diff --git a/NEWS.md b/NEWS.md
index 7534539a21d2c9..5a9277b3992411 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -184,7 +184,7 @@ The following default gems are updated.
 * io-console 0.8.1
 * io-nonblock 0.3.2
 * io-wait 0.3.2
-* json 2.13.2
+* json 2.14.1
 * optparse 0.7.0.dev.2
 * prism 1.5.1
 * psych 5.2.6
diff --git a/ext/json/generator/generator.c b/ext/json/generator/generator.c
index 8fcc980d478f80..6a38cc60a7964f 100644
--- a/ext/json/generator/generator.c
+++ b/ext/json/generator/generator.c
@@ -29,6 +29,7 @@ typedef struct JSON_Generator_StateStruct {
 
     enum duplicate_key_action on_duplicate_key;
 
+    bool as_json_single_arg;
     bool allow_nan;
     bool ascii_only;
     bool script_safe;
@@ -1033,6 +1034,13 @@ json_inspect_hash_with_mixed_keys(struct hash_foreach_arg *arg)
     }
 }
 
+static VALUE
+json_call_as_json(JSON_Generator_State *state, VALUE object, VALUE is_key)
+{
+    VALUE proc_args[2] = {object, is_key};
+    return rb_proc_call_with_block(state->as_json, 2, proc_args, Qnil);
+}
+
 static int
 json_object_i(VALUE key, VALUE val, VALUE _arg)
 {
@@ -1086,7 +1094,7 @@ json_object_i(VALUE key, VALUE val, VALUE _arg)
         default:
             if (data->state->strict) {
                 if (RTEST(data->state->as_json) && !as_json_called) {
-                    key = rb_proc_call_with_block(data->state->as_json, 1, &key, Qnil);
+                    key = json_call_as_json(data->state, key, Qtrue);
                     key_type = rb_type(key);
                     as_json_called = true;
                     goto start;
@@ -1328,7 +1336,7 @@ static void generate_json_float(FBuffer *buffer, struct generate_json_data *data
         /* for NaN and Infinity values we either raise an error or rely on Float#to_s. */
         if (!allow_nan) {
             if (data->state->strict && data->state->as_json) {
-                VALUE casted_obj = rb_proc_call_with_block(data->state->as_json, 1, &obj, Qnil);
+                VALUE casted_obj = json_call_as_json(data->state, obj, Qfalse);
                 if (casted_obj != obj) {
                     increase_depth(data);
                     generate_json(buffer, data, casted_obj);
@@ -1345,12 +1353,11 @@ static void generate_json_float(FBuffer *buffer, struct generate_json_data *data
     }
 
     /* This implementation writes directly into the buffer. We reserve
-     * the 28 characters that fpconv_dtoa states as its maximum.
+     * the 32 characters that fpconv_dtoa states as its maximum.
      */
-    fbuffer_inc_capa(buffer, 28);
+    fbuffer_inc_capa(buffer, 32);
     char* d = buffer->ptr + buffer->len;
     int len = fpconv_dtoa(value, d);
-
     /* fpconv_dtoa converts a float to its shortest string representation,
      * but it adds a ".0" if this is a plain integer.
      */
@@ -1417,7 +1424,7 @@ static void generate_json(FBuffer *buffer, struct generate_json_data *data, VALU
             general:
                 if (data->state->strict) {
                     if (RTEST(data->state->as_json) && !as_json_called) {
-                        obj = rb_proc_call_with_block(data->state->as_json, 1, &obj, Qnil);
+                        obj = json_call_as_json(data->state, obj, Qfalse);
                         as_json_called = true;
                         goto start;
                     } else {
@@ -1943,6 +1950,7 @@ static int configure_state_i(VALUE key, VALUE val, VALUE _arg)
     else if (key == sym_allow_duplicate_key)   { state->on_duplicate_key = RTEST(val) ? JSON_IGNORE : JSON_RAISE; }
     else if (key == sym_as_json)               {
         VALUE proc = RTEST(val) ? rb_convert_type(val, T_DATA, "Proc", "to_proc") : Qfalse;
+        state->as_json_single_arg = proc && rb_proc_arity(proc) == 1;
         state_write_value(data, &state->as_json, proc);
     }
     return ST_CONTINUE;
diff --git a/ext/json/lib/json/version.rb b/ext/json/lib/json/version.rb
index f9ac3e17a947a5..9c928e3940e493 100644
--- a/ext/json/lib/json/version.rb
+++ b/ext/json/lib/json/version.rb
@@ -1,5 +1,5 @@
 # frozen_string_literal: true
 
