diff --git a/build.zig b/build.zig
index 6d9fee2..c474769 100644
--- a/build.zig
+++ b/build.zig
@@ -221,11 +221,12 @@ pub fn build(b: *std.Build) void {
     // image leaves it out).
     const init_exe = b.dependency("init", .{ .serial = serial }).artifact("init");
 
-    // --- the rest of the boot tree: /system services and drivers, /test fixtures ---
+    // --- the rest of the boot tree: /system services and drivers ---
     // Each is built by the same user-binary recipe and laid out at its FHS path on
     // the boot volume (see `bundled` below). The EFI loader walks the tree at boot
     // and hands the kernel an in-RAM initial_ramdisk of it (system/initial-ramdisk.zig).
-    const vfstest_exe = b.dependency("vfs-test", .{}).artifact("vfs-test");
+    // (The /test fixtures are lazy dependencies, resolved further down only
+    // for a -Dtest-case build.)
     // The drivers, each directory its own package: the PS/2 bus family (bus +
     // keyboard + mouse from one package), the xHCI bus driver, the USB HID
     // class drivers, and USB mass storage. Their unit tests ride along.
@@ -248,47 +249,31 @@ pub fn build(b: *std.Build) void {
     const display_demo_exe = b.dependency("display-demo", .{}).artifact("display-demo");
     const virtio_gpu_package = b.dependency("virtio-gpu", .{});
     const virtio_gpu_exe = virtio_gpu_package.artifact("virtio-gpu");
-    const shared_memory_server_exe = b.dependency("shared-memory-server", .{}).artifact("shared-memory-server");
-    const shared_memory_client_exe = b.dependency("shared-memory-client", .{}).artifact("shared-memory-client");
-    const fat_test_exe = b.dependency("fat-test", .{}).artifact("fat-test");
     // The first binary package (docs/build-packages-plan.md, phase 2): pci-bus
     // builds itself against the domain packages; the root build just takes the
     // artifact for the boot image.
     const pci_bus_exe = b.dependency("pci-bus", .{}).artifact("pci-bus");
-    // crash-test is a fixture, not a real driver: it hellos to the device
-    // manager, then faults — what the driver-restart scenario drives the
-    // crash-loop cap with. pci-cap-test and iommu-fault-test exercise the
-    // driver-side PCI library surface and the VT-d rogue-DMA negative proof.
-    const crash_test_exe = b.dependency("crash-test", .{}).artifact("crash-test");
-    const device_list_exe = b.dependency("device-list", .{}).artifact("device-list");
-    const pci_cap_test_exe = b.dependency("pci-cap-test", .{}).artifact("pci-cap-test");
-    const iommu_fault_test_exe = b.dependency("iommu-fault-test", .{}).artifact("iommu-fault-test");
     // The discovery service: one swappable process per firmware
     // (docs/discovery.md), bundled under the neutral ramdisk name
     // "discovery" so the device manager never learns which firmware it is on.
     // x86 boots describe hardware with ACPI; the Raspberry Pis hand over a
     // flattened device tree — the aarch64 target flips the default when it
-    // lands (docs/arm.md). Each firmware's service is its own package; both
-    // export an artifact named "discovery", and this option picks which one
-    // ships (only the chosen one is compiled).
+    // lands (docs/arm.md). Each firmware's service is its own LAZY package;
+    // both export an artifact named "discovery", and this option picks which
+    // one ships — the unselected package's build file is never even loaded.
+    // (lazyDependency returns null only for an unfetched remote package; these
+    // are in-repo path dependencies, so a null means the directory is gone.)
     const Discovery = enum { acpi, fdt };
     const discovery = b.option(Discovery, "discovery", "Which discovery service fills the ramdisk's 'discovery' slot (default: acpi)") orelse Discovery.acpi;
     const discovery_exe = switch (discovery) {
-        .acpi => b.dependency("acpi", .{}).artifact("discovery"),
-        .fdt => b.dependency("fdt", .{}).artifact("discovery"),
+        .acpi => (b.lazyDependency("acpi", .{}) orelse @panic("system/services/acpi is missing")).artifact("discovery"),
+        .fdt => (b.lazyDependency("fdt", .{}) orelse @panic("system/services/fdt is missing")).artifact("discovery"),
     };
     const device_manager_exe = b.dependency("device-manager", .{}).artifact("device-manager");
     // The input service and its exercisers: the fan-out server, a hardware-free synthetic
     // source, and a subscriber that doubles as the `input` test's oracle. See docs/input.md.
     const input_exe = b.dependency("input", .{}).artifact("input");
-    const input_source_exe = b.dependency("input-source", .{}).artifact("input-source");
-    const input_test_exe = b.dependency("input-test", .{}).artifact("input-test");
-    const args_echo_exe = b.dependency("args-echo", .{}).artifact("args-echo");
-    const process_test_exe = b.dependency("process-test", .{}).artifact("process-test");
     const logger_exe = b.dependency("logger", .{}).artifact("logger");
-    // The first multi-threaded binary: exercises runtime.Thread over the thread ABI
-    // (docs/threading.md). Its package opts into threading (real atomics/TLS).
-    const thread_test_exe = b.dependency("thread-test", .{}).artifact("thread-test");
 
