diff --git a/canvas/src/components/tabs/ContainerConfigTab.tsx b/canvas/src/components/tabs/ContainerConfigTab.tsx
index 7a5e4dd69..389e36d1e 100644
--- a/canvas/src/components/tabs/ContainerConfigTab.tsx
+++ b/canvas/src/components/tabs/ContainerConfigTab.tsx
@@ -3,13 +3,36 @@
 import { useEffect, useMemo, useState } from "react";
 import { api } from "@/lib/api";
 import { runtimeDisplayName } from "@/lib/runtime-names";
+import { isSaaSTenant } from "@/lib/tenant";
 import { useCanvasStore, type WorkspaceNodeData } from "@/store/canvas";
 import type { WorkspaceCompute } from "@/store/socket";
 
-const INSTANCE_TYPES = ["t3.medium", "t3.large", "t3.xlarge", "t3.2xlarge", "m6i.large", "m6i.xlarge", "c6i.xlarge"];
+// Machine sizes keyed by cloud provider — an AWS t3.* is meaningless on Hetzner,
+// etc. MUST mirror the workspace-server workspaceComputeInstanceAllowlist (which
+// mirrors the CP provider configs); the PATCH validation rejects a mismatch 400.
+const INSTANCE_TYPES_BY_PROVIDER: Record<string, string[]> = {
+  aws: ["t3.medium", "t3.large", "t3.xlarge", "t3.2xlarge", "m6i.large", "m6i.xlarge", "c6i.xlarge"],
+  hetzner: ["cpx11", "cpx21", "cpx31", "cpx41", "cpx51", "cax11", "cax21", "cax31", "cax41"],
+  gcp: ["e2-small", "e2-medium", "e2-standard-2", "e2-standard-4", "e2-standard-8"],
+};
+const DEFAULT_INSTANCE_BY_PROVIDER: Record<string, string> = {
+  aws: "t3.medium", hetzner: "cpx31", gcp: "e2-standard-2",
+};
+const normalizeProvider = (p?: string): string => (p === "gcp" || p === "hetzner" ? p : "aws");
+const instanceTypesForProvider = (p?: string): string[] =>
+  INSTANCE_TYPES_BY_PROVIDER[normalizeProvider(p)] ?? INSTANCE_TYPES_BY_PROVIDER.aws;
+const defaultInstanceForProvider = (p?: string): string =>
+  DEFAULT_INSTANCE_BY_PROVIDER[normalizeProvider(p)] ?? "t3.medium";
+
+// Editable cloud-provider options (multi-provider RFC) — mirrors CreateWorkspaceDialog.
+const CLOUD_PROVIDER_OPTIONS = [
+  { value: "aws", label: "AWS (default)" },
+  { value: "gcp", label: "GCP" },
+  { value: "hetzner", label: "Hetzner" },
+];
+
 const RUNTIME_OPTIONS = ["claude-code", "codex", "hermes", "openclaw", "kimi", "kimi-cli", "external"];
 const RESOLUTIONS = ["1280x720", "1440x900", "1920x1080", "2560x1440"];
-const DEFAULT_HEADLESS_INSTANCE_TYPE = "t3.medium";
 const DEFAULT_HEADLESS_ROOT_GB = 30;
 
 type Props = {
@@ -23,6 +46,7 @@ type Props = {
 
 type FormState = {
   runtime: string;
+  provider: string; // cloud backend; editable in SaaS (in-place switch recreates the box)
   instanceType: string;
   rootGB: string;
   displayEnabled: boolean;
@@ -38,16 +62,16 @@ const DATA_PERSISTENCE_OPTIONS = ["", "persist", "ephemeral"];
 const dataPersistenceLabel = (v: string): string =>
   v === "persist" ? "Always keep (persist)" : v === "ephemeral" ? "Don't keep (ephemeral)" : "Auto";
 
-// Cloud/compute backend display name. The provider is chosen at create time and
-// is NOT editable here (changing a workspace's cloud requires a recreate), so
-// it renders as a read-only badge — but we must preserve it across Save (the
-// compute payload is rebuilt below, and dropping it would wipe the column).
+// Cloud/compute backend display name (read-only fallback for non-SaaS / legacy).
 const cloudProviderLabel = (v: string | undefined): string =>
   v === "gcp" ? "GCP" : v === "hetzner" ? "Hetzner" : "AWS";
 
 export function ContainerConfigTab({ workspaceId, data }: Props) {
+  // Provider is editable only in SaaS (CP-provisioned boxes). Local/Docker has no
+  // cloud-provider concept, so we keep the read-only badge there.
