diff --git a/bastion/src/bastion/tests/arch-dispatch.test.ts b/bastion/src/bastion/tests/arch-dispatch.test.ts
new file mode 100644
index 0000000..3d3c648
--- /dev/null
+++ b/bastion/src/bastion/tests/arch-dispatch.test.ts
@@ -0,0 +1,261 @@
+// aarch64 support in the PXE dispatch path.
+//
+// The x86_64 side is pinned separately by ipxe-x86-regression.test.ts.
+
+import { describe, it, expect, beforeEach, afterEach } from "vitest";
+import { mkdirSync, rmSync } from "node:fs";
+import { join } from "node:path";
+import { tmpdir } from "node:os";
+import type { BastionConfig, BastionState, HardwareInfo } from "@lab/shared";
+import { createApp } from "../src/server.js";
+import { resolveArch } from "../src/routes/dispatch.js";
+import { renderDnsmasqConf } from "../src/templates/dnsmasq.conf.js";
+import type { FastifyInstance } from "fastify";
+import type { StateManager } from "../src/services/state.js";
+
+function createTestConfig(testDir: string): BastionConfig {
+  return {
+    fedoraVersion: "43",
+    arch: "x86_64",
+    httpPort: 0,
+    timezone: "Europe/London",
+    locale: "en_GB.UTF-8",
+    bastionDir: testDir,
+    domain: "test.local",
+    dhcpMode: "proxy",
+    dhcpRangeStart: "",
+    dhcpRangeEnd: "",
+    ubuntuVersion: "26.04",
+    ubuntuMirror: "https://releases.ubuntu.com/26.04",
+    iface: "eth0",
+    serverIp: "10.0.0.1",
+    network: "10.0.0.0",
+    gateway: "10.0.0.1",
+    sshKeys: ["ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAITEST test@test"],
+    adminUser: "testadmin",
+    syslogPort: 15514,
+    skipDnsmasq: true,
+    skipArtifacts: true,
+    fedoraMirror: "https://download.fedoraproject.org/pub/fedora/linux/releases/43/Everything/x86_64/os",
+    tftpDir: join(testDir, "tftp"),
+    httpDir: join(testDir, "http"),
+    stateFile: join(testDir, "state.json"),
+  };
+}
+
+function hardware(mac: string, over: Partial<HardwareInfo> = {}): HardwareInfo {
+  return {
+    mac,
+    product: "TestBox",
+    board: "TestBoard",
+    serial: "SN123",
+    manufacturer: "TestCorp",
+    cpu_model: "Test CPU",
+    cpu_cores: 4,
+    memory_gb: 16,
+    arch: "x86_64",
+    disks: [],
+    nics: [],
+    first_seen: new Date().toISOString(),
+    last_seen: new Date().toISOString(),
+    ...over,
+  };
+}
+
+const emptyState = (): BastionState => ({
+  discovered: {}, install_queue: {}, installed: {}, debug: {},
+});
+
+describe("architecture resolution", () => {
+  const config = createTestConfig("/tmp/unused");
+  const mac = "aa:bb:cc:dd:ee:ff";
+
+  it("prefers the tracked record over what the client reports", () => {
+    const state = emptyState();
+    state.discovered[mac] = hardware(mac, { arch: "aarch64" });
+    // Client claims x86_64; the machine record says otherwise and wins.
