diff --git a/packages/cli/src/commands/agent.ts b/packages/cli/src/commands/agent.ts
index 73cd471b..98005a73 100644
--- a/packages/cli/src/commands/agent.ts
+++ b/packages/cli/src/commands/agent.ts
@@ -26,6 +26,7 @@ import {
   isSupervised,
   harnessReadPaths,
   harnessReadFiles,
+  BUN_TEMP_DIR,
   runtimeNonoOptions,
   runtimeNonoProfile,
   approveRuntimeNonoOptions,
@@ -909,7 +910,7 @@ export async function runAgent(args: AgentArgs): Promise<void> {
               // configured TMPDIR (cli#350 r4g). On macOS launchd sets TMPDIR
               // to /var/folders/…, so /tmp would otherwise not be granted at
               // all; on Linux TMPDIR is usually /tmp and the Set dedupes.
-              allow: [...new Set([mailDir, tmpDir, "/tmp", config.workspace, agentDir, ...(runtimeGrants.allow ?? [])])],
+              allow: [...new Set([mailDir, tmpDir, BUN_TEMP_DIR, config.workspace, agentDir, ...(runtimeGrants.allow ?? [])])],
             };
             const profile = runtimeNonoProfile(selectedRuntime);
             const approval = approveRuntimeNonoOptions(selectedRuntime, { ...launchOptions, cwd: process.cwd() }, process.env, launchId);
diff --git a/packages/cli/src/utils/nono.ts b/packages/cli/src/utils/nono.ts
index 2ee76ce6..1e5262c7 100644
--- a/packages/cli/src/utils/nono.ts
+++ b/packages/cli/src/utils/nono.ts
@@ -263,6 +263,13 @@ export function systemReadFiles(): string[] {
   return systemReadFileCandidates().filter((f) => existsSync(f));
 }
 
+/**
+ * Bun's own temp dir: `/tmp` regardless of TMPDIR (cli#350 r4g). The agent
+ * launcher grants it beside the configured TMPDIR; defined once so the launcher
+ * and its readers stay in step (cli#558).
+ */
+export const BUN_TEMP_DIR = "/tmp";
+
 /**
  * Read grants every TPS harness family needs: the agent identity dir, the bun
  * cache, the running interpreter's own directory, plus the system roots. One
diff --git a/packages/cli/test/helpers/launcher-grants.ts b/packages/cli/test/helpers/launcher-grants.ts
new file mode 100644
index 00000000..a5bdb755
--- /dev/null
+++ b/packages/cli/test/helpers/launcher-grants.ts
@@ -0,0 +1,74 @@
+/**
+ * The launcher's grants that are not under the test sandbox HOME, read from the
+ * launcher's own definitions (cli#558). A test sandbox HOME created inside any
+ * of them makes the launcher's runtime-options gate refuse before the case
+ * under test runs, so every test sandbox base must sit outside all of them.
+ *
+ * The two callers are the launch-control test and the runtime-launch fixture:
+ * one shared list and one shared chooser keep them from drifting apart.
+ */
+import { mkdtempSync, realpathSync, rmSync, statSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { basename, dirname, join, resolve } from "node:path";
+import { BUN_TEMP_DIR, harnessReadPaths } from "../../src/utils/nono.js";
+
+/** Bun's temp dir and the toolchain/interpreter read roots (cli#350 r4g, cli#341 S1b). */
+export const LAUNCHER_FIXED_GRANTS = [BUN_TEMP_DIR, ...harnessReadPaths()];
+
+function realOrResolved(path: string): string {
+  const abs = resolve(path);
+  try {
+    return realpathSync(abs);
+  } catch {
+    const parent = dirname(abs);
+    return parent === abs ? abs : join(realOrResolved(parent), basename(abs));
+  }
+}
+
+/** The launcher grant that covers `path` (equal, or an ancestor directory), or null. */
+export function launcherGrantCovering(path: string): string | null {
+  const target = realOrResolved(path);
+  for (const grant of LAUNCHER_FIXED_GRANTS) {
+    const root = realOrResolved(grant);
+    if (target === root || target.startsWith(root.endsWith("/") ? root : `${root}/`)) return grant;
+  }
+  return null;
+}
+
+function canCreateDirIn(candidate: string): boolean {
+  try {
+    if (!statSync(candidate).isDirectory()) return false;
+    rmSync(mkdtempSync(join(candidate, "lg-probe-")), { recursive: true });
+    return true;
+  } catch {
+    return false;
+  }
+}
+
+/**
+ * A base directory for a test sandbox HOME that lies outside every launcher
+ * grant (cli#558). `/var/tmp` is the sibling of the always-granted `/tmp` and
+ * sits outside it; the process temp dir is the fallback. If a TMPDIR leaves
+ * neither candidate outside the grants, refuse up front, naming TMPDIR and the
+ * grant tmpdir() falls inside, rather than let the launcher's gate report a
+ * misleading early refusal.
