fix(compaction): anchor forced manual boundary off trailing tool results

amknight · amknight · commit fd09d2e7d0cb · 2026-06-17T21:21:25.000+10:00
diff --git a/packages/agent-core/src/harness/compaction/compaction.test.ts b/packages/agent-core/src/harness/compaction/compaction.test.ts
@@ -125,6 +125,84 @@ describe("prepareCompaction", () => {
     });
   });
 
+  it("anchors a forced boundary on the assistant tool call, not a trailing tool result", () => {
+    const entries: SessionTreeEntry[] = [
+      {
+        type: "message",
+        id: "user-1",
+        parentId: null,
+        timestamp: "2026-06-17T08:45:00.000Z",
+        message: { role: "user", content: "Read the notes file.", timestamp: 1 },
+      },
+      {
+        type: "message",
+        id: "assistant-1",
+        parentId: "user-1",
+        timestamp: "2026-06-17T08:45:10.000Z",
+        message: {
+          role: "assistant",
+          content: [
+            { type: "toolCall", id: "call-1", name: "read_file", arguments: { path: "notes.md" } },
+          ],
+          api: "openai-responses",
+          provider: "openai",
+          model: "gpt-test",
+          usage: {
+            input: 625,
+            output: 6,
+            cacheRead: 172_928,
+            cacheWrite: 0,
+            totalTokens: 173_559,
+            cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0, total: 0 },
+          },
+          stopReason: "toolUse",
+          timestamp: 2,
+        },
+      },
+      {
+        type: "message",
+        id: "tool-1",
+        parentId: "assistant-1",
+        timestamp: "2026-06-17T08:45:11.000Z",
+        message: {
+          role: "toolResult",
+          toolCallId: "call-1",
+          toolName: "read_file",
+          content: [{ type: "text", text: "notes body" }],
+          isError: false,
+          timestamp: 3,
+        },
+      },
+    ];
+
+    const preparation = prepareCompaction(entries, DEFAULT_COMPACTION_SETTINGS, { force: true });
+
+    // Anchor must be the assistant that owns the tool call, never the trailing
+    // tool result, or the rebuilt context would replay an orphaned tool result.
+    expect(preparation).toEqual({
+      ok: true,
+      value: expect.objectContaining({ firstKeptEntryId: "assistant-1" }),
+    });
+
+    const compactedContext = buildSessionContext([
+      ...entries,
+      {
+        type: "compaction",
+        id: "compaction-1",
+        parentId: "tool-1",
+        timestamp: "2026-06-17T08:45:20.000Z",
+        summary: "Checkpoint of the file read.",
+        firstKeptEntryId: "assistant-1",
+        tokensBefore: 173_559,
+      },
+    ]);
+    expect(compactedContext.messages.map((message) => message.role)).toEqual([
+      "compactionSummary",
+      "assistant",
+      "toolResult",
+    ]);
+  });
+
   it("shows why the old empty-summary compaction replayed the whole transcript", () => {
     const entries: SessionTreeEntry[] = [
       {
diff --git a/packages/agent-core/src/harness/compaction/compaction.ts b/packages/agent-core/src/harness/compaction/compaction.ts
@@ -701,14 +701,20 @@ export function prepareCompaction(
           forcedMessagesToSummarize.push(msg);
         }
       }
-      if (forcedMessagesToSummarize.length > 0) {
+      // Anchor the kept tail on the last valid cut point, not the raw final entry.
+      // findValidCutPoints excludes tool results, so a forced boundary that is not
+      // collapsed to summary-only later never keeps an orphaned tool result.
+      const forcedCutPoints = findValidCutPoints(pathEntries, boundaryStart, boundaryEnd);
+      const forcedKeepIndex =
+        forcedCutPoints.length > 0 ? forcedCutPoints[forcedCutPoints.length - 1] : -1;
+      if (forcedMessagesToSummarize.length > 0 && forcedKeepIndex >= 0) {
         const forcedFileOps = extractFileOperations(
           forcedMessagesToSummarize,
           pathEntries,
           prevCompactionIndex,
         );
         return ok({
-          firstKeptEntryId: pathEntries[boundaryEnd - 1].id,
+          firstKeptEntryId: pathEntries[forcedKeepIndex].id,
           messagesToSummarize: forcedMessagesToSummarize,
           turnPrefixMessages: [],
           isSplitTurn: false,
