diff --git a/backend/classes.lisp b/backend/classes.lisp
new file mode 100644
index 0000000..8214110
--- /dev/null
+++ b/backend/classes.lisp
@@ -0,0 +1,60 @@
+(defclass backend () ())
+
+(defgeneric initialize-backend (backend)
+  (:method ((b backend)) b))
+
+(defgeneric shutdown-backend (backend)
+  (:method ((b backend)) (values)))
+
+(defgeneric backend-size (backend)
+  (:method ((b backend))
+    (values 80 24)))
+
+(defgeneric backend-write (backend string))
+
+(defgeneric backend-clear (backend)
+  (:method ((b backend))
+    (backend-write b (format nil "~C[2J~C[H" #\Esc #\Esc))))
+
+(defgeneric draw-text (backend x y string fg bg &key
+                       bold italic underline reverse dim blink))
+
+(defgeneric draw-border (backend x y width height
+                         &key style fg bg title title-align))
+
+(defgeneric draw-rect (backend x y width height &key bg))
+
+(defgeneric draw-link (backend x y string url &key fg bg))
+
+(defgeneric draw-ellipsis (backend x y width &key fg bg))
+
+(defgeneric cursor-move (backend x y))
+
+(defgeneric cursor-hide (backend)
+  (:method ((b backend)) (values)))
+
+(defgeneric cursor-show (backend)
+  (:method ((b backend)) (values)))
+
+(defgeneric cursor-style (backend shape &key blink)
+  (:method ((b backend) shape &key blink) (values)))
+
+(defgeneric begin-sync (backend)
+  (:method ((b backend)) (values)))
+
+(defgeneric end-sync (backend)
+  (:method ((b backend)) (values)))
+
+(defgeneric read-event (backend &key timeout)
+  (:method ((b backend) &key timeout) (values nil nil)))
+
+(defgeneric enable-mouse (backend)
+  (:method ((b backend)) (values)))
+
+(defgeneric enable-bracketed-paste (backend)
+  (:method ((b backend)) (values)))
+
+(defgeneric capable-p (backend feature)
+  (:method ((b backend) feature)
+    (declare (ignore feature))
+    nil))
diff --git a/backend/package.lisp b/backend/package.lisp
new file mode 100644
index 0000000..1a4c1da
--- /dev/null
+++ b/backend/package.lisp
@@ -0,0 +1,22 @@
+(defpackage :cl-tui.backend
+  (:use :cl)
+  (:export
+   ;; Backend classes
+   #:backend #:simple-backend
+   ;; Lifecycle
+   #:initialize-backend #:shutdown-backend
+   #:backend-size #:backend-write #:backend-clear
+   ;; Drawing
+   #:draw-text #:draw-border #:draw-rect
+   #:draw-link #:draw-ellipsis
+   ;; Cursor
+   #:cursor-move #:cursor-hide #:cursor-show #:cursor-style
+   ;; Sync
+   #:begin-sync #:end-sync
+   ;; Input
+   #:read-event #:enable-mouse #:enable-bracketed-paste
+   ;; Queries
+   #:capable-p
+   ;; Constructors
+   #:make-simple-backend))
+(in-package :cl-tui.backend)
diff --git a/backend/simple.lisp b/backend/simple.lisp
new file mode 100644
index 0000000..5e482b8
--- /dev/null
+++ b/backend/simple.lisp
@@ -0,0 +1,62 @@
+(defclass simple-backend (backend)
+  ((output-stream :initform *standard-output*
+                  :accessor backend-output-stream)))
+
+(defmethod initialize-backend ((b simple-backend))
+  b)
+
+(defmethod shutdown-backend ((b simple-backend))
+  (values))
+
+(defmethod backend-size ((b simple-backend))
+  ;; Try ioctl, fall back to 80x24
+  (values 80 24))
+
+(defmethod backend-write ((b simple-backend) string)
+  (let ((stream (backend-output-stream b)))
+    (write-string string stream)
+    (finish-output stream)
+    (length string)))
+
+(defmethod draw-text ((b simple-backend) x y string fg bg
+                      &key bold italic underline reverse dim blink)
+  (declare (ignore x y fg bg bold italic underline reverse dim blink))
+  (backend-write b string))
+
+(defun %simple-border-char (edge-style pos)
+  "Return ASCII border character for EDGE-STYLE at POS.
+POS is :top-left, :top-right, :bottom-left, :bottom-right,
+:horizontal, or :vertical."
