Clear TODO backlog: --debug/--profile, codex selection, MCP orientation, perf

- Add --debug[=DIR] / --profile[=DIR] flags that write run artefacts
  (patterm.log, events.jsonl, per-child raw PTY captures, CPU + heap
  + goroutine pprof) to a dir without polluting stdout/stderr.
- Strengthen vendor-TUI orientation in three places (MCP
  initialize.instructions, the spawn_agent tool description, and
  help('spawning')) to head off codex's habits of poking the Unix
  socket via perl and shelling out to launch peers — both bypass
  caller identity and produce orphaned top-level tabs.
- Fix click-and-drag text selection from alt-screen TUIs. Host SGR
  mouse reporting now follows the focused child's screen side
  instead of being permanently armed; alt-screen TUIs that need
  mouse re-enable it themselves and the toggle is forwarded.
- Move drawSidebar() off the per-PTY-chunk hot path. Long claude
  session resume was paying a full sidebar rebuild for every
  scrolled chunk; the chrome ticker now drains a dirty flag at 60 Hz.
- Gate the per-chunk Title() CGO poll on a containsOSC scan so
  codex/ratatui's many SGR-only chunks no longer pay a CGO call each.

CHANGELOG.md

@@ -6,7 +6,59 @@ loosely follows [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
 
+### Fixed
+- Long claude session resume (and codex steady-state rendering) is
+  noticeably faster. Two costs that scaled per-PTY-chunk are now
+  deferred or short-circuited: (1) `drawSidebar()` used to run
+  synchronously for every chunk that scrolled — on a session
+  resume where every chunk scrolls, this rebuilt the full sidebar
+  string hundreds of times for a frame that was almost always
+  cache-equal. The sidebar now signals dirty and the chrome ticker
+  (60 Hz) handles the repaint. (2) `pumpChild` polled the
+  emulator's OSC title after every PTY chunk via CGO, even for
+  chunks (the common case under codex/ratatui) that carry no OSC
+  bytes at all. The poll is now gated on a containsOSC scan over
+  the chunk.
+- Click-and-drag text selection from alt-screen TUIs (codex in
+  particular) now works. Patterm used to keep host SGR mouse
+  reporting armed continuously, which forced the host terminal to
+  forward every click as an escape sequence and prevented native
+  selection. The host's mouse mode now follows the focused child's
+  screen side: primary-screen children keep mouse armed (so wheel
+  scrollback works), alt-screen children get host mouse disabled by
+  default. Alt-screen TUIs that need mouse events (vim, less, etc.)
+  re-enable mouse-mode themselves; the viewport renderer forwards
+  those toggles to the host while the child is on alt. Leaving alt
+  re-arms host mouse reporting so wheel scrollback resumes.
+
 ### Added
+- MCP `initialize.instructions`, the `spawn_agent` tool description
+  (visible to LLMs via `tools/list`), and the `help('spawning')`
+  topic now spell out — in the three places vendor TUIs actually
+  consult — that the connected `patterm` MCP server is the only
+  correct way to drive the host. Anti-patterns called out by name:
+  (a) trying to launch `patterm` / `patterm mcp-stdio` themselves,
+  (b) piping JSON-RPC into the per-PID Unix socket via `perl` /
+  `nc` / `socat` / `curl`, and (c) shelling out to `claude` /
+  `codex` / `opencode` to start a peer. Each of those bypasses
+  caller identity, so a sub-agent spawned that way reads back as
+  a stray top-level tab instead of a child under the spawning
+  agent. Codex was hitting (b) and (c) in practice — this is the
+  fix.
+- `--debug[=DIR]` flag captures detailed run artefacts for offline
+  analysis: a verbose `patterm.log` (the existing `PATTERM_DEBUG_LOG`
+  stream), an `events.jsonl` lifecycle log (spawn / exit / idle-state
+  changes with timestamps), and per-child `<id>.raw` files containing
+  the raw PTY byte stream. With no argument, the dated subdir
+  `$XDG_STATE_HOME/patterm/debug/YYYYMMDD-HHMMSS` is used; pass an
+  explicit path to override. All output goes to files — stdout/stderr
+  are untouched.
+- `--profile[=DIR]` flag captures pprof data for performance work:
+  `cpu.pprof` (running for the lifetime of the session), plus
+  `heap.pprof` and `goroutine.pprof` snapshots written on shutdown.
+  Defaults to `$XDG_STATE_HOME/patterm/profile/YYYYMMDD-HHMMSS`.
+  All diagnostics (startup, errors) are written to `profile.log`
+  inside the dir, never to stdout/stderr.
 - "New Terminal" entry in the command palette spawns a bare interactive
   `$SHELL` pane (kind `terminal`). Unlike "Run process: …" presets,
   which are session-persistent and reachable via `restart_process`,

