diff --git a/src/main.zig b/src/main.zig
index 11f68ca..8b8caf6 100644
--- a/src/main.zig
+++ b/src/main.zig
@@ -1,4 +1,5 @@
 const std = @import("std");
+const session = @import("session.zig");
 
 pub const version = "0.1.0-dev";
 
@@ -62,6 +63,10 @@ pub fn main(init: std.process.Init) !u8 {
     return 0;
 }
 
+test {
+    _ = session;
+}
+
 test "regular: help text names the binary and smoke boundary" {
     try std.testing.expect(std.mem.indexOf(u8, help_text, "mim - mobile-first terminal code editor") != null);
     try std.testing.expect(std.mem.indexOf(u8, help_text, "canonical smoke path") != null);
diff --git a/src/session.zig b/src/session.zig
new file mode 100644
index 0000000..8ff9f1a
--- /dev/null
+++ b/src/session.zig
@@ -0,0 +1,267 @@
+const std = @import("std");
+
+// Headless editor state for the protocol-first core.
+// req: coding/001, session/003
+test {
+    _ = Buffer;
+    _ = Session;
+}
+
+pub const Cursor = struct {
+    byte: usize = 0,
+    cell: usize = 0,
+};
+
+pub const Selection = struct {
+    anchor: usize,
+    cursor: usize,
+};
+
+pub const Command = union(enum) {
+    move_left,
+    move_right,
+    insert: []const u8,
+    delete_backward,
+};
+
+pub const Snapshot = struct {
+    bytes: []const u8,
+    cursor_byte: usize,
+    cursor_cell: usize,
+    selection: ?Selection,
+};
+
+pub const Buffer = struct {
+    allocator: std.mem.Allocator,
+    bytes: std.ArrayList(u8),
+    cursor: Cursor = .{},
+    selection: ?Selection = null,
+
+    pub fn openFromBytes(allocator: std.mem.Allocator, fixture: []const u8) !Buffer {
+        var bytes = std.ArrayList(u8).empty;
+        try bytes.appendSlice(allocator, fixture);
+        return .{ .allocator = allocator, .bytes = bytes };
+    }
+
+    pub fn deinit(self: *Buffer) void {
+        self.bytes.deinit(self.allocator);
+        self.* = undefined;
+    }
+
+    pub fn snapshot(self: *const Buffer) Snapshot {
+        return .{
+            .bytes = self.bytes.items,
+            .cursor_byte = self.cursor.byte,
+            .cursor_cell = self.cursor.cell,
+            .selection = self.selection,
+        };
+    }
+
+    pub fn dispatch(self: *Buffer, command: Command) !void {
+        switch (command) {
+            .move_left => self.moveLeft(),
+            .move_right => self.moveRight(),
+            .insert => |text| try self.insert(text),
+            .delete_backward => self.deleteBackward(),
+        }
+    }
+
+    pub fn moveLeft(self: *Buffer) void {
+        self.cursor.byte = previousBoundary(self.bytes.items, self.cursor.byte);
+        self.refreshCell();
+    }
+
+    pub fn moveRight(self: *Buffer) void {
+        self.cursor.byte = nextBoundary(self.bytes.items, self.cursor.byte);
+        self.refreshCell();
+    }
+
+    pub fn insert(self: *Buffer, text: []const u8) !void {
+        if (!std.unicode.utf8ValidateSlice(text)) return error.InvalidUtf8Insertion;
+        try self.bytes.insertSlice(self.allocator, self.cursor.byte, text);
+        self.cursor.byte += text.len;
+        self.refreshCell();
+        self.selection = null;
+    }
+
+    pub fn deleteBackward(self: *Buffer) void {
+        if (self.cursor.byte == 0) return;
+        const start = previousBoundary(self.bytes.items, self.cursor.byte);
+        self.bytes.replaceRangeAssumeCapacity(start, self.cursor.byte - start, "");
+        self.cursor.byte = start;
+        self.refreshCell();
+        self.selection = null;
+    }
+
+    fn refreshCell(self: *Buffer) void {
+        self.cursor.cell = cellWidth(self.bytes.items[0..self.cursor.byte]);
+    }
+};
+
+pub const Session = struct {
+    allocator: std.mem.Allocator,
+    buffer: ?Buffer = null,
+
+    pub fn init(allocator: std.mem.Allocator) Session {
+        return .{ .allocator = allocator };
+    }
+
+    pub fn deinit(self: *Session) void {
+        if (self.buffer) |*buffer| buffer.deinit();
+        self.* = undefined;
+    }
+
+    pub fn openFixture(self: *Session, fixture: []const u8) !void {
+        if (self.buffer) |*buffer| buffer.deinit();
+        self.buffer = try Buffer.openFromBytes(self.allocator, fixture);
+    }
+
+    pub fn dispatch(self: *Session, command: Command) !void {
+        if (self.buffer) |*buffer| return buffer.dispatch(command);
+        return error.NoBufferOpen;
+    }
+
+    pub fn snapshot(self: *const Session) !Snapshot {
+        if (self.buffer) |*buffer| return buffer.snapshot();
+        return error.NoBufferOpen;
+    }
+};
+
+pub fn previousBoundary(bytes: []const u8, cursor: usize) usize {
+    if (cursor == 0) return 0;
+    var i = @min(cursor, bytes.len) - 1;
+    while (i > 0 and isContinuation(bytes[i])) : (i -= 1) {}
