diff --git a/README.ja.md b/README.ja.md
index 2695718..5f5c687 100644
--- a/README.ja.md
+++ b/README.ja.md
@@ -314,6 +314,22 @@ JavaScript 設定、最終 MPK ビルドを担当します。
 | `gleam run -m glendix/lint_fix` | lint 修正 |
 | `gleam run -m glendix/release` | release ビルド |
 
+### コマンド実行境界
+
+`glendix/cmd.exec` は stdin、stdout、stderr を継承する同期 shell command
+API を維持します。一般的な process 実行は `shellout` で実装し、小さな
+platform adapter が Unix では `/bin/sh -c`、Windows では
+`ComSpec /d /s /c` を選択することで、既存の command string と shell
+operator の挙動を保ちます。`plinth/node/child_process` は、現在の API が同期
+完了、exit status、標準 stream option を typed result として提供しないため、
+この境界には使用しません。
+
+`cmd_ffi.mjs` に残る custom command tooling は、意図的に Glendix 固有です。
+bridge の生成と cleanup、開発 watcher、experimental-native runtime 設定、
+Node/npm 互換 shim、Rollup/WebAssembly 処理が含まれます。Gleam build
+command も限定的な filtered runner を維持します。`shellout` では既存の stream
+継承を保ちながら stderr だけを capture して filter できないためです。
+
 ## 6.0.0 の破壊的変更
 
 `glendix/js/array` は変換を `gleam/javascript/array` に委譲するようになり、手書き
diff --git a/README.ko.md b/README.ko.md
index 8f61181..2481101 100644
--- a/README.ko.md
+++ b/README.ko.md
@@ -306,6 +306,21 @@ JavaScript 설정, 최종 MPK 빌드를 담당한다. Marketplace 위젯을 쓰
 | `gleam run -m glendix/lint_fix` | lint 수정 |
 | `gleam run -m glendix/release` | release 빌드 |
 
+### 명령 실행 경계
+
+`glendix/cmd.exec`는 stdin, stdout, stderr를 상속하는 동기식 shell command
+API를 유지한다. 일반 process 실행은 `shellout`으로 구현하고, 작은 platform
+adapter가 Unix에서는 `/bin/sh -c`, Windows에서는 `ComSpec /d /s /c`를 선택해
+기존 command string과 shell operator의 동작을 보존한다.
+`plinth/node/child_process`는 현재 API가 동기 완료, exit status, 표준 stream
+option을 typed result로 제공하지 않으므로 이 경계에는 사용하지 않는다.
+
+`cmd_ffi.mjs`에 남은 custom command tooling은 의도적으로 Glendix에 특화되어
+있다. 여기에는 bridge 생성과 정리, 개발 watcher, experimental-native runtime
+설정, Node/npm 호환 shim, Rollup/WebAssembly 처리가 포함된다. Gleam build
+command도 좁은 filtered runner를 유지한다. `shellout`은 기존의 stream 상속
+동작을 보존하면서 stderr만 별도로 capture하고 filter할 수 없기 때문이다.
+
 ## 6.0.0 호환성 변경
 
 `glendix/js/array`는 이제 변환을 `gleam/javascript/array`에 위임하며 수기
diff --git a/README.md b/README.md
index de6a278..3a088ba 100644
--- a/README.md
+++ b/README.md
@@ -403,6 +403,23 @@ Projects that do not use Marketplace widgets omit the first two steps.
 | `gleam run -m glendix/lint_fix` | Apply lint fixes |
 | `gleam run -m glendix/release` | Run the release build |
 
+### Command execution boundary
+
+`glendix/cmd.exec` remains a synchronous shell-command API with inherited
+stdin, stdout, and stderr. Its generic process execution is implemented with
+`shellout`; a small platform adapter selects `/bin/sh -c` on Unix systems and
+`ComSpec /d /s /c` on Windows so existing command strings and shell operators
+keep their behavior. `plinth/node/child_process` is not used here because its
+current API does not provide synchronous completion, exit status, and standard
+stream options as a typed result.
+
+The custom command tooling retained in `cmd_ffi.mjs` is intentionally
+Glendix-specific: bridge generation and cleanup, the development watcher,
+experimental-native runtime setup, Node/npm compatibility shims, and
+Rollup/WebAssembly handling. Gleam build commands also retain a narrow filtered
+runner because `shellout` cannot separately capture and filter stderr while
+preserving the existing inherited-stream behavior.
+
 ## Breaking changes in 6.0.0
 
