fix: null-safe compact + system-only / null payload tests

src/index.mjs

@@ -87,6 +87,12 @@ export function compact(payload, opts = {}) {
   requireNonNegativeNumber("maxToolResultTokens", maxToolResultTokens);
   if (maxTokens != null) requireNonNegativeNumber("maxTokens", maxTokens);
   requireNonNegativeNumber("keepLastTurns", keepLastTurns, { integer: true });
+  if (payload == null) {
+    return {
+      payload: { messages: [] },
+      report: { beforeTokens: 0, afterTokens: 0, savedTokens: 0, savedPct: 0, actions: [] },
+    };
+  }
   const before = analyzePayload(payload, { counter }).totalTokens;
   const out = clone(payload);
   const actions = [];

test/basic.test.mjs

@@ -93,6 +93,32 @@ test("compact rejects invalid budgets before transforming the payload", () => {
   }
 });
 
+
+test("analyzePayload null / undefined yields zero tokens", () => {
+  assert.equal(analyzePayload(null).totalTokens, 0);
+  assert.equal(analyzePayload(undefined).totalTokens, 0);
+});
+
+test("analyzePayload system-only object counts system text", () => {
+  const a = analyzePayload({ system: "You are a careful assistant." });
+  assert.ok(a.totalTokens > 0);
+  assert.ok(a.byRole.system > 0);
+  assert.equal(a.byRole.user ?? 0, 0);
+});
+
+test("compact preserves system-only payload under a tight budget", () => {
+  const payload = { system: "sys", messages: [] };
+  const { payload: out, report } = compact(payload, { maxTokens: 1, keepLastTurns: 0 });
+  assert.ok(out.system === "sys" || (out.messages && out.messages[0]?.role === "system"));
+  assert.ok(report.afterTokens >= 0);
+});
+
+test("compact null payload returns empty messages without throwing", () => {
+  const { payload: out, report } = compact(null);
+  assert.deepEqual(out, { messages: [] });
+  assert.equal(report.beforeTokens, 0);
+  assert.equal(report.afterTokens, 0);
+});
 test("analyzePayload empty messages yields zero tokens", () => {
   const a = analyzePayload({ messages: [] });
   assert.equal(a.totalTokens, 0);