fix(browser): accept HAR files that begin with a byte-order mark (#1509)

HAR 1.2 lets writers emit a UTF-8 byte-order mark and requires readers to
ignore it. The decoder passed ignoreBOM: true to TextDecoder, which keeps
the mark in the decoded text, so a BOM-prefixed export failed as "HAR is
malformed JSON." Decoding now uses the default, which skips one leading
mark. The artifact digest still covers the selected file bytes.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

src/browser/history/HarCapture.test.ts

@@ -1,7 +1,7 @@
 import { createHash } from "node:crypto";
 import { expect, it } from "vitest";
 import { historicalHar } from "../../../tests/fixtures/historicalHar.js";
-import { decodeHarCapture } from "./HarCapture.js";
+import { decodeHarCapture, decodeHarText } from "./HarCapture.js";
 
 it.each([
   { values: [] },
@@ -558,3 +558,23 @@ it.each([
     });
   },
 );
+
+it("ignores one leading byte-order mark, as HAR 1.2 requires of readers", () => {
+  const text = JSON.stringify(historicalHar());
+  const body = Buffer.from(text, "utf8");
+  const marked = Buffer.concat([Buffer.from([0xef, 0xbb, 0xbf]), body]);
+  expect(decodeHarText(marked)).toBe(text);
+  expect(decodeHarCapture(decodeHarText(marked), [])).toEqual(
+    decodeHarCapture(decodeHarText(body), []),
+  );
+  // A second mark is content, not an encoding signature.
+  expect(() =>
+    decodeHarCapture(
+      decodeHarText(Buffer.concat([Buffer.from([0xef, 0xbb, 0xbf]), marked])),
+      [],
+    ),
+  ).toThrow("HAR is malformed JSON.");
+  expect(() => decodeHarText(Buffer.from([0x7b, 0xff, 0x7d]))).toThrow(
+    "HAR is not valid UTF-8 JSON.",
+  );
+});

src/browser/history/HarCapture.ts

@@ -1,4 +1,5 @@
 import { createHash } from "node:crypto";
+import { TextDecoder } from "node:util";
 import { isSafeNumber, LosslessNumber } from "lossless-json";
 import { parseHarJson } from "./HarJson.js";
 import type { JsonValue } from "../../domain/jsonValue.js";
@@ -25,6 +26,19 @@ type Projection = Pick<
   "reported" | "numeric_literals" | "binary_fields" | "redactions"
 >;
 
+/**
+ * Decode retained HAR bytes as UTF-8. HAR 1.2 lets writers emit a byte-order
+ * mark and requires readers to ignore it, so one leading BOM is skipped.
+ */
+export const decodeHarText = (bytes: Uint8Array): string => {
+  try {
+    // ignoreBOM: false (the default) is what skips the mark; true keeps it.
+    return new TextDecoder("utf-8", { fatal: true }).decode(bytes);
+  } catch {
+    throw new CaptureFormatError("format", "HAR is not valid UTF-8 JSON.");
+  }
+};
+
 /** Decode a retained HAR with exact numeric lexemes and unchanged upstream schema validation. */
 export const decodeHarCapture = (
   text: string,

src/browser/history/HarCaptureProcess.ts

@@ -1,10 +1,9 @@
 import { readFile, writeFile } from "node:fs/promises";
-import { TextDecoder } from "node:util";
 import { z } from "zod";
 import { readStableArtifact } from "../../artifacts/readStableArtifact.js";
 import { WEB_NETWORK_CAPTURE_LIMITS } from "../../domain/webNetworkCapture.js";
 import { CaptureFormatError } from "./CaptureFormatError.js";
-import { decodeHarCapture } from "./HarCapture.js";
+import { decodeHarCapture, decodeHarText } from "./HarCapture.js";
 
 const requestSchema = z.strictObject({
   snapshot_path: z.string(),
@@ -23,17 +22,12 @@ const main = async (): Promise<void> => {
       request.snapshot_path,
       WEB_NETWORK_CAPTURE_LIMITS.inputBytes,
     );
-    let text: string;
-    try {
-      text = new TextDecoder("utf-8", { fatal: true, ignoreBOM: true }).decode(
-        snapshot.bytes,
-      );
-    } catch {
-      throw new CaptureFormatError("format", "HAR is not valid UTF-8 JSON.");
-    }
     reply = {
       ok: true,
-      value: decodeHarCapture(text, request.sensitive_values),
+      value: decodeHarCapture(
+        decodeHarText(snapshot.bytes),
+        request.sensitive_values,
+      ),
     };
   } catch (cause: unknown) {
     reply =