 `glendix/js/array` now delegates its conversions to `gleam/javascript/array`
diff --git a/gleam.toml b/gleam.toml
index f694afa..294ba30 100644
--- a/gleam.toml
+++ b/gleam.toml
@@ -25,6 +25,7 @@ term_size = ">= 1.0.1 and < 2.0.0"
 tom = ">= 2.1.0 and < 3.0.0"
 gossamer = ">= 10.0.0 and < 11.0.0"
 xmlm = ">= 1.0.1 and < 2.0.0"
+shellout = ">= 1.8.0 and < 2.0.0"
 
 [dev_dependencies]
 gleeunit = ">= 1.11.0 and < 2.0.0"
diff --git a/manifest.toml b/manifest.toml
index 451d876..fddbe2a 100644
--- a/manifest.toml
+++ b/manifest.toml
@@ -28,6 +28,7 @@ packages = [
   { name = "plinth", version = "0.11.0", build_tools = ["gleam"], requirements = ["gleam_javascript", "gleam_json", "gleam_stdlib"], otp_app = "plinth", source = "hex", outer_checksum = "D30FDE8FC11EB76538AB498ACFF5399D0AE27993C74D63BE05AFE8CE80C4C839" },
   { name = "redraw", version = "19.2.2", build_tools = ["gleam"], requirements = ["gleam_javascript", "gleam_stdlib"], otp_app = "redraw", source = "hex", outer_checksum = "B8CEEB74E8846CE10B8360B924DAD22441B61D947F9449854164F0686C4B8661" },
   { name = "redraw_dom", version = "19.2.2", build_tools = ["gleam"], requirements = ["gleam_fetch", "gleam_stdlib", "redraw"], otp_app = "redraw_dom", source = "hex", outer_checksum = "80278296AD6E3D4457D6FF6A14FEA3E90696284BEA297BDE10E430FC4CE726B8" },
+  { name = "shellout", version = "1.8.0", build_tools = ["gleam"], requirements = ["gleam_stdlib"], otp_app = "shellout", source = "hex", outer_checksum = "C416356D45151F298108C9DB9CD1EDE0313F620B5EDBB5766CD7237659D87841" },
   { name = "simplifile", version = "2.7.0", build_tools = ["gleam"], requirements = ["filepath", "gleam_stdlib"], otp_app = "simplifile", source = "hex", outer_checksum = "A2727627B063E87351934C7F7F008F2D1FDB16F6DE0B8C79F9E46459CFC9C164" },
   { name = "term_size", version = "1.0.1", build_tools = ["gleam"], requirements = ["gleam_stdlib"], otp_app = "term_size", source = "hex", outer_checksum = "D00BD2BC8FB3EBB7E6AE076F3F1FF2AC9D5ED1805F004D0896C784D06C6645F1" },
   { name = "tom", version = "2.1.0", build_tools = ["gleam"], requirements = ["gleam_stdlib", "gleam_time"], otp_app = "tom", source = "hex", outer_checksum = "DCF04CB7AB35D58CFC598C66EA2E1816D160759802C89B2BA6238780D59BC256" },
@@ -46,6 +47,7 @@ mendraw = { version = ">= 2.0.0 and < 3.0.0" }
 plinth = { version = ">= 0.11.0 and < 1.0.0" }
 redraw = { version = ">= 19.2.2 and < 20.0.0" }
 redraw_dom = { version = ">= 19.2.2 and < 20.0.0" }
+shellout = { version = ">= 1.8.0 and < 2.0.0" }
 simplifile = { version = ">= 2.6.0 and < 3.0.0" }
 term_size = { version = ">= 1.0.1 and < 2.0.0" }
 tom = { version = ">= 2.1.0 and < 3.0.0" }
diff --git a/src/glendix/cmd.gleam b/src/glendix/cmd.gleam
index a9ae71d..8e64c34 100644
--- a/src/glendix/cmd.gleam
+++ b/src/glendix/cmd.gleam
@@ -5,6 +5,7 @@ import gleam/io
 import gleam/list
 import gleam/option
 import gleam/result
+import glendix/command
 import glendix/configuration
 
 /// Describes a JavaScript tooling failure.
@@ -15,8 +16,13 @@ pub type CommandError {
 
 /// Executes a command through the JavaScript process boundary.
 pub fn exec(command command: String) -> Result(Nil, CommandError) {
-  exec_raw(command)
-  |> map_raw_error("execute command")
+  command.run(command)
+  |> result.map_error(fn(error) {
+    CommandFailed(
+      operation: "execute command",
+      reason: command.error_message(error),
+    )
+  })
 }
 
