diff --git a/packages/coding-agent/src/features/step-questionnaire.ts b/packages/coding-agent/src/features/step-questionnaire.ts
index 7301fc9a..9e198489 100644
--- a/packages/coding-agent/src/features/step-questionnaire.ts
+++ b/packages/coding-agent/src/features/step-questionnaire.ts
@@ -263,6 +263,7 @@ async function executeNativeDialog(
 		let optionIndex = 0;
 		let inputMode = questions[0]?.options.length === 0;
 		let inputQuestionId: string | null = inputMode ? (questions[0]?.id ?? null) : null;
+		let emptyAnswerError = false;
 		let cachedLines: string[] | undefined;
 		const answers = new Map<string, StepQuestionnaireAnswer>();
 		const totalTabs = questions.length + 1;
@@ -306,6 +307,7 @@ async function executeNativeDialog(
 			const question = currentQuestion();
 			inputMode = Boolean(question && question.options.length === 0);
 			inputQuestionId = inputMode ? (question?.id ?? null) : null;
+			emptyAnswerError = false;
 			editor.setText("");
 			refresh();
 		};
@@ -335,7 +337,12 @@ async function executeNativeDialog(
 		editor.onSubmit = (value) => {
 			if (!inputQuestionId) return;
 			const answer = value.trim();
-			if (!answer) return;
+			if (!answer) {
+				emptyAnswerError = true;
+				refresh();
+				return;
+			}
+			emptyAnswerError = false;
 			saveAnswer(inputQuestionId, answer, answer, true);
 			editor.setText("");
 			inputMode = false;
@@ -343,6 +350,12 @@ async function executeNativeDialog(
 			advance();
 		};
 
+		editor.onChange = () => {
+			if (!emptyAnswerError) return;
+			emptyAnswerError = false;
+			refresh();
+		};
+
 		const handleInput = (data: string): void => {
 			if (matchesKey(data, Key.escape)) {
 				if (inputMode && currentQuestion()?.options.length) {
@@ -489,6 +502,9 @@ async function executeNativeDialog(
 					for (const line of editor.render(Math.max(1, renderWidth - 2))) {
 						lines.push(` ${line}`);
 					}
+					if (emptyAnswerError) {
+						addWrapped(" ", theme.fg("warning", "Enter an answer before submitting"));
+					}
 				}
 			}
 
diff --git a/packages/coding-agent/test/suite/regressions/311-clarify-tui-empty-submission.test.ts b/packages/coding-agent/test/suite/regressions/311-clarify-tui-empty-submission.test.ts
new file mode 100644
index 00000000..8984f481
--- /dev/null
+++ b/packages/coding-agent/test/suite/regressions/311-clarify-tui-empty-submission.test.ts
@@ -0,0 +1,93 @@
+import type { AgentToolResult } from "@step-harness/agent-core";
+import { describe, expect, test, vi } from "vitest";
+import type { ExtensionContext } from "../../../src/core/extensions/index.ts";
+import {
+	registerStepClarifyUserExtension,
+	type StepQuestionnaireDetails,
+} from "../../../src/features/step-questionnaire.ts";
+import { createHarness, type Harness } from "../harness.ts";
+
+// Regression for https://github.com/stepfun-ai/Step-Code/issues/311
+
+type QuestionnaireComponent = {
+	render(width: number): string[];
+	handleInput(data: string): void;
+	invalidate(): void;
+};
+
+describe("clarify_user native TUI empty submission (#311)", () => {
+	test("shows a validation error for an empty freeform answer and keeps the dialog open", async () => {
+		let harness: Harness | undefined;
+
+		try {
+			harness = await createHarness({ extensionFactories: [registerStepClarifyUserExtension] });
+			const tool = harness.session.extensionRunner
+				.getAllRegisteredTools()
+				.find(({ definition }) => definition.name === "clarify_user");
+			if (!tool) throw new Error("clarify_user is not registered");
+
+			const baseContext = harness.session.extensionRunner.createContext();
+			let component: QuestionnaireComponent | undefined;
+			let finishDialog: ((details: StepQuestionnaireDetails) => void) | undefined;
+			const dialogResult = new Promise<StepQuestionnaireDetails>((resolve) => {
+				finishDialog = resolve;
+			});
+			const fakeTui = {
+				requestRender: () => {},
+				terminal: { rows: 24 },
+			} as unknown as never;
+			const fakeTheme = {
+				fg: (_color: string, text: string) => text,
+				bold: (text: string) => text,
+				bg: (_color: string, text: string) => text,
+			} as unknown as never;
+			const context = {
+				...baseContext,
+				mode: "tui",
+				hasUI: true,
+				ui: {
+					...baseContext.ui,
+					custom: async (
+						factory: (
+							tui: never,
+							theme: never,
+							keybindings: never,
+							done: (details: StepQuestionnaireDetails) => void,
+						) => Promise<QuestionnaireComponent>,
+					) => {
+						component = await factory(fakeTui, fakeTheme, {} as never, (details) => {
+							finishDialog?.(details);
+						});
+						return dialogResult;
+					},
+				},
+			} as unknown as ExtensionContext;
+
+			const execution = tool.definition.execute(
+				"clarify-tui-empty",
+				{ question: "Any notes?" },
+				undefined,
+				undefined,
+				context,
+			) as Promise<AgentToolResult<StepQuestionnaireDetails>>;
+
+			await vi.waitFor(() => expect(component).toBeDefined());
+			const dialog = component as QuestionnaireComponent;
+
+			dialog.handleInput("\r");
+			expect(dialog.render(80).join("\n")).toContain("Enter an answer before submitting");
+
+			for (const character of "Fixed") dialog.handleInput(character);
+			expect(dialog.render(80).join("\n")).not.toContain("Enter an answer before submitting");
+
+			dialog.handleInput("\r");
+			const result = await execution;
+			expect(result.details).toMatchObject({
+				cancelled: false,
+				answers: [{ id: "answer", value: "Fixed", label: "Fixed", wasCustom: true }],
+			});
+		} finally {
+			harness?.cleanup();
+		}
+	});
+});