feat(FN-4650): complete Step 2 — gate sandbox backend resolution

Fusion-Task-Id: FN-4650
Fusion-Task-Lineage: 3da8db80-236b-46f6-8122-644abda35ca3

packages/core/src/__tests__/sandbox-prompt-override.test.ts

@@ -29,24 +29,43 @@ describe("sandbox prompt override", () => {
   });
 
   describe("resolveSandboxBackend", () => {
-    it("prefers prompt override", () => {
+    it("ignores prompt override when sandbox experimental flag is disabled", () => {
+      expect(resolveSandboxBackend(undefined, "**Sandbox:** bubblewrap")).toEqual({
+        backend: "native",
+        source: "default",
+      });
+    });
+
+    it("ignores project setting when sandbox experimental flag is disabled", () => {
+      expect(resolveSandboxBackend({ sandbox: { backend: "podman" } }, undefined)).toEqual({
+        backend: "native",
+        source: "default",
+      });
+    });
+
+    it("prefers prompt override when sandbox experimental flag is enabled", () => {
       expect(
         resolveSandboxBackend(
-          { sandbox: { backend: "docker" } },
+          { sandbox: { backend: "docker" }, experimentalFeatures: { sandbox: true } },
           "**Sandbox:** bubblewrap",
         ),
       ).toEqual({ backend: "bubblewrap", source: "prompt" });
     });
 
-    it("falls back to project setting", () => {
-      expect(resolveSandboxBackend({ sandbox: { backend: "podman" } }, undefined)).toEqual({
+    it("falls back to project setting when sandbox experimental flag is enabled", () => {
+      expect(
+        resolveSandboxBackend(
+          { sandbox: { backend: "podman" }, experimentalFeatures: { sandbox: true } },
+          undefined,
+        ),
+      ).toEqual({
         backend: "podman",
         source: "project",
       });
     });
 
-    it("falls back to default", () => {
-      expect(resolveSandboxBackend(undefined, undefined)).toEqual({
+    it("falls back to default when enabled and no overrides are present", () => {
+      expect(resolveSandboxBackend({ experimentalFeatures: { sandbox: true } }, undefined)).toEqual({
         backend: "native",
         source: "default",
       });

packages/core/src/sandbox-prompt-override.ts

@@ -1,4 +1,5 @@
-import type { SandboxBackendName, SandboxProjectSettings } from "./types.js";
+import { isSandboxExperimentalEnabled } from "./sandbox-settings.js";
+import type { SandboxBackendName, SandboxProjectSettings, Settings } from "./types.js";
 
 /**
  * Sandbox backend override parsing for per-task PROMPT.md content (FN-4639; design from FN-4635).
@@ -22,9 +23,13 @@ export function parseSandboxPromptOverride(prompt: string | undefined): SandboxB
 }
 
 export function resolveSandboxBackend(
-  settings: { sandbox?: SandboxProjectSettings } | undefined,
+  settings: Pick<Settings, "sandbox" | "experimentalFeatures"> | { sandbox?: SandboxProjectSettings } | undefined,
   prompt: string | undefined,
 ): { backend: SandboxBackendName; source: "default" | "project" | "prompt" } {
+  if (!isSandboxExperimentalEnabled(settings)) {
+    return { backend: "native", source: "default" };
+  }
+
   const promptOverride = parseSandboxPromptOverride(prompt);
   if (promptOverride) {
     return { backend: promptOverride, source: "prompt" };