diff --git a/chrome/browser/extensions/api/declarative_net_request/declarative_net_request_browsertest.cc b/chrome/browser/extensions/api/declarative_net_request/declarative_net_request_browsertest.cc
index f94ebd9a9a7c99..308725aef8a100 100644
--- a/chrome/browser/extensions/api/declarative_net_request/declarative_net_request_browsertest.cc
+++ b/chrome/browser/extensions/api/declarative_net_request/declarative_net_request_browsertest.cc
@@ -5542,18 +5542,14 @@ IN_PROC_BROWSER_TEST_P(DeclarativeNetRequestSubresourceWebBundlesBrowserTest,
       embedded_test_server()->GetURL("/pass.js").spec();
   std::string cancel_js_url_str =
       embedded_test_server()->GetURL("/cancel.js").spec();
-  // Currently the web bundle format requires a valid GURL for the fallback URL
-  // of a web bundle. So we use |pass_js_url_str| for the fallback URL.
-  // TODO(crbug.com/966753): Stop using |pass_js_url_str| when
-  // https://github.com/WICG/webpackage/issues/590 is resolved.
-  web_package::WebBundleBuilder builder(pass_js_url_str, "");
-  auto pass_js_location = builder.AddResponse(
+  web_package::WebBundleBuilder builder;
+  builder.AddExchange(
+      pass_js_url_str,
       {{":status", "200"}, {"content-type", "application/javascript"}},
       "document.title = 'script loaded';");
-  auto cancel_js_location = builder.AddResponse(
+  builder.AddExchange(
+      cancel_js_url_str,
       {{":status", "200"}, {"content-type", "application/javascript"}}, "");
-  builder.AddIndexEntry(pass_js_url_str, "", {pass_js_location});
-  builder.AddIndexEntry(cancel_js_url_str, "", {cancel_js_location});
   std::vector<uint8_t> bundle = builder.CreateBundle();
   web_bundle = std::string(bundle.begin(), bundle.end());
 
@@ -5618,19 +5614,14 @@ IN_PROC_BROWSER_TEST_P(DeclarativeNetRequestSubresourceWebBundlesBrowserTest,
   RegisterRequestHandler("/test.html", "text/html", page_html);
   ASSERT_TRUE(embedded_test_server()->Start());
 
-  // Create a web bundle.
-  // Currently the web bundle format requires a valid GURL for the fallback
-  // URL of a web bundle. So we use |pass_js_url_str| for the fallback URL.
-  // TODO(crbug.com/966753): Stop using |pass_js_url_str| when
-  // https://github.com/WICG/webpackage/issues/590 is resolved.
-  web_package::WebBundleBuilder builder(pass_js_url, "");
-  auto pass_js_location = builder.AddResponse(
+  web_package::WebBundleBuilder builder;
+  builder.AddExchange(
+      pass_js_url,
       {{":status", "200"}, {"content-type", "application/javascript"}},
       "document.title = 'script loaded';");
-  auto cancel_js_location = builder.AddResponse(
+  builder.AddExchange(
+      cancel_js_url,
       {{":status", "200"}, {"content-type", "application/javascript"}}, "");
-  builder.AddIndexEntry(pass_js_url, "", {pass_js_location});
-  builder.AddIndexEntry(cancel_js_url, "", {cancel_js_location});
   std::vector<uint8_t> bundle = builder.CreateBundle();
   web_bundle = std::string(bundle.begin(), bundle.end());
 
@@ -5700,34 +5691,28 @@ IN_PROC_BROWSER_TEST_P(DeclarativeNetRequestSubresourceWebBundlesBrowserTest,
       embedded_test_server()->GetURL("/redirected_to_unlisted.js").spec();
   std::string redirect_to_server_js_url_str =
       embedded_test_server()->GetURL("/redirect_to_server.js").spec();
-  // Currently the web bundle format requires a valid GURL for the fallback URL
-  // of a web bundle. So we use |redirect_js_url_str| for the fallback URL.
-  // TODO(crbug.com/966753): Stop using |redirect_js_url_str| when
-  // https://github.com/WICG/webpackage/issues/590 is resolved.
-  web_package::WebBundleBuilder builder(redirect_js_url_str, "");
-  auto redirect_js_location = builder.AddResponse(
+  web_package::WebBundleBuilder builder;
+  builder.AddExchange(
+      redirect_js_url_str,
       {{":status", "200"}, {"content-type", "application/javascript"}},
       "document.title = 'redirect';");
-  auto redirected_js_location = builder.AddResponse(
+  builder.AddExchange(
+      redirected_js_url_str,
       {{":status", "200"}, {"content-type", "application/javascript"}},
       "document.title = 'redirected';");
-  auto redirect_to_unlisted_location = builder.AddResponse(
+  builder.AddExchange(
+      redirect_to_unlisted_js_url_str,
       {{":status", "200"}, {"content-type", "application/javascript"}},
       "document.title = 'redirect_to_unlisted';");
-  auto redirected_to_unlisted_js_location = builder.AddResponse(
+  builder.AddExchange(
+      redirected_to_unlisted_js_url_str,
       {{":status", "200"}, {"content-type", "application/javascript"}},
       "document.title = 'redirected_to_unlisted';");
-  auto redirect_to_server_js_location = builder.AddResponse(
+  builder.AddExchange(
+      redirect_to_server_js_url_str,
       {{":status", "200"}, {"content-type", "application/javascript"}},
       "document.title = 'redirect_to_server';");
-  builder.AddIndexEntry(redirect_js_url_str, "", {redirect_js_location});
-  builder.AddIndexEntry(redirected_js_url_str, "", {redirected_js_location});
-  builder.AddIndexEntry(redirect_to_unlisted_js_url_str, "",
-                        {redirect_to_unlisted_location});
-  builder.AddIndexEntry(redirected_to_unlisted_js_url_str, "",
-                        {redirected_to_unlisted_js_location});
-  builder.AddIndexEntry(redirect_to_server_js_url_str, "",
-                        {redirect_to_server_js_location});
+
   std::vector<uint8_t> bundle = builder.CreateBundle();
   web_bundle = std::string(bundle.begin(), bundle.end());
 
@@ -5794,7 +5779,7 @@ IN_PROC_BROWSER_TEST_P(DeclarativeNetRequestSubresourceWebBundlesBrowserTest,
 