 module JSON
-  VERSION = '2.13.2'
+  VERSION = '2.14.1'
 end
diff --git a/ext/json/parser/parser.c b/ext/json/parser/parser.c
index e34f1999d5f191..297031dcf17bb6 100644
--- a/ext/json/parser/parser.c
+++ b/ext/json/parser/parser.c
@@ -713,11 +713,16 @@ static VALUE json_string_unescape(JSON_ParserState *state, const char *string, c
                         }
                         if (pe[0] == '\\' && pe[1] == 'u') {
                             uint32_t sur = unescape_unicode(state, (unsigned char *) pe + 2);
+
+                            if ((sur & 0xFC00) != 0xDC00) {
+                                raise_parse_error_at("invalid surrogate pair at %s", state, p);
+                            }
+
                             ch = (((ch & 0x3F) << 10) | ((((ch >> 6) & 0xF) + 1) << 16)
                                     | (sur & 0x3FF));
                             pe += 5;
                         } else {
-                            unescape = (char *) "?";
+                            raise_parse_error_at("incomplete surrogate pair at %s", state, p);
                             break;
                         }
                     }
diff --git a/ext/json/vendor/fpconv.c b/ext/json/vendor/fpconv.c
index 75efd46f11e624..e91c7889c26b5d 100644
--- a/ext/json/vendor/fpconv.c
+++ b/ext/json/vendor/fpconv.c
@@ -29,6 +29,10 @@
 #include <string.h>
 #include <stdint.h>
 
+#ifdef JSON_DEBUG
+#include <assert.h>
+#endif
+
 #define npowers     87
 #define steppowers  8
 #define firstpower -348 /* 10 ^ -348 */
@@ -320,15 +324,7 @@ static int emit_digits(char* digits, int ndigits, char* dest, int K, bool neg)
 {
     int exp = absv(K + ndigits - 1);
 
-    int max_trailing_zeros = 7;
-
-    if(neg) {
-        max_trailing_zeros -= 1;
-    }
-
-    /* write plain integer */
-    if(K >= 0 && (exp < (ndigits + max_trailing_zeros))) {
-
+    if(K >= 0 && exp < 15) {
         memcpy(dest, digits, ndigits);
         memset(dest + ndigits, '0', K);
 
@@ -432,10 +428,12 @@ static int filter_special(double fp, char* dest)
  *
  * Input:
  * fp -> the double to convert, dest -> destination buffer.
- * The generated string will never be longer than 28 characters.
- * Make sure to pass a pointer to at least 28 bytes of memory.
+ * The generated string will never be longer than 32 characters.
+ * Make sure to pass a pointer to at least 32 bytes of memory.
  * The emitted string will not be null terminated.
  *
+ *
+ *
  * Output:
  * The number of written characters.
  *
@@ -474,6 +472,9 @@ static int fpconv_dtoa(double d, char dest[28])
     int ndigits = grisu2(d, digits, &K);
 
     str_len += emit_digits(digits, ndigits, dest + str_len, K, neg);
+#ifdef JSON_DEBUG
+    assert(str_len <= 32);
+#endif
 
     return str_len;
 }
diff --git a/imemo.c b/imemo.c
index 10ac960095675f..5a5ec4a4d382e8 100644
--- a/imemo.c
+++ b/imemo.c
@@ -123,7 +123,7 @@ rb_imemo_fields_new(VALUE owner, size_t capa)
 static VALUE
 imemo_fields_new_complex(VALUE owner, size_t capa)
 {
-    VALUE fields = imemo_fields_new(owner, 1);
+    VALUE fields = rb_imemo_new(imemo_fields, owner, sizeof(struct rb_fields));
     IMEMO_OBJ_FIELDS(fields)->as.complex.table = st_init_numtable_with_size(capa);
     FL_SET_RAW(fields, OBJ_FIELD_HEAP);
     return fields;
diff --git a/lib/bundler/cli.rb b/lib/bundler/cli.rb
index 0635260e95c2db..c577f4981470a4 100644
--- a/lib/bundler/cli.rb
+++ b/lib/bundler/cli.rb
@@ -24,7 +24,7 @@ class CLI < Thor
     }.freeze
 
