diff --git a/main.go b/main.go
index ab1828d940b08b..e3eb446f6d8377 100644
--- a/main.go
+++ b/main.go
@@ -58,16 +58,14 @@ func main() {
 	cwd, err := os.Getwd()
 	check(err)
 
+	var command = Msg_START // TODO use proto3
 	PubMsg("start", &Msg{
-		Payload: &Msg_Start{
-			Start: &StartMsg{
-				Cwd:       &cwd,
-				Argv:      args,
-				DebugFlag: flagDebug,
-				MainJs:    &main_js,
-				MainMap:   &main_map,
-			},
-		},
+		Command:        &command,
+		StartCwd:       &cwd,
+		StartArgv:      args,
+		StartDebugFlag: flagDebug,
+		StartMainJs:    &main_js,
+		StartMainMap:   &main_map,
 	})
 
 	DispatchLoop()
diff --git a/main.ts b/main.ts
index 4767f51ecac9e8..3cdbe6c2ba11ea 100644
--- a/main.ts
+++ b/main.ts
@@ -19,7 +19,11 @@ dispatch.sub("start", (payload: Uint8Array) => {
   startCalled = true;
 
   const msg = pb.Msg.decode(payload);
-  const { cwd, argv, debugFlag, mainJs, mainMap } = msg.start;
+  const cwd = msg.startCwd;
+  const argv = msg.startArgv;
+  const debugFlag = msg.startDebugFlag;
+  const mainJs = msg.startMainJs;
+  const mainMap = msg.startMainMap;
 
   debug = debugFlag;
   util.log("start", { cwd, argv, debugFlag });
diff --git a/msg.proto b/msg.proto
index 68ff52255ded4b..dcaa72af867c6b 100644
--- a/msg.proto
+++ b/msg.proto
@@ -8,39 +8,51 @@ message BaseMsg {
 
 message Msg {
   optional string error = 1;
-  oneof payload {
-    StartMsg start = 10;
-    SourceCodeFetchMsg source_code_fetch = 11;
-    SourceCodeFetchResMsg source_code_fetch_res = 12;
-    SourceCodeCacheMsg source_code_cache = 13;
-    ExitMsg exit = 14;
-    TimerStartMsg timer_start = 15;
-    TimerReadyMsg timer_ready = 16;
-    TimerClearMsg timer_clear = 17;
+
+  enum Command {
+    ERROR = 1;
+    START = 2;
+    SOURCE_CODE_FETCH_RES = 3;
+    ONEOF = 100;
   }
-}
+  optional Command command = 2 [ default = ONEOF ];
 
-message StartMsg {
-  optional string cwd = 1;
-  repeated string argv = 2;
-  optional bool debug_flag = 3;
-  optional string main_js = 4;  // The contents of dist/main.js
-  optional string main_map = 5; // The contents of dist/main.map
-}
+  // We avoid creating a message for each command (and use oneof or any types)
+  // In order to reduce code in the size of the generated javascript
+  // "msg.pb.js". It seems that each new message adds 20k and we want to
+  // potentially add many hundreds of commands. Therefore we just prefix command
+  // arguments by their name.
 
-message SourceCodeFetchMsg {
-  optional string module_specifier = 1;
-  optional string containing_file = 2;
-}
+  // Start
+  optional string start_cwd = 10;
+  repeated string start_argv = 11;
+  optional bool start_debug_flag = 12;
+  optional string start_main_js = 13;  // The contents of dist/main.js
+  optional string start_main_map = 14; // The contents of dist/main.map
 
-message SourceCodeFetchResMsg {
+  // SOURCE_CODE_FETCH_RES
   // If it's a non-http module, moduleName and filename will be the same.
   // For http modules, moduleName is its resolved http URL, and filename
   // is the location of the locally downloaded source code.
-  optional string moduleName = 1;
-  optional string filename = 2;
-  optional string source_code = 3;
-  optional string output_code = 4; // Non-empty only if cached.
+  optional string source_code_fetch_res_module_name = 30;
+  optional string source_code_fetch_res_filename = 31;
+  optional string source_code_fetch_res_source_code = 32;
+  optional string source_code_fetch_res_output_code =
+      33; // Non-empty only if cached.
+
+  oneof payload {
+    SourceCodeFetchMsg source_code_fetch = 100;
+    SourceCodeCacheMsg source_code_cache = 102;
+    ExitMsg exit = 103;
+    TimerStartMsg timer_start = 104;
+    TimerReadyMsg timer_ready = 105;
+    TimerClearMsg timer_clear = 106;
+  }
+}
+
+message SourceCodeFetchMsg {
+  optional string module_specifier = 1;
+  optional string containing_file = 2;
 }
 
 message SourceCodeCacheMsg {
diff --git a/os.go b/os.go
index 98997cedc18d69..fe47899619826a 100644
--- a/os.go
+++ b/os.go
@@ -99,13 +99,13 @@ func HandleSourceCodeFetch(moduleSpecifier string, containingFile string) (out [
 	}
 
 	var sourceCode = string(sourceCodeBuf)
-	res.Payload = &Msg_SourceCodeFetchRes{
-		SourceCodeFetchRes: &SourceCodeFetchResMsg{
-			ModuleName: &moduleName,
-			Filename:   &filename,
-			SourceCode: &sourceCode,
-			OutputCode: &outputCode,
-		},
+	var command = Msg_SOURCE_CODE_FETCH_RES
+	res = &Msg{
+		Command: &command,
+		SourceCodeFetchResModuleName: &moduleName,
+		SourceCodeFetchResFilename:   &filename,
+		SourceCodeFetchResSourceCode: &sourceCode,
+		SourceCodeFetchResOutputCode: &outputCode,
 	}
 	return
 }
diff --git a/os.ts b/os.ts
index 409c1186f24dab..de4bdf436962ee 100644
--- a/os.ts
+++ b/os.ts
@@ -1,5 +1,7 @@
 import { ModuleInfo } from "./types";
 import { sendMsg } from "./dispatch";
+import { main as pb } from "./msg.pb";
+import { assert } from "./util";
 
 export function exit(code = 0): void {
   sendMsg("os", { exit: { code } });
@@ -12,7 +14,13 @@ export function sourceCodeFetch(
   const res = sendMsg("os", {
     sourceCodeFetch: { moduleSpecifier, containingFile }
   });
-  return res.sourceCodeFetchRes;
+  assert(res.command === pb.Msg.Command.SOURCE_CODE_FETCH_RES);
+  return {
+    moduleName: res.sourceCodeFetchResModuleName,
+    filename: res.sourceCodeFetchResFilename,
+    sourceCode: res.sourceCodeFetchResSourceCode,
+    outputCode: res.sourceCodeFetchResOutputCode
+  };
 }
 
 export function sourceCodeCache(