diff --git a/apps/hub/src/builds/posix-sandbox.test.ts b/apps/hub/src/builds/posix-sandbox.test.ts
index ee011a64..68ce7356 100644
--- a/apps/hub/src/builds/posix-sandbox.test.ts
+++ b/apps/hub/src/builds/posix-sandbox.test.ts
@@ -1,5 +1,11 @@
 import { expect, test } from "bun:test";
-import { publicHostName } from "./posix-sandbox.ts";
+import { mkdir, mkdtemp, readFile, rm, writeFile } from "node:fs/promises";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { SandboxManager } from "@anthropic-ai/sandbox-runtime";
+import { rgPath } from "@vscode/ripgrep";
+import { buildPackageLayout } from "./package-layout.ts";
+import { createPosixBuildExecutor, publicHostName } from "./posix-sandbox.ts";
 
 test("the open network admits only public names, however a private address is spelled", () => {
   for (const host of [
@@ -28,3 +34,73 @@ test("the open network admits only public names, however a private address is sp
   ])
     expect([host, publicHostName(host)]).toEqual([host, true]);
 });
+
+// Exercise real isolation where the host has the required OS sandbox tools.
+test.skipIf(
+  !SandboxManager.isSupportedPlatform() ||
+    SandboxManager.checkDependencies({ command: rgPath }).errors.length > 0,
+)(
+  "workspace output is writable while host files and approved inputs stay protected",
+  async () => {
+    const root = await mkdtemp(join(tmpdir(), "builder-sandbox-test-"));
+    let executor:
+      | Awaited<ReturnType<typeof createPosixBuildExecutor>>
+      | undefined;
+    try {
+      const workspace = join(root, "workspace");
+      const outside = join(root, "host-only.txt");
+      await mkdir(workspace);
+      await writeFile(outside, "host-only-sentinel");
+      await writeFile(join(workspace, "brief.md"), "Approved brief");
+      const layout = await buildPackageLayout();
+      executor = await createPosixBuildExecutor(
+        workspace,
+        { commandTimeoutMs: 10_000, maxToolResultBytes: 10_000 },
+        layout,
+        { readOnly: ["brief.md"] },
+      );
+      const run = (name: string, args: Record<string, unknown>) =>
+        executor!.run(
+          { id: crypto.randomUUID(), name, arguments: args },
+          new AbortController().signal,
+        );
+      const written = await run("write_file", {
+        path: "output.txt",
+        content: "Workspace output",
+      });
+      expect(written.isError).not.toBe(true);
+      expect(await readFile(join(workspace, "output.txt"), "utf8")).toBe(
+        "Workspace output",
+      );
+      const output = await run("read_file", { path: "output.txt" });
+      expect(output.isError).not.toBe(true);
+      expect(output.content).toContain("Workspace output");
+      const brief = await run("read_file", { path: "brief.md" });
+      expect(brief.isError).not.toBe(true);
+      expect(brief.content).toContain("Approved brief");
+
+      const denied = await run("read_file", { path: outside });
+      expect(denied.isError).toBe(true);
+      expect(denied.content).not.toContain("host-only-sentinel");
+      expect((await run("read_file", { path: layout.childPath })).isError).toBe(
+        true,
+      );
+      expect(
+        (await run("write_file", { path: "brief.md", content: "changed" }))
+          .isError,
+      ).toBe(true);
+      expect(
+        (await run("write_file", { path: outside, content: "changed" }))
+          .isError,
+      ).toBe(true);
+      expect(await readFile(join(workspace, "brief.md"), "utf8")).toBe(
+        "Approved brief",
+      );
+      expect(await readFile(outside, "utf8")).toBe("host-only-sentinel");
+    } finally {
+      await executor?.dispose();
+      await rm(root, { recursive: true, force: true });
+    }
+  },
+  60_000,
+);
diff --git a/apps/hub/src/builds/posix-sandbox.ts b/apps/hub/src/builds/posix-sandbox.ts
index 3478d90d..0e7a9f15 100644
--- a/apps/hub/src/builds/posix-sandbox.ts
+++ b/apps/hub/src/builds/posix-sandbox.ts
@@ -432,6 +432,22 @@ export async function createPosixBuildExecutor(
     const nodeShim = join(shimDirectory, "node");
     await writeFile(nodeShim, `#!/bin/sh\nexec ${quote(executable)} "$@"\n`);
     await chmod(nodeShim, 0o755);
+    // Resolve workspace packages before isolation: Bun's runtime lookup scans repository paths
+    // the sandbox must keep hidden. The bundled runner needs no host package discovery.
+    const toolChild = join(shimDirectory, "posix-child.js");
+    const toolBuild = await Bun.build({
+      entrypoints: [layout.childPath],
+      outdir: shimDirectory,
+      naming: "posix-child.js",
+      target: "bun",
+      conditions: ["intx-src"],
+      packages: "bundle",
+      minify: true,
+    });
+    if (!toolBuild.success)
+      throw new BuildExecutionError(
+        `tool_runner_build_failed: ${toolBuild.logs.join("; ")}`,
+      );
     // An agent that can read check-app's source reads it instead of its own work. Each command is
     // compiled to an executable, which macOS runs with its reads denied (Seatbelt checks
     // process-exec apart from file-read*), so the source never sits where the agent can read it.
@@ -514,8 +530,6 @@ export async function createPosixBuildExecutor(
           executable,
           shimDirectory,
           ...layout.dependencyRoots,
-          // Bun lists the entry module's directory before loading it.
-          layout.assetDirectory,
           home,
           ...(options.browsers ? [options.browsers] : []),
           ...(sockets ? [sockets] : []),
@@ -580,7 +594,7 @@ export async function createPosixBuildExecutor(
       return linux ? `exec ${wrappedCommand}` : wrappedCommand;
     };
     stage = "wrap";
-    const childCommand = `exec ${[executable, "--no-env-file", "--conditions=intx-src", layout.childPath, cwd].map(quote).join(" ")}`;
+    const childCommand = `exec ${[executable, "--no-env-file", toolChild, cwd].map(quote).join(" ")}`;
     const wrapped = await wrapShell(childCommand);
     // Agent tool calls use `wrapped` above with Unix sockets blocked by seccomp;
     // sidecar-owned helpers wrapped below (dev app relay, browser, check-app)