+  const isSaaS = useMemo(() => isSaaSTenant(), []);
   const runtime = data.runtime;
-  const provider = data.compute?.provider; // read-only; set at create time
+  const provider = data.compute?.provider;
   const instanceType = data.compute?.instance_type;
   const rootGB = data.compute?.volume?.root_gb;
   const displayMode = data.compute?.display?.mode;
@@ -56,8 +80,8 @@ export function ContainerConfigTab({ workspaceId, data }: Props) {
   const displayHeight = data.compute?.display?.height;
   const dataPersistence = data.compute?.data_persistence;
   const initial = useMemo(
-    () => formFromData({ runtime, instanceType, rootGB, displayMode, displayProtocol, displayWidth, displayHeight, dataPersistence }),
-    [runtime, instanceType, rootGB, displayMode, displayProtocol, displayWidth, displayHeight, dataPersistence],
+    () => formFromData({ runtime, provider, instanceType, rootGB, displayMode, displayProtocol, displayWidth, displayHeight, dataPersistence }),
+    [runtime, provider, instanceType, rootGB, displayMode, displayProtocol, displayWidth, displayHeight, dataPersistence],
   );
   const [form, setForm] = useState<FormState>(initial);
   const [saving, setSaving] = useState(false);
@@ -87,6 +111,21 @@ export function ContainerConfigTab({ workspaceId, data }: Props) {
     try {
       let applyTemplateOnRestart = data.applyTemplateOnRestart ?? false;
       if (dirty) {
+        // In-place cloud switch is DESTRUCTIVE: changing the provider recreates the
+        // box on the new cloud (the workspace-server deprovisions the old box on
+        // its old cloud first, then the restart provisions on the new one). Confirm
+        // before doing it — the current box and any non-persisted state are lost.
+        const providerChanged = normalizeProvider(form.provider) !== normalizeProvider(initial.provider);
+        if (providerChanged && typeof window !== "undefined") {
+          const ok = window.confirm(
+            `Switch this workspace to ${cloudProviderLabel(form.provider)}? This RECREATES the box on the new cloud — the current box and any non-persisted state are replaced.`,
+          );
+          if (!ok) {
+            setSaving(false);
+            return;
+          }
+        }
+
         const rootGB = parseInt(form.rootGB, 10);
         if (!Number.isFinite(rootGB)) {
           setError("Root volume must be a number");
@@ -102,10 +141,11 @@ export function ContainerConfigTab({ workspaceId, data }: Props) {
             : { mode: "none" },
           // internal#734: omit when "auto" so the wire/default behavior is unchanged.
           ...(form.dataPersistence ? { data_persistence: form.dataPersistence } : {}),
-          // Preserve the create-time cloud provider — it's not editable here, but
-          // this PATCH rebuilds the whole compute object, so omitting it would
-          // wipe the persisted provider (and mislead the badge after a Save).
-          ...(provider ? { provider } : {}),
+          // Cloud backend: send the (possibly switched) provider. Omit for the
+          // default (aws) so a non-switching AWS save keeps the wire unchanged;
+          // a switch TO aws (omit) vs FROM aws (explicit) both register correctly
+          // because the workspace-server normalizes ""→aws when diffing.
+          ...(normalizeProvider(form.provider) !== "aws" ? { provider: normalizeProvider(form.provider) } : {}),
         };
 
         const resp = await api.patch<{ needs_restart?: boolean }>(`/workspaces/${workspaceId}`, {
@@ -140,15 +180,16 @@ export function ContainerConfigTab({ workspaceId, data }: Props) {
         <div className="mb-3 flex items-center justify-between gap-3">
           <div className="flex items-center gap-2">
             <h3 className="text-sm font-semibold text-ink">Container Config</h3>
-            {/* Read-only cloud-provider badge — which cloud this workspace's box
-                runs on (AWS/GCP/Hetzner). Defaults to AWS when unset (legacy
-                rows). Set at create time in the Create Workspace dialog. */}
-            <span
-              title="Cloud provider for this workspace's compute (set at create time)"
-              className="rounded-full border border-line/60 bg-surface-sunken px-2 py-0.5 font-mono text-[10px] uppercase tracking-wide text-ink-mid"
-            >
-              {cloudProviderLabel(provider)}
-            </span>
+            {/* Non-SaaS (local/Docker) has no cloud-provider concept → read-only
+                badge. In SaaS the provider is an editable selector in the form. */}
+            {!isSaaS && (
+              <span
+                title="Cloud provider for this workspace's compute"
+                className="rounded-full border border-line/60 bg-surface-sunken px-2 py-0.5 font-mono text-[10px] uppercase tracking-wide text-ink-mid"
+              >
+                {cloudProviderLabel(provider)}
+              </span>
+            )}
           </div>
           {data.needsRestart && <span className="text-[11px] text-warm">Restart required</span>}
         </div>
@@ -162,11 +203,32 @@ export function ContainerConfigTab({ workspaceId, data }: Props) {
             optionLabel={runtimeDisplayName}
             onChange={(runtime) => setForm((s) => ({ ...s, runtime }))}
           />
+          {isSaaS && (
+            <SelectField
+              id="cloud-provider"
+              label="Cloud provider"
+              value={normalizeProvider(form.provider)}
+              options={CLOUD_PROVIDER_OPTIONS.map((p) => p.value)}
+              optionLabel={(v) => CLOUD_PROVIDER_OPTIONS.find((p) => p.value === v)?.label ?? v}
+              // Switching cloud resets the instance type to the new provider's
+              // default (an AWS t3.* is invalid on Hetzner, etc.) — also keeps the
+              // instance-type dropdown below in sync with the provider's sizes.