   // Create a web bundle.
   std::string js_url_str = embedded_test_server()->GetURL("/script.js").spec();
-  web_package::WebBundleBuilder builder(js_url_str, "");
+  web_package::WebBundleBuilder builder;
   builder.AddExchange(
       js_url_str,
       {{":status", "200"}, {"content-type", "application/javascript"}},
diff --git a/chrome/browser/extensions/api/web_request/web_request_apitest.cc b/chrome/browser/extensions/api/web_request/web_request_apitest.cc
index 41cd947732aa32..5976f4f1ef4e7a 100644
--- a/chrome/browser/extensions/api/web_request/web_request_apitest.cc
+++ b/chrome/browser/extensions/api/web_request/web_request_apitest.cc
@@ -3588,16 +3588,15 @@ IN_PROC_BROWSER_TEST_P(SubresourceWebBundlesWebRequestApiTest,
   // Create a web bundle.
   std::string script_url_str =
       embedded_test_server()->GetURL("/test.js").spec();
-  web_package::WebBundleBuilder builder("", "");
-  auto script_location = builder.AddResponse(
+  web_package::WebBundleBuilder builder;
+  builder.AddExchange(
+      script_url_str,
       {{":status", "200"}, {"content-type", "application/javascript"}},
       "document.title = 'ScriptDone';");
-  auto uuid_in_package_script_location = builder.AddResponse(
+  builder.AddExchange(
+      uuid_in_package_script_url,
       {{":status", "200"}, {"content-type", "application/javascript"}},
       "document.title += ':UUIDInPackageScriptDone';");
-  builder.AddIndexEntry(script_url_str, "", {script_location});
-  builder.AddIndexEntry(uuid_in_package_script_url, "",
-                        {uuid_in_package_script_location});
   std::vector<uint8_t> bundle = builder.CreateBundle();
   web_bundle = std::string(bundle.begin(), bundle.end());
 
@@ -3715,23 +3714,21 @@ IN_PROC_BROWSER_TEST_P(SubresourceWebBundlesWebRequestApiTest,
       embedded_test_server()->GetURL("/pass.js").spec();
   std::string cancel_js_url_str =
       embedded_test_server()->GetURL("/cancel.js").spec();
-  web_package::WebBundleBuilder builder(pass_js_url_str, "");
-  auto pass_js_location = builder.AddResponse(
+  web_package::WebBundleBuilder builder;
+  builder.AddExchange(
+      pass_js_url_str,
       {{":status", "200"}, {"content-type", "application/javascript"}},
       "document.title = 'script loaded';");
-  auto cancel_js_location = builder.AddResponse(
+  builder.AddExchange(
+      cancel_js_url_str,
       {{":status", "200"}, {"content-type", "application/javascript"}}, "");
-  auto uuid_in_package_pass_js_location = builder.AddResponse(
+  builder.AddExchange(
+      pass_uuid_in_package_js_url,
       {{":status", "200"}, {"content-type", "application/javascript"}},
       "document.title = 'uuid-in-package script loaded';");
-  auto uuid_in_package_cancel_js_location = builder.AddResponse(
+  builder.AddExchange(
+      cancel_uuid_in_package_js_url,
       {{":status", "200"}, {"content-type", "application/javascript"}}, "");
-  builder.AddIndexEntry(pass_js_url_str, "", {pass_js_location});
-  builder.AddIndexEntry(cancel_js_url_str, "", {cancel_js_location});
-  builder.AddIndexEntry(pass_uuid_in_package_js_url, "",
-                        {uuid_in_package_pass_js_location});
-  builder.AddIndexEntry(cancel_uuid_in_package_js_url, "",
-                        {uuid_in_package_cancel_js_location});
   std::vector<uint8_t> bundle = builder.CreateBundle();
   web_bundle = std::string(bundle.begin(), bundle.end());
 
@@ -3842,11 +3839,11 @@ IN_PROC_BROWSER_TEST_P(SubresourceWebBundlesWebRequestApiTest, ChangeHeader) {
   // Create a web bundle.
   std::string target_txt_url_str =
       embedded_test_server()->GetURL("/target.txt").spec();
-  web_package::WebBundleBuilder builder(target_txt_url_str, "");
-  auto target_txt_location = builder.AddResponse(
+  web_package::WebBundleBuilder builder;
+  builder.AddExchange(
+      target_txt_url_str,
       {{":status", "200"}, {"content-type", "text/plain"}, {"foo", "bar"}},
       "Hello world");
-  builder.AddIndexEntry(target_txt_url_str, "", {target_txt_location});
   std::vector<uint8_t> bundle = builder.CreateBundle();
   web_bundle = std::string(bundle.begin(), bundle.end());
 
@@ -3946,11 +3943,11 @@ IN_PROC_BROWSER_TEST_P(SubresourceWebBundlesWebRequestApiTest,
   ASSERT_TRUE(StartEmbeddedTestServer());
 
   // Create a web bundle.
-  web_package::WebBundleBuilder builder(uuid_url, "");
-  auto uuid_script = builder.AddResponse(
+  web_package::WebBundleBuilder builder;
+  builder.AddExchange(
+      uuid_url,
       {{":status", "200"}, {"content-type", "application/javascript"}},
       "document.title = 'loaded';");
-  builder.AddIndexEntry(uuid_url, "", {uuid_script});
   std::vector<uint8_t> bundle = builder.CreateBundle();
   web_bundle = std::string(bundle.begin(), bundle.end());
 
@@ -4060,30 +4057,27 @@ IN_PROC_BROWSER_TEST_P(SubresourceWebBundlesWebRequestApiTest,
       embedded_test_server()->GetURL("/redirected_to_unlisted.js").spec();
   std::string redirect_to_server_js_url_str =
       embedded_test_server()->GetURL("/redirect_to_server.js").spec();
-  web_package::WebBundleBuilder builder(redirect_js_url_str, "");
-  auto redirect_js_location = builder.AddResponse(
+  web_package::WebBundleBuilder builder;
+  builder.AddExchange(
+      redirect_js_url_str,
       {{":status", "200"}, {"content-type", "application/javascript"}},
       "document.title = 'redirect';");
-  auto redirected_js_location = builder.AddResponse(
+  builder.AddExchange(
+      redirected_js_url_str,
       {{":status", "200"}, {"content-type", "application/javascript"}},
       "document.title = 'redirected';");
-  auto redirect_to_unlisted_location = builder.AddResponse(
+  builder.AddExchange(
+      redirect_to_unlisted_js_url_str,
       {{":status", "200"}, {"content-type", "application/javascript"}},
       "document.title = 'redirect_to_unlisted';");
-  auto redirected_to_unlisted_js_location = builder.AddResponse(
+  builder.AddExchange(
+      redirected_to_unlisted_js_url_str,
       {{":status", "200"}, {"content-type", "application/javascript"}},
       "document.title = 'redirected_to_unlisted';");
-  auto redirect_to_server_js_location = builder.AddResponse(
+  builder.AddExchange(
+      redirect_to_server_js_url_str,
       {{":status", "200"}, {"content-type", "application/javascript"}},
       "document.title = 'redirect_to_server';");
-  builder.AddIndexEntry(redirect_js_url_str, "", {redirect_js_location});
-  builder.AddIndexEntry(redirected_js_url_str, "", {redirected_js_location});
-  builder.AddIndexEntry(redirect_to_unlisted_js_url_str, "",
-                        {redirect_to_unlisted_location});
-  builder.AddIndexEntry(redirected_to_unlisted_js_url_str, "",
-                        {redirected_to_unlisted_js_location});
-  builder.AddIndexEntry(redirect_to_server_js_url_str, "",
-                        {redirect_to_server_js_location});
   std::vector<uint8_t> bundle = builder.CreateBundle();
   web_bundle = std::string(bundle.begin(), bundle.end());
 
@@ -4156,7 +4150,7 @@ IN_PROC_BROWSER_TEST_P(SubresourceWebBundlesWebRequestApiTest,
 