+ */
+export function sandboxHomeBase(candidates: string[] = ["/var/tmp", tmpdir()]): string {
+  for (const candidate of candidates) {
+    if (canCreateDirIn(candidate) && launcherGrantCovering(candidate) === null) return realpathSync(candidate);
+  }
+  const existing = candidates.filter((c) => canCreateDirIn(c));
+  if (existing.length === 0) {
+    throw new Error(
+      `no usable base directory exists or is writable (checked ${candidates.join(", ")}); ` +
+        "a test sandbox HOME needs a base outside every launcher grant.",
+    );
+  }
+  const grant = launcherGrantCovering(existing[0]);
+  const suggestion = existing.includes("/var/tmp") ? "" : " (for example /var/tmp)";
+  throw new Error(
+    `a test sandbox HOME needs a base outside every launcher grant, but '${existing[0]}' ` +
+      `(TMPDIR=${tmpdir()}) falls inside the launcher's '${grant}' grant — point TMPDIR at a ` +
+      `directory outside it${suggestion} and re-run.`,
+  );
+}
diff --git a/packages/cli/test/helpers/runtime-launch-fixture.ts b/packages/cli/test/helpers/runtime-launch-fixture.ts
index d5fedb2f..6c8e27ba 100644
--- a/packages/cli/test/helpers/runtime-launch-fixture.ts
+++ b/packages/cli/test/helpers/runtime-launch-fixture.ts
@@ -1,6 +1,6 @@
 import { mkdirSync, mkdtempSync, writeFileSync } from "node:fs";
-import { tmpdir } from "node:os";
 import { join } from "node:path";
+import { sandboxHomeBase } from "./launcher-grants.js";
 
 export interface Sandbox {
   root: string;
@@ -44,8 +44,9 @@ function seedHome(home: string, ws: string): void {
 }
 
 export function makeSandbox(): Sandbox {
-  const base = process.platform === "linux" ? "/var/tmp" : tmpdir();
-  const root = mkdtempSync(join(base, "tps-363-rt-"));
+  // The base must lie outside every launcher grant (cli#558): a HOME inside one
+  // makes the launcher's runtime-options gate refuse before the case under test.
+  const root = mkdtempSync(join(sandboxHomeBase(), "tps-363-rt-"));
   const home = join(root, "home");
   const tmp = join(root, "tmp");
   const ws = join(root, "ws");
diff --git a/packages/cli/test/launcher-grants.test.ts b/packages/cli/test/launcher-grants.test.ts
new file mode 100644
index 00000000..c66de05c
--- /dev/null
+++ b/packages/cli/test/launcher-grants.test.ts
@@ -0,0 +1,105 @@
+import { describe, expect, test } from "bun:test";
+import { chmodSync, mkdirSync, mkdtempSync, realpathSync, rmSync, symlinkSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { launcherGrantCovering, sandboxHomeBase } from "./helpers/launcher-grants.js";
+
+describe("sandboxHomeBase", () => {
+  test("refuses when every existing candidate is inside a launcher grant, naming TMPDIR and the grant", () => {
+    expect(() => sandboxHomeBase(["/tmp", "/tmp/x"])).toThrow(/TMPDIR=/);
+    expect(() => sandboxHomeBase(["/tmp", "/tmp/x"])).toThrow(/'\/tmp'/);
+  });
+
+  test("suggests /var/tmp when it was not among the rejected candidates", () => {
+    expect(() => sandboxHomeBase(["/tmp"])).toThrow(/for example \/var\/tmp/);
+  });
+
+  test("returns a candidate that lies outside every grant", () => {
+    expect(launcherGrantCovering("/var/tmp")).toBeNull();
+    expect(sandboxHomeBase(["/tmp", "/var/tmp"])).toBe(realpathSync("/var/tmp"));
+  });
+
+  test("returns the real path of a candidate reached through a symlink", () => {
+    const dir = mkdtempSync(join(tmpdir(), "lg-link-"));
+    try {
+      const link = join(dir, "link");
+      try {
+        symlinkSync(realpathSync("/var/tmp"), link);
+      } catch (err) {
+        console.warn(`symlink creation refused, case skipped: ${err}`);
+        return;
+      }
+      expect(sandboxHomeBase([link])).toBe(realpathSync("/var/tmp"));
+    } finally {
+      rmSync(dir, { recursive: true, force: true });
+    }
+  });
+
+  test("skips a non-writable directory candidate and returns the next writable uncovered one", () => {
