Windows: node_repl rejects MCP sandbox metadata with missing field sandboxPolicy #29338

Summary

On Windows, both the bundled Browser and Computer Use plugins fail before any browser/desktop automation code runs because the bundled node_repl rejects the MCP sandbox metadata:

Mcp error: -32602: js: codex/sandbox-state-meta: missing field `sandboxPolicy`

Restarting Codex, opening a fresh thread, opening the in-app browser, and using a browser-bound side chat do not resolve it.

Environment

• Windows 11 (10.0.26200), x64
• Codex Desktop package: 26.616.5445.0
• Bundled Codex CLI: 0.142.0-alpha.6
• Browser plugin: 26.616.41845
• Computer Use plugin: 26.616.41845
• Bundled node_repl archive: 20260609.1
• Permission mode: workspace write / on-request approvals
• Project is trusted

Reproduction

1. Enable the bundled Browser and Computer Use plugins.
2. Open an in-app browser tab.
3. Ask Codex to control the current tab, causing a call to node_repl/js.
4. The tool call fails immediately, before the JavaScript body executes.

The same error occurs when bootstrapping Computer Use with node_repl/js.

Expected behavior

Codex should attach sandbox metadata matching the schema expected by the bundled node_repl, and the JavaScript kernel should start inside the configured sandbox.

Actual behavior

node_repl advertises the experimental MCP capability codex/sandbox-state-meta, but rejects the metadata supplied by the Codex MCP client because sandboxPolicy is absent.

Diagnostic findings

• The desktop application's persisted thread permissions contain a valid sandboxPolicy.
• Codex logs show the active turn has a valid internal sandbox_policy=WorkspaceWrite.
• The failure occurs only at the MCP handoff to node_repl.
• The installed CUA runtime manifest and binaries exactly match the runtime bundled inside the installed Codex Desktop package, so this is not a stale local runtime.
• In current openai/codex source, codex_mcp::SandboxState serializes fields such as permissionProfile, sandboxCwd, codexLinuxSandboxExe, and useLegacyLandlock; it does not contain a sandboxPolicy field. The bundled node_repl appears to expect an older/different schema.

Relevant source:

• codex-rs/codex-mcp/src/runtime.rs
• codex-rs/codex-mcp/src/rmcp_client.rs

Attempted workaround and security impact

Launching node_repl with --disable-sandbox was tested as a temporary workaround. It did not help: metadata validation fails before the JavaScript kernel is launched. The option was immediately removed and the original sandboxed configuration restored.

Please align the bundled node_repl sandbox-state schema with the Codex MCP client schema, or make the metadata handling backward compatible.