     // Every user binary and its FHS home on the boot volume. There is no packed
     // ramdisk artifact any more: make-fat-image.py lays each binary out at this
@@ -327,32 +312,38 @@ pub fn build(b: *std.Build) void {
         .{ .path = "system/drivers/virtio-gpu", .binary = virtio_gpu_exe.getEmittedBin() },
         .{ .path = "system/drivers/pci-bus", .binary = pci_bus_exe.getEmittedBin() },
     };
-    // The userspace test fixtures under /test. A plain `zig build` produces a clean
-    // image WITHOUT them; they are bundled only for a test build — which the QEMU
-    // harness signals by passing -Dtest-case=<name> for every scenario, exactly when
-    // these fixtures must be on the boot volume. Merely building this array never
-    // forces a compile: the fixture exes build only if `bundled` (below) includes them.
-    const test_bundled = [_]images.BundledBinary{
-        .{ .path = "test/system/services/vfs-test", .binary = vfstest_exe.getEmittedBin() },
-        .{ .path = "test/system/services/fat-test", .binary = fat_test_exe.getEmittedBin() },
-        .{ .path = "test/system/services/shared-memory-server", .binary = shared_memory_server_exe.getEmittedBin() },
-        .{ .path = "test/system/services/shared-memory-client", .binary = shared_memory_client_exe.getEmittedBin() },
-        .{ .path = "test/system/services/crash-test", .binary = crash_test_exe.getEmittedBin() },
-        .{ .path = "test/system/services/device-list", .binary = device_list_exe.getEmittedBin() },
-        .{ .path = "test/system/services/pci-cap-test", .binary = pci_cap_test_exe.getEmittedBin() },
-        .{ .path = "test/system/services/iommu-fault-test", .binary = iommu_fault_test_exe.getEmittedBin() },
-        .{ .path = "test/system/services/input-source", .binary = input_source_exe.getEmittedBin() },
-        .{ .path = "test/system/services/input-test", .binary = input_test_exe.getEmittedBin() },
-        .{ .path = "test/system/services/args-echo", .binary = args_echo_exe.getEmittedBin() },
-        .{ .path = "test/system/services/process-test", .binary = process_test_exe.getEmittedBin() },
-        .{ .path = "test/system/services/thread-test", .binary = thread_test_exe.getEmittedBin() },
-    };
     // A no-option build assumes neither -Dtest-case nor -Ddiagnose: it ships the
-    // production set only. Test fixtures join in only under -Dtest-case; the
-    // diagnose display-omission is already handled by init_csv_source above.
+    // production set only. The userspace test fixtures under /test join in only
+    // for a test build — which the QEMU harness signals by passing
+    // -Dtest-case=<name> for every scenario, exactly when they must be on the
+    // boot volume. They are LAZY dependencies: a plain build neither compiles
+    // them nor loads their build files (docs/build-packages-plan.md). Fixture
+    // packages are uniform — the dependency name, the artifact name, and the
+    // boot path's leaf all match the directory — so a name is a whole entry.
     var bundled_list: std.ArrayListUnmanaged(images.BundledBinary) = .empty;
     bundled_list.appendSlice(b.allocator, &production_bundled) catch @panic("OOM");
-    if (test_case != null) bundled_list.appendSlice(b.allocator, &test_bundled) catch @panic("OOM");
+    if (test_case != null) for ([_][]const u8{
+        "vfs-test", // the user-space VFS round-trip client
+        "fat-test",
+        "shared-memory-server",
+        "shared-memory-client",
+        "crash-test", // hellos to the device manager, then faults — drives the crash-loop cap
+        "device-list",
+        "pci-cap-test", // exercises the driver-side PCI library against the pci-caps NIC
+        "iommu-fault-test", // fires the rogue DMA that VT-d must fault
+        "input-source",
+        "input-test",
+        "args-echo",
+        "process-test",
+        "thread-test", // the multi-threaded fixture (its package sets .threaded)
+    }) |fixture| {
+        const package = b.lazyDependency(fixture, .{}) orelse
+            @panic("a test fixture package is missing under test/system/services");
+        bundled_list.append(b.allocator, .{
+            .path = b.fmt("test/system/services/{s}", .{fixture}),
+            .binary = package.artifact(fixture).getEmittedBin(),
+        }) catch @panic("OOM");
+    };
     const bundled = bundled_list.items;
 
