diff --git a/src/auto-compact.ts b/src/auto-compact.ts
new file mode 100644
index 0000000..0af738e
--- /dev/null
+++ b/src/auto-compact.ts
@@ -0,0 +1,13 @@
+import type { ExtensionAPI } from "@earendil-works/pi-coding-agent";
+import type { RuntimeFlags } from "./state";
+
+/** Detect @capyup/pi-auto-compact via public getCommands API (SPEC §16.4). */
+export function detectPiAutoCompact(pi: ExtensionAPI): boolean {
+	return pi.getCommands().some((command) => command.name === "auto-compact");
+}
+
+/** Detect pi-auto-compact and cache result in runtime flags (SPEC §16.4). */
+export function detectAndCachePiAutoCompact(pi: ExtensionAPI, runtime: RuntimeFlags): boolean {
+	runtime.autoCompactDetected = detectPiAutoCompact(pi);
+	return runtime.autoCompactDetected;
+}