diff --git a/d2m/actions/create-space.js b/d2m/actions/create-space.js
index 517dad4..4218c1f 100644
--- a/d2m/actions/create-space.js
+++ b/d2m/actions/create-space.js
@@ -8,8 +8,8 @@ const api = sync.require("../../matrix/api")
 /**
  * @param {import("discord-api-types/v10").RESTGetAPIGuildResult} guild
  */
-function createSpace(guild) {
-	return api.createRoom({
+async function createSpace(guild) {
+	const roomID = api.createRoom({
 		name: guild.name,
 		preset: "private_chat",
 		visibility: "private",
@@ -37,10 +37,9 @@ function createSpace(guild) {
 				}
 			}
 		]
-	}).then(root => {
-		db.prepare("INSERT INTO guild_space (guild_id, space_id) VALUES (?, ?)").run(guild.id, root.room_id)
-		return root
 	})
+	db.prepare("INSERT INTO guild_space (guild_id, space_id) VALUES (?, ?)").run(guild.id, roomID)
+	return roomID
 }
 
 module.exports.createSpace = createSpace
diff --git a/d2m/actions/register-user.js b/d2m/actions/register-user.js
index f970218..a5fd0ef 100644
--- a/d2m/actions/register-user.js
+++ b/d2m/actions/register-user.js
@@ -1,6 +1,7 @@
 // @ts-check
 
 const assert = require("assert")
+const reg = require("../../matrix/read-registration")
 
 const passthrough = require("../../passthrough")
 const { discord, sync, db } = passthrough
@@ -14,10 +15,58 @@ const userToMxid = sync.require("../converters/user-to-mxid")
 /**
  * A sim is an account that is being simulated by the bridge to copy events from the other side.
  * @param {import("discord-api-types/v10").APIUser} user
+ * @returns mxid
  */
 async function createSim(user) {
+	// Choose sim name
 	const simName = userToMxid.userToSimName(user)
-	const appservicePrefix = "_ooye_"
-	const localpart = appservicePrefix + simName
+	const localpart = reg.namespace_prefix + simName
+	const mxid = "@" + localpart + ":cadence.moe"
+
+	// Save chosen name in the database forever
+	db.prepare("INSERT INTO sim (discord_id, sim_name, localpart, mxid) VALUES (?, ?, ?, ?)").run(user.id, simName, localpart, mxid)
+
+	// Register matrix user with that name
 	await api.register(localpart)
+	return mxid
 }
+
+/**
+ * Ensure a sim is registered for the user.
+ * If there is already a sim, use that one. If there isn't one yet, register a new sim.
+ * @returns mxid
+ */
+async function ensureSim(user) {
+	let mxid = null
+	const existing = db.prepare("SELECT mxid FROM sim WHERE discord_id = ?").pluck().get(user.id)
+	if (existing) {
+		mxid = existing
+	} else {
+		mxid = await createSim(user)
+	}
+	return mxid
+}
+
+/**
+ * Ensure a sim is registered for the user and is joined to the room.
+ * @returns mxid
+ */
+async function ensureSimJoined(user, roomID) {
+	// Ensure room ID is really an ID, not an alias
+	assert.ok(roomID[0] === "!")
+
+	// Ensure user
+	const mxid = await ensureSim(user)
+
+	// Ensure joined
+	const existing = db.prepare("SELECT * FROM sim_member WHERE room_id = ? and mxid = ?").get(roomID, mxid)
+	if (!existing) {
+		await api.inviteToRoom(roomID, mxid)
+		await api.joinRoom(roomID, mxid)
+		db.prepare("INSERT INTO sim_member (room_id, mxid) VALUES (?, ?)").run(roomID, mxid)
+	}
+	return mxid
+}
+
+module.exports.ensureSim = ensureSim
+module.exports.ensureSimJoined = ensureSimJoined
diff --git a/d2m/actions/send-message.js b/d2m/actions/send-message.js
index 1f71a66..0a425ee 100644
--- a/d2m/actions/send-message.js
+++ b/d2m/actions/send-message.js
@@ -1,27 +1,29 @@
 // @ts-check
 
-const reg = require("../../matrix/read-registration.js")
-const makeTxnId = require("../../matrix/txnid.js")
 const fetch = require("node-fetch").default
-const messageToEvent = require("../converters/message-to-event.js")
+const reg = require("../../matrix/read-registration.js")
+
+const passthrough = require("../../passthrough")
+const { discord, sync, db } = passthrough
+/** @type {import("../converters/message-to-event")} */
+const messageToEvent = sync.require("../converters/message-to-event")
+/** @type {import("../../matrix/api")} */
+const api = sync.require("../../matrix/api")
+/** @type {import("./register-user")} */
+const registerUser = sync.require("./register-user")
 