TODO.md

@@ -1,15 +1,3 @@
-- [ ] Codex seemed to think that it needed to launch patterm itself to get the mcp working
-- [ ] I cant click and drag to select text from codex
-- [ ] codex uses perl to interact with the socket rather than calling mcp tools
-  - when it _did_ open a sub claude it opened it as a separate tab rather than a sub-agent.
-- [ ] codex rendering is VERY slow
-  - maybe we need to use diffing rather than rendering the entire viewport for performance
-- We should add a --debug and --profile flag, so we can get detailed performance data and full logs of the agent output to be debugged later on.
-  - I don't mind what format this is in, ideally easy for LLMs to understand
-- [ ] Resuming a long claude session takes a couple of seconds for the entire buffer to load in, it looks like it's scrolling down for a couple seconds.
-  - In raw alacritty this is instant, so there's some sort of performance issue with patterm's terminal emulation.
-
-
 # On Hold
 - [ ] There's a unicode <?> being displayed in opencode [ON HOLD]
   - Investigated 2026-05-14: patterm passes ghostty grapheme codepoints

cmd/patterm/main.go

@@ -16,7 +16,10 @@ import (
 	"context"
 	"fmt"
 	"os"
+	"path/filepath"
+	"runtime"
 	"runtime/debug"
+	"runtime/pprof"
 	"time"
 
 	flag "github.com/spf13/pflag"
@@ -49,7 +52,13 @@ func main() {
 	var (
 		projectDir  = flag.String("project", "", "project directory (default $PWD)")
 		showVersion = flag.Bool("version", false, "print version and exit")
+		debugDir    = flag.String("debug", "", "write debug logs + per-child raw PTY output to DIR (auto-picks a dated subdir under $XDG_STATE_HOME/patterm/debug when DIR is omitted)")
+		profileDir  = flag.String("profile", "", "write CPU+heap+goroutine pprof files to DIR (auto-picks a dated subdir under $XDG_STATE_HOME/patterm/profile when DIR is omitted)")
 	)
+	// Allow bare `--debug` / `--profile` with no value — pflag treats
+	// them as boolean-shaped strings, picking a sensible default dir.
+	flag.Lookup("debug").NoOptDefVal = "auto"
+	flag.Lookup("profile").NoOptDefVal = "auto"
 	flag.Parse()
 
 	if *showVersion {
@@ -73,15 +82,103 @@ func main() {
 		die("chdir %s: %v", cwd, err)
 	}
 
+	resolvedDebug, err := resolveDiagDir(*debugDir, "debug")
+	if err != nil {
+		die("debug: %v", err)
+	}
+	resolvedProfile, err := resolveDiagDir(*profileDir, "profile")
+	if err != nil {
+		die("profile: %v", err)
+	}
+
+	stopProfile := startProfile(resolvedProfile)
+	defer stopProfile()
+
 	ctx := context.Background()
 	if err := app.Run(ctx, app.Options{
 		ProjectDir: cwd,
 		ProjectKey: key,
+		DebugDir:   resolvedDebug,
 	}); err != nil {
 		die("%v", err)
 	}
 }
 
