diff --git a/test/compaction-source-fallback.test.ts b/test/compaction-source-fallback.test.ts
new file mode 100644
index 0000000..53fcf92
--- /dev/null
+++ b/test/compaction-source-fallback.test.ts
@@ -0,0 +1,47 @@
+import { describe, expect, it } from "vitest";
+import {
+	consumeCompactionOnSessionCompact,
+	createCompactionRuntime,
+	ensureCompactionSourceMarked,
+	markManualCompactionFromInput,
+} from "../src/compaction";
+import { createDefaultSettings } from "../src/settings";
+import { createInitialState } from "../src/state";
+
+describe("compaction source fallback on session_compact", () => {
+	it("infers auto when session_compact runs without prior session_before_compact", () => {
+		const runtime = createCompactionRuntime();
+		const state = createInitialState();
+		expect(runtime.pendingCompactionSource).toBeNull();
+
+		ensureCompactionSourceMarked(runtime);
+		const shouldReinject = consumeCompactionOnSessionCompact(
+			runtime,
+			state,
+			true,
+			createDefaultSettings(),
+		);
+
+		expect(shouldReinject).toBe(true);
+		expect(state.lastCompactionSource).toBe("auto");
+		expect(runtime.pendingCompactionSource).toBeNull();
+	});
+
+	it("keeps manual source from input and skips reinject by default", () => {
+		const runtime = createCompactionRuntime();
+		const state = createInitialState();
+		markManualCompactionFromInput("/compact", runtime);
+
+		ensureCompactionSourceMarked(runtime);
+		const shouldReinject = consumeCompactionOnSessionCompact(
+			runtime,
+			state,
+			true,
+			createDefaultSettings(),
+		);
+
+		expect(shouldReinject).toBe(false);
+		expect(state.lastCompactionSource).toBe("manual");
+		expect(runtime.clearPendingReinjectOnNextUserInput).toBe(true);
+	});
+});