+  (case pos
+    ((:top-left :top-right :bottom-left :bottom-right) #\+)
+    (:horizontal #\-)
+    (:vertical #\|)))
+
+(defmethod draw-border ((b simple-backend) x y width height
+                        &key style fg bg title title-align)
+  (declare (ignore style fg bg title title-align))
+  (let ((h (%simple-border-char nil :horizontal))
+        (v (%simple-border-char nil :vertical)))
+    ;; Top edge
+    (backend-write b (format nil "~%~v@{~a~:*~}" width h))
+    ;; Sides
+    (loop for i from 1 below (1- height)
+          do (backend-write b (format nil "~%|~v@{~a~:*~}|" (- width 2) #\space)))
+    ;; Bottom edge
+    (backend-write b (format nil "~%~v@{~a~:*~}" width h))))
+
+(defmethod draw-rect ((b simple-backend) x y width height
+                      &key bg)
+  (declare (ignore x y width height bg))
+  ;; On simple backend, background fill is a no-op
+  (values))
+
+(defmethod draw-link ((b simple-backend) x y string url
+                      &key fg bg)
+  (declare (ignore url fg bg))
+  (draw-text b x y string nil nil))
+
+(defmethod draw-ellipsis ((b simple-backend) x y width
+                          &key fg bg)
+  (declare (ignore x y width fg bg))
+  (backend-write b "..."))
diff --git a/backend/tests.lisp b/backend/tests.lisp
new file mode 100644
index 0000000..fd9b7f9
--- /dev/null
+++ b/backend/tests.lisp
@@ -0,0 +1,102 @@
+(defpackage :cl-tui-backend-test
+  (:use :cl :fiveam :cl-tui.backend)
+  (:export #:run-tests))
+(in-package :cl-tui-backend-test)
+
+(def-suite backend-suite :description "Backend protocol tests")
+(in-suite backend-suite)
+
+;; ── Simple Backend ──────────────────────────────────────────────
+
+(defun run-tests ()
+  "Run all backend tests."
+  (let ((result (run 'backend-suite)))
+    (fiveam:explain! result)
+    (uiop:quit 0)))
+
+(test simple-backend-lifecycle
+  "simple-backend can be created and shut down"
+  (let ((b (make-simple-backend)))
+    (is (typep b 'simple-backend))
+    (initialize-backend b)
+    (is (capable-p b :truecolor) nil "simple backend has no truecolor")
+    (shutdown-backend b)))
+
+(test simple-backend-draw-text
+  "simple-backend renders text at position, ignoring style"
+  (let ((b (make-simple-backend)))
+    (initialize-backend b)
+    (draw-text b 0 0 "hello" nil nil)
+    (shutdown-backend b)
+    (is-t t)))
+
+(test simple-backend-border-single
+  "simple-backend draws ASCII single border"
+  (let ((b (make-simple-backend)))
+    (initialize-backend b)
+    (draw-border b 0 0 10 5 :style :single)
+    (shutdown-backend b)
+    (is-t t)))
+
+(test simple-backend-border-rounded
+  "simple-backend falls back to straight edges for rounded"
+  (let ((b (make-simple-backend)))
+    (initialize-backend b)
+    (draw-border b 0 0 10 5 :style :rounded)
+    (shutdown-backend b)
+    (is-t t)))
+
+;; ── Backend Capabilities ───────────────────────────────────────
+
+(test capable-p-known-features
+  "capable-p returns nil for all features on simple-backend"
+  (let ((b (make-simple-backend)))
+    (initialize-backend b)
+    (dolist (f '(:truecolor :osc8 :sync :mouse :bracketed-paste
+                 :kitty-keyboard :sixel :cursor-style))
+      (is (capable-p b f) nil
+          (format nil "~s should not be supported on simple-backend" f)))
+    (shutdown-backend b)))
+
+;; ── Backend Size ───────────────────────────────────────────────
+
+(test backend-size-returns-integers
+  "backend-size returns two integer values"
+  (let ((b (make-simple-backend)))
+    (initialize-backend b)
+    (multiple-value-bind (cols lines) (backend-size b)
+      (is (integerp cols))
+      (is (integerp lines))
+      (is (>= cols 10))
+      (is (>= lines 3)))
+    (shutdown-backend b)))
+
+;; ── Drawing Primitives ─────────────────────────────────────────
+
+(test draw-rect-fills-area
+  "draw-rect fills a rectangular area with background"
+  (let ((b (make-simple-backend)))
+    (initialize-backend b)
+    (draw-rect b 0 0 5 3 :bg nil)
+    (shutdown-backend b)
+    (is-t t)))
+
+(test draw-text-multi-line
+  "draw-text handles strings with newlines"
+  (let ((b (make-simple-backend)))
+    (initialize-backend b)
+    (draw-text b 0 0 "line1~%line2" nil nil)
+    (shutdown-backend b)
+    (is-t t)))
+
+;; ── Synchronization ────────────────────────────────────────────
+
+(test sync-is-noop-on-simple
+  "begin-sync and end-sync are no-ops on simple-backend"
+  (let ((b (make-simple-backend)))
+    (initialize-backend b)
+    (begin-sync b)
+    (draw-text b 0 0 "in sync" nil nil)
+    (end-sync b)
+    (shutdown-backend b)
+    (is-t t)))
diff --git a/cl-tui.asd b/cl-tui.asd
new file mode 100644
index 0000000..83034d1
--- /dev/null
+++ b/cl-tui.asd
@@ -0,0 +1,24 @@
+;;; cl-tui.asd — Common Lisp Terminal UI Framework
+(asdf:defsystem :cl-tui
+  :description "Reusable Common Lisp Terminal UI Framework"
+  :author "Amr Gharbeia"
+  :version "0.0.1"
+  :license "TBD"
+  :depends-on (:fiveam)
+  :components
+  ((:module "backend"
+    :components
+    ((:file "package")
+     (:file "classes" :depends-on ("package"))
+     (:file "simple" :depends-on ("package" "classes")))))
+  :in-order-to ((test-op (test-op :cl-tui-tests))))
+
+(asdf:defsystem :cl-tui-tests
+  :description "Test suite for cl-tui"
+  :depends-on (:cl-tui :fiveam)
+  :components
+  ((:module "backend"
+    :components
+    ((:file "tests"))))
+  :perform (test-op (o c)
+             (uiop:symbol-call :cl-tui-backend-test '#:run!)))
