mAi: #5 - tmux-window preview for generate

Adds an optional `imagen generate` post-step that opens a sibling
tmux window running tmux-img --hold <path>.

- internal/preview: Mode (auto|on|off), Resolve, and a Spawner that
  shells out to tmux new-window. Typed errors for missing tmux,
  missing tmux-img, and "preview forced on outside $TMUX".
- cmd/imagen/generate: --preview / --no-preview flags plus
  $IMAGEN_PREVIEW. Resolution chain: config -> env -> flag.
  auto requires both stdout-is-tty and $TMUX. Failures are
  warnings - the image is already on disk.
- internal/config: output.preview field, validated to auto|on|off,
  threaded into the sample.
- Tests for ParseMode, Resolve, Spawn argv (incl. shell quoting of
  paths with apostrophes), missing-binary errors, and the CLI
  resolution table.
- Docs (usage + architecture) updated.

/imagine SKILL.md edit lives in dotfiles - deferred to coordinate
with #4.

cmd/imagen/generate.go

@@ -11,6 +11,7 @@ import (
 	"mgit.msbls.de/m/ImaGen/internal/backend"
 	"mgit.msbls.de/m/ImaGen/internal/config"
 	"mgit.msbls.de/m/ImaGen/internal/output"
+	"mgit.msbls.de/m/ImaGen/internal/preview"
 	"mgit.msbls.de/m/ImaGen/internal/prompt"
 )
 
@@ -26,6 +27,8 @@ func runGenerate(ctx context.Context, args []string) error {
 		negative     string
 		configPath   string
 		noSidecar    bool
+		previewOn    bool
+		previewOff   bool
 	)
 	fs.StringVar(&backendName, "backend", "", "backend instance name (default: config.default_backend)")
 	fs.StringVar(&size, "size", "1024x1024", "WxH, e.g. 1024x1024")
@@ -36,6 +39,8 @@ func runGenerate(ctx context.Context, args []string) error {
 	fs.StringVar(&negative, "negative", "", "negative prompt (ignored by backends that don't support it)")
 	fs.StringVar(&configPath, "config", "", "config file path (default: ~/.config/imagen.yaml)")
 	fs.BoolVar(&noSidecar, "no-sidecar", false, "skip the JSON sidecar even if config enables it")
+	fs.BoolVar(&previewOn, "preview", false, "force tmux preview window on (errors outside $TMUX)")
+	fs.BoolVar(&previewOff, "no-preview", false, "skip the tmux preview window")
 	fs.Usage = func() {
 		fmt.Fprintln(fs.Output(), `Usage: imagen generate "<prompt>" [flags]`)
 		fs.PrintDefaults()
@@ -118,9 +123,70 @@ func runGenerate(ctx context.Context, args []string) error {
 	if paths.SidecarPath != "" {
 		fmt.Fprintln(os.Stderr, "sidecar:", paths.SidecarPath)
 	}
+
+	if err := maybePreview(cfg, previewOn, previewOff, paths.ImagePath, rawPrompt); err != nil {
+		// preview failures are warnings — the image already wrote.
+		fmt.Fprintln(os.Stderr, "imagen: preview:", err)
+	}
 	return nil
 }
 
