fix(sandbox): scope user-driven SANDBOX_START to the locked provider kind (#4478)

Auto-start and self-heal already passed sandboxProviderKind to
SANDBOX_START so provisioning stays pinned to the thread's locked
provider (#4025), but the user-driven start() — which retry() and
resume() also call — built its args without it. Retrying a failed
locked-provider sandbox (or resuming a suspended one) could get
re-provisioned on a different provider than the one the preview is
locked to, the exact 'different-provider sibling' the surrounding code
says should never happen.

Extract the shared arg-builder (buildSandboxStartArgs) so all three
call sites stay in sync, and add a regression test.

Author: pedrofrxncx

apps/mesh/src/web/components/sandbox/hooks/sandbox-lifecycle-context.test.ts

@@ -4,6 +4,7 @@ import {
   shouldAutoStart,
   shouldSelfHeal,
   computeDrawerStatus,
+  buildSandboxStartArgs,
   type BranchMapEntryLike,
 } from "./sandbox-lifecycle-context";
 
@@ -130,6 +131,26 @@ describe("shouldSelfHeal", () => {
   });
 });
 
+describe("buildSandboxStartArgs", () => {
+  test("includes sandboxProviderKind when the thread has a locked kind", () => {
+    // Regression: user-driven start/retry/resume used to omit
+    // sandboxProviderKind (unlike auto-start/self-heal), so retrying a
+    // failed locked-provider thread could get provisioned on the wrong
+    // provider instead of the one the preview is locked to.
+    expect(buildSandboxStartArgs("vmcp1", "main", "user-desktop")).toEqual({
+      virtualMcpId: "vmcp1",
+      branch: "main",
+      sandboxProviderKind: "user-desktop",
+    });
+  });
+
+  test("omits branch and sandboxProviderKind when absent", () => {
+    expect(buildSandboxStartArgs("vmcp1", null, null)).toEqual({
+      virtualMcpId: "vmcp1",
+    });
+  });
+});
+
 describe("computeDrawerStatus", () => {
   test("suspended → suspended", () => {
     expect(computeDrawerStatus({ kind: "suspended" })).toBe("suspended");

apps/mesh/src/web/components/sandbox/hooks/sandbox-lifecycle-context.tsx

@@ -68,6 +68,21 @@ export function shouldSelfHeal(args: ShouldSelfHealArgs): boolean {
   );
 }
 
+/** Shared SANDBOX_START arg-builder so every call site (auto-start,
+ *  self-heal, user-driven start/retry/resume) scopes provisioning to the
+ *  locked provider kind identically — a caller that omits it can get
+ *  provisioned on the wrong provider. */
+export function buildSandboxStartArgs(
+  virtualMcpId: string,
+  branch: string | null,
+  sandboxProviderKind: SandboxProviderKind | null,
+): SandboxStartArgs {
+  const args: SandboxStartArgs = { virtualMcpId };
+  if (branch) args.branch = branch;
+  if (sandboxProviderKind) args.sandboxProviderKind = sandboxProviderKind;
+  return args;
+}
+
 export function computeDrawerStatus(state: PreviewState): DrawerStatus {
   switch (state.kind) {
     case "suspended":
@@ -239,9 +254,11 @@ export function SandboxLifecycleProvider({
   useEffect(() => {
     if (!autoStartEligible || !virtualMcpId) return;
     autoStartAttemptedForBranchRef.current.add(autoStartDedupKey);
-    const args: SandboxStartArgs = { virtualMcpId };
-    if (branch) args.branch = branch;
-    if (sandboxProviderKind) args.sandboxProviderKind = sandboxProviderKind;
+    const args = buildSandboxStartArgs(
+      virtualMcpId,
+      branch,
+      sandboxProviderKind,
+    );
     startVmMutate(args, {
       onSuccess: (data) => {
         if (data?.branch && !branch) setCurrentTaskBranch(data.branch);
@@ -276,9 +293,11 @@ export function SandboxLifecycleProvider({
     if (!selfHealEligible || !deadVmId || !virtualMcpId) return;
     // oxlint-disable-next-line ban-ref-current-assignment/ban-ref-current-assignment -- record dead handle to dedup repeat 404s
     reprovisionedForVmIdRef.current = deadVmId;
-    const args: SandboxStartArgs = { virtualMcpId };
-    if (branch) args.branch = branch;
-    if (sandboxProviderKind) args.sandboxProviderKind = sandboxProviderKind;
+    const args = buildSandboxStartArgs(
+      virtualMcpId,
+      branch,
+      sandboxProviderKind,
+    );
     startVmMutate(args, {
       onSuccess: (data) => {
         if (data?.branch && !branch) setCurrentTaskBranch(data.branch);
@@ -300,8 +319,11 @@ export function SandboxLifecycleProvider({
   // User-driven actions.
   const start = () => {
     if (!virtualMcpId) return;
-    const args: SandboxStartArgs = { virtualMcpId };
-    if (branch) args.branch = branch;
+    const args = buildSandboxStartArgs(
+      virtualMcpId,
+      branch,
+      sandboxProviderKind,
+    );
     startVmMutate(args, {
       onSuccess: (data) => {
         if (data?.branch && !branch) setCurrentTaskBranch(data.branch);