diff --git a/docs/ARCHITECTURE.org b/docs/ARCHITECTURE.org
new file mode 100644
index 0000000..7234f63
--- /dev/null
+++ b/docs/ARCHITECTURE.org
@@ -0,0 +1,318 @@
+#+TITLE: cl-tui Architecture
+#+STARTUP: content
+#+FILETAGS: :project:cl-tui:architecture:
+
+* Architecture
+
+cl-tui is a layered framework. Each layer has a single responsibility
+and communicates with adjacent layers through a well-defined protocol.
+
+** Layer Diagram
+
+#+BEGIN_SRC
+  Application Code (user's CL project)
+  ┌───────────────────────────────────────────────┐
+  │  Component Tree                               │
+  │  (user constructs via macros: vbox, hbox,     │
+  │   text, box, select, markdown, etc.)          │
+  └──────────────┬────────────────────────────────┘
+                 │ defgeneric render (component backend)
+                 │ defgeneric handle-key (component event)
+                 │ defgeneric handle-mouse (component event)
+                 ▼
+  ┌───────────────────────────────────────────────┐
+  │  Rendering Pipeline                           │
+  │  1. Layout pass (constraint solve)            │
+  │  2. Dirty walk (only changed branches)        │
+  │  3. Render commands (component → cmds)        │
+  │  4. Framebuffer diff (changed cells only)     │
+  └──────────────┬────────────────────────────────┘
+                 │ Render commands:
+                 │   (:box x y w h style)
+                 │   (:text x y str fg bg attrs)
+                 │   (:rect x y w h ch)
+                 ▼
+  ┌───────────────────────────────────────────────┐
+  │  Backend Protocol                             │
+  │  ┌─────────────┐  ┌─────────────────────────┐ │
+  │  │ modern      │  │ simple                  │ │
+  │  │ truecolor   │  │ ASCII borders           │ │
+  │  │ rounded     │  │ no color                │ │
+  │  │ OSC 8 links │  │ universal compatibility │ │
+  │  │ DECICM sync │  │ SSH-safe                │ │
+  │  │ kitty proto │  │ pipe-safe               │ │
+  │  └─────────────┘  └─────────────────────────┘ │
+  └───────────────────────────────────────────────┘
+#+END_SRC
+
+** The Backend Protocol
+
+The backend protocol is the central abstraction. Every rendering
+operation is a generic function dispatched on the backend class.
+
+*** Backend Classes
+
+- =modern-backend= — raw escape sequences, truecolor, modern features
+- =simple-backend= — ASCII art, no color, universal compatibility
+- =backend= — abstract base (both inherit from this)
+
+Backend selection happens once at startup, via terminal capability
+detection. The same component tree renders correctly on both.
+
+*** Backend Generic Functions
+
+#+BEGIN_SRC
+  ;; ── Lifecycle ──
+  (initialize-backend backend)        → setup terminal, enable features
+  (shutdown-backend backend)          → restore terminal, cleanup
+  (suspend-backend backend)           → temporary suspend (SIGTSTP)
+  (resume-backend backend)            → re-initialize after resume
+
+  ;; ── Output ──
+  (backend-size backend)              → (values columns lines)
+  (backend-write backend string)      → raw output to terminal
+  (begin-sync backend)                → start synchronized update
+  (end-sync backend)                  → flush synchronized update
+  (backend-clear backend)             → clear entire screen
+
+  ;; ── Drawing primitives ──
+  (draw-rect backend x y w h ch style)           → fill rectangle
+  (draw-text backend x y str fg bg attrs)        → render text at position
+  (draw-border backend x y w h style attrs)      → draw border rectangle
+  (draw-ellipsis backend x y w)                  → truncated text marker
+  (draw-link backend x y str url fg bg attrs)    → OSC 8 hyperlink
+
+  ;; ── Cursor ──
+  (cursor-move backend x y)            → position cursor
+  (cursor-hide backend)                → hide cursor
+  (cursor-show backend)                → show cursor
+  (cursor-style backend :bar|:block|:underline &optional blink)
+
+  ;; ── Input ──
+  (read-event backend)                 → (values event-type event-data)
+  (enable-mouse backend)               → enable SGR mouse reporting
+  (enable-bracketed-paste backend)     → enable paste detection
+  (set-keyboard-mode backend :kitty|:default)
+
+  ;; ── Capability queries ──
+  (capable-p backend :truecolor|:osc8|:kitty-keyboard|:sync|:sixel|:mouse)
+#+END_SRC
+
+*** Style structure
+
+All drawing functions accept a =style= plist that is resolved through
+the theme engine before reaching the backend:
+
+#+BEGIN_SRC
+  (:fg :error                     ; semantic role from theme
+   :bg :background-panel          ; resolved to hex by theme
+   :bold t
+   :italic nil
+   :underline nil
+   :blink nil
+   :reverse nil
+   :dim nil
+   :hyperlink-url nil)            ; OSC 8 URL if set
+#+END_SRC
+
+The backend receives resolved hex colors, not semantic roles. Theme
+resolution happens in the pipeline layer, before backend dispatch.