   // Create a web bundle.
   std::string js_url_str = embedded_test_server()->GetURL("/script.js").spec();
-  web_package::WebBundleBuilder builder(js_url_str, "");
+  web_package::WebBundleBuilder builder;
   builder.AddExchange(
       js_url_str,
       {{":status", "200"}, {"content-type", "application/javascript"}},
diff --git a/chrome/browser/extensions/content_script_apitest.cc b/chrome/browser/extensions/content_script_apitest.cc
index 9ffe8abf7c8060..9144a005afcc92 100644
--- a/chrome/browser/extensions/content_script_apitest.cc
+++ b/chrome/browser/extensions/content_script_apitest.cc
@@ -2107,11 +2107,10 @@ IN_PROC_BROWSER_TEST_F(SubresourceWebBundlesContentScriptApiTest,
 
   const std::string uuid_html_url =
       "uuid-in-package:65c6f241-f6b5-4302-9f95-9a826c4dda1c";
-  web_package::WebBundleBuilder builder("", "");
-  auto html_location =
-      builder.AddResponse({{":status", "200"}, {"content-type", "text/html"}},
-                          "<script>console.error('hoge');</script>");
-  builder.AddIndexEntry(uuid_html_url, "", {html_location});
+  web_package::WebBundleBuilder builder;
+  builder.AddExchange(uuid_html_url,
+                      {{":status", "200"}, {"content-type", "text/html"}},
+                      "<script>console.error('hoge');</script>");
   std::vector<uint8_t> bundle = builder.CreateBundle();
   const std::string web_bundle = std::string(bundle.begin(), bundle.end());
 
diff --git a/components/web_package/web_bundle_builder.cc b/components/web_package/web_bundle_builder.cc
index b9f46750a9fee7..efce846e4bc29f 100644
--- a/components/web_package/web_bundle_builder.cc
+++ b/components/web_package/web_bundle_builder.cc
@@ -41,19 +41,13 @@ uint64_t GetNumUintBytes(uint64_t value) {
 
 }  // namespace
 
-WebBundleBuilder::WebBundleBuilder(const std::string& fallback_url,
-                                   const std::string& manifest_url,
-                                   BundleVersion version,
+WebBundleBuilder::WebBundleBuilder(BundleVersion version,
                                    bool allow_invalid_utf8_strings_for_testing)
-    : fallback_url_(fallback_url), version_(version) {
+    : version_(version) {
+  // Currently the only supported bundle format is b2.
+  DCHECK_EQ(version_, BundleVersion::kB2);
   writer_config_.allow_invalid_utf8_for_testing =
       allow_invalid_utf8_strings_for_testing;
-  if (!manifest_url.empty() && version == BundleVersion::kB1) {
-    AddSection("manifest", GetCborValueOfURL(manifest_url));
-  }
-  if (version == BundleVersion::kB2 && !fallback_url_.empty()) {
-    AddSection("primary", GetCborValueOfURL(fallback_url_));
-  }
 }
 WebBundleBuilder::~WebBundleBuilder() = default;
 
@@ -67,7 +61,7 @@ cbor::Value WebBundleBuilder::GetCborValueOfURL(base::StringPiece url) {
 void WebBundleBuilder::AddExchange(base::StringPiece url,
                                    const Headers& response_headers,
                                    base::StringPiece payload) {
-  AddIndexEntry(url, "", {AddResponse(response_headers, payload)});
+  AddIndexEntry(url, AddResponse(response_headers, payload));
 }
 
 WebBundleBuilder::ResponseLocation WebBundleBuilder::AddResponse(
@@ -86,15 +80,8 @@ WebBundleBuilder::ResponseLocation WebBundleBuilder::AddResponse(
 
 void WebBundleBuilder::AddIndexEntry(
     base::StringPiece url,
-    base::StringPiece variants_value,
-    std::vector<ResponseLocation> response_locations) {
-  // 'b2' version does not include |variants_value| in the response array.
-  if (version_ != BundleVersion::kB1) {
-    DCHECK_LE(response_locations.size(), 1u);
-  }
-  delayed_index_.insert(
-      {std::string(url), std::make_pair(std::string(variants_value),
-                                        std::move(response_locations))});
+    const ResponseLocation& response_location) {
+  delayed_index_.insert({std::string{url}, response_location});
 }
 
 void WebBundleBuilder::AddSection(base::StringPiece name, cbor::Value section) {
@@ -111,6 +98,10 @@ void WebBundleBuilder::AddVouchedSubset(cbor::Value::MapValue vouched_subset) {
   vouched_subsets_.emplace_back(std::move(vouched_subset));
 }
 
+void WebBundleBuilder::AddPrimaryURL(base::StringPiece url) {
+  AddSection("primary", GetCborValueOfURL(url));
+}
+
 std::vector<uint8_t> WebBundleBuilder::CreateBundle() {
   // Now that we know how many responses will be in the bundle,
   // we want to shift all the offsets by the bytes required
@@ -119,15 +110,10 @@ std::vector<uint8_t> WebBundleBuilder::CreateBundle() {
   int64_t initial_offset = 1 + GetNumUintBytes(responses_.size());
   cbor::Value::MapValue index;
   for (auto& entry : delayed_index_) {
-    auto& index_entry = entry.second;
+    const ResponseLocation& location = entry.second;
     cbor::Value::ArrayValue index_value_array;
-    if (version_ == BundleVersion::kB1) {
-      index_value_array.emplace_back(CreateByteString(index_entry.first));
-    }
-    for (auto& location : index_entry.second) {
-      index_value_array.emplace_back(location.offset + initial_offset);
-      index_value_array.emplace_back(location.length);
-    }
+    index_value_array.emplace_back(location.offset + initial_offset);
+    index_value_array.emplace_back(location.length);
     index.insert(
         {GetCborValueOfURL(entry.first), cbor::Value(index_value_array)});
   }
@@ -171,14 +157,7 @@ std::vector<uint8_t> WebBundleBuilder::CreateTopLevel() {
   cbor::Value::ArrayValue toplevel_array;
   toplevel_array.emplace_back(
       CreateByteString(u8"\U0001F310\U0001F4E6"));  // "🌐📦"
-  if (version_ == BundleVersion::kB1) {
-    toplevel_array.emplace_back(
-        CreateByteString(base::StringPiece("b1\0\0", 4)));
-    toplevel_array.emplace_back(GetCborValueOfURL(fallback_url_));
-  } else {
-    toplevel_array.emplace_back(
-        CreateByteString(base::StringPiece("b2\0\0", 4)));
-  }
+  toplevel_array.emplace_back(CreateByteString(base::StringPiece("b2\0\0", 4)));
   toplevel_array.emplace_back(Encode(cbor::Value(section_lengths_)));
   toplevel_array.emplace_back(sections_);
   // Put a dummy 8-byte bytestring.
diff --git a/components/web_package/web_bundle_builder.h b/components/web_package/web_bundle_builder.h
index b0bbe0875a5e3f..b6cbf494d9ae3a 100644
--- a/components/web_package/web_bundle_builder.h
+++ b/components/web_package/web_bundle_builder.h
@@ -16,7 +16,6 @@
 namespace web_package {
 
 enum class BundleVersion {
-  kB1,
   kB2,
 };
 