     def self.start(*)
-      check_deprecated_ext_option(ARGV) if ARGV.include?("--ext")
+      check_invalid_ext_option(ARGV) if ARGV.include?("--ext")
 
       super
     ensure
@@ -288,12 +288,11 @@ def update(*gems)
       Calling show with [GEM] will list the exact location of that gem on your machine.
     D
     method_option "paths", type: :boolean, banner: "List the paths of all gems that are required by your Gemfile."
-    method_option "outdated", type: :boolean, banner: "Show verbose output including whether gems are outdated."
+    method_option "outdated", type: :boolean, banner: "Show verbose output including whether gems are outdated (removed)."
     def show(gem_name = nil)
       if ARGV.include?("--outdated")
-        message = "the `--outdated` flag to `bundle show` will be removed in favor of `bundle show --verbose`"
         removed_message = "the `--outdated` flag to `bundle show` has been removed in favor of `bundle show --verbose`"
-        SharedHelpers.major_deprecation(2, message, removed_message: removed_message)
+        raise InvalidOption, removed_message
       end
       require_relative "cli/show"
       Show.new(options, gem_name).run
@@ -658,18 +657,15 @@ def self.reformatted_help_args(args)
       end
     end
 
-    def self.check_deprecated_ext_option(arguments)
-      # when deprecated version of `--ext` is called
-      # print out deprecation warning and pretend `--ext=c` was provided
-      if deprecated_ext_value?(arguments)
-        message = "Extensions can now be generated using C or Rust, so `--ext` with no arguments has been deprecated. Please select a language, e.g. `--ext=rust` to generate a Rust extension. This gem will now be generated as if `--ext=c` was used."
+    def self.check_invalid_ext_option(arguments)
+      # when invalid version of `--ext` is called
+      if invalid_ext_value?(arguments)
         removed_message = "Extensions can now be generated using C or Rust, so `--ext` with no arguments has been removed. Please select a language, e.g. `--ext=rust` to generate a Rust extension."
-        SharedHelpers.major_deprecation 2, message, removed_message: removed_message
-        arguments[arguments.index("--ext")] = "--ext=c"
+        raise InvalidOption, removed_message
       end
     end
 
-    def self.deprecated_ext_value?(arguments)
+    def self.invalid_ext_value?(arguments)
       index = arguments.index("--ext")
       next_argument = arguments[index + 1]
 
@@ -677,15 +673,15 @@ def self.deprecated_ext_value?(arguments)
       # for example `bundle gem hello --ext c`
       return false if EXTENSIONS.include?(next_argument)
 
-      # deprecated call when --ext is called with no value in last position
+      # invalid call when --ext is called with no value in last position
       # for example `bundle gem hello_gem --ext`
       return true if next_argument.nil?
 
-      # deprecated call when --ext is followed by other parameter
+      # invalid call when --ext is followed by other parameter
       # for example `bundle gem --ext --no-ci hello_gem`
       return true if next_argument.start_with?("-")
 
-      # deprecated call when --ext is followed by gem name
+      # invalid call when --ext is followed by gem name
       # for example `bundle gem --ext hello_gem`
       return true if next_argument
 
diff --git a/lib/bundler/cli/plugin.rb b/lib/bundler/cli/plugin.rb
index fd61ef0d954dea..32fa660fe0a3f1 100644
--- a/lib/bundler/cli/plugin.rb
+++ b/lib/bundler/cli/plugin.rb
@@ -10,11 +10,15 @@ class CLI::Plugin < Thor
     method_option "source", type: :string, default: nil, banner: "URL of the RubyGems source to fetch the plugin from"
     method_option "version", type: :string, default: nil, banner: "The version of the plugin to fetch"
     method_option "git", type: :string, default: nil, banner: "URL of the git repo to fetch from"
-    method_option "local_git", type: :string, default: nil, banner: "Path of the local git repo to fetch from (deprecated)"
+    method_option "local_git", type: :string, default: nil, banner: "Path of the local git repo to fetch from (removed)"
     method_option "branch", type: :string, default: nil, banner: "The git branch to checkout"
     method_option "ref", type: :string, default: nil, banner: "The git revision to check out"
     method_option "path", type: :string, default: nil, banner: "Path of a local gem to directly use"
     def install(*plugins)
+      if options.key?(:local_git)
+        raise InvalidOption, "--local_git has been removed, use --git"
+      end
+
       Bundler::Plugin.install(plugins, options)
     end
 
