diff --git a/internal/hook/command.go b/internal/hook/command.go
new file mode 100644
index 0000000..92e8b99
--- /dev/null
+++ b/internal/hook/command.go
@@ -0,0 +1,70 @@
+package hook
+
+import (
+	"bytes"
+	"context"
+	"fmt"
+	"os/exec"
+	"time"
+)
+
+// CommandExecutor runs a shell command and interprets its stdin/stdout.
+type CommandExecutor struct {
+	def HookDef
+}
+
+// NewCommandExecutor constructs a CommandExecutor for the given definition.
+func NewCommandExecutor(def HookDef) *CommandExecutor {
+	return &CommandExecutor{def: def}
+}
+
+// Execute runs the hook command, pipes payload to stdin, and reads stdout.
+func (c *CommandExecutor) Execute(ctx context.Context, payload []byte) (HookResult, error) {
+	timeout := c.def.timeout()
+	ctx, cancel := context.WithTimeout(ctx, timeout)
+	defer cancel()
+
+	start := time.Now()
+	cmd := exec.CommandContext(ctx, "sh", "-c", c.def.Exec)
+	cmd.Stdin = bytes.NewReader(payload)
+
+	var stdout bytes.Buffer
+	cmd.Stdout = &stdout
+
+	runErr := cmd.Run()
+	duration := time.Since(start)
+
+	// Determine exit code and whether it was a timeout.
+	exitCode := 0
+	if runErr != nil {
+		if ctx.Err() != nil {
+			// Context deadline exceeded — apply fail_open policy.
+			action := Deny
+			if c.def.FailOpen {
+				action = Allow
+			}
+			return HookResult{Action: action, Duration: duration}, fmt.Errorf("hook %q: timed out after %v", c.def.Name, timeout)
+		}
+		if exitErr, ok := runErr.(*exec.ExitError); ok {
+			exitCode = exitErr.ExitCode()
+		} else {
+			// Unexpected error launching the process.
+			action := Deny
+			if c.def.FailOpen {
+				action = Allow
+			}
+			return HookResult{Action: action, Duration: duration}, fmt.Errorf("hook %q: %w", c.def.Name, runErr)
+		}
+	}
+
+	action, transformed, err := ParseHookOutput(stdout.Bytes(), exitCode)
+	if err != nil {
+		failAction := Deny
+		if c.def.FailOpen {
+			failAction = Allow
+		}
+		return HookResult{Action: failAction, Duration: duration}, fmt.Errorf("hook %q: %w", c.def.Name, err)
+	}
+
+	return HookResult{Action: action, Output: transformed, Duration: duration}, nil
+}
diff --git a/internal/hook/command_test.go b/internal/hook/command_test.go
new file mode 100644
index 0000000..f78b5fd
--- /dev/null
+++ b/internal/hook/command_test.go
@@ -0,0 +1,235 @@
+package hook
+
+import (
+	"context"
+	"encoding/json"
+	"strings"
+	"testing"
+	"time"
+)
+
+func TestCommandExecutor_ExitZero_Allow(t *testing.T) {
+	def := HookDef{Name: "test", Event: PreToolUse, Command: CommandTypeShell, Exec: "exit 0"}
+	ex := NewCommandExecutor(def)
+	result, err := ex.Execute(context.Background(), []byte(`{}`))
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if result.Action != Allow {
+		t.Errorf("action = %v, want Allow", result.Action)
+	}
+}
+
+func TestCommandExecutor_ExitOne_Skip(t *testing.T) {
+	def := HookDef{Name: "test", Event: PreToolUse, Command: CommandTypeShell, Exec: "exit 1"}
+	ex := NewCommandExecutor(def)
+	result, err := ex.Execute(context.Background(), []byte(`{}`))
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if result.Action != Skip {
+		t.Errorf("action = %v, want Skip", result.Action)
+	}
+}
+
+func TestCommandExecutor_ExitTwo_Deny(t *testing.T) {
+	def := HookDef{Name: "test", Event: PreToolUse, Command: CommandTypeShell, Exec: "exit 2"}
+	ex := NewCommandExecutor(def)
+	result, err := ex.Execute(context.Background(), []byte(`{}`))
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if result.Action != Deny {
+		t.Errorf("action = %v, want Deny", result.Action)
+	}
+}
+
+func TestCommandExecutor_StdinDelivered(t *testing.T) {
+	// Hook reads stdin and echoes it back to stdout as JSON with action=allow
+	def := HookDef{
+		Name:    "test",
+		Event:   PreToolUse,
+		Command: CommandTypeShell,
+		Exec:    `read -r line; echo '{"action":"allow","transformed":'"$line"'}'; exit 0`,
+	}
+	ex := NewCommandExecutor(def)
+	payload := []byte(`{"tool":"bash"}`)
+	result, err := ex.Execute(context.Background(), payload)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if result.Action != Allow {
+		t.Errorf("action = %v, want Allow", result.Action)
+	}
+}
+
+func TestCommandExecutor_StdoutJSON_Parsed(t *testing.T) {
+	// Hook outputs JSON with transformed payload
+	def := HookDef{
+		Name:    "test",
+		Event:   PreToolUse,
+		Command: CommandTypeShell,
+		Exec:    `echo '{"action":"deny","transformed":{"command":"safe"}}'`,
+	}
+	ex := NewCommandExecutor(def)
+	result, err := ex.Execute(context.Background(), []byte(`{}`))
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if result.Action != Deny {