+    expect(resolveArch(state, mac, "x86_64", config)).toBe("aarch64");
+  });
+
+  it("falls back to the architecture reported at boot", () => {
+    expect(resolveArch(emptyState(), mac, "arm64", config)).toBe("aarch64");
+  });
+
+  it("normalises iPXE's arm64 spelling to aarch64", () => {
+    expect(resolveArch(emptyState(), mac, "arm64", config)).toBe("aarch64");
+    expect(resolveArch(emptyState(), mac, "x86_64", config)).toBe("x86_64");
+  });
+
+  it("falls back to the configured default for unknown architectures", () => {
+    expect(resolveArch(emptyState(), mac, "riscv64", config)).toBe("x86_64");
+    expect(resolveArch(emptyState(), mac, undefined, config)).toBe("x86_64");
+  });
+
+  it("reads arch from the installed record for already-provisioned machines", () => {
+    const state = emptyState();
+    state.installed[mac] = {
+      hostname: "spark", role: "worker", ip: "10.0.0.5",
+      installed_at: new Date().toISOString(), arch: "aarch64",
+    };
+    expect(resolveArch(state, mac, undefined, config)).toBe("aarch64");
+  });
+});
+
+describe("aarch64 dispatch", () => {
+  let testDir: string;
+  let app: FastifyInstance;
+  let state: StateManager;
+  const mac = "aa:bb:cc:dd:ee:ff";
+
+  beforeEach(() => {
+    testDir = join(tmpdir(), `bastion-arch-test-${Date.now()}-${Math.random().toString(36).slice(2)}`);
+    mkdirSync(join(testDir, "http"), { recursive: true });
+    mkdirSync(join(testDir, "tftp"), { recursive: true });
+    const result = createApp(createTestConfig(testDir));
+    app = result.app;
+    state = result.state;
+  });
+
+  afterEach(async () => {
+    await app.close();
+    rmSync(testDir, { recursive: true, force: true });
+  });
+
+  it("serves the aarch64 kernel and initrd to an arm64 client", async () => {
+    const res = await app.inject({ method: "GET", url: `/dispatch?mac=${mac}&arch=arm64` });
+    expect(res.statusCode).toBe(200);
+    expect(res.body).toContain("/vmlinuz-aarch64");
+    expect(res.body).toContain("/initrd-aarch64.img");
+    expect(res.body).not.toContain("/vmlinuz ");
+  });
+
+  it("points an arm64 client at the aarch64 Fedora mirror", async () => {
+    const res = await app.inject({ method: "GET", url: `/dispatch?mac=${mac}&arch=arm64` });
+    expect(res.body).toContain("Everything/aarch64/os");
+    expect(res.body).not.toContain("Everything/x86_64/os");
+  });
+
+  it("uses serial console arguments and not nomodeset on arm64", async () => {
+    const res = await app.inject({ method: "GET", url: `/dispatch?mac=${mac}&arch=arm64` });
+    expect(res.body).toContain("console=ttyAMA0,115200");
+    expect(res.body).not.toContain("nomodeset");
+  });
+
+  it("serves a rescue kernel for the recorded architecture, not the requester's", async () => {
+    // The Spark case: machine known to be aarch64, queued for rescue.
+    state.update((s) => {
+      s.discovered[mac] = hardware(mac, { arch: "aarch64" });
+      s.debug[mac] = { hostname: "spark-2935", queued_at: new Date().toISOString() };
+    });
+
+    const res = await app.inject({ method: "GET", url: `/dispatch?mac=${mac}` });
+    expect(res.body).toContain("DEBUG/RESCUE MODE");
+    expect(res.body).toContain("/vmlinuz-aarch64");
+    expect(res.body).toContain("inst.rescue");
+    expect(res.body).toContain("inst.sshd");
+  });
+});
+
+describe("--pxe-boot root device", () => {
+  let testDir: string;
+  let app: FastifyInstance;
+  let state: StateManager;
+  const mac = "aa:bb:cc:dd:ee:ff";
+
+  beforeEach(() => {
+    testDir = join(tmpdir(), `bastion-root-test-${Date.now()}-${Math.random().toString(36).slice(2)}`);
+    mkdirSync(join(testDir, "http"), { recursive: true });
+    mkdirSync(join(testDir, "tftp"), { recursive: true });
+    const result = createApp(createTestConfig(testDir));
+    app = result.app;
+    state = result.state;
+  });
+
+  afterEach(async () => {
+    await app.close();
+    rmSync(testDir, { recursive: true, force: true });
+  });
+
+  it("uses the root device recorded on the machine", async () => {
+    state.update((s) => {
+      s.installed[mac] = {
+        hostname: "worker-1", role: "worker", ip: "10.0.0.50",
+        installed_at: new Date().toISOString(),
+        root_device: "/dev/mapper/otherVG-root",
+        root_args: "rd.lvm.lv=otherVG/root",
+      };
+      s.debug[mac] = { hostname: "worker-1", queued_at: new Date().toISOString(), pxeBoot: true };
+    });
+
+    const res = await app.inject({ method: "GET", url: `/dispatch?mac=${mac}` });
+    expect(res.body).toContain("PXE BOOT (debug)");
+    expect(res.body).toContain("root=/dev/mapper/otherVG-root");
+    expect(res.body).toContain("rd.lvm.lv=otherVG/root");
+    // The old hardcoded layout must not leak back in.
