diff --git a/src/protocol.zig b/src/protocol.zig
new file mode 100644
index 0000000..2acf48a
--- /dev/null
+++ b/src/protocol.zig
@@ -0,0 +1,158 @@
+const std = @import("std");
+const encode = @import("encode.zig");
+const dbmod = @import("db.zig");
+const rowmod = @import("row.zig");
+const authmod = @import("auth.zig");
+const ttlmod = @import("ttl.zig");
+const collmod = @import("collections.zig");
+const aggregatemod = @import("aggregate.zig");
+const windowmod = @import("window.zig");
+const querymod = @import("query.zig");
+
+const Allocator = std.mem.Allocator;
+
+/// Guard against a corrupt or hostile length prefix causing an unbounded
+/// allocation before a single byte of the frame body has been validated.
+pub const maxFrameBytes: u32 = 64 * 1024 * 1024;
+
+pub const Op = enum(u8) {
+    ping = 0,
+    get = 1,
+    set = 2,
+    del = 3,
+    scan = 4,
+    prefix = 5,
+    stats = 6,
+    flush = 7,
+    compact = 8,
+    batch = 9,
+    table_select = 10,
+    auth = 11,
+    user_admin = 12,
+    auth_token = 13,
+    metrics = 14,
+    ttl_op = 15,
+    coll_ttl_op = 16,
+    coll_op = 17,
+    txn_op = 18,
+    table_join = 19,
+    table_aggregate = 20,
+    table_explain = 21,
+    table_update = 22,
+    table_delete = 23,
+    table_export = 24,
+    table_import = 25,
+    table_window = 26,
+    subscribe = 27,
+    publish = 28,
+    unsubscribe = 29,
+
+    pub fn fromByte(b: u8) !Op {
+        return std.meta.intToEnum(Op, b) catch error.UnknownOp;
+    }
+};
+
+pub const Status = enum(u8) {
+    ok = 0,
+    not_found = 1,
+    bad_request = 2,
+    io_error = 3,
+    conflict = 4,
+    internal = 5,
+    rate_limited = 6,
+    unauthorized = 7,
+    forbidden = 8,
+
+    pub fn fromByte(b: u8) !Status {
+        return std.meta.intToEnum(Status, b) catch error.UnknownStatus;
+    }
+};
+
+pub const ProtoError = error{
+    UnknownOp,
+    UnknownStatus,
+    FrameTooLarge,
+    Malformed,
+};
+
+/// A frame is the unit exchanged over the wire in both directions: a
+/// single tag byte (an `Op` on the request side, a `Status` on the
+/// response side) followed by a little-endian u32 payload length and the
+/// payload itself.
+pub const Frame = struct {
+    tag: u8,
+    payload: []u8,
+    allocator: Allocator,
+
+    pub fn deinit(self: *Frame) void {
+        self.allocator.free(self.payload);
+        self.* = undefined;
+    }
+};
+
+pub fn writeFrame(writer: anytype, tag: u8, payload: []const u8) !void {
+    if (payload.len > maxFrameBytes) return error.FrameTooLarge;
+    var header: [5]u8 = undefined;
+    header[0] = tag;
+    std.mem.writeInt(u32, header[1..5], @intCast(payload.len), encode.endian);
+    try writer.writeAll(&header);
+    if (payload.len > 0) try writer.writeAll(payload);
+}
+
+pub fn readFrame(allocator: Allocator, reader: anytype) !Frame {
+    var header: [5]u8 = undefined;
+    try reader.readNoEof(&header);
+    const tag = header[0];
+    const len = std.mem.readInt(u32, header[1..5], encode.endian);
+    if (len > maxFrameBytes) return error.FrameTooLarge;
+    const payload = try allocator.alloc(u8, len);
+    errdefer allocator.free(payload);
+    if (len > 0) try reader.readNoEof(payload);
+    return .{ .tag = tag, .payload = payload, .allocator = allocator };
+}
+
+/// Bounds-checked cursor over a decoded payload. Every read returns
+/// `error.Malformed` instead of panicking when the peer sent a short or
+/// truncated buffer, since payload bytes come from an untrusted socket.
+pub const Cursor = struct {
+    data: []const u8,
+    pos: usize = 0,
+
+    pub fn takeByte(self: *Cursor) !u8 {
+        if (self.pos + 1 > self.data.len) return error.Malformed;
+        const v = self.data[self.pos];
+        self.pos += 1;
+        return v;
+    }
+
+    pub fn takeU32(self: *Cursor) !u32 {
+        if (self.pos + 4 > self.data.len) return error.Malformed;
+        const v = encode.readU32(self.data, self.pos);
+        self.pos += 4;
+        return v;
+    }
+
+    pub fn takeU64(self: *Cursor) !u64 {
+        if (self.pos + 8 > self.data.len) return error.Malformed;
+        const v = encode.readU64(self.data, self.pos);
+        self.pos += 8;
+        return v;
+    }
+
+    pub fn takeBytes(self: *Cursor, n: usize) ![]const u8 {
+        if (self.pos + n > self.data.len) return error.Malformed;
+        const v = self.data[self.pos .. self.pos + n];
+        self.pos += n;
+        return v;
+    }
+
+    pub fn takeLenPrefixed(self: *Cursor) ![]const u8 {
+        const n = try self.takeU32();
+        return self.takeBytes(n);
+    }
+
+    pub fn atEnd(self: *Cursor) bool {
+        return self.pos == self.data.len;
+    }
+};
+