+		t.Errorf("action = %v, want Deny", result.Action)
+	}
+	if result.Output == nil {
+		t.Error("expected non-nil transformed output")
+	}
+}
+
+func TestCommandExecutor_StdoutJSON_ActionOverridesExitCode(t *testing.T) {
+	// exit 0, but stdout says deny
+	def := HookDef{
+		Name:    "test",
+		Event:   PreToolUse,
+		Command: CommandTypeShell,
+		Exec:    `echo '{"action":"deny"}'; exit 0`,
+	}
+	ex := NewCommandExecutor(def)
+	result, err := ex.Execute(context.Background(), []byte(`{}`))
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if result.Action != Deny {
+		t.Errorf("action = %v, want Deny", result.Action)
+	}
+}
+
+func TestCommandExecutor_EmptyStdout_ExitCodeFallback(t *testing.T) {
+	def := HookDef{
+		Name:    "test",
+		Event:   PreToolUse,
+		Command: CommandTypeShell,
+		Exec:    `exit 2`,
+	}
+	ex := NewCommandExecutor(def)
+	result, err := ex.Execute(context.Background(), []byte(`{}`))
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if result.Action != Deny {
+		t.Errorf("action = %v, want Deny", result.Action)
+	}
+	if result.Output != nil {
+		t.Error("expected nil output for empty stdout")
+	}
+}
+
+func TestCommandExecutor_Timeout_FailOpenTrue(t *testing.T) {
+	def := HookDef{
+		Name:     "test",
+		Event:    PreToolUse,
+		Command:  CommandTypeShell,
+		Exec:     "sleep 10",
+		Timeout:  50 * time.Millisecond,
+		FailOpen: true,
+	}
+	ex := NewCommandExecutor(def)
+	result, err := ex.Execute(context.Background(), []byte(`{}`))
+	if err == nil {
+		t.Fatal("expected error on timeout")
+	}
+	if result.Action != Allow {
+		t.Errorf("fail_open=true: action = %v, want Allow", result.Action)
+	}
+}
+
+func TestCommandExecutor_Timeout_FailOpenFalse(t *testing.T) {
+	def := HookDef{
+		Name:     "test",
+		Event:    PreToolUse,
+		Command:  CommandTypeShell,
+		Exec:     "sleep 10",
+		Timeout:  50 * time.Millisecond,
+		FailOpen: false,
+	}
+	ex := NewCommandExecutor(def)
+	result, err := ex.Execute(context.Background(), []byte(`{}`))
+	if err == nil {
+		t.Fatal("expected error on timeout")
+	}
+	if result.Action != Deny {
+		t.Errorf("fail_open=false: action = %v, want Deny", result.Action)
+	}
+}
+
+func TestCommandExecutor_InvalidJSON_Stdout(t *testing.T) {
+	// Hook writes garbage — should error
+	def := HookDef{
+		Name:    "test",
+		Event:   PreToolUse,
+		Command: CommandTypeShell,
+		Exec:    `echo "not valid json"`,
+	}
+	ex := NewCommandExecutor(def)
+	result, err := ex.Execute(context.Background(), []byte(`{}`))
+	if err == nil {
+		t.Fatal("expected error for invalid JSON stdout")
+	}
+	// fail_open=false (default) → Deny on error
+	if result.Action != Deny {
+		t.Errorf("action = %v, want Deny on error with fail_open=false", result.Action)
+	}
+}
+
+func TestCommandExecutor_Duration_Recorded(t *testing.T) {
+	def := HookDef{Name: "test", Event: PreToolUse, Command: CommandTypeShell, Exec: "exit 0"}
+	ex := NewCommandExecutor(def)
+	result, _ := ex.Execute(context.Background(), []byte(`{}`))
+	if result.Duration <= 0 {
+		t.Error("expected Duration > 0")
+	}
+}
+
+func TestCommandExecutor_PreToolPayload_HasToolField(t *testing.T) {
+	// Verify that a real pre_tool_use payload round-trips through the executor
+	args := json.RawMessage(`{"command":"ls"}`)
+	payload := MarshalPreToolPayload("bash", args)
+
+	def := HookDef{
+		Name:    "test",
+		Event:   PreToolUse,
+		Command: CommandTypeShell,
+		// Read tool name from stdin, allow if it's "bash"
+		Exec: `input=$(cat); tool=$(echo "$input" | grep -o '"tool":"[^"]*"' | cut -d'"' -f4); [ "$tool" = "bash" ] && exit 0 || exit 2`,
+	}
+	ex := NewCommandExecutor(def)
+	result, err := ex.Execute(context.Background(), payload)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if result.Action != Allow {
+		t.Errorf("action = %v, want Allow for tool=bash", result.Action)
+	}
+}
+
+// Verify the executor honours context cancellation in addition to its own timeout.
+func TestCommandExecutor_ContextCancelled(t *testing.T) {
+	def := HookDef{
+		Name:     "test",
+		Event:    PreToolUse,
+		Command:  CommandTypeShell,
+		Exec:     "sleep 10",
+		FailOpen: true,
+	}
+	ex := NewCommandExecutor(def)
+	ctx, cancel := context.WithTimeout(context.Background(), 50*time.Millisecond)
+	defer cancel()
+	result, err := ex.Execute(ctx, []byte(`{}`))
+	if err == nil {
+		t.Fatal("expected error on context cancellation")
+	}
+	if !strings.Contains(err.Error(), "hook") {
+		t.Logf("error = %v", err) // just informational
+	}
+	if result.Action != Allow { // fail_open=true
+		t.Errorf("action = %v, want Allow (fail_open=true)", result.Action)
+	}
+}