+    return i;
+}
+
+pub fn nextBoundary(bytes: []const u8, cursor: usize) usize {
+    if (cursor >= bytes.len) return bytes.len;
+    const len = std.unicode.utf8ByteSequenceLength(bytes[cursor]) catch 1;
+    return @min(bytes.len, cursor + len);
+}
+
+pub fn cellWidth(bytes: []const u8) usize {
+    var width: usize = 0;
+    var i: usize = 0;
+    while (i < bytes.len) {
+        const len = std.unicode.utf8ByteSequenceLength(bytes[i]) catch {
+            i += 1;
+            width += 1;
+            continue;
+        };
+        if (i + len > bytes.len) {
+            width += 1;
+            break;
+        }
+        const cp = std.unicode.utf8Decode(bytes[i .. i + len]) catch {
+            i += 1;
+            width += 1;
+            continue;
+        };
+        width += codepointCellWidth(cp);
+        i += len;
+    }
+    return width;
+}
+
+fn isContinuation(byte: u8) bool {
+    return (byte & 0b1100_0000) == 0b1000_0000;
+}
+
+fn codepointCellWidth(cp: u21) usize {
+    if (cp == 0) return 0;
+    if (cp < 0x20 or (cp >= 0x7f and cp < 0xa0)) return 0;
+    if (isCombining(cp)) return 0;
+    if (isWide(cp)) return 2;
+    return 1;
+}
+
+fn isCombining(cp: u21) bool {
+    return (cp >= 0x0300 and cp <= 0x036f) or
+        (cp >= 0x1ab0 and cp <= 0x1aff) or
+        (cp >= 0x1dc0 and cp <= 0x1dff) or
+        (cp >= 0x20d0 and cp <= 0x20ff) or
+        (cp >= 0xfe20 and cp <= 0xfe2f);
+}
+
+fn isWide(cp: u21) bool {
+    return (cp >= 0x1100 and cp <= 0x115f) or
+        (cp >= 0x2329 and cp <= 0x232a) or
+        (cp >= 0x2e80 and cp <= 0xa4cf) or
+        (cp >= 0xac00 and cp <= 0xd7a3) or
+        (cp >= 0xf900 and cp <= 0xfaff) or
+        (cp >= 0xfe10 and cp <= 0xfe19) or
+        (cp >= 0xfe30 and cp <= 0xfe6f) or
+        (cp >= 0xff00 and cp <= 0xff60) or
+        (cp >= 0xffe0 and cp <= 0xffe6) or
+        (cp >= 0x1f300 and cp <= 0x1faff) or
+        (cp >= 0x20000 and cp <= 0x3fffd);
+}
+
+test "regular: session opens fixture and dispatches multibyte edits" {
+    var session = Session.init(std.testing.allocator);
+    defer session.deinit();
+
+    try session.openFixture("let café = 1\n");
+    try session.dispatch(.move_right);
+    try session.dispatch(.move_right);
+    try session.dispatch(.move_right);
+    try session.dispatch(.{ .insert = "🔥" });
+
+    const snap = try session.snapshot();
+    try std.testing.expectEqualStrings("let🔥 café = 1\n", snap.bytes);
+    try std.testing.expectEqual(@as(usize, 7), snap.cursor_byte);
+    try std.testing.expectEqual(@as(usize, 5), snap.cursor_cell);
+}
+
+test "regular: delete backward removes whole UTF-8 codepoint" {
+    var buffer = try Buffer.openFromBytes(std.testing.allocator, "aéb");
+    defer buffer.deinit();
+
+    buffer.moveRight();
+    buffer.moveRight();
+    buffer.deleteBackward();
+
+    const snap = buffer.snapshot();
+    try std.testing.expectEqualStrings("ab", snap.bytes);
+    try std.testing.expectEqual(@as(usize, 1), snap.cursor_byte);
+    try std.testing.expectEqual(@as(usize, 1), snap.cursor_cell);
+}
+
+test "regular: cell width treats combining marks as zero and wide glyphs as two" {
+    try std.testing.expectEqual(@as(usize, 1), cellWidth("e\u{0301}"));
+    try std.testing.expectEqual(@as(usize, 2), cellWidth("界"));
+    try std.testing.expectEqual(@as(usize, 2), cellWidth("🔥"));
+}
+
+test "adversarial: cursor movement never lands inside a multibyte codepoint" {
+    var buffer = try Buffer.openFromBytes(std.testing.allocator, "aé🔥z");
+    defer buffer.deinit();
+
+    buffer.moveRight();
+    try std.testing.expectEqual(@as(usize, 1), buffer.cursor.byte);
+    buffer.moveRight();
+    try std.testing.expectEqual(@as(usize, 3), buffer.cursor.byte);
+    buffer.moveRight();
+    try std.testing.expectEqual(@as(usize, 7), buffer.cursor.byte);
+    buffer.moveLeft();
+    try std.testing.expectEqual(@as(usize, 3), buffer.cursor.byte);
+}
+
+test "adversarial: invalid inserted UTF-8 is rejected and existing bytes are preserved" {
+    var buffer = try Buffer.openFromBytes(std.testing.allocator, "safe");
+    defer buffer.deinit();
+
+    const bad = [_]u8{ 0xc3, 0x28 };
+    try std.testing.expectError(error.InvalidUtf8Insertion, buffer.insert(&bad));
+    try std.testing.expectEqualStrings("safe", buffer.snapshot().bytes);
+    try std.testing.expectEqual(@as(usize, 0), buffer.snapshot().cursor_byte);
+}
+
+test "adversarial: dispatch requires an open buffer" {
+    var session = Session.init(std.testing.allocator);
+    defer session.deinit();
+
+    try std.testing.expectError(error.NoBufferOpen, session.dispatch(.move_right));
+    try std.testing.expectError(error.NoBufferOpen, session.snapshot());
+}