+              onChange={(provider) =>
+                setForm((s) => ({
+                  ...s,
+                  provider,
+                  instanceType: instanceTypesForProvider(provider).includes(s.instanceType)
+                    ? s.instanceType
+                    : defaultInstanceForProvider(provider),
+                }))
+              }
+            />
+          )}
           <SelectField
             id="instance-type"
             label="Instance type"
             value={form.instanceType}
-            options={INSTANCE_TYPES}
+            options={instanceTypesForProvider(form.provider)}
             onChange={(instanceType) => setForm((s) => ({ ...s, instanceType }))}
           />
           <label className="grid gap-1" htmlFor="root-volume-gb">
@@ -270,6 +332,7 @@ export function ContainerConfigTab({ workspaceId, data }: Props) {
 
 function formFromData(data: {
   runtime?: string;
+  provider?: string;
   instanceType?: string;
   rootGB?: number;
   displayMode?: string;
@@ -281,9 +344,11 @@ function formFromData(data: {
   const width = data.displayWidth ?? 1920;
   const height = data.displayHeight ?? 1080;
   const resolution = `${width}x${height}`;
+  const provider = normalizeProvider(data.provider);
   return {
     runtime: data.runtime || "claude-code",
-    instanceType: data.instanceType || DEFAULT_HEADLESS_INSTANCE_TYPE,
+    provider,
+    instanceType: data.instanceType || defaultInstanceForProvider(provider),
     rootGB: String(data.rootGB || DEFAULT_HEADLESS_ROOT_GB),
     displayEnabled: !!data.displayMode && data.displayMode !== "none",
     displayMode: data.displayMode && data.displayMode !== "none" ? data.displayMode : "desktop-control",
diff --git a/canvas/src/components/tabs/__tests__/ContainerConfigTab.test.tsx b/canvas/src/components/tabs/__tests__/ContainerConfigTab.test.tsx
index 9d4beff57..45d098196 100644
--- a/canvas/src/components/tabs/__tests__/ContainerConfigTab.test.tsx
+++ b/canvas/src/components/tabs/__tests__/ContainerConfigTab.test.tsx
@@ -23,6 +23,13 @@ vi.mock("@/store/canvas", () => ({
   ),
 }));
 
+// SaaS so the editable cloud-provider selector renders (non-SaaS shows a read-only
+// badge). Existing tests keep provider=aws (default), which is omitted from the
+// PATCH payload, so their assertions are unaffected.
+vi.mock("@/lib/tenant", () => ({
+  isSaaSTenant: () => true,
+}));
+
 import { ContainerConfigTab } from "../ContainerConfigTab";
 
 afterEach(() => {
@@ -314,4 +321,67 @@ describe("ContainerConfigTab", () => {
     await waitFor(() => expect(restartWorkspace).toHaveBeenCalledWith("ws-compute", { applyTemplate: true }));
     expect(apiPatch).not.toHaveBeenCalled();
   });
+
+  it("switches cloud provider — keys the instance-type list to the provider, confirms the recreate, and PATCHes the new provider", async () => {
+    const confirmSpy = vi.spyOn(window, "confirm").mockReturnValue(true);
+    render(
+      <ContainerConfigTab
+        workspaceId="ws-switch"
+        data={{
+          runtime: "claude-code",
+          status: "online",
+          needsRestart: false,
+          activeTasks: 0,
+          maxConcurrentTasks: null,
+          workspaceAccess: "read-write",
+          deliveryMode: "push",
+          compute: { instance_type: "t3.large", provider: "aws", volume: { root_gb: 30 } },
+        }}
+      />,
+    );
+
+    const providerSel = screen.getByLabelText("Cloud provider");
+    expect(providerSel).toHaveProperty("value", "aws");
+    expect(screen.getByLabelText("Instance type")).toHaveProperty("value", "t3.large");
+
+    // Switch to Hetzner → the instance type resets to the Hetzner default (an AWS
+    // t3.* is invalid on Hetzner) and the options become Hetzner sizes.