+
+*** Backend Selection
+
+At startup:
+
+#+BEGIN_SRC
+  1. Check if stdout is a TTY (if not → simple-backend)
+  2. Send DA1 query: ESC [ c
+     - No response within 100ms → simple-backend
+     - Response parsed → check for modern features
+  3. Try DA3 (secondary device attributes):
+     - Kitty reports "Kitty" + protocol version
+     - WezTerm reports "WezTerm"
+     - iTerm2 reports specific codes
+  4. Query DECRPM for DECICM sync:
+     - ESC [?2026$p
+     - Response ESC [?2026;1$y = supported
+  5. If sync + truecolor + kitty keyboard → modern-backend
+     Otherwise → simple-backend
+#+END_SRC
+
+** Layout Engine
+
+The layout engine is pure Common Lisp — no Yoga FFI, no C dependencies.
+
+*** Constraint Model
+
+A terminal has ~200x80 cells. The constraint solver only needs to
+handle one-dimensional layout in two passes:
+
+1. **Column direction (vertical pass):** distribute Y positions, sum
+   children heights. Width is inherited from parent (minus padding).
+2. **Row direction (horizontal pass):** distribute X positions, sum
+   children widths. Height is inherited from parent.
+
+Flex properties:
+- =:grow= — proportional distribution of remaining space
+- =:shrink= — proportional reduction when content overflows
+- =:basis= — initial size before grow/shrink
+- =:wrap= — overflow moves to next row/column
+- =:gap= — spacing between children
+
+Position properties:
+- =:relative= — normal flow (default)
+- =:absolute= — positioned relative to parent's content box
+- =:top=, =:right=, =:bottom=, =:left= — offset for absolute
+
+This is a subset of CSS Flexbox. Enough for every TUI layout pattern
+(sidebar + content, toolbar + main + status, dialog overlay, tab
+navigation, split panes). ~200 lines.
+
+*** Layout Node
+
+#+BEGIN_SRC
+  (defclass layout-node ()
+    ;; Computed by solver
+    (x y width height             ; computed position + size
+     children                     ; list of child layout-nodes
+     parent                       ; parent layout-node (or nil for root)
+     ;; Style input
+     direction                    ; :row | :column | :row-reverse | :column-reverse
+     wrap                         ; :nowrap | :wrap | :wrap-reverse
+     grow shrink basis            ; flex sizing
+     align-self align-items       ; cross-axis alignment
+     justify-content              ; main-axis distribution
+     padding margin border        ; box model
+     gap                          ; spacing between children
+     position-type                ; :relative | :absolute
+     position-offset))            ; top/left for absolute
+#+END_SRC
+
+*** Composable API
+
+#+BEGIN_SRC
+  (vbox (:gap 1 :padding 1)
+    (header "Title")
+    (hbox (:grow 1)
+      (sidebar (:width 30) ...)
+      (content ...)))
+#+END_SRC
+
+Macros expand to layout-node construction + child wiring.
+
+** Component Tree
+
+Components are CLOS objects. Every component has a =layout-node=
+slot that drives positioning. Components define =render= methods.
+
+*** Base Component Class
+
+#+BEGIN_SRC
+  (defclass component ()
+    (layout-node         ; layout-node for this component
+     parent              ; parent component (or nil for root)
+     children            ; list of child components
+     dirty               ; t/nil — needs re-render
+     theme               ; theme reference
+     visible))           ; t/nil
+#+END_SRC
+
+*** Generic Functions
+
+- =(render component backend)= — returns list of render commands
+- =(handle-key component event)= — returns t if consumed
+- =(handle-mouse component event)= — returns t if consumed
+- =(measure component max-width max-height)= — returns (values w h)
+- =(children component)= — returns list of child components
+- =(find-focused component)= — returns the focused child (or nil)
+
+*** Rendering Pipeline
+
+#+BEGIN_SRC
+  1. (propagate-dirty root)     → mark ancestors dirty
+  2. (compute-layout root w h)  → pure CL constraint solve
+  3. (collect-commands root)     → walk dirty branches, call render
+  4. (diff-framebuffer prev curr) → emit only changed cells
+  5. (begin-sync backend)       → DECICM start
+  6. (flush-commands backend)   → write escape sequences
+  7. (end-sync backend)         → DECICM end
+  8. (clear-dirty root)         → mark all clean
+#+END_SRC
+
+** Input Handling
+
+Input goes through a layered keybinding system:
+
+1. Terminal emits escape sequences → parser converts to events
+2. Events dispatched through layers: =:global= → =:local= → =:focused=
+3. Focused component's =handle-key= fires first
+4. Unconsumed events bubble to =:local= keymap, then =:global=
+5. Modal layers (dialog) intercept before global
+
+Mouse events follow the same path, with hit-testing routing to the
+deepest component containing the click coordinates.
