountMatch.EXACT.test(5, 6));
+  }
+
+  @Test
+  void atLeastAcceptsMore() {
+    assertTrue(AmountMatch.AT_LEAST.test(5, 6));
+    assertTrue(AmountMatch.AT_LEAST.test(5, 5));
+    assertFalse(AmountMatch.AT_LEAST.test(5, 4));
+  }
+
+  @Test
+  void anyAmountAcceptsNothing() {
+    assertTrue(AmountMatch.ANY_AMOUNT.test(5, 0));
+  }
+
+  @Test
+  void amountsAbove2To31() {
+    assertTrue(AmountMatch.AT_LEAST.test(3_000_000_000L, 3_000_000_001L));
+    assertFalse(AmountMatch.AT_LEAST.test(3_000_000_000L, 2_999_999_999L));
+  }
+}
diff --git a/src/test/java/zone/moddev/mc/lib/machine/internals/InternalsDefinitionTest.java b/src/test/java/zone/moddev/mc/lib/machine/internals/InternalsDefinitionTest.java
new file mode 100644
index 0000000..a748c33
--- /dev/null
+++ b/src/test/java/zone/moddev/mc/lib/machine/internals/InternalsDefinitionTest.java
@@ -0,0 +1,111 @@
+package zone.moddev.mc.lib.machine.internals;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+import static zone.moddev.mc.lib.machine.MachineAssertions.assertRejected;
+
+import java.util.List;
+
+import com.google.gson.JsonParser;
+
+import com.mojang.serialization.JsonOps;
+
+import net.minecraft.resources.ResourceLocation;
+
+import org.junit.jupiter.api.Test;
+
+class InternalsDefinitionTest {
+  /** The internals example from machine-ui.md, "Sockets and stats". */
+  private static final String INTERNALS =
+      """
+      {
+        "open_with": "#c:tools/wrench",
+        "swap": "when_idle",
+        "sockets": {
+          "gears": { "count": 2, "accepts": "#c:gears" },
+          "motor": { "count": 1, "accepts": "#example:motors" },
+          "coil":  { "count": 1, "accepts": "#example:heating_coils", "optional": true }
+        },
+        "stats": {
+          "speed":           { "base": 1.0, "min": 0.25, "max": 8 },
+          "energy_per_tick": { "base": 40, "min": 1 },
+          "max_temperature": { "base": 900 }
+        }
+      }
+      """;
+
+  @Test
+  void decodesTheDocsExample() {
+    var internals = parse(INTERNALS);
+    assertEquals(SwapRule.WHEN_IDLE, internals.swap());
+    assertEquals(
+        ResourceLocation.parse("c:tools/wrench"),
+        internals.openWith().orElseThrow().value().left().orElseThrow().location());
+    assertTrue(internals.sockets().get("coil").optional());
+    assertFalse(internals.sockets().get("gears").optional());
+    assertEquals(900, internals.stats().get("max_temperature").base());
+  }
+
+  @Test
+  void laysOutSlotsSocketBySocketInNameOrder() {
+    var internals = parse(INTERNALS);
+    assertEquals(List.of("coil", "gears", "motor"), List.copyOf(internals.sockets().keySet()));
+    assertEquals(4, internals.slotCount());
+    assertEquals(List.of(0), internals.slots("coil"));
+    assertEquals(List.of(1, 2), internals.slots("gears"));
+    assertEquals(List.of(3), internals.slots("motor"));
+  }
+
+  @Test
+  void fillsInDefaults() {
+    var plain = parse("{\"sockets\":{\"motor\":{\"accepts\":\"example:motor\"}},\"stats\":{}}");
+    assertEquals(SwapRule.WHEN_IDLE, plain.swap());
+    assertTrue(plain.openWith().isEmpty());
+    assertEquals(1, plain.sockets().get("motor").count());
+    assertEquals(
+        ResourceLocation.parse("example:motor"),
+        plain.sockets().get("motor").accepts().orElseThrow().value().right().orElseThrow());
+  }
+
+  @Test
+  void swapRules() {
+    assertTrue(SwapRule.WHEN_IDLE.allows(false));
+    assertFalse(SwapRule.WHEN_IDLE.allows(true));
+    assertTrue(SwapRule.ALWAYS.allows(true));
+    assertFalse(SwapRule.NEVER.allows(false));
+  }
+
+  @Test
+  void refusesBadDefinitions() {
+    assertRejected(() -> parse(INTERNALS.replace("\"count\": 2", "\"count\": 0")));
+    assertRejected(() -> parse(INTERNALS.replace("when_idle", "sometimes")));
+    assertRejected(() -> parse(INTERNALS.replace("#c:gears", "#")));
+    assertRejected(() -> parse(INTERNALS.replace("#c:gears", "Not An Id")));
