diff --git a/crates/tui/src/sandbox/mod.rs b/crates/tui/src/sandbox/mod.rs
index 7fd278f8..cffb4f77 100644
--- a/crates/tui/src/sandbox/mod.rs
+++ b/crates/tui/src/sandbox/mod.rs
@@ -10,7 +10,9 @@
 //!
 //! - **macOS**: Uses Seatbelt (sandbox-exec) for mandatory access control
 //! - **Linux**: Uses Landlock (kernel 5.13+) for filesystem access control
-//! - **Windows**: Windows Sandbox/AppContainer/Restricted token (best-effort)
+//! - **Windows**: No OS sandbox is advertised yet. The planned first helper
+//!   contract is process-tree containment only via a Windows Job Object; it
+//!   must not claim filesystem, network, registry, or AppContainer isolation.
 //!
 //! # Usage
 //!
@@ -179,7 +181,10 @@ pub enum SandboxType {
     #[cfg(target_os = "linux")]
     LinuxLandlock,
 
-    /// Windows sandboxing (Windows Sandbox/AppContainer/Restricted token).
+    /// Windows process-containment helper.
+    ///
+    /// Not advertised until a helper enforces Job Object cleanup. This does
+    /// not imply filesystem, network, registry, or AppContainer isolation.
     #[cfg(target_os = "windows")]
     Windows,
 }
@@ -427,10 +432,12 @@ impl SandboxManager {
         }
     }
 
-    /// Prepare a Windows-sandboxed execution environment.
+    /// Prepare a Windows helper execution environment.
     ///
-    /// Note: Windows sandboxing requires a helper process for full isolation.
-    /// This implementation marks intent and defers enforcement to a helper.
+    /// Windows support is currently not advertised by `get_platform_sandbox`.
+    /// This branch only exists for forced tests and future helper wiring.
+    /// The first supported helper contract is process-tree containment only;
+    /// it must not be presented as filesystem or network isolation.
     #[cfg(target_os = "windows")]
     fn prepare_windows(spec: &CommandSpec) -> ExecEnv {
         let mut command = vec![spec.program.clone()];
diff --git a/crates/tui/src/sandbox/windows.rs b/crates/tui/src/sandbox/windows.rs
index 956f001d..5731d05e 100644
--- a/crates/tui/src/sandbox/windows.rs
+++ b/crates/tui/src/sandbox/windows.rs
@@ -1,12 +1,14 @@
-//! Windows sandbox implementation (best-effort placeholder).
+//! Windows sandbox helper contract.
 //!
-//! Windows sandboxing can be implemented using:
-//! - Windows Sandbox (full isolation)
-//! - AppContainer (process isolation)
-//! - Restricted tokens (reduced privileges)
+//! Current status: DeepSeek TUI does not advertise an in-process Windows
+//! sandbox. Future Windows support must run commands through a dedicated
+//! helper that provides process-tree containment with a Job Object and
+//! `JOB_OBJECT_LIMIT_KILL_ON_JOB_CLOSE`.
 //!
-//! This module selects a preferred approach and exposes helpers used by the
-//! sandbox manager. Full enforcement should be implemented in a helper binary.
+//! The first Windows helper slice is process containment only. It must not
+//! claim read-only filesystem isolation, workspace-write enforcement, network
+//! blocking, registry isolation, or AppContainer-level isolation until those
+//! guarantees are implemented and tested separately.
 
 use std::path::Path;
 
@@ -14,33 +16,23 @@ use super::SandboxPolicy;
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum WindowsSandboxKind {
-    WindowsSandbox,
-    AppContainer,
-    RestrictedToken,
+    ProcessContainment,
 }
 
 impl std::fmt::Display for WindowsSandboxKind {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         match self {
-            WindowsSandboxKind::WindowsSandbox => write!(f, "sandbox"),
-            WindowsSandboxKind::AppContainer => write!(f, "appcontainer"),
-            WindowsSandboxKind::RestrictedToken => write!(f, "restricted-token"),
+            WindowsSandboxKind::ProcessContainment => write!(f, "process-containment"),
         }
     }
 }
 
 pub fn is_available() -> bool {
-    windows_sandbox_available() || appcontainer_available() || restricted_token_available()
+    false
 }
 
 pub fn select_best_kind(_policy: &SandboxPolicy, _cwd: &Path) -> WindowsSandboxKind {
-    if windows_sandbox_available() {
-        WindowsSandboxKind::WindowsSandbox
-    } else if appcontainer_available() {
-        WindowsSandboxKind::AppContainer
-    } else {
-        WindowsSandboxKind::RestrictedToken
-    }
+    WindowsSandboxKind::ProcessContainment
 }
 
 pub fn detect_denial(exit_code: i32, stderr: &str) -> bool {
@@ -59,21 +51,16 @@ pub fn detect_denial(exit_code: i32, stderr: &str) -> bool {
 
     patterns.iter().any(|p| stderr.contains(p))
 }
+#[cfg(test)]
+mod tests {
+    use super::*;
 
