diff --git a/src/CodexAcpServer.ts b/src/CodexAcpServer.ts
index 3c321c86..e1998265 100644
--- a/src/CodexAcpServer.ts
+++ b/src/CodexAcpServer.ts
@@ -2382,14 +2382,26 @@ export class CodexAcpServer {
     }
 }
 
-function mergeHistoryUpdates(
+export function mergeHistoryUpdates(
     responseItemFallbackUpdates: UpdateSessionEvent[],
     threadUpdates: UpdateSessionEvent[],
 ): UpdateSessionEvent[] {
     const merged: UpdateSessionEvent[] = [];
     const seen = new Set<string>();
+    const fallbackIndicesByKey = new Map<string, number[]>();
+    const fallbackIndicesByContentKey = new Map<string, number[]>();
     let fallbackIndex = 0;
 
+    for (let index = 0; index < responseItemFallbackUpdates.length; index += 1) {
+        const update = responseItemFallbackUpdates[index]!;
+        indexHistoryUpdate(fallbackIndicesByKey, historyUpdateKey(update), index);
+        indexHistoryUpdate(
+            fallbackIndicesByContentKey,
+            historyUpdateContentKey(update),
+            index,
+        );
+    }
+
     const pushUpdate = (update: UpdateSessionEvent) => {
         const key = historyUpdateKey(update);
         if (key && seen.has(key)) {
@@ -2408,13 +2420,15 @@ function mergeHistoryUpdates(
             return;
         }
 
-        const matchIndex = responseItemFallbackUpdates.findIndex((update, index) => (
-            index >= fallbackIndex
-            && (
-                (targetKey !== null && historyUpdateKey(update) === targetKey)
-                || (targetContentKey !== null && historyUpdateContentKey(update) === targetContentKey)
-            )
-        ));
+        const keyMatchIndex = nextHistoryUpdateIndex(
+            fallbackIndicesByKey.get(targetKey ?? ""),
+            fallbackIndex,
+        );
+        const contentMatchIndex = nextHistoryUpdateIndex(
+            fallbackIndicesByContentKey.get(targetContentKey ?? ""),
+            fallbackIndex,
+        );
+        const matchIndex = minHistoryUpdateIndex(keyMatchIndex, contentMatchIndex);
         if (matchIndex === -1) {
             return;
         }
@@ -2439,6 +2453,41 @@ function mergeHistoryUpdates(
     return merged;
 }
 
+function indexHistoryUpdate(
+    target: Map<string, number[]>,
+    key: string | null,
+    index: number,
+): void {
+    if (key === null) return;
+    const indices = target.get(key);
+    if (indices) {
+        indices.push(index);
+    } else {
+        target.set(key, [index]);
+    }
+}
+
+function nextHistoryUpdateIndex(indices: number[] | undefined, minimum: number): number {
+    if (!indices || indices.length === 0) return -1;
+    let low = 0;
+    let high = indices.length;
+    while (low < high) {
+        const middle = low + Math.floor((high - low) / 2);
+        if (indices[middle]! < minimum) {
+            low = middle + 1;
+        } else {
+            high = middle;
+        }
+    }
+    return low < indices.length ? indices[low]! : -1;
+}
+
+function minHistoryUpdateIndex(left: number, right: number): number {
+    if (left === -1) return right;
+    if (right === -1) return left;
+    return Math.min(left, right);
+}
+
 function historyUpdateKey(update: UpdateSessionEvent): string | null {
     switch (update.sessionUpdate) {
         case "user_message_chunk":
diff --git a/src/__tests__/HistoryUpdateMerge.test.ts b/src/__tests__/HistoryUpdateMerge.test.ts
new file mode 100644
index 00000000..343176da
--- /dev/null
+++ b/src/__tests__/HistoryUpdateMerge.test.ts
@@ -0,0 +1,49 @@
+import {describe, expect, it} from "vitest";
+import type {UpdateSessionEvent} from "../ACPSessionConnection";
+import {mergeHistoryUpdates} from "../CodexAcpServer";
+
+describe("mergeHistoryUpdates", () => {
+    it("uses the earliest content or exact-key match", () => {
+        const earlierContentMatch = message("fallback-id", "same content", "fallback-content");
+        const laterExactMatch = message("target-id", "same content", "fallback-exact");
+        const target = message("target-id", "same content", "thread");
+
+        expect(mergeHistoryUpdates(
+            [earlierContentMatch, laterExactMatch],
+            [target],
+        )).toEqual([target]);
+    });
+
+    it("does not revisit matches before the advancing fallback cursor", () => {
+        const targetA = message("message-a", "alpha", "thread-a");
+        const between = thought("thought-between", "keep between matches");
+        const targetB = message("message-b", "beta", "thread-b");
+
+        expect(mergeHistoryUpdates(
+            [
+                message("message-a", "alpha", "fallback-a"),
+                between,
+                message("message-b", "beta", "fallback-b"),
+                message("message-a", "alpha", "fallback-a-late"),
+            ],
+            [targetA, targetB],
+        )).toEqual([targetA, between, targetB]);
+    });
+});
+
+function message(messageId: string, text: string, source: string): UpdateSessionEvent {
+    return {
+        sessionUpdate: "agent_message_chunk",
+        messageId,
+        content: {type: "text", text},
+        _meta: {source},
+    };
+}
+
+function thought(messageId: string, text: string): UpdateSessionEvent {
+    return {
+        sessionUpdate: "agent_thought_chunk",
+        messageId,
+        content: {type: "text", text},
+    };
+}