+// resolvePreviewMode applies the precedence chain config -> env -> flag.
+// Flags win, env beats config, config beats the implicit auto default.
+func resolvePreviewMode(cfg *config.Config, flagOn, flagOff bool, env string) (preview.Mode, error) {
+	mode := preview.ModeAuto
+	if cfg != nil && cfg.Output.Preview != "" {
+		m, err := preview.ParseMode(cfg.Output.Preview)
+		if err != nil {
+			return "", fmt.Errorf("config output.preview: %w", err)
+		}
+		mode = m
+	}
+	if env != "" {
+		m, err := preview.ParseMode(env)
+		if err != nil {
+			return "", fmt.Errorf("$IMAGEN_PREVIEW: %w", err)
+		}
+		mode = m
+	}
+	if flagOn && flagOff {
+		return "", userErr("--preview and --no-preview are mutually exclusive")
+	}
+	if flagOn {
+		mode = preview.ModeOn
+	}
+	if flagOff {
+		mode = preview.ModeOff
+	}
+	return mode, nil
+}
+
+// maybePreview resolves the effective preview mode and, if it says yes,
+// spawns a tmux window via tmux-img. Always non-fatal.
+func maybePreview(cfg *config.Config, flagOn, flagOff bool, imagePath, rawPrompt string) error {
+	mode, err := resolvePreviewMode(cfg, flagOn, flagOff, os.Getenv("IMAGEN_PREVIEW"))
+	if err != nil {
+		return err
+	}
+	decision, err := preview.Resolve(mode, os.Getenv("TMUX") != "", stdoutIsTTY())
+	if err != nil {
+		return err
+	}
+	if !decision.ShouldPreview {
+		return nil
+	}
+	spawner := &preview.Spawner{}
+	return spawner.Spawn(imagePath, output.Slug(rawPrompt))
+}
+
+func stdoutIsTTY() bool {
+	fi, err := os.Stdout.Stat()
+	if err != nil {
+		return false
+	}
+	return fi.Mode()&os.ModeCharDevice != 0
+}
+
 // splitLeadingPositional separates the positional args at the start of args
 // from the rest (which begins with the first flag). A literal "--" terminator
 // pushes everything after it into the positional list and out of flag parsing.

cmd/imagen/generate_test.go

@@ -0,0 +1,50 @@
+package main
+
+import (
+	"testing"
+
+	"mgit.msbls.de/m/ImaGen/internal/config"
+	"mgit.msbls.de/m/ImaGen/internal/preview"
+)
+
+func TestResolvePreviewMode(t *testing.T) {
+	type tc struct {
+		name      string
+		cfg       *config.Config
+		flagOn    bool
+		flagOff   bool
+		env       string
+		want      preview.Mode
+		wantError bool
+	}
+	cases := []tc{
+		{name: "all-empty-defaults-to-auto", want: preview.ModeAuto},
+		{name: "config-on", cfg: &config.Config{Output: config.OutputConfig{Preview: "on"}}, want: preview.ModeOn},
+		{name: "config-off", cfg: &config.Config{Output: config.OutputConfig{Preview: "off"}}, want: preview.ModeOff},
+		{name: "config-auto-explicit", cfg: &config.Config{Output: config.OutputConfig{Preview: "auto"}}, want: preview.ModeAuto},
+		{name: "env-overrides-config", cfg: &config.Config{Output: config.OutputConfig{Preview: "on"}}, env: "off", want: preview.ModeOff},
+		{name: "flag-on-overrides-env-off", env: "off", flagOn: true, want: preview.ModeOn},
+		{name: "flag-off-overrides-env-on", env: "on", flagOff: true, want: preview.ModeOff},
+		{name: "flag-off-overrides-config-on", cfg: &config.Config{Output: config.OutputConfig{Preview: "on"}}, flagOff: true, want: preview.ModeOff},
+		{name: "both-flags-error", flagOn: true, flagOff: true, wantError: true},
+		{name: "bad-env-errors", env: "yes", wantError: true},
+		{name: "bad-config-errors", cfg: &config.Config{Output: config.OutputConfig{Preview: "yes"}}, wantError: true},
+	}
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			got, err := resolvePreviewMode(c.cfg, c.flagOn, c.flagOff, c.env)
+			if c.wantError {
+				if err == nil {
+					t.Fatalf("expected error, got mode %q", got)
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("unexpected error: %v", err)
+			}
+			if got != c.want {
+				t.Errorf("mode = %q, want %q", got, c.want)
+			}
+		})
+	}
+}

