fix: preserve BOM in text tool round-trips (#23797)

packages/opencode/test/tool/edit.test.ts

@@ -96,6 +96,37 @@ describe("tool.edit", () => {
       })
     })
 
+    test("preserves BOM when oldString is empty on existing files", async () => {
+      await using tmp = await tmpdir()
+      const filepath = path.join(tmp.path, "existing.cs")
+      const bom = String.fromCharCode(0xfeff)
+      await fs.writeFile(filepath, `${bom}using System;\n`, "utf-8")
+
+      await Instance.provide({
+        directory: tmp.path,
+        fn: async () => {
+          const edit = await resolve()
+          const result = await Effect.runPromise(
+            edit.execute(
+              {
+                filePath: filepath,
+                oldString: "",
+                newString: "using Up;\n",
+              },
+              ctx,
+            ),
+          )
+
+          expect(result.metadata.diff).toContain("-using System;")
+          expect(result.metadata.diff).toContain("+using Up;")
+
+          const content = await fs.readFile(filepath, "utf-8")
+          expect(content.charCodeAt(0)).toBe(0xfeff)
+          expect(content.slice(1)).toBe("using Up;\n")
+        },
+      })
+    })
+
     test("creates new file with nested directories", async () => {
       await using tmp = await tmpdir()
       const filepath = path.join(tmp.path, "nested", "dir", "file.txt")
@@ -183,6 +214,38 @@ describe("tool.edit", () => {
       })
     })
 
+    test("replaces the first visible line in BOM files", async () => {
+      await using tmp = await tmpdir()
+      const filepath = path.join(tmp.path, "existing.cs")
+      const bom = String.fromCharCode(0xfeff)
+      await fs.writeFile(filepath, `${bom}using System;\nclass Test {}\n`, "utf-8")
+
+      await Instance.provide({
+        directory: tmp.path,
+        fn: async () => {
+          const edit = await resolve()
+          const result = await Effect.runPromise(
+            edit.execute(
+              {
+                filePath: filepath,
+                oldString: "using System;",
+                newString: "using Up;",
+              },
+              ctx,
+            ),
+          )
+
+          expect(result.metadata.diff).toContain("-using System;")
+          expect(result.metadata.diff).toContain("+using Up;")
+          expect(result.metadata.diff).not.toContain(bom)
+
+          const content = await fs.readFile(filepath, "utf-8")
+          expect(content.charCodeAt(0)).toBe(0xfeff)
+          expect(content.slice(1)).toBe("using Up;\nclass Test {}\n")
+        },
+      })
+    })
+
     test("throws error when file does not exist", async () => {
       await using tmp = await tmpdir()
       const filepath = path.join(tmp.path, "nonexistent.txt")