+// resolveDiagDir turns the raw flag value into an absolute directory
+// path. Empty string disables the feature. The sentinel "auto" (set by
+// NoOptDefVal on bare flags) picks $XDG_STATE_HOME/patterm/<kind>/<ts>.
+// Any other value is treated as a literal path.
+func resolveDiagDir(raw, kind string) (string, error) {
+	if raw == "" {
+		return "", nil
+	}
+	if raw == "auto" {
+		base := os.Getenv("XDG_STATE_HOME")
+		if base == "" {
+			home, err := os.UserHomeDir()
+			if err != nil {
+				return "", err
+			}
+			base = filepath.Join(home, ".local", "state")
+		}
+		ts := time.Now().Format("20060102-150405")
+		return filepath.Join(base, "patterm", kind, ts), nil
+	}
+	return raw, nil
+}
+
+// startProfile begins a CPU profile under dir and returns a stop func
+// that writes heap + goroutine snapshots before flushing the CPU file.
+// Returns a no-op stop func when dir is empty. All diagnostics are
+// written to <dir>/profile.log — never to stdout/stderr — so the TUI
+// stays uncluttered.
+func startProfile(dir string) func() {
+	if dir == "" {
+		return func() {}
+	}
+	if err := os.MkdirAll(dir, 0o700); err != nil {
+		return func() {}
+	}
+	logPath := filepath.Join(dir, "profile.log")
+	plog := func(format string, args ...any) {
+		f, err := os.OpenFile(logPath, os.O_CREATE|os.O_WRONLY|os.O_APPEND, 0o600)
+		if err != nil {
+			return
+		}
+		defer f.Close()
+		fmt.Fprintf(f, format+"\n", args...)
+	}
+	cpuPath := filepath.Join(dir, "cpu.pprof")
+	f, err := os.Create(cpuPath)
+	if err != nil {
+		plog("cpu open: %v", err)
+		return func() {}
+	}
+	if err := pprof.StartCPUProfile(f); err != nil {
+		plog("cpu start: %v", err)
+		_ = f.Close()
+		return func() {}
+	}
+	plog("profiling started at %s", time.Now().Format(time.RFC3339Nano))
+	return func() {
+		pprof.StopCPUProfile()
+		_ = f.Close()
+		// Heap and goroutine snapshots at exit. Heap captures
+		// steady-state allocation; goroutine catches stragglers
+		// that didn't get cleaned up.
+		runtime.GC()
+		if hf, err := os.Create(filepath.Join(dir, "heap.pprof")); err == nil {
+			_ = pprof.Lookup("heap").WriteTo(hf, 0)
+			_ = hf.Close()
+		}
+		if gf, err := os.Create(filepath.Join(dir, "goroutine.pprof")); err == nil {
+			_ = pprof.Lookup("goroutine").WriteTo(gf, 0)
+			_ = gf.Close()
+		}
+		plog("profiling stopped at %s", time.Now().Format(time.RFC3339Nano))
+	}
+}
+
 func runMCPProxy() {
 	var (
 		socket   = flag.String("socket", "", "path to the running patterm process's MCP socket")

internal/app/app.go

@@ -29,6 +29,11 @@ import (
 type Options struct {
 	ProjectDir string
 	ProjectKey string
+	// DebugDir, when non-empty, enables verbose debug logging to
+	// <DebugDir>/patterm.log and per-child raw PTY output capture to
+	// <DebugDir>/<child-id>.raw. The dir is created if missing. Events
+	// (spawn / exit / state change) land in <DebugDir>/events.jsonl.
+	DebugDir string
 }
 
 const keyCtrlK byte = 0x0b
@@ -77,6 +82,22 @@ func Run(ctx context.Context, opts Options) error {
 
 	sess := NewSession(opts.ProjectDir, opts.ProjectKey)
 	defer sess.Shutdown()
+
+	// Debug capture: when --debug=<dir> is set, write a verbose log
+	// (patterm.log), per-child raw PTY output (<id>.raw), and a
+	// JSONL event stream (events.jsonl). Installed before the TUI
+	// listener so the very first OnChildSpawned / OnPTYOut event
+	// is captured.
+	if opts.DebugDir != "" {
+		dc, err := openDebugCapture(opts.DebugDir)
+		if err != nil {
+			return fmt.Errorf("app: debug capture: %w", err)
+		}
+		os.Setenv("PATTERM_DEBUG_LOG", dc.LogPath())
+		sess.Subscribe(dc)
+		defer dc.Close()
+		logf("debug capture enabled at %s", opts.DebugDir)
+	}
 	// Snapshot persisted processes BEFORE attaching the store: Spawn
 	// mints fresh ids, so the old records would otherwise linger
 	// alongside the new ones. Drop them up front; the restore loop
@@ -248,12 +269,19 @@ func Run(ctx context.Context, opts Options) error {
 			case <-st.chromeWake:
 			case <-ticker.C:
 			}
-			if !st.chromeDirty.Swap(false) {
+			chromeChanged := st.chromeDirty.Swap(false)
+			sidebarChanged := st.sidebarDirty.Swap(false)
+			if !chromeChanged && !sidebarChanged {
 				continue
 			}
+			if chromeChanged {
 				st.drawTabBar()
 				st.drawStatusLine()
 			}
+			if sidebarChanged {
+				st.drawSidebar()
+			}
+		}
 	}()
 
 	// External termination: SPEC §2 step 4 (SIGTERM/SIGHUP → graceful exit).
@@ -372,6 +400,13 @@ type uiState struct {
 	// sensitive paths (owner flip, attention, trust, focus change)
 	// continue to call drawStatusLine / drawTabBar synchronously.
 	chromeDirty atomic.Bool
+	// sidebarDirty defers sidebar repaints off the per-chunk hot path
+	// in the same way. A long claude session resume — where every PTY
+	// chunk scrolls the viewport — used to call drawSidebar()
+	// synchronously per chunk, which dominated the resume's wall time
+	// (hundreds of full-sidebar rebuilds for a frame that was almost
+	// always cache-equal).
+	sidebarDirty atomic.Bool
 	chromeWake   chan struct{}
 
