diff --git a/src/app.c b/src/app.c
index 26026454..f9ef1d36 100644
--- a/src/app.c
+++ b/src/app.c
@@ -223,8 +223,7 @@ void app_entry_point(void)
     struct sound_startup_receipt sound_sr = sound_startup(&work_sr, &asset_cache_sr, &resource_sr);
     struct draw_startup_receipt draw_sr = draw_startup(&renderer_sr, &font_sr);
     struct phys_startup_receipt phys_sr = phys_startup();
-    struct sim_startup_receipt sim_sr = sim_startup(&mixer_sr, &sprite_sr, &sound_sr, &phys_sr, &host_sr);
-    struct user_startup_receipt user_sr = user_startup(&work_sr, &renderer_sr, &font_sr, &sprite_sr, &draw_sr, &sim_sr, &asset_cache_sr, &mixer_sr, &host_sr, &window);
+    struct user_startup_receipt user_sr = user_startup(&work_sr, &renderer_sr, &font_sr, &sprite_sr, &draw_sr, &asset_cache_sr, &sound_sr, &mixer_sr, &phys_sr, &host_sr, &window);
     struct playback_startup_receipt playback_sr = playback_startup(&mixer_sr);
 
     (UNUSED)user_sr;
diff --git a/src/config.h b/src/config.h
index 24f3ed08..3c43da26 100644
--- a/src/config.h
+++ b/src/config.h
@@ -33,7 +33,6 @@
 #define SPACE_CELL_BUCKETS_SQRT (256)
 #define SPACE_CELL_SIZE 1.0f
 
-
 #define SIM_FPS 50.0
 #define SIM_TIMESCALE 1
 
diff --git a/src/phys.c b/src/phys.c
index 1132fdc8..4790194e 100644
--- a/src/phys.c
+++ b/src/phys.c
@@ -1172,7 +1172,7 @@ u64 phys_step(struct phys_ctx *ctx, f32 timestep, u64 last_phys_iteration)
 
         if (ctx->pre_solve_callback) {
             __profscope(pre_solve_callback);
-            ctx->pre_solve_callback(collision_data);
+            ctx->pre_solve_callback(collision_data, ctx->pre_solve_callback_udata);
         }
 
         f32 substep_dt = step_dt / SIM_PHYSICS_SUBSTEPS;
@@ -1204,7 +1204,7 @@ u64 phys_step(struct phys_ctx *ctx, f32 timestep, u64 last_phys_iteration)
 
         if (ctx->post_solve_callback) {
             __profscope(post_solve_callback);
-            ctx->post_solve_callback(collision_data);
+            ctx->post_solve_callback(collision_data, ctx->post_solve_callback_udata);
         }
 
         scratch_end(scratch);
diff --git a/src/phys.h b/src/phys.h
index 9a46d489..4d04bdc3 100644
--- a/src/phys.h
+++ b/src/phys.h
@@ -25,8 +25,8 @@ struct phys_collision_data_array {
 };
 
 struct phys_collision_data;
-#define PHYS_COLLISION_CALLBACK_FUNC_DEF(name, arg) void name(struct phys_collision_data_array arg)
-typedef PHYS_COLLISION_CALLBACK_FUNC_DEF(phys_collision_callback_func, data);
+#define PHYS_COLLISION_CALLBACK_FUNC_DEF(name, arg_collision_data, arg_udata) void name(struct phys_collision_data_array arg_collision_data, void *arg_udata)
+typedef PHYS_COLLISION_CALLBACK_FUNC_DEF(phys_collision_callback_func, collision_data, udata);
 
 /* Structure containing data used for a single physics step */
 struct phys_ctx {
@@ -37,6 +37,8 @@ struct phys_ctx {
 
     phys_collision_callback_func *pre_solve_callback;
     phys_collision_callback_func *post_solve_callback;
+    void *pre_solve_callback_udata;
+    void *post_solve_callback_udata;
 
     struct sim_ent_lookup *debug_lookup;
     struct v2 dbg_cursor_pos;
diff --git a/src/sim.c b/src/sim.c
index 81ae7e03..9b4bee28 100644
--- a/src/sim.c
+++ b/src/sim.c
@@ -1,12 +1,11 @@
 #include "sim.h"
 #include "sim_ent.h"
 #include "sim_client.h"
+#include "sim_msg.h"
 #include "sys.h"
 #include "util.h"
 #include "world.h"
 #include "sprite.h"
-#include "sound.h"
-#include "mixer.h"
 #include "math.h"
 #include "scratch.h"
 #include "atomic.h"
@@ -19,7 +18,8 @@
 #include "byteio.h"
 #include "host.h"
 
-GLOBAL struct {
+struct sim_ctx {
+    struct arena arena;
     struct atomic_i32 sim_thread_shutdown;
     struct sys_thread sim_thread;
 
@@ -48,76 +48,75 @@ GLOBAL struct {
 
     /* Tick */
     struct world tick;
-} G = ZI, DEBUG_ALIAS(G, G_sim);
+};
 
 /* ========================== *
- * Startup
+ * Ctx
  * ========================== */
 
-INTERNAL APP_EXIT_CALLBACK_FUNC_DEF(sim_shutdown);
 INTERNAL SYS_THREAD_ENTRY_POINT_FUNC_DEF(sim_thread_entry_point, arg);
-INTERNAL void reset_world(void);
+INTERNAL void reset_world(struct sim_ctx *ctx);
 
-struct sim_startup_receipt sim_startup(struct mixer_startup_receipt *mixer_sr,
-                                         struct sprite_startup_receipt *sheet_sr,
-                                         struct sound_startup_receipt *sound_sr,
-                                         struct phys_startup_receipt *phys_sr,
-                                         struct host_startup_receipt *host_sr)
+struct sim_ctx *sim_ctx_alloc(struct sprite_startup_receipt *sprite_sr,
+                              struct phys_startup_receipt *phys_sr,
+                              struct host_startup_receipt *host_sr,
+                              u16 host_port)
 {
-    (UNUSED)mixer_sr;
-    (UNUSED)sheet_sr;
-    (UNUSED)sound_sr;
+    struct arena arena = arena_alloc(GIGABYTE(64));
+    struct sim_ctx *ctx = arena_push_zero(&arena, struct sim_ctx);
+    ctx->arena = arena;
+
+    (UNUSED)sprite_sr;
     (UNUSED)phys_sr;
     (UNUSED)host_sr;
 
-    G.client_store = client_store_alloc();
+    ctx->client_store = sim_client_store_alloc();
 
     /* Intialize host */
-    G.host = host_alloc(12345);
+    ctx->host = host_alloc(host_port);
 
     /* Initialize empty world */
-    reset_world();
+    reset_world(ctx);
 
-    G.sim_thread = sys_thread_alloc(&sim_thread_entry_point, NULL, LIT("[P2] Sim thread"));
-    app_register_exit_callback(&sim_shutdown);
+    ctx->sim_thread = sys_thread_alloc(&sim_thread_entry_point, ctx, LIT("[P2] Sim thread"));
 
-    return (struct sim_startup_receipt) { 0 };
+    return ctx;
 }
 
-INTERNAL APP_EXIT_CALLBACK_FUNC_DEF(sim_shutdown)
+void sim_ctx_release(struct sim_ctx *ctx)
 {
     __prof;
-    atomic_i32_eval_exchange(&G.sim_thread_shutdown, true);
-    sys_thread_wait_release(&G.sim_thread);
+    atomic_i32_eval_exchange(&ctx->sim_thread_shutdown, true);
+    sys_thread_wait_release(&ctx->sim_thread);
 }
 
 /* ========================== *
  * Reset
  * ========================== */
 
-INTERNAL void reset_world(void)
+INTERNAL void reset_world(struct sim_ctx *ctx)
 {
-    if (G.tick.ent_store) {
+    if (ctx->tick.ent_store) {
         /* Release world */
-        world_release(&G.tick);
+        world_release(&ctx->tick);
         /* Release bookkeeping */
-        space_release(G.space);
+        space_release(ctx->space);
 #if COLLIDER_DEBUG
-        sim_ent_lookup_release(&G.collision_debug_lookup);
+        sim_ent_lookup_release(&ctx->collision_debug_lookup);
 #endif
-        sim_ent_lookup_release(&G.contact_lookup);
+        sim_ent_lookup_release(&ctx->contact_lookup);
     }
 