+
+** Theme Engine
+
+Semantic tokens → hex colors → backend color pairs. No code references
+hex values directly. =:accent= resolves to gold in default theme, blue
+in nord, green in gruvbox, depending on which preset is active.
+
+Presets define both =:dark= and =:light= variants. Auto-detection
+reads terminal background color at startup.
+
+** File Structure
+
+#+BEGIN_SRC
+  cl-tui/
+  ├── cl-tui.asd
+  ├── cl-tui-tests.asd
+  ├── README.org
+  ├── LICENSE
+  ├── docs/
+  │   ├── ROADMAP.org
+  │   └── ARCHITECTURE.org          ← this file
+  ├── src/
+  │   ├── package.lisp
+  │   ├── backend/
+  │   │   ├── protocol.lisp
+  │   │   ├── detection.lisp
+  │   │   ├── simple.lisp
+  │   │   └── modern.lisp
+  │   ├── layout/
+  │   │   ├── nodes.lisp
+  │   │   ├── solver.lisp
+  │   │   └── api.lisp
+  │   ├── components/
+  │   │   ├── base.lisp
+  │   │   ├── box.lisp
+  │   │   └── text.lisp
+  │   ├── rendering/
+  │   │   ├── pipeline.lisp
+  │   │   ├── dirty.lisp
+  │   │   └── diff.lisp
+  │   └── theme/
+  │       ├── tokens.lisp
+  │       └── presets.lisp
+  └── tests/
+      ├── package.lisp
+      ├── backend.lisp
+      ├── layout.lisp
+      └── components.lisp
+#+END_SRC
+
+** Dependency Graph
+
+  backend/     (no deps)
+  layout/      (no deps — pure math)
+  theme/       (backend for color resolution)
+  components/  (layout, theme, rendering)
+  rendering/   (layout, components, backend, theme)
+  input/       (backend for raw events)
+
+Init order:
+  1. Backend — detect, initialize
+  2. Theme — load default preset
+  3. Layout — construct component tree
+  4. Render — layout → commands → flush
+  5. Input — event loop (blocks on read-event)
diff --git a/docs/ROADMAP.org b/docs/ROADMAP.org
index 1d1f0e5..c9d2bf0 100644
--- a/docs/ROADMAP.org
+++ b/docs/ROADMAP.org
@@ -5,10 +5,81 @@
 * The Roadmap
 
 Each phase is one minor release. Phases ship in dependency order — each depends on
-the components from prior phases. The layout engine ships first because everything
-else builds on it.
+the components from prior phases. The backend protocol ships first because
+everything else builds on it.
 
-Feature releases increment the minor version (v0.X.0). Bugfix releases increment
+** v0.0.1: Foundation — Backend Protocol
+
+The abstraction layer that makes everything portable. Two backends:
+=modern= (raw escape sequences, truecolor, modern features) and =simple=
+(ASCII art, universal compatibility). The component tree never touches
+the terminal directly — it dispatches through the protocol.
+
+*** TODO Backend protocol definition
+:PROPERTIES:
+:ID:       id-v000-protocol
+:CREATED:  [2026-05-10 Sat]
+:END:
+
+- Define =backend= abstract class with generic functions:
+  - =initialize-backend=, =shutdown-backend=, =suspend-backend=, =resume-backend=
+  - =backend-size=, =backend-write=, =backend-clear=
+  - =begin-sync=, =end-sync= — DECICM synchronized updates
+  - =draw-rect=, =draw-text=, =draw-border=, =draw-ellipsis=, =draw-link=
+  - =cursor-move=, =cursor-hide=, =cursor-show=, =cursor-style=
+  - =read-event=, =enable-mouse=, =enable-bracketed-paste=, =set-keyboard-mode=
+  - =capable-p= — query feature support
+- Style plist structure: ~(:fg :error :bg :background-panel :bold t :italic nil ...)~
+- ~100 lines
+
+*** TODO Simple backend
+:PROPERTIES:
+:ID:       id-v000-simple
+:CREATED:  [2026-05-10 Sat]
+:END:
+
+- =simple-backend= class — inherits =backend=
+- Borders: ASCII (~+-|~), no rounded corners
+- No color, no bold/italic — plain characters only
+- No OSC 8 links, no mouse, no synchronized updates
+- Works on any terminal, any SSH connection, piped output
+- ~100 lines
+
+*** TODO Modern backend
+:PROPERTIES:
+:ID:       id-v000-modern
+:CREATED:  [2026-05-10 Sat]
+:END:
+
+- =modern-backend= class — inherits =backend=
+- Truecolor 24-bit foreground/background
+- Rounded, single, double border styles via Unicode box-drawing
+- OSC 8 hyperlinks (clickable URLs)
+- DECICM synchronized updates (flicker-free)
+- SGR mouse tracking + kitty keyboard protocol
+- Bracketed paste detection
+- Bold, italic, underline, dim, blink, reverse, strikethrough
+- Cursor style: =:bar=, =:block=, =:underline=, with blink option
+- ~250 lines
+
+*** TODO Terminal capability detection
+:PROPERTIES:
+:ID:       id-v000-detection
+:CREATED:  [2026-05-10 Sat]
+:END:
+
+- =detect-backend= → returns =modern-backend= or =simple-backend=
+- Check if stdout is a TTY (if not → =simple-backend=)
+- Send DA1 (~ESC[c~) query, 100ms timeout
+- Send DA3 (~ESC[?c~) for kitty/wezterm identification
+- Query DECRPM (~ESC[?2026$p~) for DECICM sync support
+- Query truecolor support via =COLORTERM= env var + DA response
+- Cache detection result so subsequent calls are instant
+- ~100 lines
+
+~550 lines total. Dependencies: None (pure CL, no FFI, no external libs).
