diff --git a/site/src/pages/AgentsPage/components/AgentChatInput.stories.tsx b/site/src/pages/AgentsPage/components/AgentChatInput.stories.tsx
index f96eb754bb..baab81fcc5 100644
--- a/site/src/pages/AgentsPage/components/AgentChatInput.stories.tsx
+++ b/site/src/pages/AgentsPage/components/AgentChatInput.stories.tsx
@@ -81,6 +81,47 @@ export const SendsAndClearsInput: Story = {
 	},
 };
 
+/**
+ * CODAGT-210: On mobile viewports, Enter must insert a newline rather
+ * than submit the message, because Shift+Enter is cumbersome on
+ * on-screen keyboards. Users submit via the send button instead.
+ */
+export const MobileEnterInsertsNewline: Story = {
+	args: {
+		onSend: fn(),
+		initialValue: "Line one",
+	},
+	play: async ({ canvasElement, args }) => {
+		const originalMatchMedia = window.matchMedia;
+		window.matchMedia = ((query: string) =>
+			({
+				matches: query === "(max-width: 639px)",
+				media: query,
+				onchange: null,
+				addEventListener: () => undefined,
+				removeEventListener: () => undefined,
+				dispatchEvent: () => true,
+				addListener: () => undefined,
+				removeListener: () => undefined,
+			}) as MediaQueryList) as typeof window.matchMedia;
+
+		try {
+			const canvas = within(canvasElement);
+			const editor = canvas.getByTestId("chat-message-input");
+			await waitFor(() => {
+				expect(editor.textContent).toBe("Line one");
+			});
+
+			await userEvent.click(editor);
+			await userEvent.keyboard("{Enter}");
+
+			expect(args.onSend).not.toHaveBeenCalled();
+		} finally {
+			window.matchMedia = originalMatchMedia;
+		}
+	},
+};
+
 export const DisabledInput: Story = {
 	args: {
 		isDisabled: true,
diff --git a/site/src/pages/AgentsPage/components/ChatMessageInput/ChatMessageInput.tsx b/site/src/pages/AgentsPage/components/ChatMessageInput/ChatMessageInput.tsx
index 9135e52e86..2be8d6c671 100644
--- a/site/src/pages/AgentsPage/components/ChatMessageInput/ChatMessageInput.tsx
+++ b/site/src/pages/AgentsPage/components/ChatMessageInput/ChatMessageInput.tsx
@@ -30,6 +30,7 @@ import {
 	useRef,
 } from "react";
 import { cn } from "#/utils/cn";
+import { isMobileViewport } from "#/utils/mobile";
 import {
 	$createFileReferenceNode,
 	FileReferenceNode,
@@ -256,7 +257,9 @@ const PasteSanitizationPlugin: FC<{
 };
 
 // Handles Enter key behavior: plain Enter submits via the onEnter
-// callback, Shift+Enter inserts a newline.
+// callback, Shift+Enter inserts a newline. On mobile viewports, Enter
+// always inserts a newline; users submit via the send button because
+// Shift+Enter is cumbersome on touch keyboards (CODAGT-210).
 const EnterKeyPlugin: FC<{ onEnter?: () => void }> = function EnterKeyPlugin({
 	onEnter,
 }) {
@@ -266,7 +269,7 @@ const EnterKeyPlugin: FC<{ onEnter?: () => void }> = function EnterKeyPlugin({
 		return editor.registerCommand(
 			KEY_ENTER_COMMAND,
 			(event: KeyboardEvent | null) => {
-				if (event?.shiftKey) {
+				if (event?.shiftKey || isMobileViewport()) {
 					return false;
 				}
 				if (onEnter) {