fix(coding-agent): rechain fork paths without labels

closes #5669

packages/coding-agent/CHANGELOG.md

@@ -9,6 +9,7 @@
 ### Fixed
 
 - Fixed `--model` resolution for authenticated custom model IDs whose slash prefix matches an unauthenticated built-in provider ([#5643](https://github.com/earendil-works/pi/issues/5643)).
+- Fixed `/fork` to keep session parent chains connected when the forked path contains labels ([#5669](https://github.com/earendil-works/pi/issues/5669)).
 - Fixed `/share` and `/export` HTML exports to use the active fallback theme when the configured custom theme no longer exists ([#5596](https://github.com/earendil-works/pi/issues/5596)).
 - Fixed custom fallback model IDs with `:<thinking>` suffixes to preserve the requested thinking level when the provider template model does not advertise reasoning ([#5552](https://github.com/earendil-works/pi/issues/5552)).
 

packages/coding-agent/src/core/session-manager.ts

@@ -1290,8 +1290,16 @@ export class SessionManager {
 			throw new Error(`Entry ${leafId} not found`);
 		}
 
-		// Filter out LabelEntry from path - we'll recreate them from the resolved map
-		const pathWithoutLabels = path.filter((e) => e.type !== "label");
+		// Filter out LabelEntry from path - we'll recreate them from the resolved map.
+		// Because labels are real tree entries, later entries can be children of labels;
+		// removing labels requires re-chaining the retained path to avoid orphaned subtrees.
+		const pathWithoutLabels: SessionEntry[] = [];
+		let pathParentId: string | null = null;
+		for (const entry of path) {
+			if (entry.type === "label") continue;
+			pathWithoutLabels.push({ ...entry, parentId: pathParentId });
+			pathParentId = entry.id;
+		}
 
 		const newSessionId = createSessionId();
 		const timestamp = new Date().toISOString();

packages/coding-agent/test/session-manager/labels.test.ts

@@ -142,6 +142,19 @@ describe("SessionManager labels", () => {
 		expect(msg2Node?.labelTimestamp).toBe(msg2LabelEntry.timestamp);
 	});
 
+	it("rewires children of removed labels when forking", () => {
+		const session = SessionManager.inMemory();
+
+		const msg1Id = session.appendMessage({ role: "user", content: "hello", timestamp: 1 });
+		session.appendLabelChange(msg1Id, "checkpoint");
+		const modelChangeId = session.appendModelChange("anthropic", "claude-test");
+		const msg2Id = session.appendMessage({ role: "user", content: "followup", timestamp: 2 });
+
+		session.createBranchedSession(msg2Id);
+
+		expect(session.getEntry(modelChangeId)?.parentId).toBe(msg1Id);
+	});
+
 	it("labels not on path are not preserved in createBranchedSession", () => {
 		const session = SessionManager.inMemory();