     /* Create bookkeeping */
-    G.contact_lookup = sim_ent_lookup_alloc(4096);
+    ctx->contact_lookup = sim_ent_lookup_alloc(4096);
 #if COLLIDER_DEBUG
-    G.collision_debug_lookup = sim_ent_lookup_alloc(4096);
+    ctx->collision_debug_lookup = sim_ent_lookup_alloc(4096);
 #endif
-    G.space = space_alloc(SPACE_CELL_SIZE, SPACE_CELL_BUCKETS_SQRT);
+    ctx->space = space_alloc(SPACE_CELL_SIZE, SPACE_CELL_BUCKETS_SQRT);
 
     /* Re-create world */
-    world_alloc(&G.tick);
-    G.tick.timescale = SIM_TIMESCALE;
+    world_alloc(&ctx->tick);
+    ctx->tick.timescale = SIM_TIMESCALE;
 }
 
 /* ========================== *
@@ -126,15 +125,15 @@ INTERNAL void reset_world(void)
 
 /* TODO: Remove this */
 
-INTERNAL void spawn_test_entities(void)
+INTERNAL void spawn_test_entities(struct sim_ctx *ctx)
 {
-    struct sim_ent *root = sim_ent_from_handle(G.tick.ent_store, G.tick.ent_store->root);
+    struct sim_ent *root = sim_ent_from_handle(ctx->tick.ent_store, ctx->tick.ent_store->root);
     root->mass_unscaled = F32_INFINITY;
     root->inertia_unscaled = F32_INFINITY;
 
     /* Player */
     struct sim_ent *player_ent = sim_ent_nil();
-    //if (!G.extra_spawn) {
+    //if (!ctx->extra_spawn) {
     {
 
         struct sim_ent *e = sim_ent_alloc(root);
@@ -148,7 +147,7 @@ INTERNAL void spawn_test_entities(void)
         //f32 r = PI / 4;
         f32 r = 0;
 
-        if (!G.extra_spawn) {
+        if (!ctx->extra_spawn) {
             sim_ent_enable_prop(e, SIM_ENT_PROP_PLAYER_CONTROLLED);
             sim_ent_enable_prop(e, SIM_ENT_PROP_TEST);
             e->sprite = sprite_tag_from_path(LIT("res/graphics/tim.ase"));
@@ -280,7 +279,7 @@ INTERNAL void spawn_test_entities(void)
     }
 
     /* Camera */
-    if (!G.extra_spawn && player_ent->valid) {
+    if (!ctx->extra_spawn && player_ent->valid) {
         struct sim_ent *e = sim_ent_alloc(root);
         sim_ent_set_xform(e, XFORM_IDENT);
 
@@ -293,18 +292,18 @@ INTERNAL void spawn_test_entities(void)
         e->camera_quad_xform = XFORM_TRS(.s = V2(width, height));
     }
 
-    G.extra_spawn = true;
+    ctx->extra_spawn = true;
 }
 
 /* ========================== *
  * Release entities
  * ========================== */
 
-INTERNAL void release_entities_with_prop(enum sim_ent_prop prop)
+INTERNAL void release_entities_with_prop(struct sim_ctx *ctx, enum sim_ent_prop prop)
 {
     struct temp_arena scratch = scratch_begin_no_conflict();
-    struct sim_ent_store *store = G.tick.ent_store;
-    struct space *space = G.space;
+    struct sim_ent_store *store = ctx->tick.ent_store;
+    struct space *space = ctx->space;
 
     struct sim_ent **ents_to_release = arena_dry_push(scratch.arena, struct sim_ent *);
     u64 ents_to_release_count = 0;
@@ -347,13 +346,14 @@ INTERNAL void release_entities_with_prop(enum sim_ent_prop prop)
  * Respond to physics collisions
  * ========================== */
 
-INTERNAL PHYS_COLLISION_CALLBACK_FUNC_DEF(on_collision, array)
+INTERNAL PHYS_COLLISION_CALLBACK_FUNC_DEF(on_collision, collision_data_array, udata)
 {
-    struct sim_ent_store *store = G.tick.ent_store;
+    struct sim_ctx *ctx = (struct sim_ctx *)udata;
+    struct sim_ent_store *store = ctx->tick.ent_store;
     struct sim_ent *root = sim_ent_from_handle(store, store->root);
 
-    for (u64 i = 0; i < array.count; ++i) {
-        struct phys_collision_data *data = &array.a[i];
+    for (u64 i = 0; i < collision_data_array.count; ++i) {
+        struct phys_collision_data *data = &collision_data_array.a[i];
 
         struct phys_contact_constraint *constraint = data->constraint;
         struct sim_ent *e0 = sim_ent_from_handle(store, data->e0);
@@ -434,7 +434,7 @@ INTERNAL PHYS_COLLISION_CALLBACK_FUNC_DEF(on_collision, array)
  * Update
  * ========================== */
 
-INTERNAL void sim_update(void)
+INTERNAL void sim_update(struct sim_ctx *ctx)
 {
     __prof;
 
@@ -444,31 +444,31 @@ INTERNAL void sim_update(void)
      * Reset level if necessary
      * ========================== */
 
-    if (G.should_reset_level) {
+    if (ctx->should_reset_level) {
         logf_info("Clearing level");
-        G.should_reset_level = false;
-        G.extra_spawn = false;
-        reset_world();
+        ctx->should_reset_level = false;
+        ctx->extra_spawn = false;
+        reset_world(ctx);
     }
 
     /* ========================== *
      * Begin frame
      * ========================== */
 
-    ++G.tick.tick_id;
+    ++ctx->tick.tick_id;
 
-    G.tick.dt_ns = NS_FROM_SECONDS(max_f64(0.0, (1.0 / SIM_FPS) * G.tick.timescale));
-    G.tick.time_ns += G.tick.dt_ns;
+    ctx->tick.dt_ns = NS_FROM_SECONDS(max_f64(0.0, (1.0 / SIM_FPS) * ctx->tick.timescale));
+    ctx->tick.time_ns += ctx->tick.dt_ns;
 
-    f64 dt = SECONDS_FROM_NS(G.tick.dt_ns);
-    f64 time = SECONDS_FROM_NS(G.tick.time_ns);
-    G.sprite_frame_scope = sprite_scope_begin();
-    G.root = sim_ent_from_handle(G.tick.ent_store, G.tick.ent_store->root);
+    f64 dt = SECONDS_FROM_NS(ctx->tick.dt_ns);
+    f64 time = SECONDS_FROM_NS(ctx->tick.time_ns);
+    ctx->sprite_frame_scope = sprite_scope_begin();
+    ctx->root = sim_ent_from_handle(ctx->tick.ent_store, ctx->tick.ent_store->root);
 
-    struct sim_ent *root = G.root;
-    struct sim_ent_store *ent_store = G.tick.ent_store;
-    struct space *space = G.space;
-    struct sprite_scope *sprite_frame_scope = G.sprite_frame_scope;
+    struct sim_ent *root = ctx->root;
+    struct sim_ent_store *ent_store = ctx->tick.ent_store;
+    struct space *space = ctx->space;
+    struct sprite_scope *sprite_frame_scope = ctx->sprite_frame_scope;
 
     (UNUSED)dt;
     (UNUSED)time;
@@ -483,7 +483,7 @@ INTERNAL void sim_update(void)
         static b32 run = 0;
         if (!run) {
             run = 1;
-            spawn_test_entities();
+            spawn_test_entities(ctx);
         }
     }
 
@@ -491,7 +491,7 @@ INTERNAL void sim_update(void)
      * Release entities
      * ========================== */
 