+  }
+
+  @Test
+  void refusesAnUnknownSocket() {
+    assertRejected(() -> parse(INTERNALS).slots("pump"));
+  }
+
+  @Test
+  void findsTheSocketOfASlot() {
+    var internals = parse(INTERNALS);
+    assertEquals("coil", internals.socketAt(0));
+    assertEquals("motor", internals.socketAt(3));
+  }
+
+  @Test
+  void refusesASlotOutOfRange() {
+    var internals = parse(INTERNALS);
+    assertRejected(() -> internals.socketAt(-1));
+    assertRejected(() -> internals.socketAt(4));
+  }
+
+  private static InternalsDefinition parse(String json) {
+    return InternalsDefinition.CODEC
+        .parse(JsonOps.INSTANCE, JsonParser.parseString(json))
+        .getOrThrow();
+  }
+}
diff --git a/src/test/java/zone/moddev/mc/lib/machine/internals/MachinePartsTest.java b/src/test/java/zone/moddev/mc/lib/machine/internals/MachinePartsTest.java
new file mode 100644
index 0000000..4f54fd9
--- /dev/null
+++ b/src/test/java/zone/moddev/mc/lib/machine/internals/MachinePartsTest.java
@@ -0,0 +1,274 @@
+package zone.moddev.mc.lib.machine.internals;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertSame;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+import static zone.moddev.mc.lib.machine.MachineAssertions.assertNear;
+import static zone.moddev.mc.lib.machine.MachineAssertions.assertRejected;
+
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+import java.util.OptionalInt;
+import java.util.Set;
+import java.util.function.Function;
+
+import com.google.gson.JsonElement;
+import com.google.gson.JsonParser;
+
+import com.mojang.serialization.JsonOps;
+
+import net.minecraft.resources.ResourceLocation;
+import net.minecraft.world.entity.ai.attributes.AttributeModifier;
+
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.Test;
+
+import zone.moddev.mc.lib.common.types.MaterialProperties;
+
+class MachinePartsTest {
+  /** The gear template from machine-ui.md, "Part files". */
+  private static final String GEAR_TEMPLATE =
+      """
+{
+  "part": "mmdlib:gear",
+  "socket": "gears",
+  "modifiers": [
+    { "stat": "speed",           "op": "add_multiplied_base", "amount": "0.08 * ($material.hardness - 3)" },
+    { "stat": "energy_per_tick", "op": "add_multiplied_base", "amount": "0.03 * $material.hardness" }
+  ],
+  "durability": "500 * $material.hardness"
+}
+""";
+
+  private static final Map<ResourceLocation, PartSubject> SUBJECTS =
+      Map.of(
+          id("test:iron_gear"), gear("test:iron_gear", 4, Map.of()),
+          id("test:invar_gear"), gear("test:invar_gear", 6, Map.of("grip", 2.0)),
+          id("test:soft_gear"), gear("test:soft_gear", 0, Map.of()),
+          id("test:diamond_coated_gear"), gear("test:diamond_coated_gear", 10, Map.of()),
+          id("test:plain_rod"),
+              new PartSubject(
+                  id("test:plain_rod"), Optional.of(id("mmdlib:rod")), Optional.empty()),
+          id("test:plain_pipe"),
+              new PartSubject(id("test:plain_pipe"), Optional.empty(), Optional.empty()));
+
+  @AfterEach
+  void uninstall() {
+    MachineParts.install(MachineParts.NONE);
+  }
+
+  @Test
+  void templateAppliesWithTheMaterial() {
+    var iron = parts().resolve(id("test:iron_gear")).orElseThrow();
+    assertEquals("gears", iron.socket());
+    assertNear(.08, iron.modifiers().get(0).amount());
+    assertNear(.12, iron.modifiers().get(1).amount());
+    assertEquals(AttributeModifier.Operation.ADD_MULTIPLIED_BASE, iron.modifiers().get(0).op());
+    assertEquals(OptionalInt.of(2000), iron.durability());
+  }
+
+  @Test
+  void itemFileBeatsTheTemplate() {
+    var coated = parts().resolve(id("test:diamond_coated_gear")).orElseThrow();
+    assertEquals(AttributeModifier.Operation.ADD_MULTIPLIED_TOTAL, coated.modifiers().get(0).op());
+    assertEquals(.4, coated.modifiers().get(0).amount());
+    assertTrue(coated.durability().isEmpty());
+  }
+
+  @Test
+  void missingSpecialKeysCountAsZero() {
