diff --git a/packages/agent-core/src/harness/compaction/compaction-trailing-toolresult.test.ts b/packages/agent-core/src/harness/compaction/compaction-trailing-toolresult.test.ts
new file mode 100644
index 00000000000..4e861f291f1
--- /dev/null
+++ b/packages/agent-core/src/harness/compaction/compaction-trailing-toolresult.test.ts
@@ -0,0 +1,80 @@
+import { describe, expect, it } from "vitest";
+import type { AgentMessage } from "../../types.js";
+import type { SessionTreeEntry } from "../types.js";
+import { estimateTokens, findCutPoint } from "./compaction.js";
+
+const KEEP_RECENT_TOKENS = 20000;
+const LARGE_TOOL_OUTPUT = "x".repeat(120000);
+
+function userText(text: string, timestamp: number): AgentMessage {
+  return { role: "user", content: [{ type: "text", text }], timestamp };
+}
+
+function assistantText(text: string, timestamp: number): AgentMessage {
+  return {
+    role: "assistant",
+    content: [{ type: "text", text }],
+    api: "anthropic-messages",
+    provider: "anthropic",
+    model: "claude-fable-5",
+    usage: {
+      input: 0,
+      output: 0,
+      cacheRead: 0,
+      cacheWrite: 0,
+      totalTokens: 0,
+      cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0, total: 0 },
+    },
+    stopReason: "stop",
+    timestamp,
+  };
+}
+
+function toolResultText(text: string, timestamp: number): AgentMessage {
+  return {
+    role: "toolResult",
+    toolCallId: "call-1",
+    toolName: "bash",
+    content: [{ type: "text", text }],
+    isError: false,
+    timestamp,
+  };
+}
+
+function messageEntry(message: AgentMessage, index: number): SessionTreeEntry {
+  return {
+    type: "message",
+    id: `entry-${index}`,
+    parentId: index === 0 ? null : `entry-${index - 1}`,
+    timestamp: new Date(message.timestamp).toISOString(),
+    message,
+  };
+}
+
+function buildTranscript(): SessionTreeEntry[] {
+  const messages: AgentMessage[] = [
+    userText("start of the conversation", 1),
+    assistantText("first reply", 2),
+    userText("please run the command", 3),
+    assistantText("running it now", 4),
+    toolResultText(LARGE_TOOL_OUTPUT, 5),
+  ];
+  return messages.map((message, index) => messageEntry(message, index));
+}
+
+describe("findCutPoint with a trailing oversized tool result", () => {
+  it("counts the final tool result as larger than the keep budget", () => {
+    const trailing = toolResultText(LARGE_TOOL_OUTPUT, 5);
+
+    expect(estimateTokens(trailing)).toBeGreaterThanOrEqual(KEEP_RECENT_TOKENS);
+  });
+
+  it("trims the prefix instead of keeping the whole transcript", () => {
+    const entries = buildTranscript();
+
+    const result = findCutPoint(entries, 0, entries.length, KEEP_RECENT_TOKENS);
+
+    expect(result.firstKeptEntryIndex).toBeGreaterThan(0);
+    expect(result.firstKeptEntryIndex).toBe(3);
+  });
+});
diff --git a/packages/agent-core/src/harness/compaction/compaction.ts b/packages/agent-core/src/harness/compaction/compaction.ts
index 26e86a3f842..479f90a32cf 100644
--- a/packages/agent-core/src/harness/compaction/compaction.ts
+++ b/packages/agent-core/src/harness/compaction/compaction.ts
@@ -407,6 +407,7 @@ export function findCutPoint(
     const messageTokens = estimateTokens(entry.message);
     accumulatedTokens += messageTokens;
     if (accumulatedTokens >= keepRecentTokens) {
+      cutIndex = cutPoints[cutPoints.length - 1];
       for (const cutPoint of cutPoints) {
         if (cutPoint >= i) {
           cutIndex = cutPoint;