From cde1e2d3a1108aad02581f5b56472dfe50e95761 Mon Sep 17 00:00:00 2001
From: Ayaan Zaidi <hi@obviy.us>
Date: Sat, 4 Apr 2026 22:34:05 +0530
Subject: [PATCH] fix: preserve compaction split after trailing tool results

---
 src/agents/compaction.test.ts | 14 ++++++++++++++
 src/agents/compaction.ts      |  4 +---
 2 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/src/agents/compaction.test.ts b/src/agents/compaction.test.ts
index 9eb893eb961..94c09e240f4 100644
--- a/src/agents/compaction.test.ts
+++ b/src/agents/compaction.test.ts
@@ -171,6 +171,20 @@ describe("splitMessagesByTokenShare", () => {
     expect(parts.flat().length).toBe(messages.length);
   });
 
+  it("splits before a trailing completed tool-call pair", () => {
+    const messages: AgentMessage[] = [
+      makeMessage(1, 4000),
+      makeAssistantToolCall(2, "call_tail", "y".repeat(200)),
+      makeToolResult(3, "call_tail", "r".repeat(4000)),
+    ];
+
+    const parts = splitMessagesByTokenShare(messages, 2);
+
+    expect(parts.length).toBe(2);
+    expect(parts[0]?.map((m) => m.timestamp)).toEqual([1]);
+    expect(parts[1]?.map((m) => m.timestamp)).toEqual([2, 3]);
+  });
+
   it("does not block splits after aborted tool-call assistants", () => {
     const messages: AgentMessage[] = [
       makeAssistantToolCall(1, "call_abort", "y".repeat(4000), "aborted"),
diff --git a/src/agents/compaction.ts b/src/agents/compaction.ts
index 374cabd8b81..79c34c61a61 100644
--- a/src/agents/compaction.ts
+++ b/src/agents/compaction.ts
@@ -188,9 +188,7 @@ export function splitMessagesByTokenShare(
         chunks.length < normalizedParts - 1 &&
         currentTokens > targetTokens
       ) {
-        chunks.push(current);
-        current = [];
-        currentTokens = 0;
+        splitCurrentAtPendingBoundary();
         pendingChunkStartIndex = null;
       }
     }