+    var invar = parts().resolve(id("test:invar_gear")).orElseThrow();
+    assertEquals(1, invar.modifiers().size());
+    assertEquals(2, invar.modifiers().get(0).amount());
+    assertTrue(invar.durability().isEmpty());
+  }
+
+  @Test
+  void durabilityBelowOneIsNotAPart() {
+    assertTrue(parts().resolve(id("test:soft_gear")).isEmpty());
+  }
+
+  @Test
+  void itemsWithoutAMaterialCantUseMaterialVariables() {
+    var parts = parts();
+    assertTrue(parts.resolve(id("test:plain_pipe")).isEmpty());
+    assertTrue(parts.resolve(id("test:plain_rod")).isEmpty());
+    assertTrue(parts.resolve(id("test:unknown")).isEmpty());
+  }
+
+  @Test
+  void resolvedPartsAreCached() {
+    var parts = parts();
+    assertSame(
+        parts.resolve(id("test:iron_gear")).orElseThrow(),
+        parts.resolve(id("test:iron_gear")).orElseThrow());
+  }
+
+  @Test
+  void nonFiniteAmountIsNotAPart() {
+    var rods =
+        MachineParts.parse(
+            Map.of(
+                id("test:rod"),
+                JsonParser.parseString(
+                    """
+{"part": "mmdlib:rod", "socket": "shafts",
+ "modifiers": [{"stat": "speed", "op": "add_value", "amount": "10 / $material.hardness"}]}
+""")),
+            item ->
+                Optional.of(
+                    new PartSubject(
+                        item,
+                        Optional.of(id("mmdlib:rod")),
+                        gear(item.toString(), item.getPath().equals("zero_rod") ? 0 : 5, Map.of())
+                            .material())));
+    assertTrue(rods.resolve(id("test:zero_rod")).isEmpty());
+    assertEquals(2, rods.resolve(id("test:steel_rod")).orElseThrow().modifiers().get(0).amount());
+  }
+
+  @Test
+  void refusesBadFiles() {
+    assertRejected(() -> parsePart("{\"socket\":\"gears\",\"modifiers\":[]}"));
+    assertRejected(
+        () ->
+            parsePart(
+                "{\"part\":\"mmdlib:gear\",\"item\":\"test:x\",\"socket\":\"gears\",\"modifiers\":[]}"));
+    assertRejected(() -> parsePart(GEAR_TEMPLATE.replace("$material.hardness - 3", "$speed")));
+  }
+
+  // A misspelt key is an error, not a part that quietly never wears.
+  @Test
+  void refusesUnknownKeys() {
+    assertRejected(() -> parsePart(GEAR_TEMPLATE.replace("\"durability\"", "\"durabilty\"")));
+    assertRejected(() -> parsePart(GEAR_TEMPLATE.replace("\"stat\"", "\"note\": 1, \"stat\"")));
+  }
+
+  @Test
+  void checkReportsFilesThatDecodeButCantDoAnything() {
+    var problems =
+        filesToCheck(item -> Optional.ofNullable(SUBJECTS.get(item))).check(SUBJECTS.keySet());
+    assertEquals(3, problems.size(), problems.toString());
+    assertMentioned(problems, "test:typo_part", "mmdlib:gaer");
+    assertMentioned(problems, "test:typo_item", "test:iron_geer");
+    assertMentioned(problems, "test:special", "$material.special.gripp");
+  }
+
+  // Template errors are logged when a datapack loads, not when an item is first used.
+  @Test
+  void checkResolvesEveryItemUpFront() {
+    var lookups = new int[1];
+    var parts =
+        filesToCheck(
+            item -> {
+              lookups[0]++;
+              return Optional.ofNullable(SUBJECTS.get(item));
+            });
+    parts.check(SUBJECTS.keySet());
+    int after = lookups[0];
+    assertTrue(parts.resolve(id("test:iron_gear")).isPresent());
+    assertTrue(parts.resolve(id("test:soft_gear")).isEmpty());
+    assertEquals(after, lookups[0]);
+  }
+
+  @Test
+  void installingBumpsTheGeneration() {
+    var parts = parts();
+    int generation = MachineParts.current().generation();
+    MachineParts.install(parts);
+    assertSame(parts, MachineParts.current());
+    assertTrue(parts.generation() > generation);
+  }
+
+  /**
+   * The gear template, an item file, a second template for the same part (the lower resource ID,
+   * test:gear, wins), a file with an unknown variable (skipped, the rest still load) and two more
+   * item files.