+
+** v0.0.2: Layout Engine
 the patch version (v0.X.Y).
 
 ** File Update Checklist
diff --git a/org/backend-protocol.org b/org/backend-protocol.org
new file mode 100644
index 0000000..f1830fe
--- /dev/null
+++ b/org/backend-protocol.org
@@ -0,0 +1,382 @@
+#+TITLE: cl-tui Backend Protocol — v0.0.1
+#+STARTUP: content
+#+FILETAGS: :cl-tui:backend:v0.0.1:
+#+OPTIONS: ^:nil
+
+* Backend Protocol
+
+The backend protocol is the rendering abstraction layer. Every visual
+operation dispatches through generic functions on a backend class.
+Two implementations exist: =modern-backend= (raw escape sequences,
+truecolor, modern terminal features) and =simple-backend= (ASCII art,
+universal compatibility).
+
+** Contract
+
+*** Backend Lifecycle
+
+- =(initialize-backend backend)= → backend
+  Initialize the terminal, set raw mode, enable features.
+  Returns the backend instance.
+
+- =(shutdown-backend backend)= → nil
+  Restore terminal to cooked mode, reset colors, show cursor.
+  Must be called on exit regardless of how the image stops.
+
+- =(backend-size backend)= → (values columns lines integer integer)
+  Return terminal dimensions. First value = columns, second = lines.
+
+- =(backend-write backend string)= → integer
+  Write raw string to terminal output. Returns number of bytes written.
+
+- =(backend-clear backend)= → nil
+  Clear the entire screen and reset cursor to (0,0).
+
+*** Rendering Primitives
+
+- =(draw-text backend x y string fg bg &key bold italic underline reverse dim blink)= → nil
+  Render text at position (x, y). fg and bg are hex color strings
+  (e.g. "#FFD700") or nil for default. Attributes are booleans.
+
+- =(draw-border backend x y width height &key style fg bg title title-align)= → nil
+  Draw a border rectangle. Style is :single, :double, or :rounded.
+
+- =(draw-rect backend x y width height &key bg)= → nil
+  Fill a rectangle with background color.
+
+- =(draw-link backend x y string url &key fg bg)= → nil
+  Render clickable hyperlink (OSC 8 escape sequence).
+
+- =(draw-ellipsis backend x y width &key fg bg)= → nil
+  Render "..." truncated text marker at position.
+
+*** Cursor Operations
+
+- =(cursor-move backend x y)= → nil
+  Move cursor to position (x, y). Origin is top-left (0,0).
+
+- =(cursor-hide backend)= → nil
+- =(cursor-show backend)= → nil
+
+- =(cursor-style backend shape &key blink)= → nil
+  shape is :block, :bar, or :underline.
+
+*** Synchronization
+
+- =(begin-sync backend)= → nil
+  Start synchronized update (DECICM). All subsequent output is buffered
+  by the terminal until =end-sync=.
+
+- =(end-sync backend)= → nil
+  Flush synchronized update buffer. The entire frame appears at once.
+
+*** Input
+
+- =(read-event backend &key timeout)= → (values keyword list)
+  Read next input event. Blocks until event or timeout.
+  Returns event type keyword and event data plist.
+
+- =(enable-mouse backend)= → nil
+  Enable SGR mouse tracking (press, release, drag, scroll).
+
+- =(enable-bracketed-paste backend)= → nil
+  Enable bracketed paste mode.
+
+*** Capability Queries
+
+- =(capable-p backend feature)= → boolean
+  Feature is :truecolor, :osc8, :sync, :mouse, :bracketed-paste,
+  :kitty-keyboard, :sixel, :cursor-style.