 /**
  * @param {import("discord-api-types/v10").GatewayMessageCreateDispatchData} message
  */
-function sendMessage(message) {
+async function sendMessage(message) {
 	const event = messageToEvent.messageToEvent(message)
-	return fetch(`https://matrix.cadence.moe/_matrix/client/v3/rooms/!VwVlIAjOjejUpDhlbA:cadence.moe/send/m.room.message/${makeTxnId()}?user_id=@_ooye_example:cadence.moe`, {
-		method: "PUT",
-		body: JSON.stringify(event),
-		headers: {
-			Authorization: `Bearer ${reg.as_token}`
-		}
-	}).then(res => res.text()).then(text => {
-		// {"event_id":"$4Zxs0fMmYlbo-sTlMmSEvwIs9b4hcg6yORzK0Ems84Q"}
-		console.log(text)
-	}).catch(err => {
-		console.log(err)
-	})
+	const roomID = "!VwVlIAjOjejUpDhlbA:cadence.moe"
+	let senderMxid = null
+	if (!message.webhook_id) {
+		senderMxid = await registerUser.ensureSimJoined(message.author, roomID)
+	}
+	const eventID = api.sendEvent(roomID, "m.room.message", event, senderMxid)
+	return eventID
 }
 
 module.exports.sendMessage = sendMessage
diff --git a/d2m/converters/user-to-mxid.test.js b/d2m/converters/user-to-mxid.test.js
index 7cda6d7..4c721fc 100644
--- a/d2m/converters/user-to-mxid.test.js
+++ b/d2m/converters/user-to-mxid.test.js
@@ -13,7 +13,7 @@ test("user2name: works on normal name", t => {
 })
 
 test("user2name: works on emojis", t => {
-   t.equal(userToSimName({username: "Cookie 🍪", discriminator: "0001"}), "cookie")
+   t.equal(userToSimName({username: "🍪 Cookie Monster 🍪", discriminator: "0001"}), "cookie_monster")
 })
 
 test("user2name: works on crazy name", t => {
diff --git a/db/ooye.db b/db/ooye.db
index bb571ed..c9803f8 100644
Binary files a/db/ooye.db and b/db/ooye.db differ
diff --git a/matrix/api.js b/matrix/api.js
index 88c62d8..e3a2600 100644
--- a/matrix/api.js
+++ b/matrix/api.js
@@ -8,6 +8,15 @@ const { discord, sync, db } = passthrough
 const mreq = sync.require("./mreq")
 /** @type {import("./file")} */
 const file = sync.require("./file")
+/** @type {import("./txnid")} */
+const makeTxnId = sync.require("./txnid")
+
+function path(p, mxid = null) {
+   if (!mxid) return p
+   const u = new URL(p, "http://localhost")
+   u.searchParams.set("user_id", mxid)
+   return u.pathname + "?" + u.searchParams.toString()
+}
 
 /**
  * @param {string} username
@@ -21,10 +30,27 @@ function register(username) {
 }
 
 /**
- * @returns {Promise<import("../types").R.RoomCreated>}
+ * @returns {Promise<string>} room ID
  */
-function createRoom(content) {
-   return mreq.mreq("POST", "/client/v3/createRoom", content)
+async function createRoom(content) {
+   /** @type {import("../types").R.RoomCreated} */
+   const root = await mreq.mreq("POST", "/client/v3/createRoom", content)
+   return root.room_id
+}
+
+/**
+ * @returns {Promise<string>} room ID
+ */
+async function joinRoom(roomIDOrAlias, mxid) {
+   /** @type {import("../types").R.RoomJoined} */
+   const root = await mreq.mreq("POST", path(`/client/v3/join/${roomIDOrAlias}`, mxid))
+   return root.room_id
+}
+
+async function inviteToRoom(roomID, mxidToInvite, mxid) {
+   await mreq.mreq("POST", path(`/client/v3/rooms/${roomID}/invite`, mxid), {
+      user_id: mxidToInvite
+   })
 }
 
 /**
@@ -39,15 +65,27 @@ function getAllState(roomID) {
  * @param {string} roomID
  * @param {string} type
  * @param {string} stateKey
- * @returns {Promise<import("../types").R.EventSent>}
+ * @returns {Promise<string>} event ID
  */
-function sendState(roomID, type, stateKey, content) {
+async function sendState(roomID, type, stateKey, content, mxid) {
    assert.ok(type)
    assert.ok(stateKey)
-   return mreq.mreq("PUT", `/client/v3/rooms/${roomID}/state/${type}/${stateKey}`, content)
+   /** @type {import("../types").R.EventSent} */
+   const root = await mreq.mreq("PUT", path(`/client/v3/rooms/${roomID}/state/${type}/${stateKey}`, mxid), content)
+   return root.event_id
 }
 