+   */
+  private static MachineParts parts() {
+    var files = new HashMap<ResourceLocation, JsonElement>();
+    files.put(id("test:gear"), JsonParser.parseString(GEAR_TEMPLATE));
+    files.put(
+        id("test:diamond_coated_gear"),
+        JsonParser.parseString(
+            """
+            {
+              "item": "test:diamond_coated_gear",
+              "socket": "gears",
+              "modifiers": [
+                { "stat": "speed", "op": "add_multiplied_total", "amount": 0.4 },
+                { "stat": "energy_per_tick", "op": "add_value", "amount": 12 }
+              ]
+            }
+            """));
+    files.put(
+        id("test:zz_gear"),
+        JsonParser.parseString(GEAR_TEMPLATE.replace("\"gears\"", "\"other\"")));
+    files.put(
+        id("test:broken"),
+        JsonParser.parseString(
+            GEAR_TEMPLATE
+                .replace("mmdlib:gear", "mmdlib:plate")
+                .replace("$material.hardness - 3", "$material.colour")));
+    files.put(
+        id("test:pipe"),
+        JsonParser.parseString(
+            """
+{"item": "test:plain_pipe", "socket": "pipes",
+ "modifiers": [{"stat": "speed", "op": "add_value", "amount": "$material.special.grip + 1"}]}
+"""));
+    files.put(
+        id("test:grip"),
+        JsonParser.parseString(
+            """
+            {"item": "test:invar_gear", "socket": "gears",
+             "modifiers": [{"stat": "grip", "op": "add_value",
+                            "amount": "$material.special.grip + $material.special.none"}]}
+            """));
+    return MachineParts.parse(files, item -> Optional.ofNullable(SUBJECTS.get(item)));
+  }
+
+  private static MachineParts filesToCheck(
+      Function<ResourceLocation, Optional<PartSubject>> subjects) {
+    var files = new HashMap<ResourceLocation, JsonElement>();
+    files.put(id("test:gear"), JsonParser.parseString(GEAR_TEMPLATE));
+    files.put(
+        id("test:typo_part"),
+        JsonParser.parseString(GEAR_TEMPLATE.replace("mmdlib:gear", "mmdlib:gaer")));
+    files.put(
+        id("test:typo_item"),
+        JsonParser.parseString(
+            "{\"item\": \"test:iron_geer\", \"socket\": \"gears\", \"modifiers\": []}"));
+    files.put(
+        id("test:special"),
+        JsonParser.parseString(
+            """
+            {"item": "test:invar_gear", "socket": "gears",
+             "modifiers": [{"stat": "grip", "op": "add_value",
+                            "amount": "$material.special.grip + $material.special.gripp"}]}
+            """));
+    return MachineParts.parse(files, subjects::apply);
+  }
+
+  private static void assertMentioned(List<String> problems, String... words) {
+    assertTrue(
+        problems.stream().anyMatch(problem -> List.of(words).stream().allMatch(problem::contains)),
+        () -> "No problem mentions " + List.of(words) + " in " + problems);
+  }
+
+  private static PartSubject gear(String item, double hardness, Map<String, Double> special) {
+    return new PartSubject(
+        id(item),
+        Optional.of(id("mmdlib:gear")),
+        Optional.of(new MaterialProperties("metal", -1, hardness, 1, 0, 0, Set.of(), special)));
+  }
+
+  private static PartDefinition parsePart(String json) {
+    return PartDefinition.CODEC.parse(JsonOps.INSTANCE, JsonParser.parseString(json)).getOrThrow();
+  }
+
+  private static ResourceLocation id(String value) {
+    return ResourceLocation.parse(value);
+  }
+}
diff --git a/src/test/java/zone/moddev/mc/lib/machine/recipe/InputClaimsTest.java b/src/test/java/zone/moddev/mc/lib/machine/recipe/InputClaimsTest.java
new file mode 100644
index 0000000..2d97e14
--- /dev/null
+++ b/src/test/java/zone/moddev/mc/lib/machine/recipe/InputClaimsTest.java
@@ -0,0 +1,84 @@
+package zone.moddev.mc.lib.machine.recipe;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+
