diff --git a/src/methods/chat-session.test.ts b/src/methods/chat-session.test.ts
index c09821030..ed36c67f7 100644
--- a/src/methods/chat-session.test.ts
+++ b/src/methods/chat-session.test.ts
@@ -46,6 +46,15 @@ describe("ChatSession", () => {
         match.any,
       );
     });
+    it("sendMessage() should reset messageInProgress flag after resolving promise", async () => {
+      const mockResponse = getMockResponse(
+        "unary-success-basic-reply-short.json",
+      );
+      stub(request, "makeModelRequest").resolves(mockResponse as Response);
+      const chatSession = new ChatSession("MY_API_KEY", "a-model");
+      await chatSession.sendMessage("hello");
+      expect(chatSession["_messageInProgress"]).to.be.false;
+    });
   });
   describe("sendMessageRecitationErrorNotAddingResponseToHistory()", () => {
     it("generateContent errors should be catchable", async () => {
@@ -75,6 +84,22 @@ describe("ChatSession", () => {
       expect(consoleStub).to.not.be.called;
       clock.restore();
     });
+    it("sendMessageStream() should reset messageInProgress flag after resolving promise", async () => {
+      const consoleStub = stub(console, "error");
+      const generateContentStreamStub = stub(
+        generateContentMethods,
+        "generateContentStream",
+      ).resolves();
+      const chatSession = new ChatSession("MY_API_KEY", "a-model");
+      await expect(chatSession.sendMessageStream("hello")).to.be.fulfilled;
+      expect(generateContentStreamStub).to.be.calledWith(
+        "MY_API_KEY",
+        "a-model",
+        match.any,
+      );
+      expect(consoleStub).to.not.be.called;
+      expect(chatSession["_messageInProgress"]).to.be.false;
+    });
     it("downstream sendPromise errors should log but not throw", async () => {
       const clock = useFakeTimers();
       const consoleStub = stub(console, "error");
diff --git a/src/methods/chat-session.ts b/src/methods/chat-session.ts
index 4022e2e39..9dfc9b78f 100644
--- a/src/methods/chat-session.ts
+++ b/src/methods/chat-session.ts
@@ -45,6 +45,7 @@ export class ChatSession {
   private _apiKey: string;
   private _history: Content[] = [];
   private _sendPromise: Promise<void> = Promise.resolve();
+  private _messageInProgress: boolean = false;
 
   constructor(
     apiKey: string,
@@ -81,6 +82,12 @@ export class ChatSession {
     request: string | Array<string | Part>,
     requestOptions: SingleRequestOptions = {},
   ): Promise<GenerateContentResult> {
+    if (this._messageInProgress) {
+      throw new Error(
+        "sendMessage() was called while another message was in progress, this may lead to unexpected behavior.",
+      );
+    }
+    this._messageInProgress = true;
     await this._sendPromise;
     const newContent = formatNewContent(request);
     const generateContentRequest: GenerateContentRequest = {
@@ -126,6 +133,9 @@ export class ChatSession {
           }
         }
         finalResult = result;
+      })
+      .finally(() => {
+        this._messageInProgress = false;
       });
     await this._sendPromise;
     return finalResult;
@@ -144,6 +154,12 @@ export class ChatSession {
     request: string | Array<string | Part>,
     requestOptions: SingleRequestOptions = {},
   ): Promise<GenerateContentStreamResult> {
+    if (this._messageInProgress) {
+      throw new Error(
+        "sendMessageStream() was called while another message was in progress, this may lead to unexpected behavior.",
+      );
+    }
+    this._messageInProgress = true;
     await this._sendPromise;
     const newContent = formatNewContent(request);
     const generateContentRequest: GenerateContentRequest = {
@@ -203,6 +219,7 @@ export class ChatSession {
           console.error(e);
         }
       });
+    this._messageInProgress = false;
     return streamPromise;
   }
 }