 	// padsCacheMu guards the cached scratchpad listing. The sidebar
@@ -415,14 +450,18 @@ func (st *uiState) focusProcess(processID string) {
 		return
 	}
 	layout := st.layoutSnapshot()
+	onAlt := childIsOnAlt(c)
 	st.mu.Lock()
 	leavingPad := st.focusedPad != ""
 	st.focusedPad = ""
 	st.focusedID = c.ID
 	st.focusedName = c.DisplayName()
 	st.updateActiveAgentLocked(c)
-	st.renderer = newViewportRenderer(layout)
+	r := newViewportRenderer(layout)
+	r.SetChildOnAlt(onAlt)
+	st.renderer = r
 	st.mu.Unlock()
+	st.syncHostMouseForChild(onAlt)
 	// Wipe whatever the previous focus (PTY child or pad view) left in
 	// the viewport before painting the new child's snapshot.
 	if leavingPad {
@@ -434,6 +473,41 @@ func (st *uiState) focusProcess(processID string) {
 	st.drawStatusLine()
 }
 
+// childIsOnAlt reports whether the child's emulator is currently on
+// its alternate screen. Returns false if the emulator is gone or the
+// query fails.
+func childIsOnAlt(c *Child) bool {
+	if c == nil {
+		return false
+	}
+	em := c.Emulator()
+	if em == nil {
+		return false
+	}
+	sc, err := em.ActiveScreen()
+	if err != nil {
+		return false
+	}
+	return sc == vt.ScreenAlternate
+}
+
+// syncHostMouseForChild emits the host mouse-reporting toggle that
+// matches a newly-focused child's screen side. Primary-screen children
+// want host mouse armed so the wheel drives inline scrollback; alt-
+// screen children get host mouse disabled by default so click-and-drag
+// selection works. Alt-screen TUIs that need mouse (vim, ranger, etc.)
+// re-enable it themselves, and the viewport renderer forwards those
+// toggles back to the host.
+func (st *uiState) syncHostMouseForChild(onAlt bool) {
+	st.outMu.Lock()
+	defer st.outMu.Unlock()
+	if onAlt {
+		_, _ = os.Stdout.WriteString("\x1b[?1000l\x1b[?1006l")
+	} else {
+		_, _ = os.Stdout.WriteString("\x1b[?1000h\x1b[?1006h")
+	}
+}
+
 // focusScratchpad shifts focus to a scratchpad. The main viewport
 // renders the pad's text instead of any child PTY; PTY output for the
 // previously focused child is dropped until focus moves back to a
@@ -572,12 +646,14 @@ func (st *uiState) scratchpadsChanged() {
 // OnChildSpawned auto-focuses the new child.
 func (st *uiState) OnChildSpawned(c *Child) {
 	layout := st.layoutSnapshot()
+	onAlt := childIsOnAlt(c)
 	st.mu.Lock()
 	st.focusedPad = ""
 	st.focusedID = c.ID
 	st.focusedName = c.DisplayName()
 	st.updateActiveAgentLocked(c)
 	renderer := newViewportRenderer(layout)
+	renderer.SetChildOnAlt(onAlt)
 	st.renderer = renderer
 	palOpen := st.palette != nil
 	if palOpen {
@@ -611,6 +687,7 @@ func (st *uiState) OnChildSpawned(c *Child) {
 		st.outMu.Unlock()
 	}
 
+	st.syncHostMouseForChild(onAlt)
 	st.moveToViewportOrigin()
 	st.drawTabBar()
 	st.drawSidebar()
@@ -760,9 +837,14 @@ func (st *uiState) OnPTYOut(childID string, chunk []byte) {
 		st.chromeCacheMu.Lock()
 		st.sidebarCache = ""
 		st.chromeCacheMu.Unlock()
-		// Scrolled chunks can clobber the sidebar columns; repaint
-		// synchronously so the gap fills before the next chunk lands.
-		st.drawSidebar()
+		// Defer the sidebar repaint to the chrome ticker. On a long
+		// session resume every PTY chunk scrolls, and a synchronous
+		// drawSidebar() per chunk dominates wall time even when the
+		// frame ends up cache-equal — the rebuild work is unconditional.
+		// The chrome ticker drains the dirty flag at ~60 Hz, so the
+		// visible gap a scrolled chunk can leave in the sidebar columns
+		// is bounded by one frame.
+		st.markSidebarDirty()
 	}
 	// Defer the tab bar + status line repaint to the chrome ticker.
 	// The cached frame already short-circuits the wire write, but
@@ -866,6 +948,18 @@ func (st *uiState) markChromeDirty() {
 	}
 }
 