docs/architecture.md

@@ -15,6 +15,7 @@ upstream API. Each adapter only ever sees its own slice of `imagen.yaml`.
         │   internal/prompt     │   style preset → prompt suffix
         │   internal/output     │   filename templating, sidecar
         │   internal/config     │   YAML loader, validation
+        │   internal/preview    │   tmux-img window spawner
         └──────────┬────────────┘
                    │
         ┌──────────▼────────────┐

docs/usage.md

@@ -24,8 +24,28 @@ imagen version                      print version
 | `--negative`   | empty                        | Negative prompt (ignored by some adapters)                  |
 | `--output`     | empty (= use naming template) | Explicit path                                              |
 | `--no-sidecar` | `false`                      | Skip the JSON sidecar even if config enables it             |
+| `--preview`    | (auto)                       | Force open a tmux preview window via `tmux-img`             |
+| `--no-preview` | (auto)                       | Suppress the preview window (use for batch / CI callers)    |
 | `--config`     | `~/.config/imagen.yaml`      | Override config path                                        |
 
+### Preview window
+
+After a successful generate, imagen optionally opens a sibling tmux window
+named `img:<slug>` running `tmux-img --hold <path>`. The new window is
+spawned in the background (`tmux new-window -d`) so the generating pane
+keeps focus and its terminal output.
+
+Resolution order is **config → `$IMAGEN_PREVIEW` → flag** (later wins):
+
+- `output.preview` in `imagen.yaml`: `auto` (default) | `on` | `off`
+- `IMAGEN_PREVIEW=auto|on|off` overrides config
+- `--preview` / `--no-preview` override env
+
+`auto` previews iff stdout is a TTY *and* `$TMUX` is set. `on` previews
+unconditionally and errors outside a tmux session. `off` never previews.
+
+Preview failures are non-fatal — the image already wrote.
+
 ## Examples
 
 ```sh

internal/config/config.go

@@ -19,11 +19,16 @@ type Config struct {
 	Backends       map[string]BackendSpec `yaml:"backends"`
 }
 
-// OutputConfig controls where generated images and metadata sidecars land.
+// OutputConfig controls where generated images and metadata sidecars land,
+// and whether `imagen generate` opens a tmux preview window.
 type OutputConfig struct {
 	Directory         string `yaml:"directory"`
 	Naming            string `yaml:"naming"`
 	WriteMetadataJSON bool   `yaml:"write_metadata_json"`
+	// Preview is the tri-state preview mode: "auto" (default), "on", "off".
+	// Empty / unset is treated as "auto". $IMAGEN_PREVIEW and the
+	// --preview/--no-preview flags override this in turn.
+	Preview string `yaml:"preview"`
 }
 
 // BackendSpec is one entry under `backends:`. Type identifies the adapter;
@@ -78,6 +83,11 @@ func (c *Config) Validate() error {
 			return fmt.Errorf("default_backend %q is not defined under backends:", c.DefaultBackend)
 		}
 	}
+	switch c.Output.Preview {
+	case "", "auto", "on", "off":
+	default:
+		return fmt.Errorf("output.preview = %q (must be auto|on|off)", c.Output.Preview)
+	}
 	for name, spec := range c.Backends {
 		if name == "" {
 			return errors.New("empty backend name")
@@ -101,6 +111,11 @@ output:
