diff --git a/b3d.lua b/b3d.lua
index 4e3f61f..6787ae9 100644
--- a/b3d.lua
+++ b/b3d.lua
@@ -1,15 +1,17 @@
 -- Localize globals
-local assert, error, math, modlib, next, pairs, setmetatable, table = assert, error, math, modlib, next, pairs, setmetatable, table
+local assert, error, math, modlib, next, ipairs, pairs, setmetatable, string_char, table
+	= assert, error, math, modlib, next, ipairs, pairs, setmetatable, string.char, table
 
 local read_int, read_single = modlib.binary.read_int, modlib.binary.read_single
 
+local write_int, write_single = modlib.binary.write_int, modlib.binary.write_single
+
 -- Set environment
 local _ENV = {}
 setfenv(1, _ENV)
 
 local metatable = {__index = _ENV}
 
---! experimental
 --+ Reads a single BB3D chunk from a stream
 --+ Doing `assert(stream:read(1) == nil)` afterwards is recommended
 --+ See `b3d_specification.txt` as well as https://github.com/blitz-research/blitz3d/blob/master/blitz3d/loader_b3d.cpp
@@ -84,7 +86,9 @@ function read(stream)
 	end
 
 	local function content()
-		assert(left >= 0, stream:seek())
+		if left < 0 then
+			error(("unexpected EOF at position %d"):format(stream:seek()))
+		end
 		return left ~= 0
 	end
 
@@ -106,8 +110,8 @@ function read(stream)
 		end,
 		BRUS = function()
 			local brushes = {}
-			brushes.n_texs = int()
-			assert(brushes.n_texs <= 8)
+			local n_texs = int()
+			assert(n_texs <= 8)
 			while content() do
 				local brush = {}
 				brush.name = string()
@@ -116,7 +120,7 @@ function read(stream)
 				brush.blend = int()
 				brush.fx = int()
 				brush.texture_id = {}
-				for index = 1, brushes.n_texs do
+				for index = 1, n_texs do
 					brush.texture_id[index] = optional_id()
 				end
 				table.insert(brushes, brush)
@@ -214,8 +218,6 @@ function read(stream)
 				end
 				table.insert(bone, frame)
 			end
-			-- Ensure frames are sorted ascending
-			table.sort(bone, function(a, b) return a.frame < b.frame end)
 			return bone
 		end,
 		ANIM = function()
@@ -235,7 +237,7 @@ function read(stream)
 			node.children = {}
 			local node_type
 			-- See https://github.com/blitz-research/blitz3d/blob/master/blitz3d/loader_b3d.cpp#L263
-			-- Order is not validated; double occurences of mutually exclusive node def are
+			-- Order is not validated; double occurrences of mutually exclusive node def are
 			while content() do
 				local elem, type = chunk()
 				if type == "MESH" then
@@ -247,7 +249,6 @@ function read(stream)
 					node_type = "bone"
 					node.bone = elem
 				elseif type == "KEYS" then
-					assert((node.keys[#node.keys] or {}).frame ~= (elem[1] or {}).frame, "duplicate frame")
 					modlib.table.append(node.keys, elem)
 				elseif type == "NODE" then
 					table.insert(node.children, elem)
@@ -258,7 +259,11 @@ function read(stream)
 					node_type = "pivot"
 				end
 			end
-			-- TODO somehow merge keys
+			-- Ensure frames are sorted ascendingly
+			table.sort(node.keys, function(a, b)
+				assert(a.frame ~= b.frame, "duplicate frame")
+				return a.frame < b.frame
+			end)
 			return node
 		end,
 		BB3D = function()
@@ -267,7 +272,6 @@ function read(stream)
 				version = {
 					major = math.floor(version / 100),
 					minor = version % 100,
-					raw = version
 				},
 				textures = {},
 				brushes = {}
@@ -309,7 +313,224 @@ function read(stream)
 	return setmetatable(self, metatable)
 end
 