+
+** Backend Classes
+
+*** Simple Backend
+
+=(make-simple-backend)= → simple-backend
+
+The minimal backend. ASCII borders, no color, no modern features.
+Works everywhere — SSH, serial, pipes, ancient terminals.
+
+Borders:
+- Single: + - |
+- Double: + = |
+- Rounded: + - |  (same as single — no rounded chars)
+
+No color, no bold, no italic, no links, no mouse, no sync.
+
+*** Modern Backend
+
+=(make-modern-backend)= → modern-backend
+
+Full-featured backend. Truecolor, Unicode box-drawing, OSC 8 links,
+DECICM sync, mouse tracking, kitty keyboard protocol.
+
+Borders:
+- Single: ┌ ─ ┐ │ └ ┘
+- Double: ╔ ═ ╗ ║ ╚ ╝
+- Rounded: ╭ ─ ╮ │ ╰ ╯
+
+** Test Suite
+
+#+BEGIN_SRC lisp
+(defpackage :cl-tui-backend-test
+  (:use :cl :fiveam)
+  (:export #:run!))
+(in-package :cl-tui-backend-test)
+
+(def-suite backend-suite :description "Backend protocol tests")
+(in-suite backend-suite)
+
+;; ── Simple Backend ──────────────────────────────────────────────
+
+(test simple-backend-lifecycle
+  "simple-backend can be created and shut down"
+  (let ((b (make-simple-backend)))
+    (is (typep b 'simple-backend))
+    (initialize-backend b)
+    (is (capable-p b :truecolor) nil "simple backend has no truecolor")
+    (shutdown-backend b)))
+
+(test simple-backend-draw-text
+  "simple-backend renders text at position, ignoring style"
+  (let ((b (make-simple-backend)))
+    (initialize-backend b)
+    (draw-text b 0 0 "hello" nil nil)
+    ;; No crash = pass (simple backend writes to *standard-output*)
+    (shutdown-backend b)
+    (is-t t)))
+
+(test simple-backend-border-single
+  "simple-backend draws ASCII single border"
+  (let ((b (make-simple-backend)))
+    (initialize-backend b)
+    (draw-border b 0 0 10 5 :style :single)
+    (shutdown-backend b)
+    (is-t t)))
+
+(test simple-backend-border-rounded
+  "simple-backend falls back to straight edges for rounded"
+  (let ((b (make-simple-backend)))
+    (initialize-backend b)
+    (draw-border b 0 0 10 5 :style :rounded)
+    ;; No error — rounded falls back to single on simple
+    (shutdown-backend b)
+    (is-t t)))
+
+;; ── Backend Capabilities ───────────────────────────────────────
+
+(test capable-p-known-features
+  "capable-p returns nil for all features on simple-backend"
+  (let ((b (make-simple-backend)))
+    (initialize-backend b)
+    (dolist (f '(:truecolor :osc8 :sync :mouse :bracketed-paste
+                 :kitty-keyboard :sixel :cursor-style))
+      (is (capable-p b f) nil
+          (format nil "~s should not be supported on simple-backend" f)))
+    (shutdown-backend b)))
+
+;; ── Backend Size ───────────────────────────────────────────────
+
+(test backend-size-returns-integers
+  "backend-size returns two integer values"
+  (let ((b (make-simple-backend)))
+    (initialize-backend b)
+    (multiple-value-bind (cols lines) (backend-size b)
+      (is (integerp cols))
+      (is (integerp lines))
+      (is (>= cols 10))
+      (is (>= lines 3)))
+    (shutdown-backend b)))
+
+;; ── Drawing Primitives ─────────────────────────────────────────
+
+(test draw-rect-fills-area
+  "draw-rect fills a rectangular area with background"
+  (let ((b (make-simple-backend)))
+    (initialize-backend b)
+    (draw-rect b 0 0 5 3 :bg nil)
+    (shutdown-backend b)
+    (is-t t)))
+
+(test draw-text-multi-line
+  "draw-text handles strings with newlines"
+  (let ((b (make-simple-backend)))
+    (initialize-backend b)
+    (draw-text b 0 0 "line1~%line2" nil nil)
+    (shutdown-backend b)
+    (is-t t)))
+
+;; ── Synchronization ────────────────────────────────────────────
+
+(test sync-is-noop-on-simple
+  "begin-sync and end-sync are no-ops on simple-backend"
+  (let ((b (make-simple-backend)))
+    (initialize-backend b)
+    (begin-sync b)
+    (draw-text b 0 0 "in sync" nil nil)
+    (end-sync b)
+    (shutdown-backend b)
+    (is-t t)))
+#+END_SRC
+
+** Implementation
+
+*** Package
+
+#+BEGIN_SRC lisp
+(defpackage :cl-tui.backend
+  (:use :cl)
+  (:export
+   ;; Backend classes
+   #:backend #:simple-backend
+   ;; Lifecycle
+   #:initialize-backend #:shutdown-backend
+   #:backend-size #:backend-write #:backend-clear
+   ;; Drawing
+   #:draw-text #:draw-border #:draw-rect
+   #:draw-link #:draw-ellipsis
+   ;; Cursor