-    release_entities_with_prop(SIM_ENT_PROP_RELEASE_NEXT_TICK);
+    release_entities_with_prop(ctx, SIM_ENT_PROP_RELEASE_NEXT_TICK);
 
     /* ========================== *
      * Activate entities
@@ -503,8 +503,8 @@ INTERNAL void sim_update(void)
 
         if (!sim_ent_has_prop(ent, SIM_ENT_PROP_ACTIVE)) {
             u64 atick = ent->activation_tick;
-            if (atick != 0 || G.tick.tick_id >= atick) {
-                sim_ent_activate(ent, G.tick.tick_id);
+            if (atick != 0 || ctx->tick.tick_id >= atick) {
+                sim_ent_activate(ent, ctx->tick.tick_id);
             }
         }
     }
@@ -532,7 +532,7 @@ INTERNAL void sim_update(void)
 
     struct sim_cmd_list sim_cmds = ZI;
     {
-        struct host_event_array host_events = host_pop_events(scratch.arena, G.host);
+        struct host_event_array host_events = host_pop_events(scratch.arena, ctx->host);
         sim_cmds_from_host_events(scratch.arena, host_events, &sim_cmds);
     }
 
@@ -544,26 +544,26 @@ INTERNAL void sim_update(void)
         enum sim_cmd_kind kind = cmd->kind;
         struct host_channel_id channel_id = cmd->channel_id;
 
-        struct sim_client *client = client_from_channel_id(G.client_store, channel_id);
+        struct sim_client *client = sim_client_from_channel_id(ctx->client_store, channel_id);
         if (client->valid || host_channel_id_is_nil(channel_id)) {
             switch (kind) {
                 /* Cursor */
                 case SIM_CMD_KIND_CURSOR_MOVE:
                 {
-                    G.user_cursor = cmd->cursor_pos;
+                    ctx->user_cursor = cmd->cursor_pos;
                 } break;
 
                 /* Clear level */
                 case SIM_CMD_KIND_CLEAR_ALL:
                 {
-                    G.should_reset_level = true;
+                    ctx->should_reset_level = true;
                 } break;
 
                 /* Spawn test */
                 case SIM_CMD_KIND_SPAWN_TEST:
                 {
                     logf_info("Spawning (test)");
-                    spawn_test_entities();
+                    spawn_test_entities(ctx);
                 } break;
 
                 /* Disconnect client */
@@ -574,9 +574,9 @@ INTERNAL void sim_update(void)
                         if (client_ent->valid) {
                             sim_ent_disable_prop(client_ent, SIM_ENT_PROP_PLAYER_CONTROLLED);
                             sim_ent_enable_prop(client_ent, SIM_ENT_PROP_RELEASE_NEXT_TICK);
-                            host_queue_disconnect(G.host, channel_id);
+                            host_queue_disconnect(ctx->host, channel_id);
                         }
-                        client_release(client);
+                        sim_client_release(client);
                     }
                 } break;
 
@@ -584,7 +584,7 @@ INTERNAL void sim_update(void)
             };
         } else if (kind == SIM_CMD_KIND_SIM_CLIENT_CONNECT && !host_channel_id_is_nil(channel_id) && !client->valid) {
             /* Connect client */
-            client = client_alloc(G.client_store, channel_id);
+            client = sim_client_alloc(ctx->client_store, channel_id);
             struct sim_ent *client_ent = sim_ent_alloc(root);
             sim_ent_enable_prop(client_ent, SIM_ENT_PROP_PLAYER_CONTROLLED);
             client_ent->controlling_client = client->handle;
@@ -676,7 +676,7 @@ INTERNAL void sim_update(void)
 #endif
             ent->local_collider.count = 3;
             ent->local_collider.radius = 0.25;
-            //ent->local_collider.radius = math_fabs(math_sin(G.tick.time) / 3);
+            //ent->local_collider.radius = math_fabs(math_sin(ctx->tick.time) / 3);
 #else
                 //ent->local_collider.radius = 0.5;
             ent->local_collider.radius = 0.25;
@@ -729,7 +729,7 @@ INTERNAL void sim_update(void)
                 b32 start = cmd->state == SIM_CMD_STATE_START;
                 b32 stop = cmd->state == SIM_CMD_STATE_STOP;
 
-                /* TODO: Combine movement from multiple inputs? E.G. a sudden
+                /* TODO: Combine movement from multiple inputs? E.ctx-> a sudden
                  * start and immediate stop cmd should still move the player a
                  * tad. */
                 switch (cmd->kind) {
@@ -1071,30 +1071,31 @@ INTERNAL void sim_update(void)
      * ========================== */
 
     {
-        struct phys_ctx ctx = ZI;
-        ctx.tick_id = G.tick.tick_id;
-        ctx.store = ent_store;
-        ctx.space = space;
-        ctx.contact_lookup = &G.contact_lookup;
-        ctx.pre_solve_callback = on_collision;
+        struct phys_ctx phys = ZI;
+        phys.tick_id = ctx->tick.tick_id;
+        phys.store = ent_store;
+        phys.space = space;
+        phys.contact_lookup = &ctx->contact_lookup;
+        phys.pre_solve_callback = on_collision;
+        phys.pre_solve_callback_udata = ctx;
 #if COLLIDER_DEBUG
-        ctx.debug_lookup = &G.collision_debug_lookup;
+        phys.debug_lookup = &phys->collision_debug_lookup;
 #endif
 
         /* Mouse drag */
-        ctx.dbg_cursor_pos = G.user_cursor;
+        phys.dbg_cursor_pos = ctx->user_cursor;
         for (struct sim_cmd *cmd = sim_cmds.first; cmd; cmd = cmd->next) {
             if (cmd->kind == SIM_CMD_KIND_DRAG_OBJECT) {
                 if (cmd->state == SIM_CMD_STATE_START) {
-                    ctx.dbg_start_dragging = true;
+                    phys.dbg_start_dragging = true;
                 } else if (cmd->state == SIM_CMD_STATE_STOP) {
-                    ctx.dbg_stop_dragging = true;
+                    phys.dbg_stop_dragging = true;
                 }
             }
         }
 
         /* Step */
-        G.last_phys_iteration = phys_step(&ctx, dt, G.last_phys_iteration);
+        ctx->last_phys_iteration = phys_step(&phys, dt, ctx->last_phys_iteration);
     }
 
     /* ========================== *
@@ -1130,7 +1131,7 @@ INTERNAL void sim_update(void)
         if (!sim_ent_is_valid_and_active(ent)) continue;
         if (!sim_ent_has_prop(ent, SIM_ENT_PROP_BULLET)) continue;
 
-        if (ent->activation_tick == G.tick.tick_id) {
+        if (ent->activation_tick == ctx->tick.tick_id) {
             struct sim_ent *src = sim_ent_from_handle(ent_store, ent->bullet_src);
             struct xform src_xf = sim_ent_get_xform(src);
 
@@ -1276,6 +1277,7 @@ INTERNAL void sim_update(void)
         arena_temp_end(temp);
     }
 
+#if 0
     /* ========================== *
      * Update sound emitters
      * ========================== */
@@ -1290,7 +1292,7 @@ INTERNAL void sim_update(void)
 
         if (sim_ent_has_prop(ent, SIM_ENT_PROP_TEST_SOUND_EMITTER)) {
             struct mixer_desc desc = ent->sound_desc;
-            desc.speed = G.tick.timescale;
+            desc.speed = ctx->tick.timescale;
 
             desc.pos = sim_ent_get_xform(ent).og;
             struct sound *sound = sound_load_async(ent->sound_name, 0);
@@ -1304,12 +1306,13 @@ INTERNAL void sim_update(void)
             }
         }
     }
+#endif
 
     /* ========================== *
      * Release entities
      * ========================== */
 
-    release_entities_with_prop(SIM_ENT_PROP_RELEASE_THIS_TICK);
+    release_entities_with_prop(ctx, SIM_ENT_PROP_RELEASE_THIS_TICK);
 