diff --git a/lib/bundler/cli/show.rb b/lib/bundler/cli/show.rb
index b55eb7bad5aecc..67fdcc797e8592 100644
--- a/lib/bundler/cli/show.rb
+++ b/lib/bundler/cli/show.rb
@@ -6,7 +6,7 @@ class CLI::Show
     def initialize(options, gem_name)
       @options = options
       @gem_name = gem_name
-      @verbose = options[:verbose] || options[:outdated]
+      @verbose = options[:verbose]
       @latest_specs = fetch_latest_specs if @verbose
     end
 
diff --git a/lib/bundler/definition.rb b/lib/bundler/definition.rb
index 32dd13399ec597..49627cc56237ce 100644
--- a/lib/bundler/definition.rb
+++ b/lib/bundler/definition.rb
@@ -136,7 +136,7 @@ def initialize(lockfile, dependencies, sources, unlock, ruby_version = nil, opti
         @locked_sources = []
         @originally_locked_specs = @locked_specs
         @originally_locked_sources = @locked_sources
-        @locked_checksums = Bundler.feature_flag.lockfile_checksums?
+        @locked_checksums = Bundler.settings[:lockfile_checksums]
       end
 
       @unlocking_ruby ||= if @ruby_version && locked_ruby_version_object
diff --git a/lib/bundler/feature_flag.rb b/lib/bundler/feature_flag.rb
index 73e6ddcc68beb5..b2b134889573c1 100644
--- a/lib/bundler/feature_flag.rb
+++ b/lib/bundler/feature_flag.rb
@@ -27,9 +27,7 @@ def self.settings_method(name, key, &default)
 
     (1..10).each {|v| define_method("bundler_#{v}_mode?") { @major_version >= v } }
 
-    settings_flag(:cache_all) { bundler_4_mode? }
     settings_flag(:global_gem_cache) { bundler_5_mode? }
-    settings_flag(:lockfile_checksums) { bundler_4_mode? }
     settings_flag(:plugins) { @bundler_version >= Gem::Version.new("1.14") }
     settings_flag(:update_requires_all_flag) { bundler_5_mode? }
 