+async function sendEvent(roomID, type, content, mxid) {
+   /** @type {import("../types").R.EventSent} */
+   const root = await mreq.mreq("PUT", path(`/client/v3/rooms/${roomID}/send/${type}/${makeTxnId.makeTxnId()}`, mxid), content)
+   return root.event_id
+}
+
+module.exports.path = path
 module.exports.register = register
 module.exports.createRoom = createRoom
+module.exports.joinRoom = joinRoom
+module.exports.inviteToRoom = inviteToRoom
 module.exports.getAllState = getAllState
 module.exports.sendState = sendState
+module.exports.sendEvent = sendEvent
diff --git a/matrix/api.test.js b/matrix/api.test.js
new file mode 100644
index 0000000..f54c665
--- /dev/null
+++ b/matrix/api.test.js
@@ -0,0 +1,23 @@
+const {test} = require("supertape")
+const assert = require("assert")
+const {path} = require("./api")
+
+test("api path: no change for plain path", t => {
+	t.equal(path("/hello/world"), "/hello/world")
+})
+
+test("api path: add mxid to the URL", t => {
+	t.equal(path("/hello/world", "12345"), "/hello/world?user_id=12345")
+})
+
+test("api path: empty path with mxid", t => {
+	t.equal(path("", "12345"), "/?user_id=12345")
+})
+
+test("api path: existing query parameters with mxid", t => {
+	t.equal(path("/hello/world?foo=bar&baz=qux", "12345"), "/hello/world?foo=bar&baz=qux&user_id=12345")
+})
+
+test("api path: real world mxid", t => {
+	t.equal(path("/hello/world", "@cookie_monster:cadence.moe"), "/hello/world?user_id=%40cookie_monster%3Acadence.moe")
+})
diff --git a/matrix/mreq.js b/matrix/mreq.js
index 1345f78..6c4eaa3 100644
--- a/matrix/mreq.js
+++ b/matrix/mreq.js
@@ -16,8 +16,6 @@ class MatrixServerError extends Error {
 		this.data = data
 		/** @type {string} */
 		this.errcode = data.errcode
-		/** @type {string} */
-		this.error = data.error
 	}
 }
 
diff --git a/matrix/txnid.js b/matrix/txnid.js
index 1e26378..a3568df 100644
--- a/matrix/txnid.js
+++ b/matrix/txnid.js
@@ -2,6 +2,6 @@
 
 let now = Date.now()
 
-module.exports = function makeTxnId() {
+module.exports.makeTxnId = function makeTxnId() {
 	return now++
 }
diff --git a/stdin.js b/stdin.js
index 551d7d2..99345ab 100644
--- a/stdin.js
+++ b/stdin.js
@@ -8,6 +8,7 @@ const { discord, config, sync, db } = passthrough
 
 const createSpace = sync.require("./d2m/actions/create-space")
 const createRoom = sync.require("./d2m/actions/create-room")
+const registerUser = sync.require("./d2m/actions/register-user")
 const mreq = sync.require("./matrix/mreq")
 const api = sync.require("./matrix/api")
 const guildID = "112760669178241024"
diff --git a/test/test.js b/test/test.js
index 3008abf..1068136 100644
--- a/test/test.js
+++ b/test/test.js
@@ -13,4 +13,5 @@ const sync = new HeatSync({watchFS: false})
 Object.assign(passthrough, { config, sync, db })
 
 require("../d2m/actions/create-room.test")
-require("../d2m/converters/user-to-mxid.test")
\ No newline at end of file
+require("../d2m/converters/user-to-mxid.test")
+require("../matrix/api.test")
diff --git a/types.d.ts b/types.d.ts
index b3a9acc..bc24329 100644
--- a/types.d.ts
+++ b/types.d.ts
@@ -4,6 +4,7 @@ export type AppServiceRegistrationConfig = {
 	hs_token: string
 	url: string
 	sender_localpart: string
+	namespace_prefix: string
 	protocols: [string]
 	rate_limited: boolean
 }
@@ -43,6 +44,10 @@ namespace R {
 		room_id: string
 	}
 
+	export type RoomJoined = {
+		room_id: string
+	}
+
 	export type FileUploaded = {
 		content_uri: string
 	}