     /* ========================== *
      * Publish tick
@@ -1321,19 +1324,19 @@ INTERNAL void sim_update(void)
         /* TODO: Not like this */
         struct sim_event snapshot_event = ZI;
         snapshot_event.kind = SIM_EVENT_KIND_SNAPSHOT_FULL;
-        snapshot_event.snapshot_data = sim_string_from_tick(temp.arena, &G.tick);
+        snapshot_event.snapshot_data = sim_string_from_tick(temp.arena, &ctx->tick);
 
         struct sim_event_list l = ZI;
         l.first = &snapshot_event;
         l.last = &snapshot_event;
         struct string msg = sim_string_from_events(temp.arena, l);
 
-        host_queue_write(G.host, HOST_CHANNEL_ID_ALL, msg, 0);
+        host_queue_write(ctx->host, HOST_CHANNEL_ID_ALL, msg, 0);
 
         arena_temp_end(temp);
     }
 
-    host_update(G.host);
+    host_update(ctx->host);
     __profframe("Sim");
 
     /* ========================== *
@@ -1345,281 +1348,19 @@ INTERNAL void sim_update(void)
     scratch_end(scratch);
 }
 
-/* ========================== *
- * Sim cmd
- * ========================== */
-
-struct string sim_string_from_cmds(struct arena *arena, struct sim_cmd_list cmds)
-{
-    __prof;
-    struct byte_writer bw = bw_from_arena(arena);
-
-    for (struct sim_cmd *cmd = cmds.first; cmd; cmd = cmd->next) {
-        struct byte_writer bw_size = bw_branch(&bw, sizeof(u64));
-        u64 start = bw_pos(&bw);
-
-        bw_write_i8(&bw, cmd->kind);
-        bw_write_i8(&bw, cmd->state);
-
-#if COLLIDER_DEBUG
-        bw_write_u32(&bw, cmd->collider_gjk_steps);
-#endif
-
-        switch (cmd->kind) {
-            case SIM_CMD_KIND_PLAYER_MOVE:
-            {
-                bw_write_v2(&bw, cmd->move_dir);
-                bw_write_v2(&bw, cmd->aim_dir);
-            } break;
-
-            case SIM_CMD_KIND_CURSOR_MOVE:
-            {
-                bw_write_v2(&bw, cmd->cursor_pos);
-            } break;
-
-            default: break;
-        }
-
-        u64 size = bw_pos(&bw) - start;
-        bw_write_u64(&bw_size, size);
-    }
-
-    return bw_get_written(&bw);
-}
-
-void sim_cmds_from_host_events(struct arena *arena, struct host_event_array host_events, struct sim_cmd_list *cmds_out)
-{
-    __prof;
-    for (u64 i = 0; i < host_events.count; ++i) {
-        struct host_event host_event = host_events.events[i];
-        enum host_event_kind host_event_kind = host_event.kind;
-        switch (host_event_kind) {
-            case HOST_EVENT_KIND_CHANNEL_OPENED:
-            {
-                struct sim_cmd *cmd = arena_push_zero(arena, struct sim_cmd);
-                cmd->kind = SIM_CMD_KIND_SIM_CLIENT_CONNECT;
-                cmd->channel_id = host_event.channel_id;
-                if (cmds_out->last) {
-                    cmds_out->last->next = cmd;
-                } else {
-                    cmds_out->first = cmd;
-                }
-                cmds_out->last = cmd;
-            } break;
-
-            case HOST_EVENT_KIND_CHANNEL_CLOSED:
-            {
-                struct sim_cmd *cmd = arena_push_zero(arena, struct sim_cmd);
-                cmd->kind = SIM_CMD_KIND_SIM_CLIENT_DISCONNECT;
-                cmd->disconnect_reason = LIT("Connection lost");
-                if (cmds_out->last) {
-                    cmds_out->last->next = cmd;
-                } else {
-                    cmds_out->first = cmd;
-                }
-                cmds_out->last = cmd;
-            } break;
-
-            case HOST_EVENT_KIND_MSG:
-            {
-                struct byte_reader br = br_from_buffer(host_event.msg);
-                while (br_bytes_left(&br) > 0) {
-                    struct sim_cmd *cmd = arena_push_zero(arena, struct sim_cmd);
-                    u64 cmd_size = br_read_u64(&br);
-                    u64 cmd_pos_end = br_pos(&br) + cmd_size;
-                    cmd->kind = br_read_i8(&br);
-                    cmd->state = br_read_i8(&br);
-#if COLLIDER_DEBUG
-                    cmd->collider_gjk_steps = br_read_u32(&br);
-#endif
-
-                    switch (cmd->kind) {
-                        case SIM_CMD_KIND_PLAYER_MOVE:
-                        {
-                            cmd->move_dir = br_read_v2(&br);
-                            cmd->aim_dir = br_read_v2(&br);
-                        } break;
-
-                        case SIM_CMD_KIND_CURSOR_MOVE:
-                        {
-                            cmd->cursor_pos = br_read_v2(&br);
-                        } break;
-
-                        default: break;
-                    }
-
-                    ASSERT(br_pos(&br) == cmd_pos_end);
-                    br_seek_to(&br, cmd_pos_end);
-
-                    if (cmds_out->last) {
-                        cmds_out->last->next = cmd;
-                    } else {
-                        cmds_out->first = cmd;
-                    }
-                    cmds_out->last = cmd;
-                }
-            } break;
-
-            default: break;
-        }
-    }
-}
-
-/* ========================== *
- * Sim event
- * ========================== */
-
-struct string sim_string_from_events(struct arena *arena, struct sim_event_list events)
-{
-    __prof;
-    struct byte_writer bw = bw_from_arena(arena);
-    for (struct sim_event *event = events.first; event; event = event->next) {
-        struct byte_writer bw_size = bw_branch(&bw, sizeof(u64));
-        u64 start = bw_pos(&bw);
-        bw_write_i8(&bw, event->kind);
-
-        switch (event->kind) {
-            case SIM_EVENT_KIND_SNAPSHOT_FULL:
-            {
-                bw_write_string(&bw, event->snapshot_data);
-            } break;
-
-            default: break;
-        }
-
-        u64 size = bw_pos(&bw) - start;
-        bw_write_u64(&bw_size, size);
-    }
-    return bw_get_written(&bw);
-}
-
-void sim_events_from_host_events(struct arena *arena, struct host_event_array host_events, struct sim_event_list *events_out)
-{
-    __prof;
-    for (u64 i = 0; i < host_events.count; ++i) {
-        struct sim_event *sim_event = arena_push_zero(arena, struct sim_event);
-        struct host_event host_event = host_events.events[i];
-        enum host_event_kind host_event_kind = host_event.kind;
-        sim_event->channel_id = host_event.channel_id;
-        switch (host_event_kind) {
-            case HOST_EVENT_KIND_CHANNEL_OPENED:
-            {
-                sim_event->kind = SIM_EVENT_KIND_CONNECT;
-            } break;
-
-            case HOST_EVENT_KIND_CHANNEL_CLOSED:
-            {
-                sim_event->kind = SIM_EVENT_KIND_DISCONNECT;
-                sim_event->disconnect_reason = LIT("Connection lost");
-            } break;
-
-            case HOST_EVENT_KIND_MSG:
-            {
-                struct byte_reader br = br_from_buffer(host_event.msg);
-                while (br_bytes_left(&br) > 0) {
-                    u64 event_size = br_read_u64(&br);
-                    u64 event_pos_end = br_pos(&br) + event_size;
-
-                    sim_event->kind = br_read_i8(&br);
-                    switch (sim_event->kind) {
-                        case SIM_EVENT_KIND_SNAPSHOT_FULL:
-                        {
-                            sim_event->snapshot_data = br_read_string(arena, &br);
-                        } break;
-
-                        default: break;
-                    }
-
-                    ASSERT(br_pos(&br) == event_pos_end);
-                    br_seek_to(&br, event_pos_end);
-                }
-            } break;
-
-            default: break;
-        }
-
-        if (events_out->last) {
-            events_out->last->next = sim_event;
-        } else {
-            events_out->first = sim_event;
-        }
-        events_out->last = sim_event;
-    }
-}
-
-/* ========================== *
- * Snapshot
- * ========================== */
-
-struct string sim_string_from_tick(struct arena *arena, struct world *tick)
-{
-    __prof;
-    struct byte_writer bw = bw_from_arena(arena);
-
-    bw_write_var_uint(&bw, tick->continuity_gen);
-    bw_write_var_uint(&bw, tick->tick_id);
-    bw_write_var_sint(&bw, tick->publishtime_ns);
-
-    bw_write_f64(&bw, tick->timescale);
-    bw_write_var_sint(&bw, tick->dt_ns);
-    bw_write_var_sint(&bw, tick->time_ns);
-
-    u64 num_entities = tick->ent_store->num_reserved;
-    bw_write_var_uint(&bw, num_entities);
-
-    struct string entities_src = ZI;
-    entities_src.text = (u8 *)tick->ent_store->entities;
-    entities_src.len = sizeof(struct sim_ent) * num_entities;
-    br_write_bytes(&bw, entities_src);
-
-    return bw_get_written(&bw);
-}
-
-void sim_tick_from_string(struct string str, struct world *tick_out)
-{
-    __prof;
-    struct byte_reader br = br_from_buffer(str);
-
-    tick_out->continuity_gen = br_read_var_uint(&br);
-    tick_out->tick_id = br_read_var_uint(&br);
-    tick_out->publishtime_ns = br_read_var_sint(&br);
-
-    tick_out->timescale = br_read_f64(&br);
-    tick_out->dt_ns = br_read_var_sint(&br);
-    tick_out->time_ns = br_read_var_sint(&br);
-
-    u64 num_entities = br_read_var_uint(&br);
-    arena_push_array(&tick_out->ent_store->arena, struct sim_ent, num_entities - tick_out->ent_store->num_reserved);
-    tick_out->ent_store->num_reserved = num_entities;
-
-    tick_out->ent_store->num_allocated = 0;
-    struct sim_ent *entities_src = br_seek(&br, num_entities * sizeof(struct sim_ent));
-    if (entities_src) {
-        for (u64 i = 0; i < num_entities; ++i) {
-            struct sim_ent *src = &entities_src[i];
-            struct sim_ent *dst = &tick_out->ent_store->entities[i];
-            if (dst->valid) {
-                ++tick_out->ent_store->num_allocated;
-            }
-            *dst = *src;
-        }
-    }
-}
-
 /* ========================== *
  * Sim thread
  * ========================== */
 