+    expect(res.body).not.toContain("labvg");
+  });
+
+  it("falls back to rescue rather than guessing when no root device is known", async () => {
+    state.update((s) => {
+      s.installed[mac] = {
+        hostname: "spark-2935", role: "worker", ip: "192.168.8.12",
+        installed_at: new Date().toISOString(), arch: "aarch64",
+      };
+      s.debug[mac] = { hostname: "spark-2935", queued_at: new Date().toISOString(), pxeBoot: true };
+    });
+
+    const res = await app.inject({ method: "GET", url: `/dispatch?mac=${mac}` });
+    expect(res.body).toContain("DEBUG/RESCUE MODE");
+    expect(res.body).toContain("no root device is recorded");
+    expect(res.body).toContain("debug-setup.sh");
+    expect(res.body).not.toContain("root=");
+    // And it is still the right architecture.
+    expect(res.body).toContain("/vmlinuz-aarch64");
+  });
+
+  it("records a root device reported from a rescue shell without erasing hardware info", async () => {
+    state.update((s) => {
+      s.discovered[mac] = hardware(mac, { product: "DGX Spark", manufacturer: "NVIDIA", arch: "aarch64" });
+    });
+
+    const res = await app.inject({
+      method: "POST",
+      url: "/api/discover",
+      headers: { "Content-Type": "application/json" },
+      body: JSON.stringify({ mac, root_device: "/dev/nvme0n1p2" }),
+    });
+    expect(res.statusCode).toBe(200);
+
+    const hw = state.load().discovered[mac];
+    expect(hw?.root_device).toBe("/dev/nvme0n1p2");
+    // The partial report must not blank what we already knew.
+    expect(hw?.product).toBe("DGX Spark");
+    expect(hw?.cpu_cores).toBe(4);
+    expect(hw?.arch).toBe("aarch64");
+  });
+});
+
+describe("dnsmasq architecture detection", () => {
+  const conf = renderDnsmasqConf(createTestConfig("/tmp/unused"));
+
+  it("maps DHCP option 93 values to per-architecture bootloaders", () => {
+    // 11 = ARM 64-bit UEFI
+    expect(conf).toContain("dhcp-match=set:efi-arm64,option:client-arch,11");
+    expect(conf).toContain("dhcp-boot=tag:efi-arm64,tag:!ipxe,ipxe-arm64.efi");
+    // 7 / 9 = x64 UEFI, 0 = x86 BIOS
+    expect(conf).toContain("dhcp-match=set:efi-x86_64,option:client-arch,7");
+    expect(conf).toContain("dhcp-match=set:efi-x86_64,option:client-arch,9");
+    expect(conf).toContain("dhcp-match=set:bios,option:client-arch,0");
+  });
+
+  it("matches arm64 UEFI HTTP boot on 19, not 20", () => {
+    // IANA: 19 = arm uefi 64 boot from http, 20 = pc/at bios boot from http.
+    expect(conf).toContain("dhcp-match=set:httpboot-arm64,option:client-arch,19");
+    expect(conf).not.toContain("dhcp-match=set:httpboot-arm64,option:client-arch,20");
+    expect(conf).toContain("dhcp-match=set:httpboot-x86_64,option:client-arch,16");
+  });
+
+  it("offers an arm64 PXE service directive in proxy mode", () => {
+    expect(conf).toContain('pxe-service=tag:!ipxe,ARM64_EFI,"PXE Boot",ipxe-arm64.efi');
+  });
+});
diff --git a/bastion/src/bastion/tests/install-guard.test.ts b/bastion/src/bastion/tests/install-guard.test.ts
new file mode 100644
index 0000000..5b8d23e
--- /dev/null
+++ b/bastion/src/bastion/tests/install-guard.test.ts
@@ -0,0 +1,194 @@
+// Installs must never reach a machine running a vendor OS we cannot restore.
+//
+// This is the guardrail that stops someone reinstalling a DGX Spark at 2am. Rescue is
+// deliberately still allowed for the same machines -- that is the whole point.