+    fireEvent.change(providerSel, { target: { value: "hetzner" } });
+    expect(screen.getByLabelText("Instance type")).toHaveProperty("value", "cpx31");
+
+    fireEvent.click(screen.getByRole("button", { name: "Save" }));
+    await waitFor(() => expect(apiPatch).toHaveBeenCalledTimes(1));
+    expect(confirmSpy).toHaveBeenCalled(); // destructive recreate confirmed
+    const body = apiPatch.mock.calls[0][1] as { compute: { provider?: string; instance_type?: string } };
+    expect(body.compute.provider).toBe("hetzner");
+    expect(body.compute.instance_type).toBe("cpx31");
+    confirmSpy.mockRestore();
+  });
+
+  it("does not treat a non-provider edit as a recreate (no confirm; aws default omitted)", async () => {
+    const confirmSpy = vi.spyOn(window, "confirm").mockReturnValue(true);
+    render(
+      <ContainerConfigTab
+        workspaceId="ws-noswitch"
+        data={{
+          runtime: "claude-code",
+          status: "online",
+          needsRestart: false,
+          activeTasks: 0,
+          maxConcurrentTasks: null,
+          workspaceAccess: "read-write",
+          deliveryMode: "push",
+          compute: { instance_type: "t3.large", provider: "aws", volume: { root_gb: 30 } },
+        }}
+      />,
+    );
+
+    fireEvent.change(screen.getByLabelText("Root volume"), { target: { value: "60" } });
+    fireEvent.click(screen.getByRole("button", { name: "Save" }));
+    await waitFor(() => expect(apiPatch).toHaveBeenCalledTimes(1));
+    expect(confirmSpy).not.toHaveBeenCalled();
+    const body = apiPatch.mock.calls[0][1] as { compute: { provider?: string } };
+    expect(body.compute.provider).toBeUndefined(); // aws default omitted (wire unchanged)
+    confirmSpy.mockRestore();
+  });
 });
diff --git a/workspace-server/internal/handlers/workspace_compute.go b/workspace-server/internal/handlers/workspace_compute.go
index a8434f22f..c70785851 100644
--- a/workspace-server/internal/handlers/workspace_compute.go
+++ b/workspace-server/internal/handlers/workspace_compute.go
@@ -31,14 +31,53 @@ type workspaceDisplayResponse struct {
 	Status    string `json:"status,omitempty"`
 }
 
-var workspaceComputeInstanceAllowlist = map[string]struct{}{
-	"t3.medium":  {},
-	"t3.large":   {},
-	"t3.xlarge":  {},
-	"t3.2xlarge": {},
-	"m6i.large":  {},
-	"m6i.xlarge": {},
-	"c6i.xlarge": {},
+// workspaceComputeInstanceAllowlist is keyed by cloud provider (multi-provider /
+// in-place switch): each provider's box accepts only that provider's machine
+// sizes (an AWS t3.* is meaningless on Hetzner, and vice-versa). Mirrors the CP
+// provider SSOT — keep in lock-step with the controlplane provider configs
+// (Hetzner ServerType cpx*/cax*, GCP MachineType e2-*, AWS EC2 t3*/m6i*/c6i*).
+// TestValidateWorkspaceCompute_Provider / _InstanceTypePerProvider pin the sets.
+// "" provider = AWS default.
+var workspaceComputeInstanceAllowlist = map[string]map[string]struct{}{
+	"aws": {
+		"t3.medium": {}, "t3.large": {}, "t3.xlarge": {}, "t3.2xlarge": {},
+		"m6i.large": {}, "m6i.xlarge": {}, "c6i.xlarge": {},
+	},
+	"hetzner": {
+		"cpx11": {}, "cpx21": {}, "cpx31": {}, "cpx41": {}, "cpx51": {},
+		"cax11": {}, "cax21": {}, "cax31": {}, "cax41": {},
+	},
+	"gcp": {
+		"e2-small": {}, "e2-medium": {},
+		"e2-standard-2": {}, "e2-standard-4": {}, "e2-standard-8": {},
+	},
+}
+
+// normalizeCloudProvider maps "" → "aws" so the in-place switch comparison
+// treats the default and an explicit "aws" as the same cloud (no spurious switch).
+func normalizeCloudProvider(p string) string {
+	if p == "" {
+		return "aws"
+	}
+	return p
+}
+
+// instanceTypeAllowedForProvider reports whether instanceType is valid for the
+// given provider ("" → aws). Empty instanceType is always allowed (CP defaults).
+func instanceTypeAllowedForProvider(provider, instanceType string) bool {
+	if instanceType == "" {
+		return true
+	}
+	p := provider
+	if p == "" {
+		p = "aws"
+	}
+	set, ok := workspaceComputeInstanceAllowlist[p]
+	if !ok {
+		return false
+	}
+	_, ok = set[instanceType]
+	return ok
 }
 
 // workspaceComputeProviderAllowlist mirrors the controlplane cloud-provider SSOT
@@ -56,11 +95,24 @@ var workspaceComputeProviderAllowlist = map[string]struct{}{
 }
 
 func validateWorkspaceCompute(compute models.WorkspaceCompute) error {
-	if compute.InstanceType != "" {
-		if _, ok := workspaceComputeInstanceAllowlist[compute.InstanceType]; !ok {
-			return fmt.Errorf("unsupported compute.instance_type")
+	// Provider first (so the instance-type check below can be provider-scoped).
+	// "" = default (AWS). CP fail-closes an unwired provider with a 422; validating
+	// here gives a clean 400 before the round-trip and is the gate reused by the
+	// switch-provider flow. Mirrors the controlplane cloudprovider SSOT.
