diff --git a/docs/security.md b/docs/security.md
index e8daf59..4057bff 100644
--- a/docs/security.md
+++ b/docs/security.md
@@ -41,7 +41,10 @@ focuses on:
 
 ## Platform credential-store adapters
 
-The macOS implementation invokes the system `security` utility without a shell
-and sends new vault keys through standard input rather than the process argument
-vector. Linux similarly sends values to `secret-tool` over standard input.
-Windows calls the native Credential Manager API directly.
+The macOS implementation invokes the system `security` utility without a shell.
+Because that utility treats `-w` without a value as an interactive prompt rather
+than reading standard input, the adapter supplies the vault key directly to
+`-w`. The value can therefore be visible briefly to processes running as the
+same operating-system user, which is inside this project's trust boundary.
+Linux sends values to `secret-tool` over standard input. Windows calls the
+native Credential Manager API directly.
diff --git a/internal/secretstore/store_darwin.go b/internal/secretstore/store_darwin.go
index e43ba07..4403a28 100644
--- a/internal/secretstore/store_darwin.go
+++ b/internal/secretstore/store_darwin.go
@@ -25,14 +25,20 @@ func (darwinStore) Set(key, value string) error {
 	if err := validateKey(key); err != nil {
 		return err
 	}
-	command := exec.Command("security", "add-generic-password", "-U", "-a", target(key), "-s", serviceName, "-w")
-	command.Stdin = strings.NewReader(value + "\n")
+	command := newDarwinSetCommand(key, value)
 	if output, err := command.CombinedOutput(); err != nil {
 		return fmt.Errorf("write macOS Keychain entry: %s: %w", strings.TrimSpace(string(output)), err)
 	}
 	return nil
 }
 
+func newDarwinSetCommand(key, value string) *exec.Cmd {
+	// The security CLI treats a trailing -w as a request to prompt twice on a
+	// terminal; it does not consume a password from stdin. Supply the value as
+	// the argument to -w so unattended initialization stores the generated key.
+	return exec.Command("security", "add-generic-password", "-U", "-a", target(key), "-s", serviceName, "-w", value)
+}
+
 func (darwinStore) Get(key string) (string, error) {
 	if err := validateKey(key); err != nil {
 		return "", err
diff --git a/internal/secretstore/store_darwin_test.go b/internal/secretstore/store_darwin_test.go
new file mode 100644
index 0000000..3b98052
--- /dev/null
+++ b/internal/secretstore/store_darwin_test.go
@@ -0,0 +1,32 @@
+//go:build darwin
+
+package secretstore
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestDarwinSetSuppliesPasswordToWFlag(t *testing.T) {
+	directory := t.TempDir()
+	security := filepath.Join(directory, "security")
+	stub := `#!/bin/sh
+test "$#" -eq 8 || exit 64
+test "$1" = "add-generic-password" || exit 65
+test "$2" = "-U" || exit 66
+test "$3" = "-a" || exit 67
+test "$4" = "codex-switch/master-key/test" || exit 68
+test "$5" = "-s" || exit 69
+test "$6" = "codex-switch" || exit 70
+test "$7" = "-w" || exit 71
+test "$8" = "generated-vault-key" || exit 72
+`
+	if err := os.WriteFile(security, []byte(stub), 0o700); err != nil {
+		t.Fatal(err)
+	}
+	t.Setenv("PATH", directory)
+	if err := (darwinStore{}).Set("master-key/test", "generated-vault-key"); err != nil {
+		t.Fatal(err)
+	}
+}