diff --git a/packages/coding-agent/test/session-manager/custom-session-id.test.ts b/packages/coding-agent/test/session-manager/custom-session-id.test.ts
index 476f6d469..8ae0cb4f2 100644
--- a/packages/coding-agent/test/session-manager/custom-session-id.test.ts
+++ b/packages/coding-agent/test/session-manager/custom-session-id.test.ts
@@ -1,4 +1,4 @@
-import { existsSync, mkdtempSync, writeFileSync } from "node:fs";
+import { existsSync, mkdtempSync, readFileSync, rmSync, writeFileSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { basename, join } from "node:path";
 import { describe, expect, it } from "vitest";
@@ -167,3 +167,46 @@ describe("SessionManager.newSession with custom id", () => {
 		expect(basename(sessionFile)).toMatch(/^\d{4}-\d{2}-\d{2}T\d{2}-\d{2}-\d{2}-\d{3}Z_forked-session-id\.jsonl$/);
 	});
 });
+
+describe("SessionManager.forkFrom", () => {
+	it("copies all entries unchanged into the forked session", () => {
+		const tempDir = mkdtempSync(join(tmpdir(), "pi-session-manager-"));
+		try {
+			const sourcePath = join(tempDir, "source.jsonl");
+			const lines = [
+				JSON.stringify({
+					type: "session",
+					version: 3,
+					id: "source-id",
+					timestamp: new Date().toISOString(),
+					cwd: tempDir,
+				}),
+			];
+			let parentId: string | null = null;
+			for (let i = 0; i < 50; i++) {
+				const id = `e${i}`;
+				lines.push(
+					JSON.stringify({
+						type: "message",
+						id,
+						parentId,
+						timestamp: new Date().toISOString(),
+						message: { role: "user", content: `message ${i}`, timestamp: 0 },
+					}),
+				);
+				parentId = id;
+			}
+			writeFileSync(sourcePath, `${lines.join("\n")}\n`);
+
+			const forked = SessionManager.forkFrom(sourcePath, tempDir, join(tempDir, "sessions"));
+
+			const written = readFileSync(forked.getSessionFile()!, "utf8").trimEnd().split("\n");
+			expect(written).toHaveLength(51);
+			expect(JSON.parse(written[0]).parentSession).toBe(sourcePath);
+			expect(written.slice(1)).toEqual(lines.slice(1));
+			expect(forked.getEntries()).toHaveLength(50);
+		} finally {
+			rmSync(tempDir, { recursive: true, force: true });
+		}
+	});
+});
diff --git a/packages/coding-agent/test/session-manager/fork-from.test.ts b/packages/coding-agent/test/session-manager/fork-from.test.ts
deleted file mode 100644
index abfe959f3..000000000
--- a/packages/coding-agent/test/session-manager/fork-from.test.ts
+++ /dev/null
@@ -1,66 +0,0 @@
-import type * as Fs from "node:fs";
-import { mkdtempSync, readFileSync, writeFileSync } from "node:fs";
-import { tmpdir } from "node:os";
-import { join } from "node:path";
-import { describe, expect, it, vi } from "vitest";
-import { SessionManager } from "../../src/core/session-manager.ts";
-
-const { openSyncSpy, appendFileSyncSpy } = vi.hoisted(() => ({
-	openSyncSpy: vi.fn(),
-	appendFileSyncSpy: vi.fn(),
-}));
-
-vi.mock("fs", async (importOriginal) => {
-	const actual = await importOriginal<typeof Fs>();
-	openSyncSpy.mockImplementation(actual.openSync);
-	appendFileSyncSpy.mockImplementation(actual.appendFileSync);
-	return { ...actual, openSync: openSyncSpy, appendFileSync: appendFileSyncSpy };
-});
-
-describe("SessionManager.forkFrom", () => {
-	// Regression test for #11253: forking reopened the target file once per entry.
-	it("copies all entries while opening the new session file only once for writing", () => {
-		const tempDir = mkdtempSync(join(tmpdir(), "pi-fork-from-"));
-		const sourcePath = join(tempDir, "source.jsonl");
-		const lines = [
-			JSON.stringify({
-				type: "session",
-				version: 3,
-				id: "source-id",
-				timestamp: new Date().toISOString(),
-				cwd: tempDir,
-			}),
-		];
-		let parentId: string | null = null;
-		for (let i = 0; i < 50; i++) {
-			const id = `e${i}`;
-			lines.push(
-				JSON.stringify({
-					type: "message",
-					id,
-					parentId,
-					timestamp: new Date().toISOString(),
-					message: { role: "user", content: `message ${i}`, timestamp: 0 },
-				}),
-			);
-			parentId = id;
-		}
-		writeFileSync(sourcePath, `${lines.join("\n")}\n`);
-
-		const targetDir = join(tempDir, "sessions");
-		openSyncSpy.mockClear();
-		appendFileSyncSpy.mockClear();
-		const forked = SessionManager.forkFrom(sourcePath, tempDir, targetDir);
-		const newFile = forked.getSessionFile()!;
-
-		const writeOpens = openSyncSpy.mock.calls.filter(([path, flags]) => path === newFile && flags !== "r");
-		expect(writeOpens).toEqual([[newFile, "wx"]]);
-		expect(appendFileSyncSpy.mock.calls.filter(([path]) => path === newFile)).toHaveLength(0);
-
-		const written = readFileSync(newFile, "utf8").trimEnd().split("\n");
-		expect(written).toHaveLength(51);
-		expect(JSON.parse(written[0]).parentSession).toBe(sourcePath);
-		expect(written.slice(1)).toEqual(lines.slice(1));
-		expect(forked.getEntries()).toHaveLength(50);
-	});
-});