-fn windows_sandbox_available() -> bool {
-    let Ok(system_root) = std::env::var("SystemRoot") else {
-        return false;
-    };
-    Path::new(&system_root)
-        .join("System32")
-        .join("WindowsSandbox.exe")
-        .exists()
-}
-
-fn appcontainer_available() -> bool {
-    true
-}
-
-fn restricted_token_available() -> bool {
-    true
+    #[test]
+    fn windows_sandbox_is_not_advertised_until_helper_exists() {
+        assert!(!is_available());
+        assert_eq!(
+            select_best_kind(&SandboxPolicy::default(), Path::new(".")),
+            WindowsSandboxKind::ProcessContainment
+        );
+    }
 }
diff --git a/docs/ARCHITECTURE.md b/docs/ARCHITECTURE.md
index c31a979a..ad39a438 100644
--- a/docs/ARCHITECTURE.md
+++ b/docs/ARCHITECTURE.md
@@ -161,10 +161,13 @@ drives turns through Chat Completions.
 
 ### Security
 
-- **`sandbox/`** - macOS sandboxing support
+- **`sandbox/`** - platform sandbox policy preparation and denial reporting
   - `mod.rs` - Sandbox type definitions
   - `policy.rs` - Sandbox policy configuration
   - `seatbelt.rs` - macOS Seatbelt profile generation
+  - `landlock.rs` - Linux Landlock detection and future helper contract
+  - `windows.rs` - Windows helper contract; not advertised until a Job
+    Object process-containment helper exists
 
 ### Utilities
 
@@ -281,7 +284,10 @@ command = "echo 'Running tool: $TOOL_NAME'"
 1. **Streaming-first**: All LLM responses stream for responsiveness
 2. **Tool safety**: Non-YOLO mode requires approval for destructive operations, including side-effectful MCP tools
 3. **Extensibility**: MCP, skills, and hooks allow customization without code changes
-4. **Cross-platform**: Core works on Linux/macOS/Windows, sandboxing macOS-only
+4. **Cross-platform**: Core works on Linux/macOS/Windows. Sandbox guarantees
+   are platform-specific: macOS Seatbelt is the active policy path; Linux and
+   Windows require helper enforcement before they should be treated as full OS
+   sandboxing.
 5. **Minimal dependencies**: Careful dependency selection for build speed
 6. **Local-first runtime API**: HTTP/SSE endpoints are intended for trusted localhost access and are served by the `crates/tui` runtime today
 
diff --git a/docs/CONFIGURATION.md b/docs/CONFIGURATION.md
index 90bafbc2..46ca872a 100644
--- a/docs/CONFIGURATION.md
+++ b/docs/CONFIGURATION.md
@@ -364,6 +364,12 @@ If you are upgrading from older releases:
 - `allow_shell` (bool, optional): defaults to `true` (sandboxed).
 - `approval_policy` (string, optional): `on-request`, `untrusted`, or `never`. Runtime `approval_mode` editing in `/config` also accepts `on-request` and `untrusted` aliases.
 - `sandbox_mode` (string, optional): `read-only`, `workspace-write`, `danger-full-access`, `external-sandbox`.
+  Platform support is not identical. macOS uses Seatbelt for policy
+  enforcement. Linux support is helper-gated around Landlock. Windows does not
+  currently advertise an OS sandbox; the planned Windows helper contract starts
+  with process-tree containment only and must not be described as read-only
+  filesystem isolation, workspace-write enforcement, network blocking,
+  registry isolation, or AppContainer isolation until those are implemented.
 - `managed_config_path` (string, optional): managed config file loaded after user/env config.
 - `requirements_path` (string, optional): requirements file used to enforce allowed approval/sandbox values.
 - `max_subagents` (int, optional): defaults to `10` and is clamped to `1..=20`.