   directory: ~/Pictures/imagen
   naming: "{date}-{slug}-{seed}.png"
   write_metadata_json: true
+  # Open a tmux window with tmux-img after a successful generation.
+  # auto (default): preview iff stdout is a TTY and $TMUX is set.
+  # on: always preview (errors outside a tmux session).
+  # off: never preview (use this for batch / CI callers).
+  preview: auto
 
 backends:
   flux-schnell-local:

internal/config/config_test.go

@@ -60,6 +60,34 @@ func TestValidateRejectsMissingType(t *testing.T) {
 	}
 }
 
+func TestValidatePreviewMode(t *testing.T) {
+	for _, mode := range []string{"", "auto", "on", "off"} {
+		c := &Config{Output: OutputConfig{Preview: mode}}
+		if err := c.Validate(); err != nil {
+			t.Errorf("preview=%q: unexpected error %v", mode, err)
+		}
+	}
+	bad := &Config{Output: OutputConfig{Preview: "yes"}}
+	if err := bad.Validate(); err == nil {
+		t.Errorf("expected error for invalid preview value")
+	}
+}
+
+func TestSampleParsesPreviewAuto(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "imagen.yaml")
+	if err := os.WriteFile(path, []byte(Sample), 0o644); err != nil {
+		t.Fatalf("write sample: %v", err)
+	}
+	cfg, err := Load(path)
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	if cfg.Output.Preview != "auto" {
+		t.Errorf("Output.Preview = %q, want auto", cfg.Output.Preview)
+	}
+}
+
 func TestExpandPath(t *testing.T) {
 	home, _ := os.UserHomeDir()
 	cases := map[string]string{

internal/preview/tmux.go

@@ -0,0 +1,119 @@
+// Package preview opens a tmux window showing a generated image via tmux-img.
+// Mode resolution and the actual spawn are kept separate so the CLI can
+// decide-then-act and tests can drive each half independently.
+package preview
+
+import (
+	"errors"
+	"fmt"
+	"os/exec"
+	"strings"
+)
+
+// Mode is the tri-state preview setting: auto (default), on (force), off.
+type Mode string
+
+const (
+	ModeAuto Mode = "auto"
+	ModeOn   Mode = "on"
+	ModeOff  Mode = "off"
+)
+
+// ParseMode normalises a string into a Mode. Empty parses to ModeAuto so
+// callers can pass through unset config / env values.
+func ParseMode(s string) (Mode, error) {
+	switch strings.ToLower(strings.TrimSpace(s)) {
+	case "", "auto":
+		return ModeAuto, nil
+	case "on":
+		return ModeOn, nil
+	case "off":
+		return ModeOff, nil
+	}
+	return "", fmt.Errorf("invalid preview mode %q (auto|on|off)", s)
+}
+
+// Decision is the answer to "should we preview, and why".
+type Decision struct {
+	ShouldPreview bool
+	Reason        string
+}
+
+// Resolve maps (mode, runtime context) to a Decision.
+//
+//   - off  -> never preview
+//   - on   -> preview, but error if not in tmux (forced on outside tmux)
+//   - auto -> preview iff inTmux && stdoutTTY
+func Resolve(mode Mode, inTmux, stdoutTTY bool) (Decision, error) {
+	switch mode {
+	case ModeOff:
+		return Decision{ShouldPreview: false, Reason: "preview=off"}, nil
+	case ModeOn:
+		if !inTmux {
+			return Decision{}, ErrNoTmuxForced
+		}
+		return Decision{ShouldPreview: true, Reason: "preview=on"}, nil
+	case ModeAuto, "":
+		if !inTmux {
+			return Decision{ShouldPreview: false, Reason: "auto: $TMUX unset"}, nil
+		}
+		if !stdoutTTY {
+			return Decision{ShouldPreview: false, Reason: "auto: stdout not a tty"}, nil
+		}
