diff --git a/docs/gateway/sandboxing.md b/docs/gateway/sandboxing.md
index f1d81e4aa309..021bff86ba1d 100644
--- a/docs/gateway/sandboxing.md
+++ b/docs/gateway/sandboxing.md
@@ -149,7 +149,7 @@ Use `sandbox.backend: "podman"` to select the native `podman` CLI directly. This
 
 Podman reuses the existing `sandbox.docker.*` settings and the active native `podman` CLI context; it adds no separate connection configuration surface.
 
-Rootless Podman defaults to `--userns=keep-id` for writable workspace mounts. A long-lived sandbox can reserve subordinate IDs and block unrelated `--userns=auto` workloads; remove it before starting those workloads. Set `sandbox.docker.user` to control the container user; rootful Podman otherwise uses the workspace owner when available.
+Rootless Podman defaults to `--userns=keep-id` for writable workspace mounts. A long-lived sandbox can reserve subordinate IDs and block unrelated `--userns=auto` workloads; remove it before starting those workloads. Set `sandbox.docker.user` to a nonzero numeric UID or UID:GID to control the container user. Rootless Podman rejects UID or GID 0 because Podman 4.x cannot remap namespace root while preserving workspace bind ownership; bake root-required setup into the image or use rootful Podman. Rootful Podman otherwise uses the workspace owner when available.
 
 ```json5
 {
@@ -523,9 +523,9 @@ Paths:
     - Default `docker.network` is `"none"` (no egress), so package installs will fail.
     - `docker.network: "container:<id>"` requires `dangerouslyAllowContainerNamespaceJoin: true` and is break-glass only.
     - `readOnlyRoot: true` prevents writes; set `readOnlyRoot: false` or bake a custom image.
-    - `user` must be root for package installs. Docker can omit `user` or set
-      `user: "0:0"`; rootless Podman must set `user: "0:0"` because omission
-      defaults to the invoking user through `--userns=keep-id`.
+    - `user` must be root for package installs. Docker or rootful Podman can omit
+      `user` or set `user: "0:0"`. Rootless Podman rejects zero-valued users;
+      bake packages into the image or use rootful Podman.
     - Sandbox exec does **not** inherit host `process.env`. Use `agents.defaults.sandbox.docker.env` (or a custom image) for skill API keys.
     - Values in `agents.defaults.sandbox.docker.env` are passed as explicit container environment variables. Anyone with access to the selected container engine can inspect them with metadata commands such as `docker inspect` or `podman inspect`. Use a custom image, mounted secret file, or another secret delivery path if that metadata exposure is not acceptable.
 
diff --git a/src/agents/sandbox/docker.config-hash-recreate.test.ts b/src/agents/sandbox/docker.config-hash-recreate.test.ts
index 4c1433ae05f2..b49cf5fba1aa 100644
--- a/src/agents/sandbox/docker.config-hash-recreate.test.ts
+++ b/src/agents/sandbox/docker.config-hash-recreate.test.ts
@@ -681,20 +681,32 @@ describe("ensureSandboxContainer config-hash recreation", () => {
     expect(collectDockerFlagValues(createCall.args, "--userns")).toEqual([]);
   });
 
-  it("maps an explicit root user through rootless Podman keep-id", async () => {
-    const cfg = createSandboxConfig([]);
-    cfg.docker.user = "0:0";
-    spawnState.inspectRunning = false;
-    registryMocks.readRegistryEntry.mockResolvedValue(null);
+  it.each([{ user: "0" }, { user: "0:0" }, { user: "1001:0" }, { user: "0:1002" }])(
+    "rejects zero-valued rootless Podman user $user",
+    async ({ user }) => {
+      const cfg = createSandboxConfig([]);
+      cfg.docker.user = user;
+      spawnState.inspectRunning = false;
+      registryMocks.readRegistryEntry.mockResolvedValue(null);
 
-    const createCall = await ensureSandboxCreateCallForTest({
-      cfg,
-      engine: PODMAN_SANDBOX_ENGINE,
-    });
+      await expect(
+        ensureSandboxContainer({
+          engine: PODMAN_SANDBOX_ENGINE,
+          scopeKey: "shared",
+          workspaceDir: "/tmp/workspace",
+          agentWorkspaceDir: "/tmp/workspace",
+          cfg,
+        }),
+      ).rejects.toThrow(/cannot use UID or GID 0/iu);
 
-    expect(collectDockerFlagValues(createCall.args, "--user")).toEqual(["0:0"]);
-    expect(collectDockerFlagValues(createCall.args, "--userns")).toEqual(["keep-id:uid=0,gid=0"]);
-  });
+      expect(spawnState.calls).not.toContainEqual(
+        expect.objectContaining({
+          command: "podman",
+          args: expect.arrayContaining(["create"]),
+        }),
+      );
+    },
+  );
 
   it("rejects nonnumeric users for rootless Podman keep-id", async () => {
     const cfg = createSandboxConfig([]);
@@ -943,7 +955,7 @@ describe("ensureSandboxContainer config-hash recreation", () => {
       createArgsEpoch: SANDBOX_DOCKER_CREATE_ARGS_EPOCH,
       readOnlyWorkspaceSkillMounts: [],
     });
-    const oldHash = `${genericHash}:podman-runtime-v6:keep-id:default`;
+    const oldHash = `${genericHash}:podman-runtime-v8:keep-id:default`;
     cfg.dockerTmpfsSource = "configured";
     spawnState.inspectRunning = false;
     spawnState.labelHash = oldHash;
diff --git a/src/agents/sandbox/podman-runtime.ts b/src/agents/sandbox/podman-runtime.ts
index 2f563297f93e..ac88915f66ef 100644
--- a/src/agents/sandbox/podman-runtime.ts
+++ b/src/agents/sandbox/podman-runtime.ts
@@ -42,6 +42,11 @@ function resolvePodmanKeepIdMode(user: string | undefined): string {
     );
   }
   const [, uid, gid] = match;
+  if (uid === "0" || gid === "0") {
+    throw invalidPodmanConfig(
+      `Rootless Podman sandbox user "${normalized}" cannot use UID or GID 0 while preserving workspace bind ownership. Bake root-required setup into the image or use rootful Podman.`,
+    );
+  }
   return gid ? `keep-id:uid=${uid},gid=${gid}` : `keep-id:uid=${uid}`;
 }
 
@@ -287,7 +292,7 @@ export function resolvePodmanSandboxConfigHash(params: {
   dockerTmpfsSource: SandboxConfig["dockerTmpfsSource"];
 }): string {
   const userMode = params.configuredUser ? "configured-user" : "keep-id";
-  return `${params.genericConfigHash}:podman-runtime-v6:${userMode}:${params.dockerTmpfsSource}`;
+  return `${params.genericConfigHash}:podman-runtime-v8:${userMode}:${params.dockerTmpfsSource}`;
 }
 
 export function resolvePodmanSandboxContainerPrefix(containerPrefix: string): string {