fix(opencode): strip all leading BOMs in Bom.split (#33092)

packages/opencode/src/util/bom.ts

@@ -5,8 +5,8 @@ const BOM_CODE = 0xfeff
 const BOM = String.fromCharCode(BOM_CODE)
 
 export function split(text: string) {
-  if (text.charCodeAt(0) !== BOM_CODE) return { bom: false, text }
-  return { bom: true, text: text.slice(1) }
+  const stripped = text.replace(/^\uFEFF+/, "")
+  return { bom: stripped.length !== text.length, text: stripped }
 }
 
 export function join(text: string, bom: boolean) {

packages/opencode/test/util/bom.test.ts

@@ -0,0 +1,44 @@
+import { describe, expect, test } from "bun:test"
+import * as Bom from "../../src/util/bom"
+
+const BOM = "\uFEFF"
+
+describe("Bom.split", () => {
+  test("returns text unchanged when there is no BOM", () => {
+    expect(Bom.split("hello")).toEqual({ bom: false, text: "hello" })
+  })
+
+  test("strips a single leading BOM", () => {
+    expect(Bom.split(BOM + "hello")).toEqual({ bom: true, text: "hello" })
+  })
+
+  test("strips all consecutive leading BOMs", () => {
+    expect(Bom.split(BOM.repeat(4) + "hello")).toEqual({ bom: true, text: "hello" })
+  })
+
+  test("does not strip a BOM that is not at the start", () => {
+    expect(Bom.split("hello" + BOM)).toEqual({ bom: false, text: "hello" + BOM })
+  })
+
+  test("handles an empty string", () => {
+    expect(Bom.split("")).toEqual({ bom: false, text: "" })
+  })
+})
+
+describe("Bom.join", () => {
+  test("adds a BOM when requested", () => {
+    expect(Bom.join("hello", true)).toBe(BOM + "hello")
+  })
+
+  test("omits the BOM when not requested", () => {
+    expect(Bom.join("hello", false)).toBe("hello")
+  })
+
+  test("normalizes multiple existing leading BOMs to exactly one", () => {
+    expect(Bom.join(BOM.repeat(4) + "hello", true)).toBe(BOM + "hello")
+  })
+
+  test("strips all existing leading BOMs when none is requested", () => {
+    expect(Bom.join(BOM.repeat(3) + "hello", false)).toBe("hello")
+  })
+})