+		return Decision{ShouldPreview: true, Reason: "auto"}, nil
+	}
+	return Decision{}, fmt.Errorf("invalid preview mode %q", mode)
+}
+
+// Errors returned by Spawn and Resolve. Each names the missing piece and,
+// where relevant, where to install it.
+var (
+	ErrTmuxMissing    = errors.New("tmux: binary not found on $PATH (required for image preview)")
+	ErrTmuxImgMissing = errors.New("tmux-img: binary not found on $PATH (install at ~/.local/bin/tmux-img)")
+	ErrNoTmuxForced   = errors.New("--preview requires $TMUX (are you in a tmux session?)")
+)
+
+// Spawner spawns the tmux preview window. The exec.LookPath / cmd.Run hooks
+// exist so tests can inject fakes without touching $PATH.
+type Spawner struct {
+	LookPath func(string) (string, error)
+	Run      func(*exec.Cmd) error
+}
+
+// Spawn opens a new tmux window named img:<slug> running tmux-img --hold
+// <imagePath>. -d keeps focus in the current pane. Caller is expected to
+// have already verified that we are inside a tmux session.
+func (s *Spawner) Spawn(imagePath, slug string) error {
+	look := s.LookPath
+	if look == nil {
+		look = exec.LookPath
+	}
+	run := s.Run
+	if run == nil {
+		run = func(c *exec.Cmd) error { return c.Run() }
+	}
+
+	tmuxBin, err := look("tmux")
+	if err != nil {
+		return ErrTmuxMissing
+	}
+	tmuxImgBin, err := look("tmux-img")
+	if err != nil {
+		return ErrTmuxImgMissing
+	}
+
+	name := "img:" + slug
+	shellCmd := fmt.Sprintf("%s --hold %s",
+		shellQuote(tmuxImgBin), shellQuote(imagePath))
+	cmd := exec.Command(tmuxBin, "new-window", "-d", "-n", name, shellCmd)
+	if err := run(cmd); err != nil {
+		return fmt.Errorf("tmux new-window: %w", err)
+	}
+	return nil
+}
+
+// shellQuote single-quotes s for /bin/sh — tmux passes the trailing arg of
+// new-window through a shell.
+func shellQuote(s string) string {
+	return "'" + strings.ReplaceAll(s, "'", `'\''`) + "'"
+}

internal/preview/tmux_test.go

@@ -0,0 +1,170 @@
+package preview
+
+import (
+	"errors"
+	"os/exec"
+	"strings"
+	"testing"
+)
+
+func TestParseMode(t *testing.T) {
+	cases := map[string]Mode{
+		"":     ModeAuto,
+		"auto": ModeAuto,
+		"AUTO": ModeAuto,
+		"on":   ModeOn,
+		" on ": ModeOn,
+		"off":  ModeOff,
+	}
+	for in, want := range cases {
+		got, err := ParseMode(in)
+		if err != nil {
+			t.Errorf("ParseMode(%q) err = %v", in, err)
+			continue
+		}
+		if got != want {
+			t.Errorf("ParseMode(%q) = %q, want %q", in, got, want)
+		}
+	}
+	if _, err := ParseMode("nope"); err == nil {
+		t.Errorf("ParseMode(nope) should have errored")
+	}
+}
+
+func TestResolve(t *testing.T) {
+	type tc struct {
+		mode      Mode
+		inTmux    bool
+		stdoutTTY bool
+		want      bool
+		wantErr   error
+	}
+	cases := map[string]tc{
+		"off-anywhere":         {ModeOff, false, false, false, nil},
+		"off-in-tmux-tty":      {ModeOff, true, true, false, nil},
+		"on-in-tmux":           {ModeOn, true, false, true, nil},
+		"on-outside-tmux-errs": {ModeOn, false, true, false, ErrNoTmuxForced},
+		"auto-no-tmux":         {ModeAuto, false, true, false, nil},
+		"auto-tmux-no-tty":     {ModeAuto, true, false, false, nil},
+		"auto-tmux-and-tty":    {ModeAuto, true, true, true, nil},
+	}
+	for name, c := range cases {
+		t.Run(name, func(t *testing.T) {
+			d, err := Resolve(c.mode, c.inTmux, c.stdoutTTY)
