diff --git a/crates/tui/src/sandbox/mod.rs b/crates/tui/src/sandbox/mod.rs
index 2ba60080c8..bbb424a4c2 100644
--- a/crates/tui/src/sandbox/mod.rs
+++ b/crates/tui/src/sandbox/mod.rs
@@ -213,14 +213,21 @@ impl CommandSpec {
         self
     }
 
-    /// Add environment variables for this command.
+    /// Add environment variables without discarding shell defaults.
     pub fn with_env(mut self, env: HashMap<String, String>) -> Self {
-        self.env = env;
+        for (key, value) in env {
+            self = self.with_env_var(&key, &value);
+        }
         self
     }
 
     /// Add a single environment variable.
     pub fn with_env_var(mut self, key: &str, value: &str) -> Self {
+        // Windows environment names are case-insensitive. Remove an earlier
+        // spelling so the override cannot depend on HashMap iteration order.
+        #[cfg(windows)]
+        self.env
+            .retain(|existing, _| !existing.eq_ignore_ascii_case(key));
         self.env.insert(key.to_string(), value.to_string());
         self
     }
@@ -948,6 +955,106 @@ mod tests {
         );
     }
 
+    #[test]
+    fn command_spec_env_overlays_preserve_existing_values() {
+        let spec = CommandSpec::program("test", vec![], PathBuf::from("."), Duration::from_secs(5))
+            .with_env_var("FIRST", "original")
+            .with_env(HashMap::new())
+            .with_env(HashMap::from([("SECOND".to_string(), "added".to_string())]));
+        assert_eq!(spec.env.get("FIRST").map(String::as_str), Some("original"));
+        assert_eq!(spec.env.get("SECOND").map(String::as_str), Some("added"));
+        let spec = spec.with_env(HashMap::from([(
+            "FIRST".to_string(),
+            "override".to_string(),
+        )]));
+        assert_eq!(spec.env.get("FIRST").map(String::as_str), Some("override"));
+    }
+
+    #[cfg(windows)]
+    #[tokio::test]
+    async fn forkguard_windows_shell_env_overlays_keep_python_stdio_utf8() {
+        use crate::shell_dispatcher::ShellKind;
+
+        let _lock = crate::test_support::lock_test_env();
+        let _legacy_python = crate::test_support::EnvVarGuard::set("PYTHONIOENCODING", "gbk");
+        let kind = crate::shell_dispatcher::global_dispatcher().kind();
+        let command = if kind.is_powershell() {
+            "Write-Output $env:PYTHONIOENCODING; Write-Output '中文-output'"
+        } else if matches!(kind, ShellKind::Cmd) {
+            "echo %PYTHONIOENCODING%& echo 中文-output"
+        } else {
+            "printf '%s\\n' \"$PYTHONIOENCODING\" '中文-output'"
+        };
+        for encoded in [false, true] {
+            if encoded && !kind.is_powershell() {
+                continue;
+            }
+            for extra_env in [
+                HashMap::new(),
+                HashMap::from([(
+                    "CODEWHALE_ENCODING_PROBE".to_string(),
+                    "present".to_string(),
+                )]),
+            ] {
+                let cwd = std::env::temp_dir();
+                let timeout = Duration::from_secs(30);
+                let spec = if encoded {
+                    CommandSpec::powershell_encoded_shell(command, cwd, timeout)
+                        .expect("Windows PowerShell encoded form")
+                } else {
+                    CommandSpec::shell(command, cwd, timeout)
+                }
+                .with_env(extra_env)
+                .with_policy(SandboxPolicy::DangerFullAccess);
+                let exec = SandboxManager::new().prepare(&spec);
+                let mut cmd = tokio::process::Command::new(exec.program());
+                cmd.args(exec.args())
+                    .current_dir(&spec.cwd)
+                    .kill_on_drop(true);
+                crate::utils::suppress_console_window(cmd.as_std_mut());
+                crate::child_env::apply_to_tokio_command(
+                    &mut cmd,
+                    crate::child_env::string_map_env(&exec.env),
+                );
+                let output = tokio::time::timeout(spec.timeout, cmd.output())
+                    .await
+                    .expect("shell environment probe timed out")
+                    .expect("run shell with overlaid environment");
+                assert!(output.status.success(), "{output:?}");
+                let stdout = String::from_utf8(output.stdout).expect("UTF-8 shell output");
+                assert!(stdout.lines().any(|line| line == "utf-8"), "{stdout:?}");
+                assert!(stdout.contains("中文-output"), "{stdout:?}");
+            }
+        }
+    }
+
+    #[cfg(windows)]
+    #[test]
+    fn windows_shell_env_explicit_override_is_case_insensitive() {
+        let spec = CommandSpec::shell(
+            "Write-Output 'probe'",
+            std::env::temp_dir(),
+            Duration::from_secs(5),
+        )
+        .with_env(HashMap::from([(
+            "pythonioencoding".to_string(),
+            "gbk".to_string(),
+        )]));
+        let matches: Vec<_> = spec
+            .env
+            .iter()
+            .filter(|(key, _)| key.eq_ignore_ascii_case("PYTHONIOENCODING"))
+            .collect();
+        assert_eq!(matches.len(), 1);
+        assert_eq!(matches[0].1, "gbk");
+        let spec = spec.with_env_var("PYTHONIOENCODING", "utf-8");
+        assert_eq!(
+            spec.env.get("PYTHONIOENCODING").map(String::as_str),
+            Some("utf-8")
+        );
+        assert!(!spec.env.contains_key("pythonioencoding"));
+    }
+
     // The UTF-8 output contract keys on the PowerShell family: a custom
     // PowerShell path is accepted by `is_powershell`, so it must get the prefix
     // on both the normal shell spec and the encoded fallback.
diff --git a/crates/tui/src/tools/shell/tests.rs b/crates/tui/src/tools/shell/tests.rs
index 6894081aaf..3fd3ede155 100644
--- a/crates/tui/src/tools/shell/tests.rs
+++ b/crates/tui/src/tools/shell/tests.rs
@@ -210,6 +210,113 @@ async fn lowercase_bash_returns_one_ordered_stream() {
     assert_eq!(result.content, "out-1err-2out-3");
 }
 