+// markSidebarDirty schedules a sidebar repaint on the next ticker
+// frame. Hot path — every scrolled PTY chunk lands here. Synchronous
+// repaints from latency-sensitive sites (spawn, exit, focus, state
+// change, trust) keep calling drawSidebar directly.
+func (st *uiState) markSidebarDirty() {
+	st.sidebarDirty.Store(true)
+	select {
+	case st.chromeWake <- struct{}{}:
+	default:
+	}
+}
+
 func (st *uiState) invalidateChromeCache() {
 	st.chromeCacheMu.Lock()
 	st.tabBarCache = ""

internal/app/debug.go

@@ -0,0 +1,155 @@
+package app
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+	"sync"
+	"time"
+)
+
+// debugCapture implements ChildEventListener and writes structured
+// debug artefacts under a single directory:
+//
+//   - patterm.log    — the existing logf() stream
+//   - events.jsonl   — one JSON object per lifecycle event
+//   - <id>.raw       — raw PTY bytes for each child, by id+name
+//
+// The capture is installed only when --debug=<dir> is set, so default
+// runs pay nothing.
+type debugCapture struct {
+	dir     string
+	logPath string
+
+	mu      sync.Mutex
+	events  *os.File
+	rawByID map[string]*os.File
+}
+
+func openDebugCapture(dir string) (*debugCapture, error) {
+	if err := os.MkdirAll(dir, 0o700); err != nil {
+		return nil, err
+	}
+	logPath := filepath.Join(dir, "patterm.log")
+	// Truncate-style fresh log per run is friendlier for grep'ing one
+	// session. The existing logf opens O_APPEND though, so concurrent
+	// runs against the same dir would interleave — that's on the user.
+	if f, err := os.Create(logPath); err != nil {
+		return nil, err
+	} else {
+		_ = f.Close()
+	}
+	ev, err := os.Create(filepath.Join(dir, "events.jsonl"))
+	if err != nil {
+		return nil, err
+	}
+	dc := &debugCapture{
+		dir:     dir,
+		logPath: logPath,
+		events:  ev,
+		rawByID: make(map[string]*os.File),
+	}
+	dc.writeEvent("session_start", map[string]any{
+		"time": time.Now().Format(time.RFC3339Nano),
+		"pid":  os.Getpid(),
+	})
+	return dc, nil
+}
+
+func (d *debugCapture) LogPath() string { return d.logPath }
+
+func (d *debugCapture) Close() error {
+	d.mu.Lock()
+	defer d.mu.Unlock()
+	d.writeEventLocked("session_end", map[string]any{
+		"time": time.Now().Format(time.RFC3339Nano),
+	})
+	for _, f := range d.rawByID {
+		_ = f.Close()
+	}
+	d.rawByID = nil
+	if d.events != nil {
+		_ = d.events.Close()
+		d.events = nil
+	}
+	return nil
+}
+
+func (d *debugCapture) OnChildSpawned(c *Child) {
+	d.writeEvent("child_spawned", map[string]any{
+		"time":      time.Now().Format(time.RFC3339Nano),
+		"id":        c.ID,
+		"name":      c.Name,
+		"kind":      string(c.Kind),
+		"parent_id": c.ParentID,
+		"preset":    c.PresetRef,
+		"argv":      c.Argv,
+	})
+}
+
+func (d *debugCapture) OnChildExited(c *Child) {
+	d.writeEvent("child_exited", map[string]any{
+		"time":      time.Now().Format(time.RFC3339Nano),
+		"id":        c.ID,
+		"name":      c.Name,
+		"exit_code": c.ExitCode(),
+	})
+	d.mu.Lock()
+	defer d.mu.Unlock()
+	if f, ok := d.rawByID[c.ID]; ok {
+		_ = f.Close()
+		delete(d.rawByID, c.ID)
+	}
+}
+
+func (d *debugCapture) OnChildStateChanged(id string, state IdleState) {
+	d.writeEvent("child_state", map[string]any{
+		"time":  time.Now().Format(time.RFC3339Nano),
+		"id":    id,
+		"state": string(state),
+	})
+}
+
+func (d *debugCapture) OnPTYOut(childID string, chunk []byte) {
+	if len(chunk) == 0 {
+		return
+	}
+	d.mu.Lock()
+	defer d.mu.Unlock()
+	f, ok := d.rawByID[childID]
+	if !ok {
+		path := filepath.Join(d.dir, childID+".raw")
+		nf, err := os.Create(path)
+		if err != nil {
+			return
+		}
+		f = nf
+		d.rawByID[childID] = nf
+	}
+	// Listener contract: don't retain chunk past return. Writing now
+	// is fine; the slice's backing buffer is reused for the next read
+	// only after this listener chain completes.
+	_, _ = f.Write(chunk)
+}
+
+func (d *debugCapture) writeEvent(kind string, fields map[string]any) {
+	d.mu.Lock()
+	defer d.mu.Unlock()
+	d.writeEventLocked(kind, fields)
+}
+
+func (d *debugCapture) writeEventLocked(kind string, fields map[string]any) {
+	if d.events == nil {
+		return
+	}
+	if fields == nil {
+		fields = map[string]any{}
+	}
+	fields["event"] = kind
+	enc, err := json.Marshal(fields)
+	if err != nil {
+		return
+	}
+	_, _ = fmt.Fprintln(d.events, string(enc))
+}