--- TODO function write(self, stream)
+-- Writer
+
+local function write_rope(self)
+	local rope = {}
+
+	local written_len = 0
+	local function write(str)
+		written_len = written_len + #str
+		table.insert(rope, str)
+	end
+
+	local function byte(val)
+		write(string_char(val))
+	end
+
+	local function int(val)
+		write_int(byte, val, 4)
+	end
+
+	local function id(val)
+		int(val - 1)
+	end
+
+	local function optional_id(val)
+		int(val and (val - 1) or -1)
+	end
+
+	local function string(val)
+		write(val)
+		write"\0"
+	end
+
+	local function float(val)
+		write_single(byte, val)
+	end
+
+	local function float_array(arr, len)
+		assert(#arr == len)
+		for i = 1, len do
+			float(arr[i])
+		end
+	end
+
+	local function color(val)
+		float(val.r)
+		float(val.g)
+		float(val.b)
+		float(val.a)
+	end
+
+	local function vector3(val)
+		float_array(val, 3)
+	end
+
+	local function quaternion(quat)
+		float(quat[4])
+		float(quat[1])
+		float(quat[2])
+		float(quat[3])
+	end
+
+	local function chunk(name, write_func)
+		write(name)
+
+		-- Insert placeholder for the 4-bit len
+		table.insert(rope, false)
+		written_len = written_len + 4
+		local len_idx = #rope -- save index of placeholder
+
+		local prev_written_len = written_len
+		write_func()
+
+		-- Write the length of this chunk
+		local chunk_len = written_len - prev_written_len
+		local len_binary = {}
+		write_int(function(byte)
+			table.insert(len_binary, string_char(byte))
+		end, chunk_len, 4)
+		rope[len_idx] = table.concat(len_binary)
+	end
+
+	local function NODE(node)
+		chunk("NODE", function()
+			string(node.name)
+			vector3(node.position)
+			vector3(node.scale)
+			quaternion(node.rotation)
+			local mesh = node.mesh
+			if mesh then
+				chunk("MESH", function()
+					optional_id(mesh.brush_id)
+					local vertices = mesh.vertices
+					chunk("VRTS", function()
+						int(vertices.flags)
+						int(vertices.tex_coord_sets)
+						int(vertices.tex_coord_set_size)
+						for _, vertex in ipairs(vertices) do
+							if vertex.pos then vector3(vertex.pos) end
+							if vertex.normal then vector3(vertex.normal) end
+							if vertex.color then color(vertex.color) end
+							for tex_coord_set = 1, vertices.tex_coord_sets do
+								local tex_coords = vertex.tex_coords[tex_coord_set]
+								for tex_coord = 1, vertices.tex_coord_set_size do
+									float(tex_coords[tex_coord])
+								end
+							end
+						end
+					end)
+					for _, triangle_set in ipairs(mesh.triangle_sets) do
+						chunk("TRIS", function()
+							id(triangle_set.brush_id)
+							for _, tri in ipairs(triangle_set.vertex_ids) do
+								id(tri[1])
+								id(tri[2])
+								id(tri[3])
+							end
+						end)
+					end
+				end)
+			end
+			if node.bone then
+				chunk("BONE", function()
+					for vertex_id, weight in pairs(node.bone) do
+						id(vertex_id)
+						float(weight)
+					end
+				end)
+			end
+			if node.keys then
+				local keys_by_flags = {}
+				for _, key in ipairs(node.keys) do
+					local flags = 0
+					flags = flags
+						+ (key.position and 1 or 0)
+						+ (key.scale and 2 or 0)
+						+ (key.rotation and 4 or 0)
+					keys_by_flags[flags] = keys_by_flags[flags] or {}
+					table.insert(keys_by_flags[flags], key)
+				end
+				for flags, keys in pairs(keys_by_flags) do
+					chunk("KEYS", function()
+						int(flags)
+						for _, frame in ipairs(keys) do
+							int(frame.frame)
+							if frame.position then vector3(frame.position) end
+							if frame.scale then vector3(frame.scale) end
+							if frame.rotation then quaternion(frame.rotation) end
+						end
+					end)
+				end
+			end
+			local anim = node.animation
+			if anim then
+				chunk("ANIM", function()
+					int(anim.flags)
+					int(anim.frames)
+					float(anim.fps)
+				end)
+			end
+			for _, child in ipairs(node.children) do
+				NODE(child)
+			end
+		end)
+	end
+
+	chunk("BB3D", function()
+		int(self.version.major * 100 + self.version.minor)
+		if self.textures[1] then
+			chunk("TEXS", function()
+				for _, tex in ipairs(self.textures) do
+					string(tex.file)
+					int(tex.flags)
+					int(tex.blend)
+					float_array(tex.pos, 2)
+					float_array(tex.scale, 2)
+					float(tex.rotation)
+				end
+			end)
+		end
+		if self.brushes[1] then
+			local max_n_texs = 0
+			for _, brush in ipairs(self.brushes) do
+				for n in pairs(brush.texture_id) do
+					if n > max_n_texs then
+						max_n_texs = n
+					end
+				end
+			end
+			chunk("BRUS", function()
+				int(max_n_texs)
+				for _, brush in ipairs(self.brushes) do
+					string(brush.name)
+					color(brush.color)
+					float(brush.shininess)
+					int(brush.blend)
+					int(brush.fx)
+					for index = 1, max_n_texs do
+						optional_id(brush.texture_id[index])
+					end
+				end
+			end)
+		end
+		if self.node then
+			NODE(self.node)
+		end
+	end)
+	return rope
+end
+
+function write_string(self)
+	return table.concat(write_rope(self))
+end
+
+function write(self, stream)
+	for _, str in ipairs(write_rope(self)) do
+		stream:write(str)
+	end
+end
 
 local binary_search_frame = modlib.table.binary_search_comparator(function(a, b)
 	return modlib.table.default_comparator(a, b.frame)
diff --git a/test.lua b/test.lua
index f6bfc41..8ba60b7 100644
--- a/test.lua
+++ b/test.lua
@@ -480,9 +480,9 @@ local tests = {
 }
 if tests.b3d then
 	local stream = assert(io.open(mod.get_resource("player_api", "models", "character.b3d"), "r"))
-	local b3d = b3d.read(stream)
+	local character = assert(b3d.read(stream))
 	stream:close()
-	--! dirty helper method to create truncate tables with 10+ number keys
+	--! dirty helper method to truncate tables with 10+ number keys
 	local function _b3d_truncate(table)
 		local count = 1
 		for key, value in pairs(table) do
@@ -503,7 +503,10 @@ if tests.b3d then
 		end
 		return table
 	end
-	file.write(mod.get_resource"character.b3d.lua", "return " .. dump(_b3d_truncate(table.copy(b3d))))
+	local str = character:write_string()
+	local read = b3d.read(text.inputstream(str))
+	assert(modlib.table.equals_noncircular(character, read))
+	file.write(mod.get_resource"character.b3d.lua", "return " .. dump(_b3d_truncate(table.copy(character))))
 end
 local vector, minetest, ml_mt = _G.vector, _G.minetest, minetest
 if tests.liquid_dir then