diff --git a/lib/bundler/man/bundle-config.1 b/lib/bundler/man/bundle-config.1
index b7276daa89195a..29e830a3b0e03b 100644
--- a/lib/bundler/man/bundle-config.1
+++ b/lib/bundler/man/bundle-config.1
@@ -146,7 +146,7 @@ Generate a \fBgems\.rb\fR instead of a \fBGemfile\fR when running \fBbundle init
 The number of gems Bundler can install in parallel\. Defaults to the number of available processors\.
 .TP
 \fBlockfile_checksums\fR (\fBBUNDLE_LOCKFILE_CHECKSUMS\fR)
-Whether Bundler should include a checksums section in new lockfiles, to protect from compromised gem sources\.
+Whether Bundler should include a checksums section in new lockfiles, to protect from compromised gem sources\. Defaults to true\.
 .TP
 \fBno_install\fR (\fBBUNDLE_NO_INSTALL\fR)
 Whether \fBbundle package\fR should skip installing gems\.
diff --git a/lib/bundler/man/bundle-config.1.ronn b/lib/bundler/man/bundle-config.1.ronn
index 18260c6c931f09..62fce8fa919aa0 100644
--- a/lib/bundler/man/bundle-config.1.ronn
+++ b/lib/bundler/man/bundle-config.1.ronn
@@ -190,7 +190,7 @@ learn more about their operation in [bundle install(1)](bundle-install.1.html).
    The number of gems Bundler can install in parallel. Defaults to the number of
    available processors.
 * `lockfile_checksums` (`BUNDLE_LOCKFILE_CHECKSUMS`):
-   Whether Bundler should include a checksums section in new lockfiles, to protect from compromised gem sources.
+   Whether Bundler should include a checksums section in new lockfiles, to protect from compromised gem sources. Defaults to true.
 * `no_install` (`BUNDLE_NO_INSTALL`):
    Whether `bundle package` should skip installing gems.
 * `no_prune` (`BUNDLE_NO_PRUNE`):
diff --git a/lib/bundler/man/bundle-plugin.1 b/lib/bundler/man/bundle-plugin.1
index 9da6927dc4d8f0..25da56247598b5 100644
--- a/lib/bundler/man/bundle-plugin.1
+++ b/lib/bundler/man/bundle-plugin.1
@@ -4,7 +4,7 @@
 .SH "NAME"
 \fBbundle\-plugin\fR \- Manage Bundler plugins
 .SH "SYNOPSIS"
-\fBbundle plugin\fR install PLUGINS [\-\-source=SOURCE] [\-\-version=VERSION] [\-\-git=GIT] [\-\-branch=BRANCH|\-\-ref=REF] [\-\-local\-git=LOCAL_GIT] [\-\-path=PATH]
+\fBbundle plugin\fR install PLUGINS [\-\-source=SOURCE] [\-\-version=VERSION] [\-\-git=GIT] [\-\-branch=BRANCH|\-\-ref=REF] [\-\-path=PATH]
 .br
 \fBbundle plugin\fR uninstall PLUGINS [\-\-all]
 .br
@@ -54,13 +54,6 @@ When you specify \fB\-\-git\fR, you can use \fB\-\-ref\fR to specify any tag, or
 Install the plugin gem from a local path\.
 .IP
 Example: \fBbundle plugin install bundler\-graph \-\-path \.\./bundler\-graph\fR
-.TP
-\fB\-\-local\-git=LOCAL_GIT\fR
-Install the plugin gem from a local Git repository\.
-.IP
-Example: \fBbundle plugin install bundler\-graph \-\-local\-git \.\./bundler\-graph\fR\.
-.IP
-This option is deprecated in favor of \fB\-\-git\fR\.
 .SS "uninstall"
 Uninstall the plugin(s) specified in PLUGINS\.
 .P
diff --git a/lib/bundler/man/bundle-plugin.1.ronn b/lib/bundler/man/bundle-plugin.1.ronn
index efb42407611836..b54e0c08b4e424 100644
--- a/lib/bundler/man/bundle-plugin.1.ronn
+++ b/lib/bundler/man/bundle-plugin.1.ronn
@@ -5,7 +5,6 @@ bundle-plugin(1) -- Manage Bundler plugins
 
 `bundle plugin` install PLUGINS [--source=SOURCE] [--version=VERSION]
                               [--git=GIT] [--branch=BRANCH|--ref=REF]
-                              [--local-git=LOCAL_GIT]
                               [--path=PATH]<br>
 `bundle plugin` uninstall PLUGINS [--all]<br>
 `bundle plugin` list<br>
@@ -59,13 +58,6 @@ global source specified in Gemfile is ignored.
 
   Example: `bundle plugin install bundler-graph --path ../bundler-graph`
 
-* `--local-git=LOCAL_GIT`:
-  Install the plugin gem from a local Git repository.
-
-  Example: `bundle plugin install bundler-graph --local-git ../bundler-graph`.
-
-  This option is deprecated in favor of `--git`.
-
 ### uninstall
 
 Uninstall the plugin(s) specified in PLUGINS.
diff --git a/lib/bundler/man/bundle-show.1 b/lib/bundler/man/bundle-show.1
index 3a6d157903d6bb..901460962cfed6 100644
--- a/lib/bundler/man/bundle-show.1
+++ b/lib/bundler/man/bundle-show.1
@@ -4,7 +4,7 @@
 .SH "NAME"
 \fBbundle\-show\fR \- Shows all the gems in your bundle, or the path to a gem
 .SH "SYNOPSIS"
-\fBbundle show\fR [GEM] [\-\-paths] [\-\-outdated]
+\fBbundle show\fR [GEM] [\-\-paths]
 .SH "DESCRIPTION"
 Without the [GEM] option, \fBshow\fR will print a list of the names and versions of all gems that are required by your [\fBGemfile(5)\fR][Gemfile(5)], sorted by name\.
 .P
@@ -13,7 +13,4 @@ Calling show with [GEM] will list the exact location of that gem on your machine
 .TP
 \fB\-\-paths\fR
 List the paths of all gems that are required by your [\fBGemfile(5)\fR][Gemfile(5)], sorted by gem name\.
-.TP
-\fB\-\-outdated\fR
-Show verbose output including whether gems are outdated\.
 