+	if compute.Provider != "" {
+		if _, ok := workspaceComputeProviderAllowlist[compute.Provider]; !ok {
+			return fmt.Errorf("unsupported compute.provider (want aws|gcp|hetzner)")
 		}
 	}
+	// Instance type must belong to the chosen provider (an AWS t3.* is invalid on
+	// Hetzner, etc.). Empty = CP default for the provider.
+	if !instanceTypeAllowedForProvider(compute.Provider, compute.InstanceType) {
+		prov := compute.Provider
+		if prov == "" {
+			prov = "aws"
+		}
+		return fmt.Errorf("unsupported compute.instance_type %q for provider %q", compute.InstanceType, prov)
+	}
 	if compute.Volume.RootGB != 0 {
 		if compute.Volume.RootGB < workspaceComputeDiskFloorGB || compute.Volume.RootGB > workspaceComputeDiskCeilingGB {
 			return fmt.Errorf("compute.volume.root_gb must be between %d and %d", workspaceComputeDiskFloorGB, workspaceComputeDiskCeilingGB)
@@ -87,15 +139,6 @@ func validateWorkspaceCompute(compute models.WorkspaceCompute) error {
 	default:
 		return fmt.Errorf("unsupported compute.data_persistence (want persist|ephemeral)")
 	}
-	// Cloud backend for the box (multi-provider). "" = default (AWS). CP fail-
-	// closes an unwired provider with a 422 (PROVIDER_UNAVAILABLE); validating
-	// here gives a clean 400 before the round-trip and is the gate reused by the
-	// switch-provider flow. Mirrors the controlplane cloudprovider SSOT.
-	if compute.Provider != "" {
-		if _, ok := workspaceComputeProviderAllowlist[compute.Provider]; !ok {
-			return fmt.Errorf("unsupported compute.provider (want aws|gcp|hetzner)")
-		}
-	}
 	return nil
 }
 
diff --git a/workspace-server/internal/handlers/workspace_compute_test.go b/workspace-server/internal/handlers/workspace_compute_test.go
index 26d23ebce..27e255cbd 100644
--- a/workspace-server/internal/handlers/workspace_compute_test.go
+++ b/workspace-server/internal/handlers/workspace_compute_test.go
@@ -64,6 +64,40 @@ func TestValidateWorkspaceCompute_Provider(t *testing.T) {
 	}
 }
 
+// Multi-provider / in-place switch: an instance type must belong to the chosen
+// provider — an AWS t3.* is meaningless on Hetzner, a cpx* on AWS, etc. Pins the
+// provider-keyed allowlist (mirrors the CP provider configs).
+func TestValidateWorkspaceCompute_InstanceTypePerProvider(t *testing.T) {
+	good := []struct{ provider, instance string }{
+		{"", "t3.medium"}, {"aws", "t3.2xlarge"}, {"aws", "c6i.xlarge"},
+		{"hetzner", "cpx31"}, {"hetzner", "cax41"},
+		{"gcp", "e2-standard-2"}, {"gcp", "e2-small"},
+		{"hetzner", ""}, {"gcp", ""}, // empty instance = CP default, always ok
+	}
+	for _, g := range good {
+		c := models.WorkspaceCompute{Provider: g.provider, InstanceType: g.instance}
+		if err := validateWorkspaceCompute(c); err != nil {
+			t.Errorf("provider=%q instance=%q must be accepted: %v", g.provider, g.instance, err)
+		}
+	}
+	bad := []struct{ provider, instance string }{
+		{"hetzner", "t3.medium"}, // AWS type on Hetzner
+		{"aws", "cpx31"},         // Hetzner type on AWS
+		{"gcp", "t3.large"},      // AWS type on GCP
+		{"hetzner", "e2-small"},  // GCP type on Hetzner
+		{"", "cpx31"},            // default(aws) + Hetzner type
+	}
+	for _, b := range bad {
+		c := models.WorkspaceCompute{Provider: b.provider, InstanceType: b.instance}
+		if err := validateWorkspaceCompute(c); err == nil {
+			t.Errorf("provider=%q instance=%q must be rejected (cross-provider instance type)", b.provider, b.instance)
+		}
+	}
+	if normalizeCloudProvider("") != "aws" || normalizeCloudProvider("hetzner") != "hetzner" {
+		t.Fatal("normalizeCloudProvider: \"\" must map to aws; explicit providers unchanged")
+	}
+}
+
 // internal#734: data_persistence enum. "" (auto), "persist", "ephemeral" are
 // the only accepted values; anything else is a clear 400 before the CP call.