-
 INTERNAL SYS_THREAD_ENTRY_POINT_FUNC_DEF(sim_thread_entry_point, arg)
 {
-    (UNUSED)arg;
+    struct sim_ctx *ctx = (struct sim_ctx *)arg;
     i64 last_frame_ns = 0;
     i64 target_dt_ns = NS_FROM_SECONDS(SIM_FPS > (0) ? (1.0 / SIM_FPS) : 0);
-    while (!atomic_i32_eval(&G.sim_thread_shutdown)) {
+    while (!atomic_i32_eval(&ctx->sim_thread_shutdown)) {
         __profscope(sim_update_w_sleep);
         sleep_frame(last_frame_ns, target_dt_ns);
         last_frame_ns = sys_time_ns();
-        sim_update();
+        sim_update(ctx);
     }
 }
diff --git a/src/sim.h b/src/sim.h
index 19e42f51..372afaca 100644
--- a/src/sim.h
+++ b/src/sim.h
@@ -1,12 +1,8 @@
 #ifndef SIM_H
 #define SIM_H
 
-#include "host.h"
-
-struct world;
-struct mixer_startup_receipt;
+struct sim_ctx;
 struct sprite_startup_receipt;
-struct sound_startup_receipt;
 struct phys_startup_receipt;
 struct host_startup_receipt;
 
@@ -20,116 +16,13 @@ struct host_startup_receipt;
 #define SIM_LAYER_RELATIVE_DEFAULT     0
 #define SIM_LAYER_RELATIVE_WEAPON      1
 
-struct sim_startup_receipt { i32 _; };
-struct sim_startup_receipt sim_startup(struct mixer_startup_receipt *mixer_sr,
-                                         struct sprite_startup_receipt *sheet_sr,
-                                         struct sound_startup_receipt *sound_sr,
-                                         struct phys_startup_receipt *phys_sr,
-                                         struct host_startup_receipt *host_sr);
+/* TODO: Get rid of startup receipts */
+struct sim_ctx *sim_ctx_alloc(struct sprite_startup_receipt *sprite_sr,
+                              struct phys_startup_receipt *phys_sr,
+                              struct host_startup_receipt *host_sr,
+                              u16 host_port);
 
+void sim_ctx_release(struct sim_ctx *ctx);
 
-/* ========================== *
- * Sim cmd
- * ========================== */
-
-enum sim_cmd_state {
-    SIM_CMD_STATE_STOP = -1,
-    SIM_CMD_STATE_NO_CHANGE = 0,
-    SIM_CMD_STATE_START = 1
-};
-
-enum sim_cmd_kind {
-    SIM_CMD_KIND_NONE,
-
-    SIM_CMD_KIND_PLAYER_MOVE,
-    SIM_CMD_KIND_PLAYER_FIRE,
-
-    SIM_CMD_KIND_SIM_CLIENT_CONNECT,
-    SIM_CMD_KIND_SIM_CLIENT_DISCONNECT,
-
-    /* Testing */
-    SIM_CMD_KIND_CLEAR_ALL,
-    SIM_CMD_KIND_SPAWN_TEST,
-    SIM_CMD_KIND_PAUSE,
-    SIM_CMD_KIND_STEP,
-    SIM_CMD_KIND_DRAG_OBJECT,
-    SIM_CMD_KIND_CURSOR_MOVE,
-
-    SIM_CMD_KIND_COUNT
-};
-
-struct sim_cmd {
-    enum sim_cmd_kind kind;
-    enum sim_cmd_state state;
-    struct host_channel_id channel_id;
-
-    /* SIM_CMD_KIND_PLAYER_MOVE */
-    struct v2 move_dir;
-    struct v2 aim_dir;
-
-    /* SIM_CMD_KIND_CURSOR_MOVE */
-    struct v2 cursor_pos;
-
-    /* SIM_CMD_KIND_PLAYER_DISCONNECT */
-    struct string disconnect_reason;
-
-#if RTC
-    u32 collider_gjk_steps;
-#endif
-
-    struct sim_cmd *next;
-};
-
-struct sim_cmd_list {
-    struct sim_cmd *first;
-    struct sim_cmd *last;
-};
-
-struct string sim_string_from_cmds(struct arena *arena, struct sim_cmd_list cmds);
-void sim_cmds_from_host_events(struct arena *arena, struct host_event_array host_events, struct sim_cmd_list *cmds_out);
-
-/* ========================== *
- * Sim event
- * ========================== */
-
-enum sim_event_kind {
-    SIM_EVENT_KIND_NONE,
-
-    SIM_EVENT_KIND_CONNECT,
-    SIM_EVENT_KIND_DISCONNECT,
-    SIM_EVENT_KIND_SNAPSHOT_FULL,
-
-    //SIM_EVENT_KIND_ENTITY_UPDATE,
-    //SIM_EVENT_KIND_ENTITY_CREATE,
-    //SIM_EVENT_KIND_ENTITY_DESTROY
-};
-
-struct sim_event {
-    enum sim_event_kind kind;
-    struct host_channel_id channel_id;
-
-    struct string snapshot_data;
-    struct string disconnect_reason;
-
-    //struct sim_ent_handle entity;
-    //struct string update_data;
-
-    struct sim_event *next;
-};
-
-struct sim_event_list {
-    struct sim_event *first;
-    struct sim_event *last;
-};
-
-struct string sim_string_from_events(struct arena *arena, struct sim_event_list events);
-void sim_events_from_host_events(struct arena *arena, struct host_event_array host_events, struct sim_event_list *events_out);
-
-/* ========================== *
- * Snapshot
- * ========================== */
-
-struct string sim_string_from_tick(struct arena *arena, struct world *tick);
-void sim_tick_from_string(struct string str, struct world *tick_out);
 
