diff --git a/src/client/sse.ts b/src/client/sse.ts
index 2ebf23e8..e7f0dc6b 100644
--- a/src/client/sse.ts
+++ b/src/client/sse.ts
@@ -22,6 +22,12 @@ export class SSEClientTransport implements Transport {
   }
 
   start(): Promise<void> {
+    if (this._eventSource) {
+      throw new Error(
+        "SSEClientTransport already started! If using Client class, note that connect() calls start() automatically.",
+      );
+    }
+
     return new Promise((resolve, reject) => {
       this._eventSource = new EventSource(this._url.href);
       this._abortController = new AbortController();
diff --git a/src/client/stdio.ts b/src/client/stdio.ts
index 36d4f206..98f5b77f 100644
--- a/src/client/stdio.ts
+++ b/src/client/stdio.ts
@@ -45,6 +45,12 @@ export class StdioClientTransport implements Transport {
    * Starts the server process and prepares to communicate with it.
    */
   async start(): Promise<void> {
+    if (this._process) {
+      throw new Error(
+        "StdioClientTransport already started! If using Client class, note that connect() calls start() automatically.",
+      );
+    }
+
     return new Promise((resolve, reject) => {
       this._process = spawn(
         this._serverParams.command,
diff --git a/src/client/websocket.ts b/src/client/websocket.ts
index 97dc5529..3ca76082 100644
--- a/src/client/websocket.ts
+++ b/src/client/websocket.ts
@@ -19,6 +19,12 @@ export class WebSocketClientTransport implements Transport {
   }
 
   start(): Promise<void> {
+    if (this._socket) {
+      throw new Error(
+        "WebSocketClientTransport already started! If using Client class, note that connect() calls start() automatically.",
+      );
+    }
+
     return new Promise((resolve, reject) => {
       this._socket = new WebSocket(this._url, SUBPROTOCOL);
 
diff --git a/src/server/sse.ts b/src/server/sse.ts
index 9afbea79..1342fcc3 100644
--- a/src/server/sse.ts
+++ b/src/server/sse.ts
@@ -37,7 +37,9 @@ export class SSEServerTransport implements Transport {
    */
   async start(): Promise<void> {
     if (this._sseResponse) {
-      throw new Error("Already connected!");
+      throw new Error(
+        "SSEServerTransport already started! If using Server class, note that connect() calls start() automatically.",
+      );
     }
 
     this.res.writeHead(200, {
diff --git a/src/server/stdio.ts b/src/server/stdio.ts
index d5f3eee4..f0eff82c 100644
--- a/src/server/stdio.ts
+++ b/src/server/stdio.ts
@@ -11,6 +11,7 @@ import { Transport } from "../shared/transport.js";
  */
 export class StdioServerTransport implements Transport {
   private _readBuffer: ReadBuffer = new ReadBuffer();
+  private _started = false;
 
   constructor(
     private _stdin: Readable = process.stdin,
@@ -34,6 +35,13 @@ export class StdioServerTransport implements Transport {
    * Starts listening for messages on stdin.
    */
   async start(): Promise<void> {
+    if (this._started) {
+      throw new Error(
+        "StdioServerTransport already started! If using Server class, note that connect() calls start() automatically.",
+      );
+    }
+
+    this._started = true;
     this._stdin.on("data", this._ondata);
     this._stdin.on("error", this._onerror);
   }