@@ -30,10 +29,9 @@ class WebBundleBuilder {
     int64_t length;
   };
 
-  WebBundleBuilder(const std::string& fallback_url,
-                   const std::string& manifest_url,
-                   BundleVersion version = BundleVersion::kB2,
-                   bool allow_invalid_utf8_strings_for_testing = false);
+  explicit WebBundleBuilder(
+      BundleVersion version = BundleVersion::kB2,
+      bool allow_invalid_utf8_strings_for_testing = false);
 
   ~WebBundleBuilder();
 
@@ -45,11 +43,11 @@ class WebBundleBuilder {
                                base::StringPiece payload);
 
   void AddIndexEntry(base::StringPiece url,
-                     base::StringPiece variants_value,
-                     std::vector<ResponseLocation> response_locations);
+                     const ResponseLocation& response_location);
   void AddSection(base::StringPiece name, cbor::Value section);
   void AddAuthority(cbor::Value::MapValue authority);
   void AddVouchedSubset(cbor::Value::MapValue vouched_subset);
+  void AddPrimaryURL(base::StringPiece url);
 
   std::vector<uint8_t> CreateBundle();
 
@@ -71,11 +69,9 @@ class WebBundleBuilder {
   int64_t EncodedLength(const cbor::Value& value);
 
   cbor::Writer::Config writer_config_;
-  std::string fallback_url_;
   cbor::Value::ArrayValue section_lengths_;
   cbor::Value::ArrayValue sections_;
-  std::map<std::string, std::pair<std::string, std::vector<ResponseLocation>>>
-      delayed_index_;
+  std::map<std::string, ResponseLocation> delayed_index_;
   cbor::Value::ArrayValue responses_;
   cbor::Value::ArrayValue authorities_;
   cbor::Value::ArrayValue vouched_subsets_;
diff --git a/components/web_package/web_bundle_builder_unittest.cc b/components/web_package/web_bundle_builder_unittest.cc
index 034b361f25cb32..44491fd67d5ca8 100644
--- a/components/web_package/web_bundle_builder_unittest.cc
+++ b/components/web_package/web_bundle_builder_unittest.cc
@@ -42,7 +42,7 @@ class WebBundleBuilderTest : public testing::Test {
 };
 
 TEST_F(WebBundleBuilderTest, CorrectWebBundleSizeIsWritten) {
-  WebBundleBuilder builder(kFallbackUrl, "");
+  WebBundleBuilder builder;
   builder.AddExchange("https://test.example.com/",
                       {{":status", "200"}, {"content-type", "text/plain"}},
                       "payload");
@@ -55,7 +55,7 @@ TEST_F(WebBundleBuilderTest, CorrectWebBundleSizeIsWritten) {
 }
 
 TEST_F(WebBundleBuilderTest, ByteByByteComparison) {
-  WebBundleBuilder builder("", "");
+  WebBundleBuilder builder;
   builder.AddExchange(
       "https://test.example.org/",
       {{":status", "200"}, {"content-type", "text/html; charset=UTF-8"}},
@@ -71,7 +71,7 @@ TEST_F(WebBundleBuilderTest, ByteByByteComparison) {
 }
 
 TEST_F(WebBundleBuilderTest, MoreThan23ResponsesInABundle) {
-  WebBundleBuilder builder("", "");
+  WebBundleBuilder builder;
   for (int i = 0; i < 24; ++i) {
     builder.AddExchange("https://test.example.org/" + base::NumberToString(i),
                         {{":status", "200"}, {"content-type", "text/html;"}},
diff --git a/components/web_package/web_bundle_parser_unittest.cc b/components/web_package/web_bundle_parser_unittest.cc
index 53b23be3ca9079..4095159bf2a6fa 100644
--- a/components/web_package/web_bundle_parser_unittest.cc
+++ b/components/web_package/web_bundle_parser_unittest.cc
@@ -19,8 +19,7 @@ namespace web_package {
 
 namespace {
 
-constexpr char kFallbackUrl[] = "https://test.example.com/";
-constexpr char kManifestUrl[] = "https://test.example.com/manifest";
+constexpr char kPrimaryUrl[] = "https://test.example.com/";
 constexpr char kValidityUrl[] =
     "https://test.example.org/resource.validity.msg";
 const uint64_t kSignatureDate = 1564272000;  // 2019-07-28T00:00:00Z
@@ -152,8 +151,8 @@ std::string AsString(const std::vector<uint8_t>& data) {
 }
 
 std::vector<uint8_t> CreateSmallBundle() {
-  web_package::WebBundleBuilder builder(kFallbackUrl, "" /* manifest_url */);
-  builder.AddExchange(kFallbackUrl,
+  web_package::WebBundleBuilder builder;
+  builder.AddExchange(kPrimaryUrl,
                       {{":status", "200"}, {"content-type", "text/plain"}},
                       "payload");
   return builder.CreateBundle();
@@ -167,7 +166,7 @@ class WebBundleParserTest : public testing::Test {
 };
 
 TEST_F(WebBundleParserTest, WrongMagic) {
-  WebBundleBuilder builder(kFallbackUrl, kManifestUrl);
+  WebBundleBuilder builder;
   std::vector<uint8_t> bundle = builder.CreateBundle();
   bundle[3] ^= 1;
   TestDataSource data_source(bundle);
@@ -178,9 +177,9 @@ TEST_F(WebBundleParserTest, WrongMagic) {
 }
 
 TEST_F(WebBundleParserTest, UnknownVersion) {
-  WebBundleBuilder builder(kFallbackUrl, kManifestUrl);
+  WebBundleBuilder builder;
   std::vector<uint8_t> bundle = builder.CreateBundle();
-  // Modify the version string from "b1\0\0" to "q1\0\0".
+  // Modify the version string from "b2\0\0" to "q2\0\0".
   ASSERT_EQ(bundle[11], 'b');
   bundle[11] = 'q';
   TestDataSource data_source(bundle);
@@ -191,7 +190,7 @@ TEST_F(WebBundleParserTest, UnknownVersion) {
 }
 
 TEST_F(WebBundleParserTest, SectionLengthsTooLarge) {
-  WebBundleBuilder builder(kFallbackUrl, kManifestUrl);
+  WebBundleBuilder builder;
   std::string too_long_section_name(8192, 'x');
   builder.AddSection(too_long_section_name, cbor::Value(0));
   TestDataSource data_source(builder.CreateBundle());
@@ -200,7 +199,7 @@ TEST_F(WebBundleParserTest, SectionLengthsTooLarge) {
 }
 
 TEST_F(WebBundleParserTest, DuplicateSectionName) {
-  WebBundleBuilder builder(kFallbackUrl, kManifestUrl);
+  WebBundleBuilder builder;
   builder.AddSection("foo", cbor::Value(0));
   builder.AddSection("foo", cbor::Value(0));
   TestDataSource data_source(builder.CreateBundle());
@@ -209,7 +208,7 @@ TEST_F(WebBundleParserTest, DuplicateSectionName) {
 }
 