+#[cfg(windows)]
+fn windows_python_stdio_fixture() -> (tempfile::TempDir, String) {
+    assert!(
+        crate::shell_dispatcher::global_dispatcher()
+            .kind()
+            .is_powershell(),
+        "requires a PowerShell dispatcher; run from PowerShell or cmd with SHELL unset"
+    );
+    let workspace = tempdir().expect("workspace");
+    let program = std::env::var_os("CODEWHALE_TEST_PYTHON")
+        .unwrap_or_else(|| std::ffi::OsString::from("python"));
+    let mut probe = std::process::Command::new(program);
+    crate::utils::suppress_console_window(&mut probe);
+    let resolved = probe
+        .args(["-c", "import sys; print(sys.executable)"])
+        .env("PYTHONIOENCODING", "utf-8")
+        .output()
+        .expect("native Python on PATH for this opt-in regression");
+    assert!(
+        resolved.status.success(),
+        "Python resolution failed: {resolved:?}"
+    );
+    let python = PathBuf::from(
+        String::from_utf8(resolved.stdout)
+            .expect("UTF-8 Python path")
+            .trim(),
+    );
+    let script = workspace.path().join("verify.py");
+    std::fs::write(
+        &script,
+        concat!(
+            "import sys\n",
+            "sys.stdout.write('x' * 65536 + '\\n')\n",
+            "print('STDOUT-\\u5927\\u5c0f\\u4e0d\\u5339\\u914d: 0 []', flush=True)\n",
+            "print('STDERR-\\u6821\\u9a8c\\u901a\\u8fc7: 0 GB', file=sys.stderr, flush=True)\n",
+        ),
+    )
+    .expect("write Python output fixture");
+    let quote = |path: &Path| path.to_string_lossy().replace('\'', "''");
+    let command = format!(
+        "& '{}' '{}'; Write-Output 'PowerShell-中文正常'",
+        quote(&python),
+        quote(&script)
+    );
+    (workspace, command)
+}
+
+#[cfg(windows)]
+fn assert_windows_python_stdio(output: &str) {
+    assert!(
+        output.contains("STDOUT-大小不匹配: 0 []"),
+        "Python stdout missing or garbled: {output}"
+    );
+    assert!(
+        output.contains("STDERR-校验通过: 0 GB"),
+        "Python stderr missing or garbled: {output}"
+    );
+    assert!(
+        output.contains("PowerShell-中文正常"),
+        "PowerShell output missing or garbled: {output}"
+    );
+    assert!(
+        !output.contains('\u{fffd}'),
+        "output contains replacement characters"
+    );
+}
+
+#[cfg(windows)]
+#[tokio::test]
+#[ignore = "requires native Windows Python on PATH and PowerShell"]
+async fn forkguard_windows_python_stdio_lowercase_bash() {
+    let _lock = crate::test_support::lock_test_env();
+    let _legacy_python = crate::test_support::EnvVarGuard::set("PYTHONIOENCODING", "gbk");
+    let (workspace, command) = windows_python_stdio_fixture();
+    let _home = crate::test_support::EnvVarGuard::set("CODEWHALE_HOME", workspace.path());
+    let context = ToolContext::new(workspace.path());
+    let result = LowercaseBashTool
+        .execute(json!({"command": command}), &context)
+        .await
+        .expect("execute actual lowercase bash tool");
+    assert!(result.success, "{}", result.content);
+    assert_windows_python_stdio(&result.content);
+}
+
+#[cfg(windows)]
+#[test]
+#[ignore = "requires native Windows Python on PATH and PowerShell"]
+fn forkguard_windows_python_stdio_background_shell() {
+    let _lock = crate::test_support::lock_test_env();
+    let _legacy_python = crate::test_support::EnvVarGuard::set("PYTHONIOENCODING", "gbk");
+    let (workspace, command) = windows_python_stdio_fixture();
+    let _home = crate::test_support::EnvVarGuard::set("CODEWHALE_HOME", workspace.path());
+    for extra_env in [
+        HashMap::new(),
+        HashMap::from([("ENCODING_HOOK_PROBE".to_string(), "present".to_string())]),
+    ] {
+        let mut manager = ShellManager::new(workspace.path().to_path_buf());
+        let started = manager
+            .execute_with_options_env(&command, None, 30_000, true, None, false, None, extra_env)
+            .expect("start real background shell");
+        let task_id = started.task_id.expect("background task id");
+        let result = wait_for_completed_shell(&mut manager, &task_id);
+        assert_eq!(result.status, ShellStatus::Completed, "{result:?}");
+        assert_windows_python_stdio(&format!("{}\n{}", result.stdout, result.stderr));
+    }
+}
+
 #[cfg(unix)]
 #[tokio::test]
 async fn lowercase_bash_keeps_raw_command_under_readonly_policy() {