 #endif
diff --git a/src/sim_client.c b/src/sim_client.c
index 15312a06..f13cc23f 100644
--- a/src/sim_client.c
+++ b/src/sim_client.c
@@ -17,7 +17,7 @@ READONLY struct sim_client_store _g_sim_client_store_nil = { .valid = false };
  * Store
  * ========================== */
 
-struct sim_client_store *client_store_alloc(void)
+struct sim_client_store *sim_client_store_alloc(void)
 {
     struct arena arena = arena_alloc(GIGABYTE(64));
     u64 num_channel_lookup_buckets = CHANNEL_LOOKUP_BUCKETS;
@@ -34,7 +34,7 @@ struct sim_client_store *client_store_alloc(void)
     return store;
 }
 
-void client_store_release(struct sim_client_store *store)
+void sim_client_store_release(struct sim_client_store *store)
 {
     arena_release(&store->arena);
 }
@@ -60,7 +60,7 @@ INTERNAL u64 hash_from_channel_id(struct host_channel_id channel_id)
     return hash_fnv64(HASH_FNV64_BASIS, STRING_FROM_STRUCT(&channel_id));
 }
 
-struct sim_client *client_from_handle(struct sim_client_store *store, struct client_handle handle)
+struct sim_client *sim_client_from_handle(struct sim_client_store *store, struct client_handle handle)
 {
     if (handle.gen != 0 && handle.idx < store->clients_reserved) {
         struct sim_client *client = &store->clients[handle.idx];
@@ -71,7 +71,7 @@ struct sim_client *client_from_handle(struct sim_client_store *store, struct cli
     return client_nil();
 }
 
-struct sim_client *client_from_channel_id(struct sim_client_store *store, struct host_channel_id channel_id)
+struct sim_client *sim_client_from_channel_id(struct sim_client_store *store, struct host_channel_id channel_id)
 {
     struct sim_client *res = client_nil();
     u64 channel_hash = hash_from_channel_id(channel_id);
@@ -86,7 +86,7 @@ struct sim_client *client_from_channel_id(struct sim_client_store *store, struct
     return res;
 }
 
-struct sim_client *client_alloc(struct sim_client_store *store, struct host_channel_id channel_id)
+struct sim_client *sim_client_alloc(struct sim_client_store *store, struct host_channel_id channel_id)
 {
     struct sim_client *client = NULL;
     struct client_handle handle = ZI;
@@ -122,7 +122,7 @@ struct sim_client *client_alloc(struct sim_client_store *store, struct host_chan
     return client;
 }
 
-void client_release(struct sim_client *client)
+void sim_client_release(struct sim_client *client)
 {
     struct sim_client_store *store = client_store_from_client(client);
     client->valid = false;
diff --git a/src/sim_client.h b/src/sim_client.h
index c5c22a3e..c82fde87 100644
--- a/src/sim_client.h
+++ b/src/sim_client.h
@@ -50,12 +50,12 @@ INLINE struct sim_client_store *client_store_nil(void)
     return &_g_sim_client_store_nil;
 }
 
-struct sim_client_store *client_store_alloc(void);
-void client_store_release(struct sim_client_store *store);
+struct sim_client_store *sim_client_store_alloc(void);
+void sim_client_store_release(struct sim_client_store *store);
 struct sim_client_store *client_store_from_client(struct sim_client *client);
-struct sim_client *client_from_handle(struct sim_client_store *store, struct client_handle handle);
-struct sim_client *client_from_channel_id(struct sim_client_store *store, struct host_channel_id channel_id);
-struct sim_client *client_alloc(struct sim_client_store *store, struct host_channel_id channel_id);
-void client_release(struct sim_client *client);
+struct sim_client *sim_client_from_handle(struct sim_client_store *store, struct client_handle handle);
+struct sim_client *sim_client_from_channel_id(struct sim_client_store *store, struct host_channel_id channel_id);
+struct sim_client *sim_client_alloc(struct sim_client_store *store, struct host_channel_id channel_id);
+void sim_client_release(struct sim_client *client);
 