internal/app/host.go

@@ -1111,7 +1111,7 @@ func helpFor(topic string) mcp.HelpResponse {
 	case "spawning":
 		return mcp.HelpResponse{
 			Topic:        "spawning",
-			Content:      "spawn_agent launches another vendor LLM CLI as a sub-agent (orchestrator only). spawn_process(kind: command, preset: …) starts a stored command; spawn_process(kind: terminal) opens a shell. Command presets need trust the first time — you'll get needs_trust until the human accepts. Whatever you spawn is yours to clean up — see help('lifecycle').",
+			Content:      "spawn_agent launches another vendor LLM CLI as a sub-agent (orchestrator only). spawn_process(kind: command, preset: …) starts a stored command; spawn_process(kind: terminal) opens a shell. Command presets need trust the first time — you'll get needs_trust until the human accepts. ANTI-PATTERNS: do not shell out to `claude` / `codex` / `opencode` (or any other agent CLI) yourself, and do not pipe JSON-RPC into patterm's Unix socket via perl / nc / socat / curl. Either path bypasses caller-identity and the new agent reads back as a stray top-level tab instead of your child — call spawn_agent through the MCP transport you were initialised on. Whatever you spawn is yours to clean up — see help('lifecycle').",
 			RelatedTools: []string{"spawn_agent", "spawn_process", "start_process", "restart_process", "close_process"},
 		}
 	case "lifecycle":

internal/app/session.go

@@ -397,14 +397,17 @@ func (s *Session) pumpChild(c *Child, runID uint64) {
 				}
 				// OSC 0/2 title updates ride on the same byte stream as
 				// the rest of the output. Polling the emulator after each
-				// Write is cheap (one cgo call returning a borrowed
-				// string) and lets the classifier treat title changes as
-				// an activity signal — even when the title isn't visible
-				// in the rendered grid.
+				// chunk is cheap on its own (one CGO call) but codex/
+				// ratatui sends so many small chunks that the per-chunk
+				// CGO cost becomes measurable. Skip the Title poll when
+				// the chunk doesn't carry an OSC start byte at all; the
+				// title can only change on chunks that include one.
+				if containsOSC(chunk) {
 					if t, terr := em.Title(); terr == nil {
 						c.recordTitle(t)
 					}
 				}
+			}
 			c.recordWrite(chunk)
 			s.emitPTYOut(c.ID, chunk)
 		}
@@ -679,6 +682,24 @@ func (s *Session) Shutdown() {
 	}
 }
 
+// containsOSC reports whether chunk holds a sequence that could begin
+// an OSC. OSC starts as ESC ] (0x1b 0x5d) or the bare C1 ] (0x9d),
+// so a chunk without either cannot have changed the emulator's OSC
+// title state. Used to short-circuit the per-chunk Title() poll from
+// pumpChild, which otherwise pays a CGO call for every chunk even
+// when codex/ratatui is just emitting SGR-styled output.
+func containsOSC(chunk []byte) bool {
+	for i, b := range chunk {
+		if b == 0x9d {
+			return true
+		}
+		if b == 0x1b && i+1 < len(chunk) && chunk[i+1] == ']' {
+			return true
+		}
+	}
+	return false
+}
+
 func logf(format string, args ...any) {
 	if os.Getenv("PATTERM_DEBUG_LOG") == "" {
 		return

internal/app/viewport_renderer.go

@@ -33,6 +33,14 @@ type viewportRenderer struct {
 	// cache so the next drawSidebar repaints over the clobber.
 	scrolled bool
 
+	// childOnAlt tracks whether the focused child has entered its
+	// alternate screen (via ?47 / ?1047 / ?1049). Used to gate mouse-
+	// tracking-mode forwarding to the host: filter on primary so
+	// patterm's wheel-scrollback stays armed, forward on alt so codex
+	// (which disables mouse) lets the user select text and vim (which
+	// enables it) still gets mouse events.
+	childOnAlt bool
+
 	// skipUTF8 is set when the current multi-byte UTF-8 character started
 	// past the viewport's right edge. The starter byte was dropped, so
 	// the remaining continuation bytes must be dropped too instead of
@@ -65,6 +73,16 @@ func newViewportRenderer(l terminalLayout) *viewportRenderer {
 	return vr
 }
 
