diff --git a/packages/tui/src/terminal.ts b/packages/tui/src/terminal.ts
index c7cfbc155..2465c76e6 100644
--- a/packages/tui/src/terminal.ts
+++ b/packages/tui/src/terminal.ts
@@ -337,8 +337,9 @@ export interface Terminal {
 	get kittyEnableSequence(): string | null;
 
 	// The active modified-key reporting sequence to reassert after terminal buffer
-	// switches, or null when no enhanced keyboard mode is active.
-	get keyboardEnhancementSequence(): string | null;
+	// switches, or null when no enhanced keyboard mode is active. Optional so
+	// custom Terminals built against older pi-tui versions keep working.
+	readonly keyboardEnhancementSequence?: string | null;
 
 	// Cursor positioning (relative to current position)
 	moveBy(lines: number): void; // Move cursor up (negative) or down (positive) by N lines
diff --git a/packages/tui/src/tui.ts b/packages/tui/src/tui.ts
index 28c4eafaa..cb7b69887 100644
--- a/packages/tui/src/tui.ts
+++ b/packages/tui/src/tui.ts
@@ -2492,7 +2492,7 @@ export class TUI extends Container {
 			// modified-key reporting sequence on the freshly entered alternate
 			// screen, or Esc/modified keys revert to legacy encoding inside
 			// fullscreen overlays (Ghostty/kitty/iTerm2).
-			this.terminal.write(`\x1b[?1049h${this.terminal.keyboardEnhancementSequence ?? ""}${MOUSE_TRACKING_ON}`);
+			this.terminal.write(`\x1b[?1049h${this.#keyboardEnhancementSequence()}${MOUSE_TRACKING_ON}`);
 			setAltScreenActive(true);
 			this.terminal.hideCursor();
 			this.#forgetHardwareCursorState();
@@ -3340,13 +3340,17 @@ export class TUI extends Container {
 	}
 
 	/** Enter or leave the alternate screen borrowed for transient resize frames. */
+	#keyboardEnhancementSequence(): string {
+		return this.terminal.keyboardEnhancementSequence ?? this.terminal.kittyEnableSequence ?? "";
+	}
+
 	#enterResizeAltSequence(): string {
 		if (this.#resizeAltActive || this.#altActive) return "";
 		this.#resizeAltActive = true;
 		setAltScreenActive(true);
 		this.#forgetHardwareCursorState();
 		this.#recordHardwareCursorHidden();
-		return `${ALT_SCREEN_ENTER}${this.terminal.keyboardEnhancementSequence ?? ""}`;
+		return `${ALT_SCREEN_ENTER}${this.#keyboardEnhancementSequence()}`;
 	}
 
 	#leaveResizeAltSequence(): string {
diff --git a/packages/tui/test/render-regressions.test.ts b/packages/tui/test/render-regressions.test.ts
index 8d45683ca..e9e127225 100644
--- a/packages/tui/test/render-regressions.test.ts
+++ b/packages/tui/test/render-regressions.test.ts
@@ -117,6 +117,12 @@ class CountingViewportTerminal extends VirtualTerminal {
 	}
 }
 
+class LegacyKeyboardVirtualTerminal extends VirtualTerminal {
+	get keyboardEnhancementSequence(): string | null {
+		return undefined as unknown as string | null;
+	}
+}
+
 function rows(prefix: string, count: number): string[] {
 	return Array.from({ length: count }, (_v, i) => `${prefix}${i}`);
 }
@@ -3281,6 +3287,32 @@ describe("TUI terminal-state regressions", () => {
 			}
 		});
 
+		it("falls back to kittyEnableSequence for legacy custom terminals", async () => {
+			const term = new LegacyKeyboardVirtualTerminal(40, 8, 200);
+			const writes = captureWrites(term);
+			const tui = new TUI(term);
+			tui.addChild(new MutableLinesComponent(rows("base-", 8)));
+
+			try {
+				tui.start();
+				await settle(term);
+
+				const showFrom = writes.length;
+				tui.showOverlay(new MutableLinesComponent(["MODAL-0"]), {
+					width: "100%",
+					maxHeight: "100%",
+					margin: 0,
+					fullscreen: true,
+				});
+				await settle(term);
+
+				const modalWrites = writes.slice(showFrom).join("");
+				expect(modalWrites).toContain("\x1b[?1049h\x1b[>1u");
+			} finally {
+				tui.stop();
+			}
+		});
+
 		it("leaves native scrollback untouched across the modal lifetime", async () => {
 			const term = new VirtualTerminal(40, 6, 200);
 			const tui = new TUI(term);