fix(upgrade): resolve CONFIRM_DOWNGRADE deterministically when no TTY

An override value the prompt library rejects (its pattern is lowercase
yes/no only — YES, true, 1, or a typo all fail) was deleted by the
library and fell through to the blocking stdin read, reintroducing the
hang the previous commit fixed. With no TTY, resolve the answer without
ever reaching prompt.get(): case-insensitive yes/no, and an unrecognized
value throws a descriptive error.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FNbH9xnNHcd7V1a58ho2CJ

Author: heskew

unitTests/upgrade/upgradePrompt.test.js

@@ -51,8 +51,21 @@ describe('forceDowngradePrompt — non-interactive starts', () => {
 		assert.strictEqual(await forceDowngradePrompt(upgradeObj), true);
 	});
 
+	it('accepts the override case-insensitively (the prompt library would reject YES and hang)', async () => {
+		process.env.CONFIRM_DOWNGRADE = 'YES';
+		assert.strictEqual(await forceDowngradePrompt(upgradeObj), true);
+	});
+
 	it('declines without a TTY when CONFIRM_DOWNGRADE=no', async () => {
 		process.env.CONFIRM_DOWNGRADE = 'no';
 		assert.strictEqual(await forceDowngradePrompt(upgradeObj), false);
 	});
+
+	it('throws on an unrecognized override value instead of falling through to the blocking prompt', async () => {
+		process.env.CONFIRM_DOWNGRADE = 'true';
+		await assert.rejects(forceDowngradePrompt(upgradeObj), (error) => {
+			assert.ok(error.message.includes("Unrecognized CONFIRM_DOWNGRADE value 'true'"));
+			return true;
+		});
+	});
 });

upgrade/upgradePrompt.ts

@@ -14,15 +14,23 @@ const UPGRADE_PROCEED = ['yes', 'y'];
  */
 export async function forceDowngradePrompt(upgradeObj: any) {
 	const override = assignCMDENVVariables(['CONFIRM_DOWNGRADE']);
-	// Without a terminal the prompt below blocks on stdin forever (systemd, containers, CI), so
-	// refuse to start instead unless a CONFIRM_DOWNGRADE override answers it (#2046).
-	if (override.CONFIRM_DOWNGRADE === undefined && !process.stdin.isTTY) {
-		throw new Error(
-			`This instance's data was last run on Harper ${upgradeObj.data_version}, which is newer than this installed version ${upgradeObj.upgrade_version}.` +
-				' Running the older version requires confirmation, and there is no interactive terminal to ask on.' +
-				' Set CONFIRM_DOWNGRADE=yes (environment variable or --CONFIRM_DOWNGRADE yes) to proceed with the downgrade,' +
-				` or run Harper ${upgradeObj.data_version} or newer.`
-		);
+	// Without a terminal, prompt.get() blocks on stdin forever (systemd, containers, CI) — and an
+	// override value the prompt library rejects (its pattern is lowercase-only) is deleted and
+	// falls through to that same blocking read. So with no TTY, resolve the answer here and never
+	// reach the prompt (#2046).
+	if (!process.stdin.isTTY) {
+		if (override.CONFIRM_DOWNGRADE === undefined) {
+			throw new Error(
+				`This instance's data was last run on Harper ${upgradeObj.data_version}, which is newer than this installed version ${upgradeObj.upgrade_version}.` +
+					' Running the older version requires confirmation, and there is no interactive terminal to ask on.' +
+					' Set CONFIRM_DOWNGRADE=yes (environment variable or --CONFIRM_DOWNGRADE yes) to proceed with the downgrade,' +
+					` or run Harper ${upgradeObj.data_version} or newer.`
+			);
+		}
+		const answer = override.CONFIRM_DOWNGRADE.toLowerCase();
+		if (UPGRADE_PROCEED.includes(answer)) return true;
+		if (answer === 'no' || answer === 'n') return false;
+		throw new Error(`Unrecognized CONFIRM_DOWNGRADE value '${override.CONFIRM_DOWNGRADE}'; use yes or no.`);
 	}
 	let downgradeMessage =
 		`${os.EOL}` +