+
+import { describe, it, expect, beforeEach, afterEach } from "vitest";
+import { mkdirSync, rmSync } from "node:fs";
+import { join } from "node:path";
+import { tmpdir } from "node:os";
+import type { BastionConfig, BastionState, HardwareInfo } from "@lab/shared";
+import { classifyOnboard } from "@lab/shared";
+import { createApp } from "../src/server.js";
+import { checkInstallAllowed } from "../src/services/install-guard.js";
+import type { FastifyInstance } from "fastify";
+import type { StateManager } from "../src/services/state.js";
+
+// The real machines this exists to protect.
+const SPARK_2935 = "4c:bb:47:7f:29:35";
+const SPARK_3A1C = "48:21:0b:96:3a:1c";
+const ORDINARY = "aa:bb:cc:dd:ee:ff";
+
+function createTestConfig(testDir: string): BastionConfig {
+  return {
+    fedoraVersion: "43", arch: "x86_64", httpPort: 0,
+    timezone: "Europe/London", locale: "en_GB.UTF-8", bastionDir: testDir,
+    domain: "test.local", dhcpMode: "proxy", dhcpRangeStart: "", dhcpRangeEnd: "",
+    ubuntuVersion: "26.04", ubuntuMirror: "https://releases.ubuntu.com/26.04",
+    iface: "eth0", serverIp: "10.0.0.1", network: "10.0.0.0", gateway: "10.0.0.1",
+    sshKeys: [], adminUser: "testadmin", syslogPort: 15514,
+    skipDnsmasq: true, skipArtifacts: true,
+    fedoraMirror: "https://download.fedoraproject.org/pub/fedora/linux/releases/43/Everything/x86_64/os",
+    tftpDir: join(testDir, "tftp"), httpDir: join(testDir, "http"),
+    stateFile: join(testDir, "state.json"),
+  };
+}
+
+function hardware(mac: string, over: Partial<HardwareInfo> = {}): HardwareInfo {
+  return {
+    mac, product: "TestBox", board: "TestBoard", serial: "SN1",
+    manufacturer: "TestCorp", cpu_model: "Test CPU", cpu_cores: 4, memory_gb: 16,
+    arch: "x86_64", disks: [], nics: [],
+    first_seen: new Date().toISOString(), last_seen: new Date().toISOString(),
+    ...over,
+  };
+}
+
+const emptyState = (): BastionState => ({
+  discovered: {}, install_queue: {}, installed: {}, debug: {},
+});
+
+describe("classifyOnboard", () => {
+  it("recognises a DGX Spark from its DMI identity", () => {
+    expect(classifyOnboard({
+      mac: ORDINARY, manufacturer: "NVIDIA", product: "NVIDIA DGX Spark", board: "GB10",
+    })).toEqual({ onboard: "ssh", vendor_os: "dgx-os" });
+  });
+
+  it("recognises the known Sparks even with no DMI recorded", () => {
+    // Neither Spark has hardware info in bastion state today. A DMI-only rule would
+    // fail open on exactly the machines this protects.
+    expect(classifyOnboard({ mac: SPARK_2935 }).onboard).toBe("ssh");
+    expect(classifyOnboard({ mac: SPARK_3A1C }).onboard).toBe("ssh");
+  });
+
+  it("treats ordinary hardware as PXE-installable", () => {
+    expect(classifyOnboard({
+      mac: ORDINARY, manufacturer: "Beelink", product: "SER9", board: "SER9",
+    })).toEqual({ onboard: "pxe" });
+  });
+
+  it("does not override an explicit classification already on the record", () => {
+    expect(classifyOnboard({
+      mac: SPARK_2935, onboard: "pxe",
+    })).toEqual({ onboard: "pxe" });
+  });
+});
+
+describe("checkInstallAllowed", () => {
+  it("refuses a DGX Spark and explains why", () => {
+    const state = emptyState();
+    state.installed[SPARK_2935] = {
+      hostname: "spark-2935", role: "worker", ip: "192.168.8.12",
+      installed_at: new Date().toISOString(), arch: "aarch64",
+    };
+
+    const result = checkInstallAllowed(state, SPARK_2935, "fedora-43");
+    expect(result.allowed).toBe(false);
+    if (result.allowed === false) {
+      expect(result.error).toContain("spark-2935");
+      expect(result.error).toContain("DGX OS");
+      expect(result.error).toContain("provision debug");
+    }
+  });
+
+  it("refuses a Spark that is only known by MAC", () => {
