diff --git a/src/memory/mmr.test.ts b/src/memory/mmr.test.ts
index 0891133525..9df2da3eea 100644
--- a/src/memory/mmr.test.ts
+++ b/src/memory/mmr.test.ts
@@ -148,7 +148,7 @@ describe("mmrRerank", () => {
     });
 
     it("lambda=0 maximizes diversity", () => {
-      const result = mmrRerank(diverseItems, { lambda: 0 });
+      const result = mmrRerank(diverseItems, { enabled: true, lambda: 0 });
       // First item is still highest score (no penalty yet)
       expect(result[0].id).toBe("1");
       // Second should be most different from first
@@ -161,7 +161,7 @@ describe("mmrRerank", () => {
     });
 
     it("clamps lambda < 0 to 0", () => {
-      const result = mmrRerank(diverseItems, { lambda: -0.5 });
+      const result = mmrRerank(diverseItems, { enabled: true, lambda: -0.5 });
       expect(result[0].id).toBe("1");
       expect(result[1].id).toBe("3");
     });
@@ -176,7 +176,7 @@ describe("mmrRerank", () => {
         { id: "4", score: 0.85, content: "machine learning algorithms" },
       ];
 
-      const result = mmrRerank(items, { lambda: 0.5 });
+      const result = mmrRerank(items, { enabled: true, lambda: 0.5 });
 
       // First is always highest score
       expect(result[0].id).toBe("1");
@@ -191,7 +191,7 @@ describe("mmrRerank", () => {
         { id: "3", score: 0.8, content: "different stuff" },
       ];
 
-      const result = mmrRerank(items, { lambda: 0.5 });
+      const result = mmrRerank(items, { enabled: true, lambda: 0.5 });
       expect(result[0].id).toBe("1");
       // Second should be different, not identical duplicate
       expect(result[1].id).toBe("3");
@@ -344,7 +344,7 @@ describe("applyMMRToHybridResults", () => {
       },
     ];
 
-    const reranked = applyMMRToHybridResults(results, { lambda: 0.5 });
+    const reranked = applyMMRToHybridResults(results, { enabled: true, lambda: 0.5 });
 
     // First stays the same (highest score)
     expect(reranked[0].path).toBe("/a.ts");
@@ -365,7 +365,7 @@ describe("applyMMRToHybridResults", () => {
 
 describe("DEFAULT_MMR_CONFIG", () => {
   it("has expected default values", () => {
-    expect(DEFAULT_MMR_CONFIG.enabled).toBe(true);
+    expect(DEFAULT_MMR_CONFIG.enabled).toBe(false);
     expect(DEFAULT_MMR_CONFIG.lambda).toBe(0.7);
   });
 });
diff --git a/src/memory/mmr.ts b/src/memory/mmr.ts
index 52b0d9e280..ec87b9e3ff 100644
--- a/src/memory/mmr.ts
+++ b/src/memory/mmr.ts
@@ -14,14 +14,14 @@ export type MMRItem = {
 };
 
 export type MMRConfig = {
-  /** Enable/disable MMR re-ranking. Default: true */
+  /** Enable/disable MMR re-ranking. Default: false (opt-in) */
   enabled: boolean;
   /** Lambda parameter: 0 = max diversity, 1 = max relevance. Default: 0.7 */
   lambda: number;
 };
 
 export const DEFAULT_MMR_CONFIG: MMRConfig = {
-  enabled: true,
+  enabled: false,
   lambda: 0.7,
 };
 
@@ -163,7 +163,7 @@ export function mmrRerank<T extends MMRItem>(items: T[], config: Partial<MMRConf
       // Use original score as tiebreaker (higher is better)
       if (
         mmrScore > bestMMRScore ||
-        (mmrScore === bestMMRScore && bestItem && candidate.score > bestItem.score)
+        (mmrScore === bestMMRScore && (bestItem === null || candidate.score > bestItem.score))
       ) {
         bestMMRScore = mmrScore;
         bestItem = candidate;