 func TestValidateWorkspaceCompute_DataPersistence(t *testing.T) {
diff --git a/workspace-server/internal/handlers/workspace_crud.go b/workspace-server/internal/handlers/workspace_crud.go
index 4faaf1a8d..b22886a27 100644
--- a/workspace-server/internal/handlers/workspace_crud.go
+++ b/workspace-server/internal/handlers/workspace_crud.go
@@ -164,6 +164,7 @@ func (h *WorkspaceHandler) Update(c *gin.Context) {
 		}
 	}
 	var computeJSON string
+	var newComputeProvider string // hoisted: drives the cloud-provider switch detection below
 	computePatch := false
 	if rawCompute, ok := body["compute"]; ok {
 		computePatch = true
@@ -184,6 +185,7 @@ func (h *WorkspaceHandler) Update(c *gin.Context) {
 				c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
 				return
 			}
+			newComputeProvider = compute.Provider
 			encoded, err := workspaceComputeJSON(compute)
 			if err != nil {
 				log.Printf("Update compute encode error for %s: %v", id, err)
@@ -262,6 +264,55 @@ func (h *WorkspaceHandler) Update(c *gin.Context) {
 		needsRestart = true
 	}
 	if computePatch {
+		// Cloud-provider SWITCH (in-place): if the incoming provider differs from
+		// the one currently stored, the existing box lives on the OLD cloud. We
+		// MUST deprovision it on the OLD provider BEFORE overwriting compute —
+		// otherwise the subsequent "Save & Restart" restart's provider-aware
+		// deprovision (cpProv.Stop → resolveProvider reads compute->>'provider')
+		// would target the NEW cloud and ORPHAN the old box (a silently-billing
+		// leak). Cloud mode only (the local Docker provisioner has no cross-cloud
+		// concept; provider stays "" there so this never fires). After this, the
+		// canvas's restart provisions the box on the new cloud; its own Stop is a
+		// safe no-op (the box is already gone).
+		if h.cpProv != nil {
+			var oldProvider sql.NullString
+			err := db.DB.QueryRowContext(ctx, `SELECT compute->>'provider' FROM workspaces WHERE id = $1`, id).Scan(&oldProvider)
+			// FAIL-CLOSED on the read. The earlier `err == nil` gate was fail-OPEN:
+			// a transient/unexpected DB error here skipped the whole switch block and
+			// fell through to the compute UPDATE — so during a real switch the later
+			// provider-aware restart deprovision would target the NEW cloud and ORPHAN
+			// the old box (silent billing, unrecoverable). We cannot tell whether this
+			// is a cross-cloud switch without the old provider, so on any error other
+			// than "no such row" we abort exactly like a failed deprovision: compute
+			// untouched, old box still recoverable, user retries. (sql.ErrNoRows means
+			// there is genuinely no prior box — nothing to orphan — so it's safe to
+			// skip the switch and let the UPDATE proceed.)
+			if err != nil && !errors.Is(err, sql.ErrNoRows) {
+				log.Printf("Update: provider-switch precheck for %s ABORTED — could not read current cloud provider (provider left unchanged): %v", id, err)
+				c.JSON(http.StatusBadGateway, gin.H{"error": "could not read the current cloud provider; provider unchanged — please retry"})
+				return
+			}
+			if err == nil && normalizeCloudProvider(oldProvider.String) != normalizeCloudProvider(newComputeProvider) {
+				log.Printf("Update: cloud-provider switch for %s: %q -> %q; deprovisioning old box on old provider before overwriting compute",
+					id, normalizeCloudProvider(oldProvider.String), normalizeCloudProvider(newComputeProvider))
+				// Use the ERROR-returning variant and ABORT before overwriting
+				// compute if the old-box deprovision fails. If we proceeded, the
+				// old box would keep running on the OLD cloud while the row now
+				// records the NEW provider+instance — stranding it with no DB
+				// pointer (an UNRECOVERABLE cross-cloud orphan that no reconciler
+				// can map back). Aborting leaves the row pointing at the
+				// still-recoverable old box; the user can retry the switch. (The
+				// restart paths' void cpStopWithRetry is fine there because the
+				// box stays on the SAME cloud, so the provider record is unchanged
+				// and a provider-scoped sweep can still find it.)
+				if err := h.cpStopWithRetryErr(ctx, id, "provider-switch", false); err != nil {
+					log.Printf("Update: provider-switch for %s ABORTED — could not deprovision old box on %q (provider left unchanged, old box recoverable): %v",
+						id, normalizeCloudProvider(oldProvider.String), err)
+					c.JSON(http.StatusBadGateway, gin.H{"error": "could not deprovision the current cloud box; provider unchanged — please retry"})
+					return
+				}
+			}
+		}
 		if _, err := db.DB.ExecContext(ctx, `UPDATE workspaces SET compute = $2::jsonb, updated_at = now() WHERE id = $1`, id, computeJSON); err != nil {
 			log.Printf("Update compute error for %s: %v", id, err)
 			c.JSON(http.StatusInternalServerError, gin.H{"error": "failed to save compute config"})
diff --git a/workspace-server/internal/handlers/workspace_provider_switch_test.go b/workspace-server/internal/handlers/workspace_provider_switch_test.go
new file mode 100644
index 000000000..ea19aa31d
--- /dev/null
+++ b/workspace-server/internal/handlers/workspace_provider_switch_test.go
@@ -0,0 +1,180 @@
+package handlers
+
+// workspace_provider_switch_test.go — deterministic coverage for the in-place
+// cloud-provider switch in the Update (PATCH /workspaces/:id) handler.
