feat: add Codex CLI backend support

internal/backend/codex.go

@@ -0,0 +1,100 @@
+package backend
+
+import (
+	"bufio"
+	"fmt"
+	"os/exec"
+	"strings"
+)
+
+// CodexBackend uses the OpenAI Codex CLI tool as the backend.
+// It execs `codex -q <prompt>` and streams stdout.
+type CodexBackend struct {
+	binaryPath string
+}
+
+// NewCodexBackend creates a backend that delegates to the Codex CLI.
+// If binaryPath is empty, it looks for "codex" in PATH.
+func NewCodexBackend(binaryPath string) (*CodexBackend, error) {
+	if binaryPath == "" {
+		path, err := exec.LookPath("codex")
+		if err != nil {
+			return nil, fmt.Errorf("codex CLI not found in PATH: %w", err)
+		}
+		binaryPath = path
+	}
+	return &CodexBackend{binaryPath: binaryPath}, nil
+}
+
+func (b *CodexBackend) Query(messages []Message, model string, onChunk func(text string)) (string, error) {
+	// Build the prompt: use the last user message as the primary prompt.
+	// Pass conversation history as context prepended to the prompt.
+	var prompt string
+	var history []string
+
+	for i, m := range messages {
+		if i == len(messages)-1 && m.Role == "user" {
+			prompt = m.Content
+		} else {
+			prefix := "User"
+			if m.Role == "assistant" {
+				prefix = "Assistant"
+			}
+			history = append(history, fmt.Sprintf("%s: %s", prefix, m.Content))
+		}
+	}
+
+	if prompt == "" {
+		return "", fmt.Errorf("no user message found")
+	}
+
+	// Codex CLI uses -q (--quiet) for non-interactive mode that streams to stdout,
+	// and --approval-mode suggest for safe operation (suggest changes, don't auto-apply).
+	args := []string{"-q", "--approval-mode", "suggest"}
+
+	// Pass model if specified.
+	if model != "" {
+		args = append(args, "--model", model)
+	}
+
+	// Prepend conversation history to the prompt for context.
+	fullPrompt := prompt
+	if len(history) > 0 {
+		fullPrompt = "Previous conversation:\n" + strings.Join(history, "\n") + "\n\nCurrent request:\n" + prompt
+	}
+
+	args = append(args, fullPrompt)
+
+	cmd := exec.Command(b.binaryPath, args...)
+
+	stdout, err := cmd.StdoutPipe()
+	if err != nil {
+		return "", fmt.Errorf("failed to create stdout pipe: %w", err)
+	}
+
+	if err := cmd.Start(); err != nil {
+		return "", fmt.Errorf("failed to start codex CLI: %w", err)
+	}
+
+	var fullResponse strings.Builder
+	scanner := bufio.NewScanner(stdout)
+	scanner.Split(scanChunks)
+
+	for scanner.Scan() {
+		chunk := scanner.Text()
+		fullResponse.WriteString(chunk)
+		if onChunk != nil {
+			onChunk(chunk)
+		}
+	}
+
+	if err := cmd.Wait(); err != nil {
+		// If we already got some output, return it with the error.
+		if fullResponse.Len() > 0 {
+			return fullResponse.String(), fmt.Errorf("codex CLI exited with error: %w", err)
+		}
+		return "", fmt.Errorf("codex CLI failed: %w", err)
+	}
+
+	return fullResponse.String(), nil
+}

internal/config/config.go

@@ -22,6 +22,9 @@ type Config struct {
 	// CLI backend settings
 	CLIPath string `toml:"cli_path"` // Path to claude binary (default: auto-detect)
 
+	// Codex backend settings
+	CodexPath string `toml:"codex_path"` // Path to codex binary (default: auto-detect)
+
 	// General settings
 	SocketPath            string `toml:"socket_path"`
 	PIDFile               string `toml:"pid_file"`
@@ -85,6 +88,9 @@ func Load() (Config, error) {
 	if v := os.Getenv("INLINE_CLI_CLI_PATH"); v != "" {
 		cfg.CLIPath = v
 	}
+	if v := os.Getenv("INLINE_CLI_CODEX_PATH"); v != "" {
+		cfg.CodexPath = v
+	}
 	if v := os.Getenv("INLINE_CLI_MAX_IDLE"); v != "" {
 		if n, err := strconv.Atoi(v); err == nil {
 			cfg.MaxSessionIdleMinutes = n

internal/daemon/server.go

@@ -44,12 +44,14 @@ func createBackend(cfg config.Config) (backend.Backend, error) {
 	switch cfg.Backend {
 	case "cli":
 		return backend.NewCLIBackend(cfg.CLIPath)
+	case "codex":
+		return backend.NewCodexBackend(cfg.CodexPath)
 	case "acp":
 		return backend.NewACPBackend()
 	case "api", "":
 		return backend.NewAPIBackend(cfg.APIKey, cfg.APIBaseURL)
 	default:
-		return nil, fmt.Errorf("unknown backend: %q (supported: api, cli, acp)", cfg.Backend)
+		return nil, fmt.Errorf("unknown backend: %q (supported: api, cli, codex, acp)", cfg.Backend)
 	}
 }