+			if c.wantErr != nil {
+				if !errors.Is(err, c.wantErr) {
+					t.Fatalf("err = %v, want %v", err, c.wantErr)
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("err = %v", err)
+			}
+			if d.ShouldPreview != c.want {
+				t.Errorf("ShouldPreview = %v, want %v (reason: %s)", d.ShouldPreview, c.want, d.Reason)
+			}
+		})
+	}
+}
+
+func TestSpawn_BuildsCorrectCommand(t *testing.T) {
+	var captured *exec.Cmd
+	s := &Spawner{
+		LookPath: func(name string) (string, error) {
+			switch name {
+			case "tmux":
+				return "/usr/bin/tmux", nil
+			case "tmux-img":
+				return "/home/m/.local/bin/tmux-img", nil
+			}
+			return "", exec.ErrNotFound
+		},
+		Run: func(c *exec.Cmd) error {
+			captured = c
+			return nil
+		},
+	}
+	if err := s.Spawn("/tmp/imagen/cat.png", "cat-in-a-fishbowl"); err != nil {
+		t.Fatalf("Spawn: %v", err)
+	}
+	if captured == nil {
+		t.Fatal("Run was not called")
+	}
+	if captured.Path != "/usr/bin/tmux" {
+		t.Errorf("Path = %q, want /usr/bin/tmux", captured.Path)
+	}
+	args := captured.Args
+	if len(args) < 6 {
+		t.Fatalf("args = %v (need at least 6)", args)
+	}
+	// tmux new-window -d -n img:<slug> '<shell-cmd>'
+	if args[1] != "new-window" {
+		t.Errorf("args[1] = %q, want new-window", args[1])
+	}
+	if args[2] != "-d" {
+		t.Errorf("args[2] = %q, want -d", args[2])
+	}
+	if args[3] != "-n" {
+		t.Errorf("args[3] = %q, want -n", args[3])
+	}
+	if args[4] != "img:cat-in-a-fishbowl" {
+		t.Errorf("args[4] = %q, want img:cat-in-a-fishbowl", args[4])
+	}
+	shellCmd := args[5]
+	if !strings.Contains(shellCmd, "tmux-img") || !strings.Contains(shellCmd, "--hold") || !strings.Contains(shellCmd, "/tmp/imagen/cat.png") {
+		t.Errorf("shell cmd %q missing expected pieces", shellCmd)
+	}
+}
+
+func TestSpawn_PathWithSpacesAndQuotes(t *testing.T) {
+	var captured *exec.Cmd
+	s := &Spawner{
+		LookPath: func(name string) (string, error) {
+			if name == "tmux" {
+				return "/usr/bin/tmux", nil
+			}
+			if name == "tmux-img" {
+				return "/usr/local/bin/tmux-img", nil
+			}
+			return "", exec.ErrNotFound
+		},
+		Run: func(c *exec.Cmd) error { captured = c; return nil },
+	}
+	weird := "/tmp/imagen/o'malley's cat.png"
+	if err := s.Spawn(weird, "slug"); err != nil {
+		t.Fatalf("Spawn: %v", err)
+	}
+	shellCmd := captured.Args[5]
+	// Single-quoted with the embedded apostrophe escaped via the
+	// '\'' shell idiom — confirm we did not just splice the raw path.
+	if strings.Contains(shellCmd, "o'malley's") {
+		t.Errorf("shell cmd %q contains unescaped apostrophes", shellCmd)
+	}
+}
+
+func TestSpawn_MissingTmux(t *testing.T) {
+	s := &Spawner{
+		LookPath: func(string) (string, error) { return "", exec.ErrNotFound },
+		Run:      func(*exec.Cmd) error { return nil },
+	}
+	err := s.Spawn("/x.png", "s")
+	if !errors.Is(err, ErrTmuxMissing) {
+		t.Errorf("err = %v, want ErrTmuxMissing", err)
+	}
+}
+
+func TestSpawn_MissingTmuxImg(t *testing.T) {
+	s := &Spawner{
+		LookPath: func(name string) (string, error) {
+			if name == "tmux" {
+				return "/usr/bin/tmux", nil
+			}
+			return "", exec.ErrNotFound
+		},
+		Run: func(*exec.Cmd) error { return nil },
+	}
+	err := s.Spawn("/x.png", "s")
+	if !errors.Is(err, ErrTmuxImgMissing) {
+		t.Errorf("err = %v, want ErrTmuxImgMissing", err)
+	}
+}