diff --git a/nodejs/src/session.ts b/nodejs/src/session.ts
index 8ae19755a..b529408cb 100644
--- a/nodejs/src/session.ts
+++ b/nodejs/src/session.ts
@@ -254,6 +254,7 @@ export class CopilotSession {
      * @param timeout - Timeout in milliseconds (default: 60000). Controls how long to wait; does not abort in-flight agent work.
      * @returns A promise that resolves with the final assistant message when the session becomes idle,
      *          or undefined if no assistant message was received
+     * @throws TypeError if `timeout` is provided but is not a non-negative finite number of milliseconds
      * @throws Error if the timeout is reached before the session becomes idle
      * @throws Error if the session has been disconnected or the connection fails
      *
@@ -275,6 +276,26 @@ export class CopilotSession {
     ): Promise<AssistantMessageEvent | undefined> {
         const options: MessageOptions =
             typeof optionsOrPrompt === "string" ? { prompt: optionsOrPrompt } : optionsOrPrompt;
+
+        // Guard against a non-numeric timeout reaching setTimeout(). Without this,
+        // an object or NaN is coerced to a 0ms delay, producing a spurious
+        // immediate timeout with a malformed "Timeout after [object Object]ms"
+        // message instead of a clear, actionable error.
+        const isValidTimeout =
+            timeout === undefined ||
+            (typeof timeout === "number" && Number.isFinite(timeout) && timeout >= 0);
+        if (!isValidTimeout) {
+            const received =
+                timeout === null
+                    ? "null"
+                    : typeof timeout === "number"
+                      ? `${timeout}`
+                      : typeof timeout;
+            throw new TypeError(
+                `sendAndWait timeout must be a non-negative number of milliseconds (got ${received})`
+            );
+        }
+
         const effectiveTimeout = timeout ?? 60_000;
 
         let resolveIdle: () => void;
diff --git a/nodejs/test/client.test.ts b/nodejs/test/client.test.ts
index 42c0ff18e..8e15234d5 100644
--- a/nodejs/test/client.test.ts
+++ b/nodejs/test/client.test.ts
@@ -24,6 +24,54 @@ describe("CopilotClient", () => {
         expect(spy).not.toHaveBeenCalled();
     });
 
+    describe("sendAndWait timeout validation", () => {
+        // Regression for #915: a non-numeric timeout used to reach setTimeout(),
+        // which coerced it to a 0ms delay and rejected with a malformed
+        // "Timeout after [object Object]ms" message instead of a clear error.
+        async function captureRejection(promise: Promise<unknown>): Promise<Error> {
+            return promise.then(
+                () => {
+                    throw new Error("expected sendAndWait to reject");
+                },
+                (error) => error
+            );
+        }
+
+        it("rejects an object timeout with a clear TypeError", async () => {
+            const session = new CopilotSession("session-1", {} as any);
+            const error = await captureRejection(
+                session.sendAndWait("hello", { ms: 30000 } as any)
+            );
+            expect(error).toBeInstanceOf(TypeError);
+            expect(error.message).toBe(
+                "sendAndWait timeout must be a non-negative number of milliseconds (got object)"
+            );
+            expect(error.message).not.toContain("[object Object]");
+        });
+
+        it("reports a null timeout as null rather than object", async () => {
+            const session = new CopilotSession("session-1", {} as any);
+            const error = await captureRejection(session.sendAndWait("hello", null as any));
+            expect(error.message).toBe(
+                "sendAndWait timeout must be a non-negative number of milliseconds (got null)"
+            );
+        });
+
+        it("rejects when timeout is NaN", async () => {
+            const session = new CopilotSession("session-1", {} as any);
+            await expect(session.sendAndWait("hello", NaN)).rejects.toThrowError(
+                "sendAndWait timeout must be a non-negative number of milliseconds (got NaN)"
+            );
+        });
+
+        it("rejects when timeout is negative", async () => {
+            const session = new CopilotSession("session-1", {} as any);
+            await expect(session.sendAndWait("hello", -1)).rejects.toThrowError(
+                "sendAndWait timeout must be a non-negative number of milliseconds (got -1)"
+            );
+        });
+    });
+
     it("forwards canvas declarations and request flags in session.create", async () => {
         const client = new CopilotClient();
         await client.start();