diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..d89fdd1
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,53 @@
+.PHONY: test test-short test-cover build install clean help
+
+SHELL := /bin/bash
+
+# Versioning (override on command line or via env for releases)
+VERSION ?= dev-$(shell git describe --tags --always --dirty 2>/dev/null || echo unknown)
+COMMIT ?= $(shell git rev-parse --short HEAD 2>/dev/null || echo unknown)
+DATE ?= $(shell date -u +%Y-%m-%dT%H:%M:%SZ 2>/dev/null || echo unknown)
+
+MODULE = github.com/gmgauthier/gralculator
+LDFLAGS = -s -w \
+	-X '$(MODULE)/internal/version.Version=$(VERSION)' \
+	-X '$(MODULE)/internal/version.Commit=$(COMMIT)' \
+	-X '$(MODULE)/internal/version.BuildDate=$(DATE)' \
+	-X 'main.version=$(VERSION)'
+
+test: deps
+	go test ./... -v -race
+
+test-short: deps
+	go test -short ./... -v -race
+
+test-cover: deps
+	@mkdir -p build
+	go test ./... -coverprofile=build/coverage.out
+	go tool cover -html=build/coverage.out -o build/coverage.html
+	@echo "✅ Coverage report: open build/coverage.html in your browser"
+
+deps:
+	go mod download
+
+build: deps
+	@mkdir -p build
+	go build -trimpath -ldflags "$(LDFLAGS)" -o build/gralculator .
+	@echo "✅ Dev build: VERSION=$(VERSION) COMMIT=$(COMMIT) DATE=$(DATE)"
+	@build/gralculator -v || true
+
+install: build
+	mkdir -p ~/.local/bin
+	cp build/gralculator ~/.local/bin/gralculator
+	chmod +x ~/.local/bin/gralculator
+	@echo "✅ gralculator installed to ~/.local/bin/gralculator"
+
+clean:
+	rm -rf build/
+
+help:
+	@echo "Common targets:"
+	@echo "  make build       - build dev binary to build/gralculator"
+	@echo "  make install     - build and install to ~/.local/bin/gralculator"
+	@echo "  make test        - run all tests with race detector"
+	@echo "  make test-short  - short tests with race"
+	@echo "  make clean       - remove build/"
diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..0ec1a3d
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,8 @@
+module github.com/gmgauthier/gralculator
+
+go 1.24.2
+
+require (
+	github.com/charmbracelet/bubbletea v0.26.6
+	github.com/charmbracelet/lipgloss v0.11.0
+)
diff --git a/internal/calc/calc.go b/internal/calc/calc.go
new file mode 100644
index 0000000..2f0099c
--- /dev/null
+++ b/internal/calc/calc.go
@@ -0,0 +1,87 @@
+package calc
+
+import (
+	"errors"
+	"math"
+)
+
+// Base represents the current display formatting mode.
+type Base string
+
+const (
+	BaseDEC Base = "DEC"
+	BaseHEX Base = "HEX"
+	BaseBIN Base = "BIN"
+	BaseOCT Base = "OCT"
+)
+
+var basesCycle = []Base{BaseDEC, BaseHEX, BaseBIN, BaseOCT}
+
+// ErrConversionNotPossible is returned by CycleBase when the current value
+// has a fractional part and cannot be cleanly displayed in a non-DEC base.
+var ErrConversionNotPossible = errors.New("conversion not possible (CERR)")
+
+// Engine holds the calculator state. All math is decimal (float64).
+// Bases affect only formatting via FormatForDisplay and the small row.
+type Engine struct {
+	value     float64
+	base      Base
+	pendingOp string // "+", "-", "*", "/", "mod", or ""
+	pendingVal float64
+	// entry buffer / more state will be added in engine implementation
+}
+
+// NewEngine creates a fresh calculator engine starting in DEC.
+func NewEngine() *Engine {
+	return &Engine{
+		value: 0,
+		base:  BaseDEC,
+	}
+}
+
+// CurrentBase returns the active display base for the small row.
+func (e *Engine) CurrentBase() Base {
+	return e.base
+}
+
+// IsInteger reports whether the current value is effectively an integer
+// (within epsilon to tolerate fp noise from operations like 1/3).
+func (e *Engine) IsInteger() bool {
+	_, frac := math.Modf(e.value)
+	return math.Abs(frac) < 1e-10
+}
+
+// CycleBase advances to the next base in the cycle (DEC→HEX→BIN→OCT→DEC).
+// Returns ErrConversionNotPossible (and does not change base) if the value
+// has a fractional part. The caller (UI) should trigger a "CERR" flash.
+func (e *Engine) CycleBase() error {
+	if !e.IsInteger() {
+		return ErrConversionNotPossible
+	}
+	for i, b := range basesCycle {
+		if b == e.base {
+			e.base = basesCycle[(i+1)%len(basesCycle)]
+			return nil
+		}
+	}
+	// fallback
+	e.base = BaseDEC
+	return nil
+}
+
+// FormatForDisplay returns the string to show in the large number area
+// according to the current base. For MVP only integer values are expected
+// in non-DEC bases (callers should have checked via CycleBase or IsInteger).
+func (e *Engine) FormatForDisplay() string {
+	// Placeholder implementation. Real version in phase 2 will handle
+	// proper integer formatting for hex/bin/oct (no 0x prefix, etc.)
+	// and fall back gracefully.