+class InputClaimsTest {
+  @Test
+  void broadClaimMovesOffTheOnlyEntryALaterClaimAccepts() {
+    var claims = new InputClaims(new long[] {1, 1});
+    assertEquals(0, claims.claim(1, any -> true));
+    assertEquals(0, claims.claim(1, entry -> entry == 0));
+    assertEquals(1, claims.claimed(0));
+    assertEquals(1, claims.claimed(1));
+  }
+
+  @Test
+  void orderOfTheClaimsDoesntMatter() {
+    var claims = new InputClaims(new long[] {1, 1});
+    assertEquals(0, claims.claim(1, entry -> entry == 0));
+    assertEquals(0, claims.claim(1, any -> true));
+    assertEquals(1, claims.claimed(0));
+    assertEquals(1, claims.claimed(1));
+  }
+
+  @Test
+  void movesFollowAChain() {
+    var claims = new InputClaims(new long[] {1, 1, 1});
+    assertEquals(0, claims.claim(1, entry -> entry != 1));
+    assertEquals(0, claims.claim(1, entry -> entry != 2));
+    assertEquals(0, claims.claim(1, entry -> entry == 0));
+    assertEquals(1, claims.claimed(0));
+    assertEquals(1, claims.claimed(1));
+    assertEquals(1, claims.claimed(2));
+  }
+
+  @Test
+  void partOfAStackMovesAndTheRestStays() {
+    var claims = new InputClaims(new long[] {3, 3});
+    assertEquals(0, claims.claim(4, any -> true));
+    assertEquals(0, claims.claim(2, entry -> entry == 0));
+    assertEquals(3, claims.claimed(0));
+    assertEquals(3, claims.claimed(1));
+  }
+
+  @Test
+  void nothingCountsTwiceAndAShortClaimReportsWhatsMissing() {
+    var claims = new InputClaims(new long[] {3});
+    assertEquals(0, claims.claim(2, any -> true));
+    assertEquals(1, claims.claim(2, any -> true));
+    assertEquals(3, claims.claimed(0));
+  }
+
+  @Test
+  void claimsOnTheSameSingleEntryCantBothBeMet() {
+    var claims = new InputClaims(new long[] {1, 1});
+    assertEquals(0, claims.claim(1, entry -> entry == 0));
+    assertEquals(1, claims.claim(1, entry -> entry == 0));
+    assertEquals(1, claims.claimed(0));
+    assertEquals(0, claims.claimed(1));
+  }
+
+  // So an ingredient never tests an empty stack.
+  @Test
+  void emptyEntriesAreNotAskedAbout() {
+    var claims = new InputClaims(new long[] {0, 2});
+    assertEquals(
+        0,
+        claims.claim(
+            1,
+            entry -> {
+              assertEquals(1, entry);
+              return true;
+            }));
+  }
+
+  // Energy amounts don't fit an int.
+  @Test
+  void amountsLargerThanAnInt() {
+    var claims = new InputClaims(new long[] {3_000_000_000L});
+    assertEquals(0, claims.claim(2_999_999_999L, any -> true));
+    assertEquals(1, claims.claim(2, any -> true));
+  }
+}
diff --git a/src/test/java/zone/moddev/mc/lib/machine/stat/MachineStatsTest.java b/src/test/java/zone/moddev/mc/lib/machine/stat/MachineStatsTest.java
new file mode 100644
index 0000000..ca6107f
--- /dev/null
+++ b/src/test/java/zone/moddev/mc/lib/machine/stat/MachineStatsTest.java
@@ -0,0 +1,133 @@
+package zone.moddev.mc.lib.machine.stat;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+import static zone.moddev.mc.lib.machine.MachineAssertions.assertNear;
+import static zone.moddev.mc.lib.machine.MachineAssertions.assertRejected;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+import java.util.Map;
+import java.util.OptionalDouble;
+
+import com.google.gson.JsonParser;
+
+import com.mojang.serialization.JsonOps;
+
+import net.minecraft.world.entity.ai.attributes.AttributeModifier;
+
+import org.junit.jupiter.api.Test;
+
+class MachineStatsTest {
+  private static final Map<String, StatDefinition> DEFS =
+      Map.of(
+          "speed", new StatDefinition(1, OptionalDouble.of(.25), OptionalDouble.of(8)),
+          "energy_per_tick", new StatDefinition(40, OptionalDouble.of(1), OptionalDouble.empty()));
+
+  // The worked table in machine-ui.md, with example hardness iron 4 and invar 6.