 /// Detects the configured or lockfile-selected JavaScript package manager.
@@ -331,9 +337,6 @@ fn map_configuration_error(
 }
 
 // -- FFI --
-@external(javascript, "./cmd_ffi.mjs", "exec")
-fn exec_raw(command command: String) -> Result(Nil, RawCommandError)
-
 @external(javascript, "./cmd_ffi.mjs", "file_exists")
 fn file_exists(path: String) -> Bool
 
diff --git a/src/glendix/cmd_ffi.mjs b/src/glendix/cmd_ffi.mjs
index 42d85eb..96e3968 100644
--- a/src/glendix/cmd_ffi.mjs
+++ b/src/glendix/cmd_ffi.mjs
@@ -1,4 +1,4 @@
-import { execSync, spawnSync } from "node:child_process";
+import { spawnSync } from "node:child_process";
 import { createHash } from "node:crypto";
 import {
   chmodSync,
@@ -17,6 +17,7 @@ import { tmpdir } from "node:os";
 import { basename, delimiter, dirname, extname, join, resolve } from "node:path";
 import { fileURLToPath } from "node:url";
 import { Ok, Error as GleamError } from "../gleam.mjs";
+import { runFilteredCommandOrThrow } from "./command_ffi.mjs";
 
 const EXPERIMENTAL_NATIVE_RUNNER = "--glendix-experimental-native-runner";
 const EXPERIMENTAL_NATIVE_SHIM = "--glendix-experimental-native-shim";
@@ -30,62 +31,6 @@ function errorMessage(error) {
 export function command_error_message(error) {
   return errorMessage(error);
 }
-function filterErlangWarnings(stderr) {
-  const lines = stderr.split(/\r?\n/);
-  const result = [];
-  let skip = false;
-  let skipNextEmpty = false;
-  for (let i = 0; i < lines.length; i++) {
-    if (!skip && lines[i] === "warning: Unused value") {
-      if (i + 1 < lines.length && lines[i + 1].includes("gleam_erlang")) {
-        skip = true;
-        continue;
-      }
-    }
-    if (skip) {
-      if (lines[i].includes("not needed")) {
-        skip = false;
-        skipNextEmpty = true;
-      }
-      continue;
-    }
-    if (skipNextEmpty) {
-      skipNextEmpty = false;
-      if (lines[i].trim() === "") continue;
-    }
-    result.push(lines[i]);
-  }
-  return result.join("\n").replace(/\n{3,}/g, "\n\n").trim();
-}
-function execGleamFiltered(command) {
-  const result = spawnSync(command, { shell: true, stdio: ["inherit", "pipe", "pipe"] });
-  if (result.stdout && result.stdout.length > 0) process.stdout.write(result.stdout);
-  if (result.stderr && result.stderr.length > 0) {
-    const filtered = filterErlangWarnings(result.stderr.toString());
-    if (filtered) process.stderr.write(filtered + "\n");
-  }
-  if (result.status !== 0) {
-    const err = new Error("Command failed: " + command);
-    err.status = result.status;
-    throw err;
-  }
-}
-function execOrThrow(command) {
-  if (command.startsWith("gleam ")) {
-    execGleamFiltered(command);
-  } else {
-    execSync(command, { stdio: "inherit", shell: true });
-  }
-}
-
-export function exec(command) {
-  try {
-    execOrThrow(command);
-    return new Ok(undefined);
-  } catch (error) {
-    return new GleamError(error);
-  }
-}
 export function file_exists(path) {
   return existsSync(path);
 }
@@ -554,7 +499,7 @@ export function fail_process() {
 