diff --git a/lib/bundler/man/bundle-show.1.ronn b/lib/bundler/man/bundle-show.1.ronn
index 6e80b046964704..a6a59a1445dcd3 100644
--- a/lib/bundler/man/bundle-show.1.ronn
+++ b/lib/bundler/man/bundle-show.1.ronn
@@ -5,7 +5,6 @@ bundle-show(1) -- Shows all the gems in your bundle, or the path to a gem
 
 `bundle show` [GEM]
               [--paths]
-              [--outdated]
 
 ## DESCRIPTION
 
@@ -20,6 +19,3 @@ machine.
 * `--paths`:
   List the paths of all gems that are required by your [`Gemfile(5)`][Gemfile(5)],
   sorted by gem name.
-
-* `--outdated`:
-  Show verbose output including whether gems are outdated.
diff --git a/lib/bundler/materialization.rb b/lib/bundler/materialization.rb
index 43124f25fbedbf..82e48464a73bd9 100644
--- a/lib/bundler/materialization.rb
+++ b/lib/bundler/materialization.rb
@@ -29,7 +29,7 @@ def specs
     end
 
     def dependencies
-      specs.first.runtime_dependencies.map {|d| [d, platform] }
+      (materialized_spec || specs.first).runtime_dependencies.map {|d| [d, platform] }
     end
 
     def materialized_spec
diff --git a/lib/bundler/plugin/installer.rb b/lib/bundler/plugin/installer.rb
index ac3c3ea7f31216..853ad9edca4f6f 100644
--- a/lib/bundler/plugin/installer.rb
+++ b/lib/bundler/plugin/installer.rb
@@ -43,16 +43,6 @@ def definition.lock(*); end
       private
 
       def check_sources_consistency!(options)
-        if options.key?(:git) && options.key?(:local_git)
-          raise InvalidOption, "Remote and local plugin git sources can't be both specified"
-        end
-
-        # back-compat; local_git is an alias for git
-        if options.key?(:local_git)
-          Bundler::SharedHelpers.major_deprecation(2, "--local_git is deprecated, use --git")
-          options[:git] = options.delete(:local_git)
-        end
-
         if (options.keys & [:source, :git, :path]).length > 1
           raise InvalidOption, "Only one of --source, --git, or --path may be specified"
         end
diff --git a/lib/bundler/settings.rb b/lib/bundler/settings.rb
index ecc3ee8080a239..7923ba51c36630 100644
--- a/lib/bundler/settings.rb
+++ b/lib/bundler/settings.rb
@@ -81,6 +81,8 @@ class Settings
       "BUNDLE_RETRY" => 3,
       "BUNDLE_TIMEOUT" => 10,
       "BUNDLE_VERSION" => "lockfile",
+      "BUNDLE_LOCKFILE_CHECKSUMS" => true,
+      "BUNDLE_CACHE_ALL" => true,
     }.freeze
 
     def initialize(root = nil)
diff --git a/lib/bundler/source/git.rb b/lib/bundler/source/git.rb
index ddea7714a16f5d..bb12ff52f514fb 100644
--- a/lib/bundler/source/git.rb
+++ b/lib/bundler/source/git.rb
@@ -268,7 +268,7 @@ def local?
       private
 
       def cache_to(custom_path, try_migrate: false)
-        return unless Bundler.feature_flag.cache_all?
+        return unless Bundler.settings[:cache_all]
 
         app_cache_path = app_cache_path(custom_path)
 
