fix(compaction): drop label/branch_summary entries referencing removed messages

Addresses Codex review feedback: label and branch_summary entries whose
parentId points to a removed message are now also dropped during
truncation to avoid dangling metadata. This is consistent with the
approach in tool-result-truncation.ts.

src/agents/pi-embedded-runner/session-truncation.test.ts

@@ -229,6 +229,55 @@ describe("truncateSessionAfterCompaction", () => {
     expect(ctx.thinkingLevel).toBe("high");
   });
 
+  it("drops label and branch_summary entries that reference removed messages", async () => {
+    const dir = await createTmpDir();
+    const sm = SessionManager.create(dir, dir);
+
+    // Messages before compaction
+    sm.appendMessage({ role: "user", content: "hello", timestamp: 1 });
+    sm.appendMessage(makeAssistant("hi", 2));
+    sm.appendMessage({ role: "user", content: "do task", timestamp: 3 });
+    sm.appendMessage(makeAssistant("done", 4));
+
+    // Get a pre-compaction message ID to attach label to
+    const branch = sm.getBranch();
+    const preCompactionMsgId = branch[1].id; // "hi" message
+
+    // Compaction summarizing the conversation
+    const firstKeptId = branch[branch.length - 1].id;
+    sm.appendCompaction("Summary.", firstKeptId, 5000);
+
+    // Post-compaction messages
+    sm.appendMessage({ role: "user", content: "next", timestamp: 5 });
+
+    const sessionFile = sm.getSessionFile()!;
+
+    // Inject a label entry referencing a pre-compaction message into the
+    // raw JSONL before the compaction line (SessionManager has no appendLabel).
+    const lines = (await fs.readFile(sessionFile, "utf-8")).trimEnd().split("\n");
+    const compactionLineIdx = lines.findIndex((l) => l.includes('"compaction"'));
+    const labelLine = JSON.stringify({
+      type: "label",
+      id: "label-001",
+      parentId: preCompactionMsgId,
+      timestamp: 2500,
+      targetId: preCompactionMsgId,
+      label: "my-label",
+    });
+    lines.splice(compactionLineIdx, 0, labelLine);
+    await fs.writeFile(sessionFile, lines.join("\n") + "\n", "utf-8");
+
+    const result = await truncateSessionAfterCompaction({ sessionFile });
+
+    expect(result.truncated).toBe(true);
+
+    // Verify label entry was dropped (its parent message was removed)
+    const smAfter = SessionManager.open(sessionFile);
+    const allAfter = smAfter.getEntries();
+    const labels = allAfter.filter((e) => e.type === "label");
+    expect(labels).toHaveLength(0);
+  });
+
   it("preserves unsummarized sibling branches during truncation", async () => {
     const dir = await createTmpDir();
     const sm = SessionManager.create(dir, dir);

src/agents/pi-embedded-runner/session-truncation.ts

@@ -16,7 +16,9 @@ import { log } from "./logger.js";
  * This function rewrites the file keeping:
  * 1. The session header
  * 2. All non-message session state (custom, model_change, thinking_level_change,
- *    session_info, label, branch_summary, custom_message, compaction entries)
+ *    session_info, custom_message, compaction entries)
+ *    Note: label and branch_summary entries referencing removed messages are
+ *    also dropped to avoid dangling metadata.
  * 3. All entries from sibling branches not covered by the compaction
  * 4. The compaction entry and all entries after it in the current branch
  *
@@ -81,9 +83,13 @@ export async function truncateSessionAfterCompaction(params: {
   const allEntries = sm.getEntries();
 
   // Only remove message-type entries that the compaction actually summarized.
-  // Non-message entries (custom, model_change, thinking_level_change,
-  // session_info, label, branch_summary, custom_message) are preserved even
-  // if they sit in the summarized portion of the branch.
+  // Non-message session state (custom, model_change, thinking_level_change,
+  // session_info, custom_message) is preserved even if it sits in the
+  // summarized portion of the branch.
+  //
+  // label and branch_summary entries that reference removed message IDs are
+  // also dropped to avoid dangling metadata (consistent with the approach in
+  // tool-result-truncation.ts).
   const removedIds = new Set<string>();
   for (const entry of allEntries) {
     if (summarizedBranchIds.has(entry.id) && entry.type === "message") {
@@ -91,6 +97,17 @@ export async function truncateSessionAfterCompaction(params: {
     }
   }
 
+  // Drop label/branch_summary entries whose parent points to a removed message
+  for (const entry of allEntries) {
+    if (
+      (entry.type === "label" || entry.type === "branch_summary") &&
+      entry.parentId !== null &&
+      removedIds.has(entry.parentId)
+    ) {
+      removedIds.add(entry.id);
+    }
+  }
+
   if (removedIds.size === 0) {
     return { truncated: false, entriesRemoved: 0, reason: "no entries to remove" };
   }