fix(core): decode UTF-16 BOMs in readJson

packages/core/src/fs-util.ts

@@ -100,9 +100,9 @@ export namespace FSUtil {
       })
 
       const readJson = Effect.fn("FileSystem.readJson")(function* (path: string) {
-        const text = yield* fs.readFileString(path)
+        const bytes = yield* fs.readFile(path)
         return yield* Effect.try({
-          try: () => JSON.parse(text),
+          try: () => JSON.parse(decodeJsonBytes(bytes)),
           catch: (cause) => new FileSystemError({ method: "readJson", cause }),
         })
       })
@@ -272,3 +272,13 @@ export namespace FSUtil {
     return result === "" || (!isAbsolute(result) && result !== ".." && !result.startsWith(`..${sep}`))
   }
 }
+
+// Windows shells commonly write redirected output as UTF-16 with a BOM
+// (PowerShell's `>` uses UTF-16LE), so accept BOM-prefixed JSON files.
+// TextDecoder's UTF-8 mode strips a UTF-8 BOM by default.
+function decodeJsonBytes(bytes: Uint8Array): string {
+  if (bytes[0] === 0xff && bytes[1] === 0xfe) return Buffer.from(bytes.subarray(2)).toString("utf16le")
+  // UTF-16BE: swap to little-endian, then decode.
+  if (bytes[0] === 0xfe && bytes[1] === 0xff) return Buffer.from(bytes.subarray(2)).swap16().toString("utf16le")
+  return new TextDecoder("utf-8").decode(bytes)
+}

packages/core/test/filesystem/filesystem.test.ts

@@ -125,6 +125,54 @@ describe("FSUtil", () => {
         expect(result).toHaveProperty("_tag", "FileSystemError")
       }),
     )
+
+    it(
+      "reads UTF-16LE JSON with BOM (Windows PowerShell redirection)",
+      Effect.gen(function* () {
+        const fs = yield* FSUtil.Service
+        const filesys = yield* FileSystem.FileSystem
+        const tmp = yield* filesys.makeTempDirectoryScoped()
+        const file = path.join(tmp, "utf16le.json")
+        const json = JSON.stringify({ name: "test", count: 42 })
+
+        yield* filesys.writeFile(file, Buffer.concat([Buffer.from([0xff, 0xfe]), Buffer.from(json, "utf16le")]))
+
+        expect(yield* fs.readJson(file)).toEqual({ name: "test", count: 42 })
+      }),
+    )
+
+    it(
+      "reads UTF-16BE JSON with BOM",
+      Effect.gen(function* () {
+        const fs = yield* FSUtil.Service
+        const filesys = yield* FileSystem.FileSystem
+        const tmp = yield* filesys.makeTempDirectoryScoped()
+        const file = path.join(tmp, "utf16be.json")
+        const json = JSON.stringify({ name: "test", count: 42 })
+
+        yield* filesys.writeFile(
+          file,
+          Buffer.concat([Buffer.from([0xfe, 0xff]), Buffer.from(json, "utf16le").swap16()]),
+        )
+
+        expect(yield* fs.readJson(file)).toEqual({ name: "test", count: 42 })
+      }),
+    )
+
+    it(
+      "reads UTF-8 JSON with BOM",
+      Effect.gen(function* () {
+        const fs = yield* FSUtil.Service
+        const filesys = yield* FileSystem.FileSystem
+        const tmp = yield* filesys.makeTempDirectoryScoped()
+        const file = path.join(tmp, "utf8bom.json")
+        const json = JSON.stringify({ name: "test", count: 42 })
+
+        yield* filesys.writeFile(file, Buffer.concat([Buffer.from([0xef, 0xbb, 0xbf]), Buffer.from(json, "utf8")]))
+
+        expect(yield* fs.readJson(file)).toEqual({ name: "test", count: 42 })
+      }),
+    )
   })
 
   describe("ensureDir", () => {