diff --git a/lib/bundler/source/path.rb b/lib/bundler/source/path.rb
index 76bd1c66c19f08..d258270fe09dd9 100644
--- a/lib/bundler/source/path.rb
+++ b/lib/bundler/source/path.rb
@@ -83,7 +83,7 @@ def install(spec, options = {})
 
       def cache(spec, custom_path = nil)
         app_cache_path = app_cache_path(custom_path)
-        return unless Bundler.feature_flag.cache_all?
+        return unless Bundler.settings[:cache_all]
         return if expand(@original_path).to_s.index(root_path.to_s + "/") == 0
 
         unless @original_path.exist?
diff --git a/lib/rubygems.rb b/lib/rubygems.rb
index f8f1451ee661a7..8bb8cdfc0486de 100644
--- a/lib/rubygems.rb
+++ b/lib/rubygems.rb
@@ -16,6 +16,7 @@ module Gem
 require_relative "rubygems/deprecate"
 require_relative "rubygems/errors"
 require_relative "rubygems/target_rbconfig"
+require_relative "rubygems/win_platform"
 
 ##
 # RubyGems is the Ruby standard for publishing and managing third party
@@ -113,18 +114,6 @@ module Gem
 module Gem
   RUBYGEMS_DIR = __dir__
 
-  ##
-  # An Array of Regexps that match windows Ruby platforms.
-
-  WIN_PATTERNS = [
-    /bccwin/i,
-    /cygwin/i,
-    /djgpp/i,
-    /mingw/i,
-    /mswin/i,
-    /wince/i,
-  ].freeze
-
   GEM_DEP_FILES = %w[
     gem.deps.rb
     gems.rb
@@ -160,8 +149,6 @@ module Gem
 
   DEFAULT_SOURCE_DATE_EPOCH = 315_619_200
 
-  @@win_platform = nil
-
   @configuration = nil
   @gemdeps = nil
   @loaded_specs = {}
@@ -1091,18 +1078,6 @@ def self.use_paths(home, *paths)
     self.paths = hash
   end
 
-  ##
-  # Is this a windows platform?
-
-  def self.win_platform?
-    if @@win_platform.nil?
-      ruby_platform = RbConfig::CONFIG["host_os"]
-      @@win_platform = !WIN_PATTERNS.find {|r| ruby_platform =~ r }.nil?
-    end
-
-    @@win_platform
-  end
-
   ##
   # Is this a java platform?
 
diff --git a/lib/rubygems/package.rb b/lib/rubygems/package.rb
index cd8dfdf37d1a15..b56d68cc45bb15 100644
--- a/lib/rubygems/package.rb
+++ b/lib/rubygems/package.rb
@@ -7,6 +7,7 @@
 
 # rubocop:enable Style/AsciiComments
 
+require_relative "win_platform"
 require_relative "security"
 require_relative "user_interaction"
 
@@ -518,10 +519,12 @@ def install_location(filename, destination_dir) # :nodoc:
     destination
   end
 
-  def normalize_path(pathname)
-    if Gem.win_platform?
+  if Gem.win_platform?
+    def normalize_path(pathname) # :nodoc:
       pathname.downcase
-    else
+    end
+  else
+    def normalize_path(pathname) # :nodoc:
       pathname
     end
   end
diff --git a/lib/rubygems/win_platform.rb b/lib/rubygems/win_platform.rb
new file mode 100644
index 00000000000000..78e968fe493084
--- /dev/null
+++ b/lib/rubygems/win_platform.rb
@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+require "rbconfig"
+
+module Gem
+  ##
+  # An Array of Regexps that match windows Ruby platforms.
+
+  WIN_PATTERNS = [
+    /bccwin/i,
+    /cygwin/i,
+    /djgpp/i,
+    /mingw/i,
+    /mswin/i,
+    /wince/i,
+  ].freeze
+
+  @@win_platform = nil
+
+  ##
+  # Is this a windows platform?
+
+  def self.win_platform?
+    if @@win_platform.nil?
+      ruby_platform = RbConfig::CONFIG["host_os"]
+      @@win_platform = !WIN_PATTERNS.find {|r| ruby_platform =~ r }.nil?
+    end
+
+    @@win_platform
+  end
+end
diff --git a/spec/bundler/cache/git_spec.rb b/spec/bundler/cache/git_spec.rb
index 66eaf65dd1201d..149fad28384446 100644
--- a/spec/bundler/cache/git_spec.rb
+++ b/spec/bundler/cache/git_spec.rb
@@ -13,6 +13,22 @@
 end
 