 TEST_F(WebBundleParserTest, InvalidRequestURL) {
-  WebBundleBuilder builder(kFallbackUrl, kManifestUrl);
+  WebBundleBuilder builder;
   builder.AddExchange("", {{":status", "200"}, {"content-type", "text/plain"}},
                       "payload");
   TestDataSource data_source(builder.CreateBundle());
@@ -218,8 +217,8 @@ TEST_F(WebBundleParserTest, InvalidRequestURL) {
 }
 
 TEST_F(WebBundleParserTest, RequestURLIsNotUTF8) {
-  WebBundleBuilder builder(kFallbackUrl, kManifestUrl, BundleVersion::kB1,
-                           true);
+  WebBundleBuilder builder(BundleVersion::kB2,
+                           /*allow_invalid_utf8_strings_for_testing*/ true);
   builder.AddExchange("https://test.example.com/\xcc",
                       {{":status", "200"}, {"content-type", "text/plain"}},
                       "payload");
@@ -229,7 +228,7 @@ TEST_F(WebBundleParserTest, RequestURLIsNotUTF8) {
 }
 
 TEST_F(WebBundleParserTest, RequestURLHasBadScheme) {
-  WebBundleBuilder builder(kFallbackUrl, kManifestUrl);
+  WebBundleBuilder builder;
   builder.AddExchange("file:///tmp/foo",
                       {{":status", "200"}, {"content-type", "text/plain"}},
                       "payload");
@@ -239,7 +238,7 @@ TEST_F(WebBundleParserTest, RequestURLHasBadScheme) {
 }
 
 TEST_F(WebBundleParserTest, RequestURLHasCredentials) {
-  WebBundleBuilder builder(kFallbackUrl, kManifestUrl);
+  WebBundleBuilder builder;
   builder.AddExchange("https://user:passwd@test.example.com/",
                       {{":status", "200"}, {"content-type", "text/plain"}},
                       "payload");
@@ -249,7 +248,7 @@ TEST_F(WebBundleParserTest, RequestURLHasCredentials) {
 }
 
 TEST_F(WebBundleParserTest, RequestURLHasFragment) {
-  WebBundleBuilder builder(kFallbackUrl, kManifestUrl);
+  WebBundleBuilder builder;
   builder.AddExchange("https://test.example.com/#fragment",
                       {{":status", "200"}, {"content-type", "text/plain"}},
                       "payload");
@@ -261,7 +260,7 @@ TEST_F(WebBundleParserTest, RequestURLHasFragment) {
 TEST_F(WebBundleParserTest, RequestURLIsValidUuidInPackage) {
   const char uuid_in_package[] =
       "uuid-in-package:f81d4fae-7dec-11d0-a765-00a0c91e6bf6";
-  WebBundleBuilder builder(kFallbackUrl, kManifestUrl);
+  WebBundleBuilder builder;
   builder.AddExchange(uuid_in_package,
                       {{":status", "200"}, {"content-type", "text/plain"}},
                       "payload");
@@ -276,7 +275,7 @@ TEST_F(WebBundleParserTest, RequestURLIsValidUuidInPackage) {
 
 TEST_F(WebBundleParserTest, RequestURLIsInvalidUuidInPackage) {
   const char uuid_in_package[] = "uuid-in-package:invalid";
-  WebBundleBuilder builder(kFallbackUrl, kManifestUrl);
+  WebBundleBuilder builder;
   builder.AddExchange(uuid_in_package,
                       {{":status", "200"}, {"content-type", "text/plain"}},
                       "payload");
@@ -286,7 +285,7 @@ TEST_F(WebBundleParserTest, RequestURLIsInvalidUuidInPackage) {
 }
 
 TEST_F(WebBundleParserTest, NoStatusInResponseHeaders) {
-  WebBundleBuilder builder(kFallbackUrl, kManifestUrl);
+  WebBundleBuilder builder;
   builder.AddExchange("https://test.example.com/",
                       {{"content-type", "text/plain"}},
                       "payload");  // ":status" is missing.
@@ -300,7 +299,7 @@ TEST_F(WebBundleParserTest, NoStatusInResponseHeaders) {
 }
 
 TEST_F(WebBundleParserTest, InvalidResponseStatus) {
-  WebBundleBuilder builder(kFallbackUrl, kManifestUrl);
+  WebBundleBuilder builder;
   builder.AddExchange("https://test.example.com/",
                       {{":status", "0200"}, {"content-type", "text/plain"}},
                       "payload");
@@ -314,7 +313,7 @@ TEST_F(WebBundleParserTest, InvalidResponseStatus) {
 }
 
 TEST_F(WebBundleParserTest, ExtraPseudoInResponseHeaders) {
-  WebBundleBuilder builder(kFallbackUrl, kManifestUrl);
+  WebBundleBuilder builder;
   builder.AddExchange(
       "https://test.example.com/",
       {{":status", "200"}, {":foo", ""}, {"content-type", "text/plain"}},
@@ -329,7 +328,7 @@ TEST_F(WebBundleParserTest, ExtraPseudoInResponseHeaders) {
 }
 
 TEST_F(WebBundleParserTest, UpperCaseCharacterInHeaderName) {
-  WebBundleBuilder builder(kFallbackUrl, kManifestUrl);
+  WebBundleBuilder builder;
   builder.AddExchange("https://test.example.com/",
                       {{":status", "200"}, {"Content-Type", "text/plain"}},
                       "payload");
@@ -343,7 +342,7 @@ TEST_F(WebBundleParserTest, UpperCaseCharacterInHeaderName) {
 }
 
 TEST_F(WebBundleParserTest, InvalidHeaderValue) {
-  WebBundleBuilder builder(kFallbackUrl, kManifestUrl);
+  WebBundleBuilder builder;
   builder.AddExchange("https://test.example.com/",
                       {{":status", "200"}, {"content-type", "\n"}}, "payload");
   TestDataSource data_source(builder.CreateBundle());
@@ -356,7 +355,7 @@ TEST_F(WebBundleParserTest, InvalidHeaderValue) {
 }
 
 TEST_F(WebBundleParserTest, NoContentTypeWithNonEmptyContent) {
-  WebBundleBuilder builder(kFallbackUrl, kManifestUrl);
+  WebBundleBuilder builder;
   builder.AddExchange("https://test.example.com/", {{":status", "200"}},
                       "payload");
   TestDataSource data_source(builder.CreateBundle());
@@ -369,7 +368,7 @@ TEST_F(WebBundleParserTest, NoContentTypeWithNonEmptyContent) {
 }
 
 TEST_F(WebBundleParserTest, NoContentTypeWithEmptyContent) {
-  WebBundleBuilder builder(kFallbackUrl, kManifestUrl);
+  WebBundleBuilder builder;
   builder.AddExchange("https://test.example.com/", {{":status", "301"}}, "");
   TestDataSource data_source(builder.CreateBundle());
 
@@ -380,16 +379,8 @@ TEST_F(WebBundleParserTest, NoContentTypeWithEmptyContent) {
   ASSERT_TRUE(ParseResponse(&data_source, location));
 }
 