 #endif
diff --git a/src/sim_msg.c b/src/sim_msg.c
new file mode 100644
index 00000000..d93de2b9
--- /dev/null
+++ b/src/sim_msg.c
@@ -0,0 +1,265 @@
+#include "sim_msg.h"
+#include "arena.h"
+#include "byteio.h"
+#include "world.h"
+
+/* ========================== *
+ * Sim cmd
+ * ========================== */
+
+struct string sim_string_from_cmds(struct arena *arena, struct sim_cmd_list cmds)
+{
+    __prof;
+    struct byte_writer bw = bw_from_arena(arena);
+
+    for (struct sim_cmd *cmd = cmds.first; cmd; cmd = cmd->next) {
+        struct byte_writer bw_size = bw_branch(&bw, sizeof(u64));
+        u64 start = bw_pos(&bw);
+
+        bw_write_i8(&bw, cmd->kind);
+        bw_write_i8(&bw, cmd->state);
+
+#if COLLIDER_DEBUG
+        bw_write_u32(&bw, cmd->collider_gjk_steps);
+#endif
+
+        switch (cmd->kind) {
+            case SIM_CMD_KIND_PLAYER_MOVE:
+            {
+                bw_write_v2(&bw, cmd->move_dir);
+                bw_write_v2(&bw, cmd->aim_dir);
+            } break;
+
+            case SIM_CMD_KIND_CURSOR_MOVE:
+            {
+                bw_write_v2(&bw, cmd->cursor_pos);
+            } break;
+
+            default: break;
+        }
+
+        u64 size = bw_pos(&bw) - start;
+        bw_write_u64(&bw_size, size);
+    }
+
+    return bw_get_written(&bw);
+}
+
+void sim_cmds_from_host_events(struct arena *arena, struct host_event_array host_events, struct sim_cmd_list *cmds_out)
+{
+    __prof;
+    for (u64 i = 0; i < host_events.count; ++i) {
+        struct host_event host_event = host_events.events[i];
+        enum host_event_kind host_event_kind = host_event.kind;
+        switch (host_event_kind) {
+            case HOST_EVENT_KIND_CHANNEL_OPENED:
+            {
+                struct sim_cmd *cmd = arena_push_zero(arena, struct sim_cmd);
+                cmd->kind = SIM_CMD_KIND_SIM_CLIENT_CONNECT;
+                cmd->channel_id = host_event.channel_id;
+                if (cmds_out->last) {
+                    cmds_out->last->next = cmd;
+                } else {
+                    cmds_out->first = cmd;
+                }
+                cmds_out->last = cmd;
+            } break;
+
+            case HOST_EVENT_KIND_CHANNEL_CLOSED:
+            {
+                struct sim_cmd *cmd = arena_push_zero(arena, struct sim_cmd);
+                cmd->kind = SIM_CMD_KIND_SIM_CLIENT_DISCONNECT;
+                cmd->disconnect_reason = LIT("Connection lost");
+                if (cmds_out->last) {
+                    cmds_out->last->next = cmd;
+                } else {
+                    cmds_out->first = cmd;
+                }
+                cmds_out->last = cmd;
+            } break;
+
+            case HOST_EVENT_KIND_MSG:
+            {
+                struct byte_reader br = br_from_buffer(host_event.msg);
+                while (br_bytes_left(&br) > 0) {
+                    struct sim_cmd *cmd = arena_push_zero(arena, struct sim_cmd);
+                    u64 cmd_size = br_read_u64(&br);
+                    u64 cmd_pos_end = br_pos(&br) + cmd_size;
+                    cmd->kind = br_read_i8(&br);
+                    cmd->state = br_read_i8(&br);
+#if COLLIDER_DEBUG
+                    cmd->collider_gjk_steps = br_read_u32(&br);
+#endif
+
+                    switch (cmd->kind) {
+                        case SIM_CMD_KIND_PLAYER_MOVE:
+                        {
+                            cmd->move_dir = br_read_v2(&br);
+                            cmd->aim_dir = br_read_v2(&br);
+                        } break;
+
+                        case SIM_CMD_KIND_CURSOR_MOVE:
+                        {
+                            cmd->cursor_pos = br_read_v2(&br);
+                        } break;
+
+                        default: break;
+                    }
+
+                    ASSERT(br_pos(&br) == cmd_pos_end);
+                    br_seek_to(&br, cmd_pos_end);
+
+                    if (cmds_out->last) {
+                        cmds_out->last->next = cmd;
+                    } else {
+                        cmds_out->first = cmd;
+                    }
+                    cmds_out->last = cmd;
+                }
+            } break;
+
+            default: break;
+        }
+    }
+}
+
+/* ========================== *
+ * Sim event
+ * ========================== */
+
+struct string sim_string_from_events(struct arena *arena, struct sim_event_list events)
+{
+    __prof;
+    struct byte_writer bw = bw_from_arena(arena);
+    for (struct sim_event *event = events.first; event; event = event->next) {
+        struct byte_writer bw_size = bw_branch(&bw, sizeof(u64));
+        u64 start = bw_pos(&bw);
+        bw_write_i8(&bw, event->kind);
+
+        switch (event->kind) {
+            case SIM_EVENT_KIND_SNAPSHOT_FULL:
+            {
+                bw_write_string(&bw, event->snapshot_data);
+            } break;
+
+            default: break;
+        }
+
+        u64 size = bw_pos(&bw) - start;
+        bw_write_u64(&bw_size, size);
+    }
+    return bw_get_written(&bw);
+}
+
+void sim_events_from_host_events(struct arena *arena, struct host_event_array host_events, struct sim_event_list *events_out)
+{
+    __prof;
+    for (u64 i = 0; i < host_events.count; ++i) {
+        struct sim_event *sim_event = arena_push_zero(arena, struct sim_event);
+        struct host_event host_event = host_events.events[i];
+        enum host_event_kind host_event_kind = host_event.kind;
+        sim_event->channel_id = host_event.channel_id;
+        switch (host_event_kind) {
+            case HOST_EVENT_KIND_CHANNEL_OPENED:
+            {
+                sim_event->kind = SIM_EVENT_KIND_CONNECT;
+            } break;
+
+            case HOST_EVENT_KIND_CHANNEL_CLOSED:
+            {
+                sim_event->kind = SIM_EVENT_KIND_DISCONNECT;
+                sim_event->disconnect_reason = LIT("Connection lost");
+            } break;
+
+            case HOST_EVENT_KIND_MSG:
+            {
+                struct byte_reader br = br_from_buffer(host_event.msg);
+                while (br_bytes_left(&br) > 0) {
+                    u64 event_size = br_read_u64(&br);
+                    u64 event_pos_end = br_pos(&br) + event_size;
+
+                    sim_event->kind = br_read_i8(&br);
+                    switch (sim_event->kind) {
+                        case SIM_EVENT_KIND_SNAPSHOT_FULL:
+                        {
+                            sim_event->snapshot_data = br_read_string(arena, &br);
+                        } break;
+
+                        default: break;
+                    }
+
+                    ASSERT(br_pos(&br) == event_pos_end);
+                    br_seek_to(&br, event_pos_end);
+                }
+            } break;
+
+            default: break;
+        }
+
+        if (events_out->last) {
+            events_out->last->next = sim_event;
+        } else {
+            events_out->first = sim_event;
+        }
+        events_out->last = sim_event;
+    }
+}
+
+/* ========================== *
+ * Snapshot
+ * ========================== */
+
+struct string sim_string_from_tick(struct arena *arena, struct world *tick)
+{
+    __prof;
+    struct byte_writer bw = bw_from_arena(arena);
+
+    bw_write_var_uint(&bw, tick->continuity_gen);
+    bw_write_var_uint(&bw, tick->tick_id);
+    bw_write_var_sint(&bw, tick->publishtime_ns);
+
+    bw_write_f64(&bw, tick->timescale);
+    bw_write_var_sint(&bw, tick->dt_ns);
+    bw_write_var_sint(&bw, tick->time_ns);
+
+    u64 num_entities = tick->ent_store->num_reserved;
+    bw_write_var_uint(&bw, num_entities);
+
+    struct string entities_src = ZI;
+    entities_src.text = (u8 *)tick->ent_store->entities;
+    entities_src.len = sizeof(struct sim_ent) * num_entities;
+    br_write_bytes(&bw, entities_src);
+
+    return bw_get_written(&bw);
+}
+
+void sim_tick_from_string(struct string str, struct world *tick_out)
+{
+    __prof;
+    struct byte_reader br = br_from_buffer(str);
+
+    tick_out->continuity_gen = br_read_var_uint(&br);
+    tick_out->tick_id = br_read_var_uint(&br);
+    tick_out->publishtime_ns = br_read_var_sint(&br);
+
+    tick_out->timescale = br_read_f64(&br);
+    tick_out->dt_ns = br_read_var_sint(&br);
+    tick_out->time_ns = br_read_var_sint(&br);
+
+    u64 num_entities = br_read_var_uint(&br);
+    arena_push_array(&tick_out->ent_store->arena, struct sim_ent, num_entities - tick_out->ent_store->num_reserved);
+    tick_out->ent_store->num_reserved = num_entities;
+
+    tick_out->ent_store->num_allocated = 0;
+    struct sim_ent *entities_src = br_seek(&br, num_entities * sizeof(struct sim_ent));
+    if (entities_src) {
+        for (u64 i = 0; i < num_entities; ++i) {
+            struct sim_ent *src = &entities_src[i];
+            struct sim_ent *dst = &tick_out->ent_store->entities[i];
+            if (dst->valid) {
+                ++tick_out->ent_store->num_allocated;
+            }
+            *dst = *src;
+        }
+    }
+}
diff --git a/src/sim_msg.h b/src/sim_msg.h
new file mode 100644
index 00000000..4a877a69
--- /dev/null
+++ b/src/sim_msg.h
@@ -0,0 +1,112 @@
+#ifndef SIM_MSG_H
+#define SIM_MSG_H
+
+#include "host.h"
+
+struct world;
+
+/* ========================== *
+ * Sim cmd
+ * ========================== */
+
+enum sim_cmd_state {
+    SIM_CMD_STATE_STOP = -1,
+    SIM_CMD_STATE_NO_CHANGE = 0,
+    SIM_CMD_STATE_START = 1
+};
+
+enum sim_cmd_kind {
+    SIM_CMD_KIND_NONE,
+
+    SIM_CMD_KIND_PLAYER_MOVE,
+    SIM_CMD_KIND_PLAYER_FIRE,
+
+    SIM_CMD_KIND_SIM_CLIENT_CONNECT,
+    SIM_CMD_KIND_SIM_CLIENT_DISCONNECT,
+
+    /* Testing */
+    SIM_CMD_KIND_CLEAR_ALL,
+    SIM_CMD_KIND_SPAWN_TEST,
+    SIM_CMD_KIND_PAUSE,
+    SIM_CMD_KIND_STEP,
+    SIM_CMD_KIND_DRAG_OBJECT,
+    SIM_CMD_KIND_CURSOR_MOVE,
+
+    SIM_CMD_KIND_COUNT
+};
+
+struct sim_cmd {
+    enum sim_cmd_kind kind;
+    enum sim_cmd_state state;
+    struct host_channel_id channel_id;
+
+    /* SIM_CMD_KIND_PLAYER_MOVE */
+    struct v2 move_dir;
+    struct v2 aim_dir;
+
+    /* SIM_CMD_KIND_CURSOR_MOVE */
+    struct v2 cursor_pos;
+
+    /* SIM_CMD_KIND_PLAYER_DISCONNECT */
+    struct string disconnect_reason;
+
+#if RTC
+    u32 collider_gjk_steps;
+#endif
+
+    struct sim_cmd *next;
+};
+
+struct sim_cmd_list {
+    struct sim_cmd *first;
+    struct sim_cmd *last;
+};
+
+struct string sim_string_from_cmds(struct arena *arena, struct sim_cmd_list cmds);
+void sim_cmds_from_host_events(struct arena *arena, struct host_event_array host_events, struct sim_cmd_list *cmds_out);
+
+/* ========================== *
+ * Sim event
+ * ========================== */
+
+enum sim_event_kind {
+    SIM_EVENT_KIND_NONE,
+
+    SIM_EVENT_KIND_CONNECT,
+    SIM_EVENT_KIND_DISCONNECT,
+    SIM_EVENT_KIND_SNAPSHOT_FULL,
+
+    //SIM_EVENT_KIND_ENTITY_UPDATE,
+    //SIM_EVENT_KIND_ENTITY_CREATE,
+    //SIM_EVENT_KIND_ENTITY_DESTROY
+};
+
+struct sim_event {
+    enum sim_event_kind kind;
+    struct host_channel_id channel_id;
+
+    struct string snapshot_data;
+    struct string disconnect_reason;
+
+    //struct sim_ent_handle entity;
+    //struct string update_data;
+
+    struct sim_event *next;
+};
+
+struct sim_event_list {
+    struct sim_event *first;
+    struct sim_event *last;
+};
+
+struct string sim_string_from_events(struct arena *arena, struct sim_event_list events);
+void sim_events_from_host_events(struct arena *arena, struct host_event_array host_events, struct sim_event_list *events_out);
+
+/* ========================== *
+ * Snapshot
+ * ========================== */
+
+struct string sim_string_from_tick(struct arena *arena, struct world *tick);
+void sim_tick_from_string(struct string str, struct world *tick_out);
+
+#endif
diff --git a/src/sock.h b/src/sock.h
index 9ecc5a6c..7a4f7c1f 100644
--- a/src/sock.h
+++ b/src/sock.h
@@ -8,13 +8,8 @@ enum sock_address_family {
     SOCK_ADDRESS_FAMILY_IPV6
 };
 