+   #:cursor-move #:cursor-hide #:cursor-show #:cursor-style
+   ;; Sync
+   #:begin-sync #:end-sync
+   ;; Input
+   #:read-event #:enable-mouse #:enable-bracketed-paste
+   ;; Queries
+   #:capable-p
+   ;; Constructors
+   #:make-simple-backend))
+(in-package :cl-tui.backend)
+#+END_SRC
+
+*** Backend Base Class
+
+#+BEGIN_SRC lisp
+(defclass backend () ())
+
+(defgeneric initialize-backend (backend)
+  (:method ((b backend)) b))
+
+(defgeneric shutdown-backend (backend)
+  (:method ((b backend)) (values)))
+
+(defgeneric backend-size (backend)
+  (:method ((b backend))
+    (values 80 24)))
+
+(defgeneric backend-write (backend string))
+
+(defgeneric backend-clear (backend)
+  (:method ((b backend))
+    (backend-write b (string #\escape) "[2J")
+    (cursor-move b 0 0)))
+
+(defgeneric draw-text (backend x y string fg bg &key
+                       bold italic underline reverse dim blink))
+
+(defgeneric draw-border (backend x y width height
+                         &key style fg bg title title-align))
+
+(defgeneric draw-rect (backend x y width height &key bg))
+
+(defgeneric draw-link (backend x y string url &key fg bg))
+
+(defgeneric draw-ellipsis (backend x y width &key fg bg))
+
+(defgeneric cursor-move (backend x y))
+
+(defgeneric cursor-hide (backend)
+  (:method ((b backend)) (values)))
+
+(defgeneric cursor-show (backend)
+  (:method ((b backend)) (values)))
+
+(defgeneric cursor-style (backend shape &key blink)
+  (:method ((b backend) shape &key blink) (values)))
+
+(defgeneric begin-sync (backend)
+  (:method ((b backend)) (values)))
+
+(defgeneric end-sync (backend)
+  (:method ((b backend)) (values)))
+
+(defgeneric read-event (backend &key timeout)
+  (:method ((b backend) &key timeout) (values nil nil)))
+
+(defgeneric enable-mouse (backend)
+  (:method ((b backend)) (values)))
+
+(defgeneric enable-bracketed-paste (backend)
+  (:method ((b backend)) (values)))
+
+(defgeneric capable-p (backend feature)
+  (:method ((b backend) feature)
+    (declare (ignore feature))
+    nil))
+#+END_SRC
+
+*** Simple Backend
+
+#+BEGIN_SRC lisp
+(defclass simple-backend (backend)
+  ((output-stream :initform *standard-output*
+                  :accessor backend-output-stream)))
+
+(defmethod initialize-backend ((b simple-backend))
+  b)
+
+(defmethod shutdown-backend ((b simple-backend))
+  (values))
+
+(defmethod backend-size ((b simple-backend))
+  ;; Try ioctl, fall back to 80x24
+  (values 80 24))
+
+(defmethod backend-write ((b simple-backend) string)
+  (let ((stream (backend-output-stream b)))
+    (write-string string stream)
+    (finish-output stream)
+    (length string)))
+
+(defmethod draw-text ((b simple-backend) x y string fg bg
+                      &key bold italic underline reverse dim blink)
+  (declare (ignore x y fg bg bold italic underline reverse dim blink))
+  (backend-write b string))
+
+(defun %simple-border-char (edge-style pos)
+  "Return ASCII border character for EDGE-STYLE at POS.
+POS is :top-left, :top-right, :bottom-left, :bottom-right,
+:horizontal, or :vertical."
+  (case pos
+    ((:top-left :top-right :bottom-left :bottom-right) #\+)
+    (:horizontal #\-)
+    (:vertical #\|)))
+
+(defmethod draw-border ((b simple-backend) x y width height
+                        &key style fg bg title title-align)
+  (declare (ignore style fg bg title title-align))
+  (let ((h (%simple-border-char nil :horizontal))
+        (v (%simple-border-char nil :vertical)))
+    ;; Top edge
+    (backend-write b (format nil "~%~v@{~a~:*~}" width h))
+    ;; Sides
+    (loop for i from 1 below (1- height)
+          do (backend-write b (format nil "~%|~v@{~a~:*~}|" (- width 2) #\space)))
+    ;; Bottom edge
+    (backend-write b (format nil "~%~v@{~a~:*~}" width h))))
+
+(defmethod draw-rect ((b simple-backend) x y width height
+                      &key bg)
+  (declare (ignore x y width height bg))
+  ;; On simple backend, background fill is a no-op
+  (values))
+
+(defmethod draw-link ((b simple-backend) x y string url
+                      &key fg bg)
+  (declare (ignore url fg bg))
+  (draw-text b x y string nil nil))
+
+(defmethod draw-ellipsis ((b simple-backend) x y width
+                          &key fg bg)
+  (declare (ignore x y width fg bg))
+  (backend-write b "..."))
+#+END_SRC