+//
+// The switch is DESTRUCTIVE (it recreates the box on a new cloud) and its
+// safety hinges on ORDER + ABORT, which these tests pin without touching a real
+// cloud (sqlmock DB + the scriptedCPStop fake from workspace_restart_stop_retry_test):
+//
+//   1. On a provider change, the OLD box is deprovisioned (cpProv.Stop) BEFORE
+//      the compute row is overwritten — otherwise the later restart's
+//      provider-aware deprovision would target the NEW cloud and ORPHAN the old
+//      (still-billing) box. The sqlmock query ORDER pins "read old provider →
+//      [Stop] → UPDATE compute".
+//   2. If the old-box deprovision FAILS, the handler ABORTS (502) and does NOT
+//      overwrite compute — leaving the row pointed at the recoverable old box
+//      (an unexpected UPDATE would fail sqlmock's expectations).
+//   3. A non-switch compute edit (same provider) does NOT deprovision anything.
+//   4. If the old-provider READ errors (transient DB fault, not sql.ErrNoRows),
+//      the handler FAILS CLOSED: aborts (502), deprovisions nothing, and does NOT
+//      overwrite compute — closing the fail-open read path that would otherwise
+//      orphan the old box on a real switch (security review RC 9895).
+
+import (
+	"bytes"
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/DATA-DOG/go-sqlmock"
+	"github.com/gin-gonic/gin"
+)
+
+func newPatchContext(t *testing.T, id, body string) (*gin.Context, *httptest.ResponseRecorder) {
+	t.Helper()
+	w := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(w)
+	c.Params = gin.Params{{Key: "id", Value: id}}
+	req := httptest.NewRequest("PATCH", "/workspaces/"+id, bytes.NewBufferString(body))
+	req.Header.Set("Content-Type", "application/json")
+	c.Request = req
+	return c, w
+}
+
+const switchTestWSID = "cccccccc-0001-0000-0000-000000000000"
+
+func newSwitchTestHandler(t *testing.T, cp *scriptedCPStop) *WorkspaceHandler {
+	t.Helper()
+	h := NewWorkspaceHandler(newTestBroadcaster(), nil, "http://localhost:8080", t.TempDir())
+	h.cpProv = cp
+	return h
+}
+
+// 1. aws → hetzner: deprovision the OLD box, THEN overwrite compute (200).
+func TestWorkspaceUpdate_ProviderSwitch_DeprovisionsOldBeforeUpdate(t *testing.T) {
+	mock := setupTestDB(t)
+	cp := &scriptedCPStop{} // Stop succeeds
+	h := newSwitchTestHandler(t, cp)
+
+	// Ordered expectations pin: EXISTS → read OLD provider (aws) → UPDATE compute.
+	// The cpProv.Stop deprovision runs (in code) AFTER the provider read and
+	// BEFORE the UPDATE — exactly the orphan-safe order.
+	mock.ExpectQuery("SELECT EXISTS").WithArgs(switchTestWSID).
+		WillReturnRows(sqlmock.NewRows([]string{"exists"}).AddRow(true))
+	mock.ExpectQuery("compute->>'provider'").WithArgs(switchTestWSID).
+		WillReturnRows(sqlmock.NewRows([]string{"provider"}).AddRow("aws"))
+	mock.ExpectExec("UPDATE workspaces SET compute").
+		WillReturnResult(sqlmock.NewResult(0, 1))
+
+	c, w := newPatchContext(t, switchTestWSID,
+		`{"compute":{"instance_type":"cpx31","provider":"hetzner","volume":{"root_gb":30}}}`)
+	h.Update(c)
+
+	if w.Code != http.StatusOK {
+		t.Fatalf("expected 200 on a successful switch, got %d: %s", w.Code, w.Body.String())
+	}
+	if cp.calls != 1 {
+		t.Fatalf("expected the OLD box to be deprovisioned exactly once on a provider switch; got %d Stop calls", cp.calls)
+	}
+	if err := mock.ExpectationsWereMet(); err != nil {
+		t.Fatalf("unmet/unexpected DB queries (ordering broken?): %v", err)
+	}
+}
+
+// 2. Deprovision FAILS → abort (502) + compute NOT overwritten (no UPDATE).
+func TestWorkspaceUpdate_ProviderSwitch_AbortsWhenDeprovisionFails(t *testing.T) {
+	shrinkRetryBackoff(t) // don't burn the 1s/2s/4s retry backoff
+	mock := setupTestDB(t)
+	// All retry attempts fail → cpStopWithRetryErr returns an error → abort.