-TEST_F(WebBundleParserTest, EmptyIndexEntry) {
-  WebBundleBuilder builder(kFallbackUrl, kManifestUrl);
-  builder.AddIndexEntry("https://test.example.com/", "", {});
-  TestDataSource data_source(builder.CreateBundle());
-
-  ExpectFormatError(ParseBundle(&data_source));
-}
-
 TEST_F(WebBundleParserTest, AllKnownSectionInCritical) {
-  WebBundleBuilder builder(kFallbackUrl, kManifestUrl);
+  WebBundleBuilder builder;
   builder.AddExchange("https://test.example.com/",
                       {{":status", "200"}, {"content-type", "text/plain"}},
                       "payload");
@@ -405,7 +396,7 @@ TEST_F(WebBundleParserTest, AllKnownSectionInCritical) {
 }
 
 TEST_F(WebBundleParserTest, UnknownSectionInCritical) {
-  WebBundleBuilder builder(kFallbackUrl, kManifestUrl);
+  WebBundleBuilder builder;
   builder.AddExchange("https://test.example.com/",
                       {{":status", "200"}, {"content-type", "text/plain"}},
                       "payload");
@@ -418,7 +409,7 @@ TEST_F(WebBundleParserTest, UnknownSectionInCritical) {
 }
 
 TEST_F(WebBundleParserTest, EmptySignaturesSection) {
-  WebBundleBuilder builder(kFallbackUrl, kManifestUrl);
+  WebBundleBuilder builder;
   builder.AddExchange("https://test.example.com/",
                       {{":status", "200"}, {"content-type", "text/plain"}},
                       "payload");
@@ -438,7 +429,7 @@ TEST_F(WebBundleParserTest, EmptySignaturesSection) {
 }
 
 TEST_F(WebBundleParserTest, SignaturesSection) {
-  WebBundleBuilder builder(kFallbackUrl, kManifestUrl);
+  WebBundleBuilder builder;
   builder.AddExchange("https://test.example.com/",
                       {{":status", "200"}, {"content-type", "text/plain"}},
                       "payload");
@@ -497,7 +488,7 @@ TEST_F(WebBundleParserTest, SignaturesSection) {
 }
 
 TEST_F(WebBundleParserTest, MultipleSignatures) {
-  WebBundleBuilder builder(kFallbackUrl, kManifestUrl);
+  WebBundleBuilder builder;
   builder.AddExchange("https://test.example.com/",
                       {{":status", "200"}, {"content-type", "text/plain"}},
                       "payload");
@@ -559,36 +550,6 @@ TEST_F(WebBundleParserTest, MultipleSignatures) {
 TEST_F(WebBundleParserTest, ParseGoldenFile) {
   TestDataSource data_source(base::FilePath(FILE_PATH_LITERAL("hello_b2.wbn")));
 
-  mojom::BundleMetadataPtr metadata = ParseBundle(&data_source).first;
-  ASSERT_TRUE(metadata);
-  ASSERT_EQ(metadata->requests.size(), 4u);
-
-  std::map<std::string, mojom::BundleResponsePtr> responses;
-  for (const auto& item : metadata->requests) {
-    auto location = FindResponse(metadata, item.first);
-    ASSERT_TRUE(location);
-    auto resp = ParseResponse(&data_source, location);
-    ASSERT_TRUE(resp);
-    responses[item.first.spec()] = std::move(resp);
-  }
-
-  ASSERT_TRUE(responses["https://test.example.org/"]);
-  EXPECT_EQ(responses["https://test.example.org/"]->response_code, 200);
-  EXPECT_EQ(
-      responses["https://test.example.org/"]->response_headers["content-type"],
-      "text/html; charset=utf-8");
-  EXPECT_EQ(data_source.GetPayload(responses["https://test.example.org/"]),
-            GetTestFileContents(
-                base::FilePath(FILE_PATH_LITERAL("hello/index.html"))));
-
-  EXPECT_TRUE(responses["https://test.example.org/index.html"]);
-  EXPECT_TRUE(responses["https://test.example.org/manifest.webmanifest"]);
-  EXPECT_TRUE(responses["https://test.example.org/script.js"]);
-}
-
-TEST_F(WebBundleParserTest, ParseGoldenFileB2Version) {
-  TestDataSource data_source(base::FilePath(FILE_PATH_LITERAL("hello_b2.wbn")));
-
   mojom::BundleMetadataPtr metadata = ParseBundle(&data_source).first;
   ASSERT_TRUE(metadata);
   ASSERT_EQ(metadata->requests.size(), 4u);
@@ -643,7 +604,8 @@ TEST_F(WebBundleParserTest, ParseSignedFile) {
 }
 
 TEST_F(WebBundleParserTest, SingleEntry) {
-  WebBundleBuilder builder(kFallbackUrl, kManifestUrl);
+  WebBundleBuilder builder;
+  builder.AddPrimaryURL(kPrimaryUrl);
   builder.AddExchange("https://test.example.com/",
                       {{":status", "200"}, {"content-type", "text/plain"}},
                       "payload");
@@ -661,11 +623,11 @@ TEST_F(WebBundleParserTest, SingleEntry) {
   EXPECT_EQ(response->response_headers.size(), 1u);
   EXPECT_EQ(response->response_headers["content-type"], "text/plain");
   EXPECT_EQ(data_source.GetPayload(response), "payload");
-  EXPECT_EQ(metadata->primary_url, kFallbackUrl);
+  EXPECT_EQ(metadata->primary_url, kPrimaryUrl);
 }
 
-TEST_F(WebBundleParserTest, B2BundleNoPrimaryUrlSingleEntry) {
-  WebBundleBuilder builder("", kManifestUrl, BundleVersion::kB2);
+TEST_F(WebBundleParserTest, NoPrimaryUrlSingleEntry) {
+  WebBundleBuilder builder;
   builder.AddExchange("https://test.example.com/",
                       {{":status", "200"}, {"content-type", "text/plain"}},
                       "payload");
@@ -686,8 +648,8 @@ TEST_F(WebBundleParserTest, B2BundleNoPrimaryUrlSingleEntry) {
 }
 
 TEST_F(WebBundleParserTest, RelativeURL) {
-  WebBundleBuilder builder("path/to/primary_url", "path/to/manifest",
-                           BundleVersion::kB2);
+  WebBundleBuilder builder;
+  builder.AddPrimaryURL("path/to/primary_url");
   builder.AddExchange("path/to/file.txt",
                       {{":status", "200"}, {"content-type", "text/plain"}},
                       "payload");
diff --git a/content/browser/web_package/web_bundle_browsertest_base.cc b/content/browser/web_package/web_bundle_browsertest_base.cc
index 90b8b0b5f255aa..0f109b8ed1c6f2 100644
--- a/content/browser/web_package/web_bundle_browsertest_base.cc
+++ b/content/browser/web_package/web_bundle_browsertest_base.cc
@@ -345,7 +345,8 @@ FrameTreeNode* GetFirstChild(WebContents* web_contents) {
 }
 
 std::string CreateSimpleWebBundle(const GURL& primary_url) {
-  web_package::WebBundleBuilder builder(primary_url.spec(), "");
+  web_package::WebBundleBuilder builder;
+  builder.AddPrimaryURL(primary_url.spec());
   builder.AddExchange(primary_url.spec(),
                       {{":status", "200"}, {"content-type", "text/html"}},
                       "<title>Ready</title>");
@@ -374,8 +375,8 @@ void AddScriptFile(web_package::WebBundleBuilder* builder,
 
 std::string CreatePathTestWebBundle(const GURL& base_url) {
   const std::string primary_url_path = "/web_bundle/path_test/in_scope/";
-  web_package::WebBundleBuilder builder(
-      base_url.Resolve(primary_url_path).spec(), "");
+  web_package::WebBundleBuilder builder;
+  builder.AddPrimaryURL(base_url.Resolve(primary_url_path).spec());
   AddHtmlFile(&builder, base_url, primary_url_path, "<title>Ready</title>");
   AddHtmlFile(
       &builder, base_url, "/web_bundle/path_test/in_scope/page.html",
@@ -453,8 +454,8 @@ void SetUpSubPageTest(net::EmbeddedTestServer* primary_server,
   *primary_url_origin = primary_server->GetURL("/");
   *third_party_origin = third_party_server->GetURL("/");
 