     // Boot methods live in boot/, one per way of getting the kernel running.
diff --git a/build.zig.zon b/build.zig.zon
index 3f708cd..c1b043a 100644
--- a/build.zig.zon
+++ b/build.zig.zon
@@ -51,26 +51,29 @@
         .@"device-manager" = .{ .path = "system/services/device-manager" },
         .input = .{ .path = "system/services/input" },
         .logger = .{ .path = "system/services/logger" },
-        .acpi = .{ .path = "system/services/acpi" },
-        .fdt = .{ .path = "system/services/fdt" },
+        // The discovery pair and the /test fixtures are lazy: only what a
+        // given build actually ships gets its build file loaded and compiled
+        // (-Ddiscovery picks one of the pair; -Dtest-case pulls the fixtures).
+        .acpi = .{ .path = "system/services/acpi", .lazy = true },
+        .fdt = .{ .path = "system/services/fdt", .lazy = true },
         .@"ps2-bus" = .{ .path = "system/drivers/ps2-bus" },
         .@"usb-xhci-bus" = .{ .path = "system/drivers/usb-xhci-bus" },
         .@"usb-hid" = .{ .path = "system/drivers/usb-hid" },
         .@"usb-storage" = .{ .path = "system/drivers/usb-storage" },
         .@"virtio-gpu" = .{ .path = "system/drivers/virtio-gpu" },
-        .@"vfs-test" = .{ .path = "test/system/services/vfs-test" },
-        .@"fat-test" = .{ .path = "test/system/services/fat-test" },
-        .@"shared-memory-server" = .{ .path = "test/system/services/shared-memory-server" },
-        .@"shared-memory-client" = .{ .path = "test/system/services/shared-memory-client" },
-        .@"crash-test" = .{ .path = "test/system/services/crash-test" },
-        .@"device-list" = .{ .path = "test/system/services/device-list" },
-        .@"pci-cap-test" = .{ .path = "test/system/services/pci-cap-test" },
-        .@"iommu-fault-test" = .{ .path = "test/system/services/iommu-fault-test" },
-        .@"input-source" = .{ .path = "test/system/services/input-source" },
-        .@"input-test" = .{ .path = "test/system/services/input-test" },
-        .@"args-echo" = .{ .path = "test/system/services/args-echo" },
-        .@"process-test" = .{ .path = "test/system/services/process-test" },
-        .@"thread-test" = .{ .path = "test/system/services/thread-test" },
+        .@"vfs-test" = .{ .path = "test/system/services/vfs-test", .lazy = true },
+        .@"fat-test" = .{ .path = "test/system/services/fat-test", .lazy = true },
+        .@"shared-memory-server" = .{ .path = "test/system/services/shared-memory-server", .lazy = true },
+        .@"shared-memory-client" = .{ .path = "test/system/services/shared-memory-client", .lazy = true },
+        .@"crash-test" = .{ .path = "test/system/services/crash-test", .lazy = true },
+        .@"device-list" = .{ .path = "test/system/services/device-list", .lazy = true },
+        .@"pci-cap-test" = .{ .path = "test/system/services/pci-cap-test", .lazy = true },
+        .@"iommu-fault-test" = .{ .path = "test/system/services/iommu-fault-test", .lazy = true },
+        .@"input-source" = .{ .path = "test/system/services/input-source", .lazy = true },
+        .@"input-test" = .{ .path = "test/system/services/input-test", .lazy = true },
+        .@"args-echo" = .{ .path = "test/system/services/args-echo", .lazy = true },
+        .@"process-test" = .{ .path = "test/system/services/process-test", .lazy = true },
+        .@"thread-test" = .{ .path = "test/system/services/thread-test", .lazy = true },
         // See `zig fetch --save <url>` for a command-line interface for adding dependencies.
         //.example = .{
         //    // When updating this field to a new URL, be sure to delete the corresponding
diff --git a/docs/build-packages-plan.md b/docs/build-packages-plan.md
index 37d5a1c..22b4955 100644
--- a/docs/build-packages-plan.md
+++ b/docs/build-packages-plan.md
@@ -9,7 +9,9 @@ named "discovery" that the root's -Ddiscovery picks between), and phase 3 (the
 root split into `build/images.zig` + `build/qemu.zig`; the root `build.zig` is
 ~460 lines of orchestration, down from ~1,250). Every phase landed green: unit
 tests, the QEMU suite at parity with main, boot-image file list unchanged.
-Still future: `lazyDependency` for image-specific builds (What-this-buys #4).
+The `lazyDependency` payoff (What-this-buys #4) is in too: the /test fixtures
+and the unselected discovery package are lazy — a build loads and compiles
+only what it ships.
 
 ## Why
 
@@ -76,7 +78,9 @@ Rules:
    stability testing in isolation.
 3. Adding a binary = adding a directory (source + two small files), not editing
    three places in a 1,250-line file.
-4. Later: `lazyDependency` lets an image target build only what it ships.
+4. `lazyDependency` lets an image target build only what it ships: the /test
+   fixtures resolve only under -Dtest-case, and only the -Ddiscovery-selected
+   discovery package ever loads.
 
 ## Phases