+    expect(checkInstallAllowed(emptyState(), SPARK_3A1C, "fedora-43").allowed).toBe(false);
+  });
+
+  it("allows an ordinary discovered machine", () => {
+    const state = emptyState();
+    state.discovered[ORDINARY] = hardware(ORDINARY);
+    expect(checkInstallAllowed(state, ORDINARY, "fedora-43").allowed).toBe(true);
+  });
+
+  it("allows Fedora on aarch64", () => {
+    const state = emptyState();
+    state.discovered[ORDINARY] = hardware(ORDINARY, { arch: "aarch64" });
+    expect(checkInstallAllowed(state, ORDINARY, "fedora-43").allowed).toBe(true);
+  });
+
+  it("refuses Ubuntu on aarch64 -- no netboot artifacts are published", () => {
+    const state = emptyState();
+    state.discovered[ORDINARY] = hardware(ORDINARY, { arch: "aarch64" });
+    const result = checkInstallAllowed(state, ORDINARY, "ubuntu-26.04");
+    expect(result.allowed).toBe(false);
+    if (result.allowed === false) {
+      expect(result.error).toContain("aarch64");
+    }
+  });
+
+  it("allows Ubuntu on x86_64", () => {
+    const state = emptyState();
+    state.discovered[ORDINARY] = hardware(ORDINARY, { arch: "x86_64" });
+    expect(checkInstallAllowed(state, ORDINARY, "ubuntu-26.04").allowed).toBe(true);
+  });
+});
+
+describe("install route enforces the guard", () => {
+  let testDir: string;
+  let app: FastifyInstance;
+  let state: StateManager;
+
+  beforeEach(() => {
+    testDir = join(tmpdir(), `bastion-guard-test-${Date.now()}-${Math.random().toString(36).slice(2)}`);
+    mkdirSync(join(testDir, "http"), { recursive: true });
+    mkdirSync(join(testDir, "tftp"), { recursive: true });
+    const result = createApp(createTestConfig(testDir));
+    app = result.app;
+    state = result.state;
+  });
+
+  afterEach(async () => {
+    await app.close();
+    rmSync(testDir, { recursive: true, force: true });
+  });
+
+  it("rejects POST /api/install for a Spark and queues nothing", async () => {
+    const res = await app.inject({
+      method: "POST",
+      url: "/api/install",
+      headers: { "Content-Type": "application/json" },
+      body: JSON.stringify({ mac: SPARK_2935, hostname: "spark-2935", role: "worker" }),
+    });
+
+    expect(res.statusCode).toBe(409);
+    expect(JSON.parse(res.body).error).toContain("Refusing to install");
+    expect(state.load().install_queue[SPARK_2935]).toBeUndefined();
+  });
+
+  it("still serves rescue to a Spark -- debug is never guarded", async () => {
+    state.update((s) => {
+      s.installed[SPARK_2935] = {
+        hostname: "spark-2935", role: "worker", ip: "192.168.8.12",
+        installed_at: new Date().toISOString(), arch: "aarch64",
+      };
+      s.debug[SPARK_2935] = { hostname: "spark-2935", queued_at: new Date().toISOString() };
+    });
+
+    const res = await app.inject({ method: "GET", url: `/dispatch?mac=${SPARK_2935}` });
+    expect(res.statusCode).toBe(200);
+    expect(res.body).toContain("DEBUG/RESCUE MODE");
+    expect(res.body).toContain("/vmlinuz-aarch64");
+  });
+
+  it("a Spark that PXE boots unqueued gets discovery, never an install", async () => {
+    const res = await app.inject({ method: "GET", url: `/dispatch?mac=${SPARK_2935}&arch=arm64` });
+    expect(res.body).toContain("DISCOVERY MODE");
+    expect(res.body).not.toContain("INSTALLING");
+  });
+
+  it("still accepts an ordinary machine", async () => {
+    state.update((s) => { s.discovered[ORDINARY] = hardware(ORDINARY); });
+
+    const res = await app.inject({
+      method: "POST",
+      url: "/api/install",
+      headers: { "Content-Type": "application/json" },
+      body: JSON.stringify({ mac: ORDINARY, hostname: "worker-1", role: "worker" }),
+    });
+
+    expect(res.statusCode).toBe(200);
+    expect(state.load().install_queue[ORDINARY]).toBeDefined();
+  });
+});