+    const parent = mkdtempSync(join(sandboxHomeBase(), "lg-skip-"));
+    const dir = join(parent, "ro");
+    const next = join(parent, "rw");
+    try {
+      mkdirSync(dir);
+      mkdirSync(next);
+      chmodSync(dir, 0o500);
+      let writable = false;
+      try {
+        rmSync(mkdtempSync(join(dir, "w-")), { recursive: true });
+        writable = true;
+      } catch {}
+      if (writable) {
+        console.warn("a 0o500 directory is still writable (running as root?), case skipped");
+        return;
+      }
+      expect(sandboxHomeBase([dir, next])).toBe(realpathSync(next));
+    } finally {
+      chmodSync(dir, 0o700);
+      rmSync(parent, { recursive: true, force: true });
+    }
+  });
+
+  test("skips a candidate that is a regular file and returns the next writable directory", () => {
+    const parent = mkdtempSync(join(sandboxHomeBase(), "lg-file-"));
+    try {
+      const file = join(parent, "file");
+      const next = join(parent, "dir");
+      writeFileSync(file, "");
+      mkdirSync(next);
+      expect(sandboxHomeBase([file, next])).toBe(realpathSync(next));
+    } finally {
+      rmSync(parent, { recursive: true, force: true });
+    }
+  });
+
+  test("names the missing candidates when none of them exists", () => {
+    const missing = ["/var/tmp/launcher-grants-missing-a", "/var/tmp/launcher-grants-missing-b"];
+    expect(() => sandboxHomeBase(missing)).toThrow(/no usable base directory exists/);
+    expect(() => sandboxHomeBase(missing)).toThrow(/missing-a, \/var\/tmp\/launcher-grants-missing-b/);
+    expect(() => sandboxHomeBase(missing)).not.toThrow(/'null'/);
+  });
+});
+
+describe("launcherGrantCovering", () => {
+  test("returns the grant for a path under it and null across the prefix boundary", () => {
+    expect(launcherGrantCovering("/tmp/a")).toBe("/tmp");
+    expect(launcherGrantCovering("/tmpfoo")).toBeNull();
+  });
+
+  test("recognises a path under a symlink to /tmp as covered by /tmp", () => {
+    const dir = mkdtempSync(join(tmpdir(), "lg-link-"));
+    try {
+      const link = join(dir, "tmplink");
+      try {
+        symlinkSync("/tmp", link);
+      } catch (err) {
+        console.warn(`symlink creation refused, case skipped: ${err}`);
+        return;
+      }
+      expect(launcherGrantCovering(join(link, "a"))).toBe("/tmp");
+      expect(launcherGrantCovering(realpathSync("/tmp"))).toBe("/tmp");
+    } finally {
+      rmSync(dir, { recursive: true, force: true });
+    }
+  });
+});
diff --git a/packages/cli/test/sandbox-launch-control.test.ts b/packages/cli/test/sandbox-launch-control.test.ts
index 4d1bbfb3..a69352c2 100644
--- a/packages/cli/test/sandbox-launch-control.test.ts
+++ b/packages/cli/test/sandbox-launch-control.test.ts
@@ -31,6 +31,7 @@ import {
 } from "node:fs";
 import { spawnSync } from "node:child_process";
 import { evaluateLaunchControl } from "../src/utils/nono.js";
+import { sandboxHomeBase } from "./helpers/launcher-grants.js";
 import meow from "meow";
 import { buildPlist } from "../src/commands/mail-watch.js";
 import { generateOfficePlist, generateTunnelPlist } from "../src/commands/office-supervision.js";
@@ -117,9 +118,9 @@ describe("T3 — missing --sandbox-required is refused in non-TTY", () => {
 
 describe("T5 — the pinned-path launch spawns nono and the child argv asserts the flags", () => {
   test("agent start --sandbox-required with a fake nono at NONO_BIN: the run argv carries both flags", () => {
-    // OUTSIDE /tmp: the launch grants /tmp too (cli#350 r4g), so a /tmp HOME would
-    // sit inside that grant and the overlap assert would refuse before spawning.
-    const base = "/var/tmp";
+    // A HOME inside a launcher grant makes the runtime-options gate refuse
+    // before the case under test, whatever TMPDIR is (cli#558).
+    const base = sandboxHomeBase();
     const home = mkdtempSync(join(base, "tps-reexec-argv-"));
     try {
       const nonoDir = join(home, "nono");