+// SetChildOnAlt seeds the renderer's view of the focused child's screen
+// side. Used when a new renderer is constructed for an already-running
+// child whose alt-screen transition we missed, so subsequent mouse-mode
+// toggles are filtered/forwarded according to the right side.
+func (vr *viewportRenderer) SetChildOnAlt(onAlt bool) {
+	vr.mu.Lock()
+	defer vr.mu.Unlock()
+	vr.childOnAlt = onAlt
+}
+
 func (vr *viewportRenderer) SetLayout(l terminalLayout) {
 	vr.mu.Lock()
 	defer vr.mu.Unlock()
@@ -236,15 +254,36 @@ func (vr *viewportRenderer) emitCSI() {
 			return
 		}
 		if isAltScreenMode(params) {
+			// Track the child's screen side so we know whether to filter
+			// or forward subsequent mouse-mode toggles. Entering alt
+			// disables host mouse reporting by default so codex (and
+			// any other alt-screen TUI that doesn't request mouse)
+			// allows the user to click-drag to select text. Alt-screen
+			// TUIs that want mouse (vim, less with -X) re-enable it
+			// via ?1000h after switching to alt — the forwarder below
+			// passes that through. Leaving alt re-arms host mouse for
+			// primary-screen wheel-scrollback.
+			wasAlt := vr.childOnAlt
+			vr.childOnAlt = final == 'h'
+			if !wasAlt && vr.childOnAlt {
+				vr.pending.WriteString("\x1b[?1000l\x1b[?1006l")
+			}
+			if wasAlt && !vr.childOnAlt {
+				vr.pending.WriteString("\x1b[?1000h\x1b[?1006h")
+			}
 			return
 		}
 		if isMouseTrackingMode(params) {
-			// Patterm owns mouse reporting on the host so wheel events keep
-			// flowing for scroll-viewport. The child's own emulator still
-			// observes the mode set/reset (it processes the same bytes we
-			// hand to ghostty_terminal_vt_write), so we know whether the
-			// child wants mouse input — we just don't let it disarm our
-			// host listener.
+			// On the child's primary screen patterm owns mouse reporting so
+			// wheel events keep flowing for in-pane scrollback — drop the
+			// child's toggle. On the alt screen the child should be free
+			// to enable mouse (vim, less) or disable it (codex); we forward
+			// the toggle to the host so click-and-drag selection works for
+			// alt-screen TUIs that don't want mouse, and mouse-aware ones
+			// still see the events they need.
+			if vr.childOnAlt {
+				vr.pending.Write(vr.buf)
+			}
 			return
 		}
 	}

internal/app/viewport_renderer_test.go

@@ -24,8 +24,36 @@ func TestViewportRendererShiftsCursor(t *testing.T) {
 func TestViewportRendererSwallowsAltScreenToggles(t *testing.T) {
 	vr := newViewportRenderer(newTerminalLayout(120, 40))
 	got := string(vr.Render([]byte("a\x1b[?1049hb\x1b[?1049lc")))
+	// The ?1049h/l toggles themselves must not reach the host (patterm
+	// owns its own alt screen). On the transition we re-sync host mouse
+	// reporting so codex (which doesn't request mouse) lets the user
+	// drag-select; leaving alt re-arms it for primary-screen wheel
+	// scrollback.
+	want := "a\x1b[?1000l\x1b[?1006lb\x1b[?1000h\x1b[?1006hc"
+	if got != want {
+		t.Fatalf("alt-screen toggles: got %q want %q", got, want)
+	}
+}
+
+func TestViewportRendererMouseTrackingFilteredOnPrimary(t *testing.T) {
+	vr := newViewportRenderer(newTerminalLayout(120, 40))
+	got := string(vr.Render([]byte("a\x1b[?1000lb\x1b[?1000hc")))
 	if got != "abc" {
-		t.Fatalf("alt-screen toggles: got %q", got)
+		t.Fatalf("mouse mode on primary should be filtered: got %q", got)
+	}
+}
+
+func TestViewportRendererMouseTrackingForwardedOnAlt(t *testing.T) {
+	vr := newViewportRenderer(newTerminalLayout(120, 40))
+	// Enter alt; subsequent mouse-mode toggles should reach the host so
+	// alt-screen TUIs (vim, less) can run with mouse on, and selection-
+	// using ones (codex) stay with mouse off.
+	got := string(vr.Render([]byte("\x1b[?1049h\x1b[?1000lx\x1b[?1000hy")))
+	if !strings.Contains(got, "\x1b[?1000l") {
+		t.Fatalf("alt-screen mouse disable should reach host: %q", got)
+	}
+	if !strings.Contains(got, "\x1b[?1000h") {
+		t.Fatalf("alt-screen mouse enable should reach host: %q", got)
 	}
 }
 