-  web_package::WebBundleBuilder builder(
-      primary_url_origin->Resolve("/top").spec(), "");
+  web_package::WebBundleBuilder builder;
+  builder.AddPrimaryURL(primary_url_origin->Resolve("/top").spec());
   AddHtmlFile(&builder, *primary_url_origin, "/top", R"(
     <script>
     window.addEventListener('message',
@@ -666,8 +667,8 @@ void SetUpSharedNavigationsTest(net::EmbeddedTestServer* server,
                                 GURL* url_origin,
                                 std::string* web_bundle_content) {
   SetUpNavigationTestServer(server, url_origin);
-  web_package::WebBundleBuilder builder(
-      url_origin->Resolve("/top-page/").spec(), "");
+  web_package::WebBundleBuilder builder;
+  builder.AddPrimaryURL(url_origin->Resolve("/top-page/").spec());
   for (const auto& path : pathes)
     AddHtmlAndScriptForNavigationTest(&builder, *url_origin, path, "");
 
@@ -1034,8 +1035,8 @@ void SetUpIframeNavigationTest(net::EmbeddedTestServer* server,
                                GURL* url_origin,
                                std::string* web_bundle_content) {
   SetUpNavigationTestServer(server, url_origin);
-  web_package::WebBundleBuilder builder(
-      url_origin->Resolve("/top-page/").spec(), "");
+  web_package::WebBundleBuilder builder;
+  builder.AddPrimaryURL(url_origin->Resolve("/top-page/").spec());
   const std::vector<std::string> pathes = {"/top-page/", "/1-page/",
                                            "/2-page/"};
   for (const auto& path : pathes)
diff --git a/content/browser/web_package/web_bundle_element_browsertest.cc b/content/browser/web_package/web_bundle_element_browsertest.cc
index 825028ca17d649..ff5d54e27e62df 100644
--- a/content/browser/web_package/web_bundle_element_browsertest.cc
+++ b/content/browser/web_package/web_bundle_element_browsertest.cc
@@ -144,9 +144,6 @@ class WebBundleElementBrowserTest : public ContentBrowserTest {
     mock_cert_verifier_.mock_cert_verifier()->set_default_result(net::OK);
     original_client_ = SetBrowserClientForTesting(&browser_client_);
     host_resolver()->AddRule("*", "127.0.0.1");
-    https_server_.RegisterRequestHandler(base::BindRepeating(
-        &WebBundleElementBrowserTest::HandleHugeWebBundleRequest,
-        base::Unretained(this)));
     https_server_.RegisterRequestHandler(base::BindRepeating(
         &WebBundleElementBrowserTest::HandleTestWebBundleRequest,
         base::Unretained(this)));
@@ -206,41 +203,13 @@ class WebBundleElementBrowserTest : public ContentBrowserTest {
     EXPECT_EQ("\"iframe.onload\"", message);
   }
 
-  std::unique_ptr<net::test_server::HttpResponse> HandleHugeWebBundleRequest(
-      const net::test_server::HttpRequest& request) {
-    // Handler should return huge bundles only for "/web_bundle/huge.wbn" and
-    // "/web_bundle/huge2.wbn".
-    if (!(request.relative_url == "/web_bundle/huge.wbn" ||
-          request.relative_url == "/web_bundle/huge2.wbn"))
-      return nullptr;
-    GURL primary_url(https_server_.GetURL("/web_bundle/huge.txt"));
-    web_package::WebBundleBuilder builder(primary_url.spec(),
-                                          "" /* manifest_url */);
-    builder.AddExchange(
-        primary_url.spec(),
-        {{":status", "200"}, {"content-type", "text/plain"}},
-        // The body size should be greater than kDefaultMaxMemoryPerProcess / 2.
-        std::string(web_package::kDefaultMaxMemoryPerProcess / 2 + 1000, 'X'));
-    auto bundle = builder.CreateBundle();
-    std::string body(reinterpret_cast<const char*>(bundle.data()),
-                     bundle.size());
-    auto http_response =
-        std::make_unique<net::test_server::BasicHttpResponse>();
-    http_response->set_code(net::HTTP_OK);
-    http_response->set_content(body);
-    http_response->set_content_type("application/webbundle");
-    http_response->AddCustomHeader("X-Content-Type-Options", "nosniff");
-    return http_response;
-  }
-
   std::unique_ptr<net::test_server::HttpResponse> HandleTestWebBundleRequest(
       const net::test_server::HttpRequest& request) {
     if (request.relative_url != "/web_bundle/test.wbn")
       return nullptr;
     GURL test1_url(https_server_.GetURL("/web_bundle/test1.txt"));
     GURL test2_url(https_server_.GetURL("/web_bundle/test2.txt"));
-    web_package::WebBundleBuilder builder("" /* fallback_url */,
-                                          "" /* manifest_url */);
+    web_package::WebBundleBuilder builder;
     builder.AddExchange(test1_url.spec(),
                         {{":status", "200"}, {"content-type", "text/plain"}},
                         "test1");
diff --git a/content/browser/web_package/web_bundle_network_browsertest.cc b/content/browser/web_package/web_bundle_network_browsertest.cc
index e2997f6a330b20..916f7dbeee685f 100644
--- a/content/browser/web_package/web_bundle_network_browsertest.cc
+++ b/content/browser/web_package/web_bundle_network_browsertest.cc
@@ -135,7 +135,8 @@ IN_PROC_BROWSER_TEST_F(WebBundleNetworkBrowserTest, SimpleWithScript) {
   const GURL script_url =
       embedded_test_server()->GetURL("/web_bundle/script.js");
 