+	cp := &scriptedCPStop{errs: []error{
+		fmt.Errorf("cp 503"), fmt.Errorf("cp 503"), fmt.Errorf("cp 503"),
+	}}
+	h := newSwitchTestHandler(t, cp)
+
+	mock.ExpectQuery("SELECT EXISTS").WithArgs(switchTestWSID).
+		WillReturnRows(sqlmock.NewRows([]string{"exists"}).AddRow(true))
+	mock.ExpectQuery("compute->>'provider'").WithArgs(switchTestWSID).
+		WillReturnRows(sqlmock.NewRows([]string{"provider"}).AddRow("aws"))
+	// NO UPDATE expectation: if the handler overwrote compute after a failed
+	// deprovision (the orphan bug), sqlmock would flag the unexpected query.
+
+	c, w := newPatchContext(t, switchTestWSID,
+		`{"compute":{"instance_type":"cpx31","provider":"hetzner","volume":{"root_gb":30}}}`)
+	h.Update(c)
+
+	if w.Code != http.StatusBadGateway {
+		t.Fatalf("expected 502 when the old-box deprovision fails, got %d: %s", w.Code, w.Body.String())
+	}
+	if cp.calls == 0 {
+		t.Fatal("expected at least one Stop attempt before aborting")
+	}
+	if err := mock.ExpectationsWereMet(); err != nil {
+		// A failure here means an UNEXPECTED UPDATE ran — i.e. compute was
+		// overwritten after a failed deprovision → the orphan bug is back.
+		t.Fatalf("compute must NOT be overwritten when deprovision fails (orphan-prevention): %v", err)
+	}
+}
+
+// 3. Same provider (no switch): no deprovision; compute is updated normally.
+func TestWorkspaceUpdate_NoProviderSwitch_DoesNotDeprovision(t *testing.T) {
+	mock := setupTestDB(t)
+	cp := &scriptedCPStop{}
+	h := newSwitchTestHandler(t, cp)
+
+	mock.ExpectQuery("SELECT EXISTS").WithArgs(switchTestWSID).
+		WillReturnRows(sqlmock.NewRows([]string{"exists"}).AddRow(true))
+	mock.ExpectQuery("compute->>'provider'").WithArgs(switchTestWSID).
+		WillReturnRows(sqlmock.NewRows([]string{"provider"}).AddRow("aws"))
+	mock.ExpectExec("UPDATE workspaces SET compute").
+		WillReturnResult(sqlmock.NewResult(0, 1))
+
+	// provider stays aws (only the instance size changes) → no switch, no Stop.
+	c, w := newPatchContext(t, switchTestWSID,
+		`{"compute":{"instance_type":"t3.large","provider":"aws","volume":{"root_gb":60}}}`)
+	h.Update(c)
+
+	if w.Code != http.StatusOK {
+		t.Fatalf("expected 200, got %d: %s", w.Code, w.Body.String())
+	}
+	if cp.calls != 0 {
+		t.Fatalf("a non-switching compute edit must NOT deprovision the box; got %d Stop calls", cp.calls)
+	}
+	if err := mock.ExpectationsWereMet(); err != nil {
+		t.Fatalf("unmet/unexpected DB queries: %v", err)
+	}
+}
+
+// 4. Provider READ errors (transient DB fault) → fail-CLOSED: abort 502, no
+//    deprovision, no compute overwrite. A fail-open read (the old `err == nil`
+//    gate) would skip switch detection and overwrite compute → orphan the old
+//    cloud box. sqlmock has NO UPDATE/Stop expectations, so either an overwrite
+//    or a stray deprovision trips it.
+func TestWorkspaceUpdate_ProviderSwitch_AbortsOnProviderReadError(t *testing.T) {
+	mock := setupTestDB(t)
+	cp := &scriptedCPStop{}
+	h := newSwitchTestHandler(t, cp)
+
+	mock.ExpectQuery("SELECT EXISTS").WithArgs(switchTestWSID).
+		WillReturnRows(sqlmock.NewRows([]string{"exists"}).AddRow(true))
+	// The old-provider read hits a transient error (NOT sql.ErrNoRows).
+	mock.ExpectQuery("compute->>'provider'").WithArgs(switchTestWSID).
+		WillReturnError(fmt.Errorf("connection reset by peer"))
+
+	c, w := newPatchContext(t, switchTestWSID,
+		`{"compute":{"instance_type":"cpx31","provider":"hetzner","volume":{"root_gb":30}}}`)
+	h.Update(c)
+
+	if w.Code != http.StatusBadGateway {
+		t.Fatalf("expected 502 when the provider read fails (fail-closed), got %d: %s", w.Code, w.Body.String())
+	}
+	if cp.calls != 0 {
+		t.Fatalf("must NOT deprovision when the current provider can't be read; got %d Stop calls", cp.calls)
+	}
+	if err := mock.ExpectationsWereMet(); err != nil {
+		// An unexpected UPDATE here = compute was overwritten despite an unreadable
+		// provider → the fail-open orphan path is back.
+		t.Fatalf("compute must NOT be overwritten on a provider read error (fail-closed): %v", err)
+	}
+}