+  private static final List<StatModifier> IRON =
+      List.of(base("speed", .08), base("energy_per_tick", .12));
+  private static final List<StatModifier> INVAR =
+      List.of(base("speed", .24), base("energy_per_tick", .18));
+  private static final List<StatModifier> MOTOR =
+      List.of(total("speed", .15), value("energy_per_tick", 8));
+  private static final List<StatModifier> UPGRADE =
+      List.of(total("speed", .5), total("energy_per_tick", .5));
+
+  @Test
+  void twoIronGears() {
+    var stats = MachineStats.compute(DEFS, concat(IRON, IRON));
+    assertNear(1.16, stats.get("speed"));
+    assertNear(49.6, stats.get("energy_per_tick"));
+  }
+
+  @Test
+  void twoInvarGears() {
+    var stats = MachineStats.compute(DEFS, concat(INVAR, INVAR));
+    assertNear(1.48, stats.get("speed"));
+    assertNear(54.4, stats.get("energy_per_tick"));
+  }
+
+  @Test
+  void invarGearsAndAMotor() {
+    var stats = MachineStats.compute(DEFS, concat(INVAR, INVAR, MOTOR));
+    assertNear(1.702, stats.get("speed"));
+    assertNear(65.28, stats.get("energy_per_tick"));
+  }
+
+  @Test
+  void invarGearsAMotorAndAnUpgrade() {
+    var stats = MachineStats.compute(DEFS, concat(INVAR, INVAR, MOTOR, UPGRADE));
+    assertNear(2.553, stats.get("speed"));
+    assertNear(97.92, stats.get("energy_per_tick"));
+  }
+
+  @Test
+  void orderDoesNotMatter() {
+    var all = concat(INVAR, INVAR, MOTOR, UPGRADE);
+    var reversed = new ArrayList<>(all);
+    Collections.reverse(reversed);
+    assertEquals(
+        MachineStats.compute(DEFS, all).values(), MachineStats.compute(DEFS, reversed).values());
+  }
+
+  @Test
+  void noModifiersGiveTheBase() {
+    assertEquals(1, MachineStats.compute(DEFS, List.of()).get("speed"));
+  }
+
+  @Test
+  void clampsToMinAndMax() {
+    assertEquals(8, MachineStats.compute(DEFS, List.of(total("speed", 10))).get("speed"));
+    assertEquals(.25, MachineStats.compute(DEFS, List.of(value("speed", -5))).get("speed"));
+  }
+
+  @Test
+  void ignoresUndeclaredStats() {
+    var stats = MachineStats.compute(DEFS, List.of(value("max_temperature", 400)));
+    assertEquals(2, stats.values().size());
+    assertEquals(1, stats.get("speed"));
+  }
+
+  @Test
+  void refusesUnknownStatsAndNaN() {
+    assertRejected(() -> MachineStats.EMPTY.get("speed"));
+    assertRejected(() -> value("speed", Double.NaN));
+  }
+
+  @Test
+  void decodesDefinitions() {
+    assertEquals(DEFS.get("speed"), parse("{\"base\":1,\"min\":0.25,\"max\":8}"));
+    assertTrue(parse("{\"base\":900}").min().isEmpty());
+  }
+
+  @Test
+  void refusesBadDefinitions() {
+    assertRejected(() -> parse("{\"base\":1,\"min\":5,\"max\":2}"));
+    assertRejected(() -> parse("{\"min\":1}"));
+  }
+
+  private static StatDefinition parse(String json) {
+    return StatDefinition.CODEC.parse(JsonOps.INSTANCE, JsonParser.parseString(json)).getOrThrow();
+  }
+
+  private static StatModifier value(String stat, double amount) {
+    return new StatModifier(stat, AttributeModifier.Operation.ADD_VALUE, amount);
+  }
+
+  private static StatModifier base(String stat, double amount) {
+    return new StatModifier(stat, AttributeModifier.Operation.ADD_MULTIPLIED_BASE, amount);
+  }
+
+  private static StatModifier total(String stat, double amount) {
+    return new StatModifier(stat, AttributeModifier.Operation.ADD_MULTIPLIED_TOTAL, amount);
+  }
+
+  @SafeVarargs
+  private static List<StatModifier> concat(List<StatModifier>... lists) {
+    var all = new ArrayList<StatModifier>();
+    for (var list : lists) all.addAll(list);
+    return all;
+  }
+}