 function setupBridge(bindings) {
   generateBindingsOrThrow(bindings);
-  execGleamFiltered("gleam build");
+  runFilteredCommandOrThrow("gleam build");
   const pkg = JSON.parse(readFileSync("package.json", "utf-8"));
   const widgetName = pkg.widgetName;
   const widgets = pkg.widgets;
@@ -1174,7 +1119,7 @@ function runDevWithBridgeUsing(execBuild, bindings) {
     if (!hasChanges()) return;
     console.log("\n[glendix] 변경 감지 → 리빌드");
     try {
-      execGleamFiltered("gleam build");
+      runFilteredCommandOrThrow("gleam build");
       execBuild();
       console.log("[glendix] 빌드 완료");
     } catch (error) {
diff --git a/src/glendix/command.gleam b/src/glendix/command.gleam
new file mode 100644
index 0000000..77dab89
--- /dev/null
+++ b/src/glendix/command.gleam
@@ -0,0 +1,101 @@
+//// Runs synchronous shell commands while preserving Glendix CLI semantics.
+////
+
+import gleam/result
+import gleam/string
+import shellout
+
+/// Describes a synchronous command execution failure.
+@internal
+pub type CommandFailure {
+  /// A command failed with the supplied process-boundary reason.
+  CommandFailed(command: String, reason: String)
+}
+
+/// Runs a shell command synchronously with inherited standard streams.
+@internal
+pub fn run(command command: String) -> Result(Nil, CommandFailure) {
+  case string.starts_with(command, "gleam ") {
+    True -> run_filtered(command)
+    False -> run_with_shellout(command)
+  }
+}
+
+/// Returns the compatibility error message for a failed command.
+@internal
+pub fn error_message(failure failure: CommandFailure) -> String {
+  let CommandFailed(reason:, ..) = failure
+  reason
+}
+
+type RawCommandError
+
+type SigintListeners
+
+fn run_with_shellout(command: String) -> Result(Nil, CommandFailure) {
+  let listeners = capture_sigint_listeners()
+  let execution =
+    shellout.command(
+      run: shell_executable(),
+      with: shell_arguments(command),
+      in: ".",
+      opt: [shellout.LetBeStdout, shellout.LetBeStderr],
+    )
+  restore_sigint_listeners(listeners)
+  execution
+  |> result.map(fn(_) { Nil })
+  |> result.map_error(fn(error) {
+    let #(_status, output) = error
+    CommandFailed(
+      command: command,
+      reason: shellout_error_message(command, output),
+    )
+  })
+}
+
+fn shell_executable() -> String {
+  case is_windows() {
+    True -> windows_shell()
+    False -> "/bin/sh"
+  }
+}
+
+fn shell_arguments(command: String) -> List(String) {
+  case is_windows() {
+    True -> ["/d", "/s", "/c", command]
+    False -> ["-c", command]
+  }
+}
+
+fn shellout_error_message(command: String, output: String) -> String {
+  let summary = "Command failed: " <> command
+  case string.trim(output) {
+    "" -> summary
+    detail -> summary <> "\n" <> detail
+  }
+}
+
+fn run_filtered(command: String) -> Result(Nil, CommandFailure) {
+  run_filtered_raw(command)
+  |> result.map_error(fn(error) {
+    CommandFailed(command: command, reason: raw_error_message(error))
+  })
+}
+
+@external(javascript, "./command_ffi.mjs", "is_windows")
+fn is_windows() -> Bool
+
+@external(javascript, "./command_ffi.mjs", "windows_shell")
+fn windows_shell() -> String
+
+@external(javascript, "./command_ffi.mjs", "capture_sigint_listeners")
+fn capture_sigint_listeners() -> SigintListeners
+
+@external(javascript, "./command_ffi.mjs", "restore_sigint_listeners")
+fn restore_sigint_listeners(listeners: SigintListeners) -> Nil
+
+@external(javascript, "./command_ffi.mjs", "run_filtered")
+fn run_filtered_raw(command: String) -> Result(Nil, RawCommandError)
+
+@external(javascript, "./command_ffi.mjs", "error_message")
+fn raw_error_message(error: RawCommandError) -> String
diff --git a/src/glendix/command_ffi.mjs b/src/glendix/command_ffi.mjs
new file mode 100644
index 0000000..91fda49
--- /dev/null
+++ b/src/glendix/command_ffi.mjs
@@ -0,0 +1,87 @@
+import { spawnSync } from "node:child_process";
+import process from "node:process";
+import { Ok, Error as GleamError } from "../gleam.mjs";
+
+function filterErlangWarnings(stderr) {
+  const lines = stderr.split(/\r?\n/);
+  const result = [];
+  let skip = false;
+  let skipNextEmpty = false;
+  for (let index = 0; index < lines.length; index += 1) {
+    if (!skip && lines[index] === "warning: Unused value") {
+      if (
+        index + 1 < lines.length
+        && lines[index + 1].includes("gleam_erlang")
+      ) {
+        skip = true;
+        continue;
+      }
+    }
+    if (skip) {
+      if (lines[index].includes("not needed")) {