-  web_package::WebBundleBuilder builder(primary_url.spec(), "");
+  web_package::WebBundleBuilder builder;
+  builder.AddPrimaryURL(primary_url.spec());
   builder.AddExchange(primary_url.spec(),
                       {{":status", "200"}, {"content-type", "text/html"}},
                       "<script src=\"script.js\"></script>");
@@ -230,7 +231,8 @@ IN_PROC_BROWSER_TEST_F(WebBundleNetworkBrowserTest,
   const GURL primary_url = embedded_test_server()->GetURL(primary_url_path);
   const GURL inner_url =
       embedded_test_server()->GetURL("/web_bundle/inner.html");
-  web_package::WebBundleBuilder builder(primary_url.spec(), "");
+  web_package::WebBundleBuilder builder;
+  builder.AddPrimaryURL(primary_url.spec());
   builder.AddExchange(inner_url.spec(),
                       {{":status", "200"}, {"content-type", "text/html"}},
                       "<title>Ready</title>");
@@ -248,7 +250,7 @@ IN_PROC_BROWSER_TEST_F(WebBundleNetworkBrowserTest, PrimaryURLNotFound) {
   const GURL wbn_url = embedded_test_server()->GetURL(wbn_path);
   const GURL inner_url =
       embedded_test_server()->GetURL("/web_bundle/inner.html");
-  web_package::WebBundleBuilder builder("", "");
+  web_package::WebBundleBuilder builder;
   builder.AddExchange(inner_url.spec(),
                       {{":status", "200"}, {"content-type", "text/html"}},
                       "<title>Ready</title>");
@@ -715,8 +717,8 @@ IN_PROC_BROWSER_TEST_F(WebBundleNetworkBrowserTest, OutScopeSubPage) {
       embedded_test_server(), "");
   ASSERT_TRUE(embedded_test_server()->Start());
   const GURL origin = embedded_test_server()->GetURL("/");
-  web_package::WebBundleBuilder builder(origin.Resolve(primary_url_path).spec(),
-                                        "");
+  web_package::WebBundleBuilder builder;
+  builder.AddPrimaryURL(origin.Resolve(primary_url_path).spec());
   web_bundle_browsertest_utils::AddHtmlFile(&builder, origin, primary_url_path,
                                             R"(
     <script>
diff --git a/services/data_decoder/web_bundler.cc b/services/data_decoder/web_bundler.cc
index 5d0578933ef397..e67e9ece8f5c3a 100644
--- a/services/data_decoder/web_bundler.cc
+++ b/services/data_decoder/web_bundler.cc
@@ -114,8 +114,8 @@ void WebBundler::WriteWebBundleIndex() {
   }
 
   CHECK_EQ(resources.size(), bodies.size());
-  web_package::WebBundleBuilder builder(url.spec(), "",
-                                        web_package::BundleVersion::kB2);
+  web_package::WebBundleBuilder builder(web_package::BundleVersion::kB2);
+  builder.AddPrimaryURL(url.spec());
   for (size_t i = 0; i < resources.size(); ++i) {
     const auto& info = resources[i];
     const auto& body = bodies[i];
@@ -131,7 +131,7 @@ void WebBundler::WriteWebBundleIndex() {
     GURL::Replacements replacements;
     replacements.ClearRef();
     url = url.ReplaceComponents(replacements);
-    builder.AddIndexEntry(url.spec(), "", {response_location});
+    builder.AddIndexEntry(url.spec(), response_location);
   }
   std::vector<uint8_t> bundle = builder.CreateBundle();
   int written_size = file_.WriteAtCurrentPos(
diff --git a/services/network/web_bundle/web_bundle_manager_unittest.cc b/services/network/web_bundle/web_bundle_manager_unittest.cc
index 88b59ef91af671..af5658ba65484c 100644
--- a/services/network/web_bundle/web_bundle_manager_unittest.cc
+++ b/services/network/web_bundle/web_bundle_manager_unittest.cc
@@ -35,7 +35,7 @@ const int32_t process_id1 = 100;
 const int32_t process_id2 = 200;
 
 std::string CreateSmallBundleString() {
-  web_package::WebBundleBuilder builder(kResourceUrl, "" /* manifest_url */);
+  web_package::WebBundleBuilder builder;
   builder.AddExchange(kResourceUrl,
                       {{":status", "200"}, {"content-type", "text/plain"}},
                       "body");
@@ -387,7 +387,7 @@ TEST_F(WebBundleManagerTest, MemoryQuota_QuotaErrorWhileReadingBody) {
 
   // Create a not small size bundle to trigger the quota error while reading the
   // body of the subresource.
-  web_package::WebBundleBuilder builder(kResourceUrl, "" /* manifest_url */);
+  web_package::WebBundleBuilder builder;
   builder.AddExchange(kResourceUrl,
                       {{":status", "200"}, {"content-type", "text/plain"}},
                       std::string(10000, 'X'));
diff --git a/services/network/web_bundle/web_bundle_url_loader_factory_unittest.cc b/services/network/web_bundle/web_bundle_url_loader_factory_unittest.cc
index b0d05e2563f3fa..3714d224e85f91 100644
--- a/services/network/web_bundle/web_bundle_url_loader_factory_unittest.cc
+++ b/services/network/web_bundle/web_bundle_url_loader_factory_unittest.cc
@@ -45,7 +45,7 @@ using ::testing::Optional;
 using ::testing::Pointee;
 
 std::vector<uint8_t> CreateSmallBundle() {
-  web_package::WebBundleBuilder builder(kResourceUrl, "" /* manifest_url */);
+  web_package::WebBundleBuilder builder;
   builder.AddExchange(kResourceUrl,
                       {{":status", "200"}, {"content-type", "text/plain"}},
                       "body");
@@ -53,7 +53,7 @@ std::vector<uint8_t> CreateSmallBundle() {
 }
 
 std::vector<uint8_t> CreateLargeBundle() {
-  web_package::WebBundleBuilder builder(kResourceUrl, "" /* manifest_url */);
+  web_package::WebBundleBuilder builder;
   builder.AddExchange(kResourceUrl,
                       {{":status", "200"}, {"content-type", "text/plain"}},
                       "body");
@@ -67,8 +67,7 @@ std::vector<uint8_t> CreateLargeBundle() {
 }
 
 std::vector<uint8_t> CreateCrossOriginBundle() {
-  web_package::WebBundleBuilder builder(kCrossOriginJsonUrl,
-                                        "" /* manifest_url */);
+  web_package::WebBundleBuilder builder;
   builder.AddExchange(
       kCrossOriginJsonUrl,
       {{":status", "200"}, {"content-type", "application/json"}},
@@ -302,7 +301,7 @@ TEST_F(WebBundleURLLoaderFactoryTest, MetadataParseError) {
 }
 
 TEST_F(WebBundleURLLoaderFactoryTest, ResponseParseError) {
-  web_package::WebBundleBuilder builder(kResourceUrl, "" /* manifest_url */);
+  web_package::WebBundleBuilder builder;
   // An invalid response.
   builder.AddExchange(kResourceUrl, {{":status", "0"}}, "body");
   WriteBundle(builder.CreateBundle());
@@ -354,7 +353,7 @@ TEST_F(WebBundleURLLoaderFactoryTest, ResourceNotFoundInBundle) {
 }
 
 TEST_F(WebBundleURLLoaderFactoryTest, RedirectResponseIsNotAllowed) {
-  web_package::WebBundleBuilder builder(kResourceUrl, "" /* manifest_url */);
+  web_package::WebBundleBuilder builder;
   builder.AddExchange(kResourceUrl,
                       {{":status", "301"}, {"location", kResourceUrl2}}, "");
   builder.AddExchange(kResourceUrl2,