diff --git a/src/core/openai-history.ts b/src/core/openai-history.ts
index 9b5136e0..7609c390 100644
--- a/src/core/openai-history.ts
+++ b/src/core/openai-history.ts
@@ -784,8 +784,12 @@ async function planResponsesMixedCollapse(
     messageIndices.push(i);
     if (msg.role === 'user') latestUserIndex = i;
   }
+  // slice(-0) returns the WHOLE array, so guard keepTail===0 explicitly —
+  // otherwise "protect 0 tail messages" protects every message and blocks
+  // all collapse. latestUserIndex is added separately below.
+  const tail = Math.max(0, Math.floor(o.keepTail));
   const protectedMessages = new Set(
-    messageIndices.slice(-Math.max(0, Math.floor(o.keepTail))),
+    tail > 0 ? messageIndices.slice(-tail) : [],
   );
   if (latestUserIndex >= 0) protectedMessages.add(latestUserIndex);
 
diff --git a/tests/openai-history.test.ts b/tests/openai-history.test.ts
index 60fa95e0..6a8af550 100644
--- a/tests/openai-history.test.ts
+++ b/tests/openai-history.test.ts
@@ -558,6 +558,25 @@ describe('planResponsesPairCollapse — native state classification', () => {
     expect(plan.baselineTokens).toBeLessThan(o200k(plan.text));
   });
 
+  it('mixed mode with keepTail 0 collapses old messages (no slice(-0) whole-array protect)', async () => {
+    // Regression: keepTail:0 must protect NO tail messages. slice(-0) returns
+    // the whole array, which would protect every message and block all collapse.
+    const items: Array<Record<string, unknown>> = [
+      { role: 'user', content: `old request ${'alpha '.repeat(500)}` },   // 0
+      { role: 'assistant', content: `old answer ${'beta '.repeat(500)}` }, // 1
+      { role: 'user', content: 'current live request' },                   // 2 (latest user)
+    ];
+    const plan = await planResponsesPairCollapse(items, yes, {
+      responsesMode: 'mixed', keepTail: 0, keepRecentPairs: 0,
+      minCollapseTokens: 1, maxImages: 100,
+    });
+    const selected = new Set(plan.selectedIndices);
+    // Only the latest user turn stays native; the old turns collapse.
+    expect(selected.has(0)).toBe(true);
+    expect(selected.has(1)).toBe(true);
+    expect(selected.has(2)).toBe(false);
+  });
+
   it('with a one-item Sol tail, protects the latest user and newest closed pair only', async () => {
     const items: Array<Record<string, unknown>> = [
       { role: 'user', content: `old request ${'alpha '.repeat(500)}` },