+        skip = false;
+        skipNextEmpty = true;
+      }
+      continue;
+    }
+    if (skipNextEmpty) {
+      skipNextEmpty = false;
+      if (lines[index].trim() === "") continue;
+    }
+    result.push(lines[index]);
+  }
+  return result.join("\n").replace(/\n{3,}/g, "\n\n").trim();
+}
+
+function runFilteredCommandOrThrow(command) {
+  const result = spawnSync(command, {
+    shell: true,
+    stdio: ["inherit", "pipe", "pipe"],
+  });
+  if (result.stdout && result.stdout.length > 0) {
+    process.stdout.write(result.stdout);
+  }
+  if (result.stderr && result.stderr.length > 0) {
+    const filtered = filterErlangWarnings(result.stderr.toString());
+    if (filtered) process.stderr.write(filtered + "\n");
+  }
+  if (result.status !== 0) {
+    const error = new Error("Command failed: " + command);
+    error.status = result.status;
+    throw error;
+  }
+}
+
+export { runFilteredCommandOrThrow };
+
+export function is_windows() {
+  return globalThis.Deno?.build?.os === "windows" || process.platform === "win32";
+}
+
+export function windows_shell() {
+  return process.env.ComSpec ?? process.env.COMSPEC ?? "cmd.exe";
+}
+
+export function capture_sigint_listeners() {
+  return process.rawListeners("SIGINT");
+}
+
+export function restore_sigint_listeners(listeners) {
+  const retained = new Set(listeners);
+  for (const listener of process.rawListeners("SIGINT")) {
+    if (!retained.has(listener)) process.removeListener("SIGINT", listener);
+  }
+}
+
+export function run_filtered(command) {
+  try {
+    runFilteredCommandOrThrow(command);
+    return new Ok(undefined);
+  } catch (error) {
+    return new GleamError(error);
+  }
+}
+
+export function error_message(error) {
+  return error instanceof globalThis.Error ? error.message : String(error);
+}
diff --git a/test/glendix/command_test.gleam b/test/glendix/command_test.gleam
new file mode 100644
index 0000000..9d171cf
--- /dev/null
+++ b/test/glendix/command_test.gleam
@@ -0,0 +1,69 @@
+//// Verifies synchronous command execution and error contracts.
+////
+
+import gleam/string
+import gleeunit/should
+import glendix/cmd
+
+/// Verifies an empty shell command preserves its successful no-op behavior.
+pub fn empty_command_succeeds_test() -> Nil {
+  cmd.exec(command: "")
+  |> should.be_ok
+}
+
+/// Verifies command strings retain shell parsing and operator behavior.
+pub fn shell_command_string_succeeds_test() -> Nil {
+  cmd.exec(
+    command: "node -e \"process.exit(0)\" && node -e \"process.exit(0)\"",
+  )
+  |> should.be_ok
+}
+
+/// Verifies a nonzero generic command returns the public command error.
+pub fn shell_command_failure_surfaces_reason_test() -> Nil {
+  let source = "node -e \"process.exit(7)\""
+  case cmd.exec(command: source) {
+    Error(cmd.CommandFailed(operation, reason)) -> {
+      operation
+      |> should.equal("execute command")
+      reason
+      |> should.equal("Command failed: " <> source)
+    }
+    Ok(_) -> should.fail()
+  }
+}
+
+/// Verifies shellout's inherited-stream listener does not leak after commands.
+pub fn shell_command_cleans_up_sigint_listeners_test() -> Nil {
+  let initial_count = sigint_listener_count()
+  cmd.exec(command: "")
+  |> should.be_ok
+  cmd.exec(command: "node -e \"process.exit(9)\"")
+  |> should.be_error
+  sigint_listener_count()
+  |> should.equal(initial_count)
+}
+
+/// Verifies the filtered Gleam command path remains synchronous and successful.
+pub fn filtered_gleam_command_succeeds_test() -> Nil {
+  cmd.exec(command: "gleam --version")
+  |> should.be_ok
+}
+
+/// Verifies the filtered Gleam command path preserves its failure message.
+pub fn filtered_gleam_command_failure_surfaces_reason_test() -> Nil {
+  let source = "gleam command-that-does-not-exist"
+  case cmd.exec(command: source) {
+    Error(cmd.CommandFailed(operation, reason)) -> {
+      operation
+      |> should.equal("execute command")
+      reason
+      |> string.contains(source)
+      |> should.be_true
+    }
+    Ok(_) -> should.fail()
+  }
+}
+
+@external(javascript, "./command_test_ffi.mjs", "sigint_listener_count")
+fn sigint_listener_count() -> Int
diff --git a/test/glendix/command_test_ffi.mjs b/test/glendix/command_test_ffi.mjs
new file mode 100644
index 0000000..9f446d7
--- /dev/null
+++ b/test/glendix/command_test_ffi.mjs
@@ -0,0 +1,5 @@
+import process from "node:process";
+
+export function sigint_listener_count() {
+  return process.listenerCount("SIGINT");
+}