+	if e.base == BaseDEC {
+		// simple for skeleton
+		return "0"
+	}
+	return "0"
+}
+
+// TODO (phase 2): Add EnterDigit, SetOperator, Equals, Mod, ClearEntry,
+// AllClear, ChangeSign, Backspace, etc. + full FormatForDisplay.
diff --git a/internal/calc/calc_test.go b/internal/calc/calc_test.go
new file mode 100644
index 0000000..75da8ab
--- /dev/null
+++ b/internal/calc/calc_test.go
@@ -0,0 +1,44 @@
+package calc
+
+import (
+	"errors"
+	"testing"
+)
+
+func TestNewEngine_DefaultsToDEC(t *testing.T) {
+	e := NewEngine()
+	if e.CurrentBase() != BaseDEC {
+		t.Errorf("expected DEC, got %s", e.CurrentBase())
+	}
+	if !e.IsInteger() {
+		t.Error("0 should be integer")
+	}
+}
+
+func TestCycleBase_IntegerOK(t *testing.T) {
+	e := NewEngine()
+	e.value = 42 // integer
+
+	if err := e.CycleBase(); err != nil {
+		t.Fatalf("unexpected error cycling from integer: %v", err)
+	}
+	if e.CurrentBase() != BaseHEX {
+		t.Errorf("expected HEX after one cycle, got %s", e.CurrentBase())
+	}
+}
+
+func TestCycleBase_FractionalCERR(t *testing.T) {
+	e := NewEngine()
+	e.value = 23.0 / 6.0 // 3.8333...
+
+	err := e.CycleBase()
+	if !errors.Is(err, ErrConversionNotPossible) {
+		t.Fatalf("expected ErrConversionNotPossible, got %v", err)
+	}
+	if e.CurrentBase() != BaseDEC {
+		t.Error("base must not change on CERR")
+	}
+}
+
+// TODO (phase 2): more tests for FormatForDisplay (hex/bin/oct of integers),
+// ops, clears, entry, combined fractional + multiple BASE presses, etc.
diff --git a/internal/ui/ui.go b/internal/ui/ui.go
new file mode 100644
index 0000000..551a352
--- /dev/null
+++ b/internal/ui/ui.go
@@ -0,0 +1,67 @@
+package ui
+
+import (
+	tea "github.com/charmbracelet/bubbletea"
+	"github.com/charmbracelet/lipgloss"
+
+	"github.com/gmgauthier/gralculator/internal/calc"
+)
+
+// App is the root Bubble Tea model for the gralculator TUI.
+// Phase 1/3 skeleton only — real implementation in phase 3.
+type App struct {
+	engine *calc.Engine
+	width  int
+	height int
+
+	// flash state etc. will be added during spike (see gostations patterns)
+}
+
+func NewApp() *App {
+	return &App{
+		engine: calc.NewEngine(),
+	}
+}
+
+func (a *App) Init() tea.Cmd {
+	return nil
+}
+
+func (a *App) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
+	switch msg := msg.(type) {
+	case tea.WindowSizeMsg:
+		a.width = msg.Width
+		a.height = msg.Height
+	case tea.KeyMsg:
+		switch msg.String() {
+		case "q", "ctrl+c", "esc":
+			return a, tea.Quit
+		case "tab":
+			// Will drive engine.CycleBase() + possible CERR flash
+			_ = a.engine.CycleBase()
+			// TODO (phase 3): trigger flash on error, re-render small row
+		}
+	}
+	return a, nil
+}
+
+func (a *App) View() string {
+	// Placeholder view. Real two-row display + keypad grid in phase 3.
+	// Will use lipgloss heavily for:
+	// - large number area (tall + styled)
+	// - small current-base row (highlighted label)
+	// - centered content card (lipgloss.Place)
+	// - button cells for keypad (including single "BASE")
+	// - flash styles (140ms color 63)
+	// - minimal hint row
+	base := a.engine.CurrentBase()
+	return lipgloss.NewStyle().Padding(1, 2).Render(
+		"gralculator skeleton\n\n" +
+			"Large number area (TODO)\n" +
+			"Current base: " + string(base) + "   (press Tab to cycle)\n\n" +
+			"See docs/UI_DESIGN.md and spec.md",
+	)
+}
+
+// TODO (phase 3): full model with flashes, proper display rendering,
+// keypad grid, Tab + other key handling wired to engine, resize/centering.
diff --git a/internal/version/version.go b/internal/version/version.go
new file mode 100644
index 0000000..4fcca69
--- /dev/null
+++ b/internal/version/version.go
@@ -0,0 +1,17 @@
+package version
+
+// These vars are set at build time via -ldflags (see Makefile), e.g.
+// -ldflags "-X github.com/gmgauthier/gralculator/internal/version.Version=0.1.0 -X .../Commit=... -X .../BuildDate=..."
+var (
+	Version   = "dev"
+	Commit    = ""
+	BuildDate = ""
+)
+
+// String returns a human-friendly version string.
+func String() string {
+	if Commit != "" {
+		return Version + "-" + Commit
+	}
+	return Version
+}
diff --git a/main.go b/main.go
new file mode 100644
index 0000000..56014ce
--- /dev/null
+++ b/main.go
@@ -0,0 +1,32 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"os"
+
+	"github.com/gmgauthier/gralculator/internal/version"
+)
+
+var versionFlag bool
+
+func init() {
+	flag.BoolVar(&versionFlag, "v", false, "print version and exit")
+	flag.BoolVar(&versionFlag, "version", false, "print version and exit")
+}
+
+func main() {
+	flag.Parse()
+
+	if versionFlag {
+		fmt.Println(version.String())
+		return
+	}
+
+	// TODO (phase 3): wire up tea.NewProgram with ui model
+	fmt.Println("gralculator (skeleton)")
+	fmt.Println("Version:", version.String())
+	fmt.Println("Tab will eventually cycle bases (DEC/HEX/BIN/OCT).")
+	fmt.Println("See docs/ and spec.md for design.")
+	os.Exit(0)
+}