internal/mcp/protocol.go

@@ -27,6 +27,24 @@ var serverInfo = map[string]any{
 	"version": "0.1.0",
 }
 
+// serverInstructions is returned in the MCP `initialize` response. MCP
+// clients show this to the underlying LLM as context for how to use
+// the server. Failure modes we've seen and want to head off:
+//   - The agent assumes patterm is something it has to launch (running
+//     `patterm` or `patterm mcp-stdio` from its own shell). It's
+//     already attached — it just calls the tools.
+//   - The agent reaches for shell tools (perl / nc / socat / curl) to
+//     poke patterm's Unix socket directly. That socket connection
+//     carries no caller identity, so any sub-agent the agent spawns
+//     that way ends up as a stray top-level tab instead of a child
+//     under the spawning agent. Always go through the MCP tools.
+//   - The agent shells out to `claude` / `codex` / `opencode` to start
+//     a peer instead of calling `spawn_agent`. Those peers won't show
+//     up as sub-agents and won't be tied into the patterm lifecycle.
+//
+// Keep this short — clients vary in how much they surface to the LLM.
+const serverInstructions = "You are already running INSIDE patterm; the `patterm` MCP server is connected over the same stdio MCP transport you use for any other MCP server. Use the MCP tools you see in tools/list — do NOT (a) try to launch `patterm` or `patterm mcp-stdio` yourself, (b) poke the Unix socket through perl / nc / socat / curl, or (c) shell out to `claude` / `codex` / `opencode` to start a peer. Any of those bypasses caller-identity and the new agent will land as a stray top-level tab instead of a child under you. Start with `whoami` for your role and the full tool list, then `help('topics')` for orientation. `spawn_agent` is the only correct way to start a sub-agent; `spawn_process` is for non-LLM commands; `list_processes` / `get_process_output` inspect them; `send_input` / `send_message` drive them. Whatever you spawn is yours to `close_process` when done."
+
 // toolDescriptor is the shape returned by `tools/list`. inputSchema is
 // a JSON Schema object — we provide a minimal `{type: "object"}` schema
 // for each tool, which lets MCP clients accept arbitrary arguments and
@@ -88,7 +106,7 @@ func toolCatalog() []toolDescriptor {
 	return []toolDescriptor{
 		{
 			Name:        "spawn_agent",
-			Description: "Spawn a sub-agent from an agent preset and optionally seed it with initial instructions. Caller owns lifecycle: when the sub-agent's work is done (it reports back via send_message, or you no longer need it), call close_process on its process_id to free the pane and tear down the PTY. See help('lifecycle').",
+			Description: "Spawn a sub-agent from an agent preset and optionally seed it with initial instructions. This is the ONLY correct way to start a sub-agent under you — do not shell out to `claude` / `codex` / `opencode` and do not poke patterm's Unix socket via perl / nc / socat. Either bypasses caller identity and the new agent lands as a stray top-level tab instead of your child. Caller owns lifecycle: when the sub-agent's work is done (it reports back via send_message, or you no longer need it), call close_process on its process_id to free the pane and tear down the PTY. See help('spawning') and help('lifecycle').",
 			InputSchema: objectSchema(map[string]any{
 				"agent":              stringProp("Preset name (e.g. \"claude\", \"codex\")."),
 				"agent_instructions": stringProp("Initial prompt typed into the agent after it's ready."),
@@ -378,6 +396,7 @@ func (s *Server) handleProtocolMethod(callerID, method string, params json.RawMe
 				"tools": map[string]any{"listChanged": false},
 			},
 			"serverInfo":   serverInfo,
+			"instructions": serverInstructions,
 		}
 		return result, true, 0, "", nil
 

internal/mcp/protocol_test.go

@@ -36,6 +36,13 @@ func TestInitializeReturnsCapabilities(t *testing.T) {
 	if caps["tools"] == nil {
 		t.Fatalf("tools capability missing: %+v", caps)
 	}
+	// patterm-specific orientation: clients show this to the underlying
+	// LLM, so it's our primary hook for steering vendor TUIs (codex in
+	// particular) toward the MCP tool surface instead of shell-ing out.
+	instructions, ok := parsed.Result["instructions"].(string)
+	if !ok || instructions == "" {
+		t.Fatalf("instructions missing or wrong type: %+v", parsed.Result)
+	}
 }
 
 func TestInitializedNotificationSuppressesResponse(t *testing.T) {