-struct sock {
-    i32 _;
-};
-
-struct sock_signal {
-    i32 _;
-};
+struct sock;
+struct sock_signal;
 
 struct sock_array {
     struct sock **socks;
diff --git a/src/user.c b/src/user.c
index 92d23ba0..7edbcde6 100644
--- a/src/user.c
+++ b/src/user.c
@@ -2,6 +2,7 @@
 #include "app.h"
 #include "sim.h"
 #include "sim_ent.h"
+#include "sim_msg.h"
 #include "renderer.h"
 #include "font.h"
 #include "sprite.h"
@@ -46,9 +47,8 @@ GLOBAL struct {
     struct sys_thread user_thread;
 
     struct arena arena;
-
     struct sys_window *window;
-
+    struct sim_ctx *sim_ctx;
     struct host *host;
 
     /* Usage stats */
@@ -148,9 +148,10 @@ struct user_startup_receipt user_startup(struct work_startup_receipt *work_sr,
                                          struct font_startup_receipt *font_sr,
                                          struct sprite_startup_receipt *sprite_sr,
                                          struct draw_startup_receipt *draw_sr,
-                                         struct sim_startup_receipt *sim_sr,
                                          struct asset_cache_startup_receipt *asset_cache_sr,
+                                         struct sound_startup_receipt *sound_sr,
                                          struct mixer_startup_receipt *mixer_sr,
+                                         struct phys_startup_receipt *phys_sr,
                                          struct host_startup_receipt *host_sr,
                                          struct sys_window *window)
 {
@@ -159,9 +160,10 @@ struct user_startup_receipt user_startup(struct work_startup_receipt *work_sr,
     (UNUSED)font_sr;
     (UNUSED)sprite_sr;
     (UNUSED)draw_sr;
-    (UNUSED)sim_sr;
     (UNUSED)asset_cache_sr;
+    (UNUSED)sound_sr;
     (UNUSED)mixer_sr;
+    (UNUSED)phys_sr;
     (UNUSED)host_sr;
 
     G.arena = arena_alloc(GIGABYTE(64));
@@ -170,8 +172,7 @@ struct user_startup_receipt user_startup(struct work_startup_receipt *work_sr,
     world_alloc(&G.world);
 
     //struct sock_address bind_addr = sock_address_from_any_local_interface_with_dynamic_port();
-    //G.host = host_alloc(0);
-    G.host = host_alloc(5920);
+    G.host = host_alloc(0);
 
     G.world_to_ui_xf = XFORM_IDENT;
     G.world_cmd_buffer = renderer_cmd_buffer_alloc();
@@ -182,6 +183,8 @@ struct user_startup_receipt user_startup(struct work_startup_receipt *work_sr,
     G.window = window;
     sys_window_register_event_callback(G.window, &window_event_callback);
 
+    G.sim_ctx = sim_ctx_alloc(sprite_sr, phys_sr, host_sr, 12345);
+
     G.debug_draw = true;
 
     G.user_thread = sys_thread_alloc(&user_thread_entry_point, NULL, LIT("[P1] User thread"));
@@ -195,6 +198,10 @@ INTERNAL APP_EXIT_CALLBACK_FUNC_DEF(user_shutdown)
     __prof;
     atomic_i32_eval_exchange(&G.user_thread_shutdown, true);
     sys_thread_wait_release(&G.user_thread);
+
+    if (G.sim_ctx) {
+        sim_ctx_release(G.sim_ctx);
+    }
 }
 
 /* ========================== *
@@ -1620,10 +1627,10 @@ INTERNAL void user_update(void)
             pos.y += spacing;
 
             pos.y += spacing;
-            draw_text(G.ui_cmd_buffer, font, pos, string_format(temp.arena, LIT("Client data read: %F mbit/s"), FMT_FLOAT_P((f64)G.client_bytes_read.last_second * 8 / 1024 / 1024, 2)));
+            draw_text(G.ui_cmd_buffer, font, pos, string_format(temp.arena, LIT("Client data read: %F mbit/s"), FMT_FLOAT_P((f64)G.client_bytes_read.last_second * 8 / 1000 / 1000, 2)));
             pos.y += spacing;
 
-            draw_text(G.ui_cmd_buffer, font, pos, string_format(temp.arena, LIT("Client data sent: %F mbit/s"), FMT_FLOAT_P((f64)G.client_bytes_sent.last_second * 8 / 1024 / 1024, 2)));
+            draw_text(G.ui_cmd_buffer, font, pos, string_format(temp.arena, LIT("Client data sent: %F mbit/s"), FMT_FLOAT_P((f64)G.client_bytes_sent.last_second * 8 / 1000 / 1000, 2)));
             pos.y += spacing;
 
             draw_text(G.ui_cmd_buffer, font, pos, string_format(temp.arena, LIT("Memory usage: %F MiB"), FMT_FLOAT_P((f64)atomic_u64_eval(&app_statistics()->memory_committed) / 1024 / 1024, 2)));
diff --git a/src/user.h b/src/user.h
index a80739c3..6163c4b7 100644
--- a/src/user.h
+++ b/src/user.h
@@ -7,9 +7,10 @@ struct renderer_startup_receipt;
 struct font_startup_receipt;
 struct sprite_startup_receipt;
 struct draw_startup_receipt;
-struct sim_startup_receipt;
 struct asset_cache_startup_receipt;
+struct sound_startup_receipt;
 struct mixer_startup_receipt;
+struct phys_startup_receipt;
 struct host_startup_receipt;
 
 enum user_bind_kind {
@@ -54,9 +55,10 @@ struct user_startup_receipt user_startup(struct work_startup_receipt *work_sr,
                                          struct font_startup_receipt *font_sr,
                                          struct sprite_startup_receipt *sprite_sr,
                                          struct draw_startup_receipt *draw_sr,
-                                         struct sim_startup_receipt *sim_sr,
                                          struct asset_cache_startup_receipt *asset_cache_sr,
+                                         struct sound_startup_receipt *sound_sr,
                                          struct mixer_startup_receipt *mixer_sr,
+                                         struct phys_startup_receipt *phys_sr,
                                          struct host_startup_receipt *host_sr,
                                          struct sys_window *window);