 RSpec.describe "bundle cache with git" do
+  it "does not copy repository to vendor cache when cache_all set to false" do
+    git = build_git "foo"
+    ref = git.ref_for("main", 11)
+
+    install_gemfile <<-G
+      source "https://gem.repo1"
+      gem "foo", :git => '#{lib_path("foo-1.0")}'
+    G
+
+    bundle "config cache_all false"
+    bundle :cache
+    expect(bundled_app("vendor/cache/foo-1.0-#{ref}")).not_to exist
+
+    expect(the_bundle).to include_gems "foo 1.0"
+  end
+
   it "copies repository to vendor cache and uses it" do
     git = build_git "foo"
     ref = git.ref_for("main", 11)
@@ -22,7 +38,6 @@
       gem "foo", :git => '#{lib_path("foo-1.0")}'
     G
 
-    bundle "config set cache_all true"
     bundle :cache
     expect(bundled_app("vendor/cache/foo-1.0-#{ref}")).to exist
     expect(bundled_app("vendor/cache/foo-1.0-#{ref}/.git")).not_to exist
@@ -43,7 +58,6 @@
 
     bundle "config set --local path vendor/bundle"
     bundle "install"
-    bundle "config set cache_all true"
     bundle :cache
 
     expect(bundled_app("vendor/cache/foo-1.0-#{ref}")).to exist
@@ -61,7 +75,6 @@
       gem "foo", :git => '#{lib_path("foo-1.0")}'
     G
 
-    bundle "config set cache_all true"
     bundle :cache
     bundle :cache
 
@@ -79,7 +92,6 @@
       gem "foo", :git => '#{lib_path("foo-1.0")}'
     G
 
-    bundle "config set cache_all true"
     bundle :cache
 
     update_git "foo" do |s|
@@ -109,7 +121,6 @@
       gem "foo", :git => '#{lib_path("foo-1.0")}'
     G
 
-    bundle "config set cache_all true"
     bundle :cache
 
     update_git "foo" do |s|
@@ -140,7 +151,6 @@
 
     bundle %(config set local.foo #{lib_path("foo-1.0")})
     bundle "install"
-    bundle "config set cache_all true"
     bundle :cache
 
     expect(bundled_app("vendor/cache/foo-invalid-#{ref}")).to exist
@@ -179,7 +189,6 @@
       source "https://gem.repo1"
       gem "foo", :git => '#{lib_path("foo-1.0")}'
     G
-    bundle "config set cache_all true"
     bundle :cache, "all-platforms" => true
 
     pristine_system_gems
@@ -196,7 +205,6 @@
       source "https://gem.repo1"
       gem "foo", :git => '#{lib_path("foo-1.0")}'
     G
-    bundle "config set cache_all true"
     bundle :cache, "all-platforms" => true
 
     pristine_system_gems
@@ -213,7 +221,6 @@
       source "https://gem.repo1"
       gem "foo", :git => '#{lib_path("foo-1.0")}'
     G
-    bundle "config set cache_all true"
     bundle :cache, "all-platforms" => true
 
     pristine_system_gems
@@ -242,7 +249,6 @@
       gem "foo", :git => '#{lib_path("foo-1.0")}'
     G
     bundle "config set global_gem_cache false"
-    bundle "config set cache_all true"
     bundle "config path vendor/bundle"
     bundle :install
 
@@ -274,7 +280,6 @@
       gem "foo", :git => '#{lib_path("foo-1.0")}'
     G
     bundle "config set global_gem_cache false"
-    bundle "config set cache_all true"
     bundle "config path vendor/bundle"
     bundle :install
 
@@ -304,7 +309,6 @@
       gem "foo", :git => '#{lib_path("foo-1.0")}'
     G
     bundle "config set global_gem_cache false"
-    bundle "config set cache_all true"
     bundle "config path vendor/bundle"
     bundle :install
 
@@ -342,7 +346,6 @@
     G
 
     ref = git.ref_for("main", 11)
-    bundle "config set cache_all true"
     bundle :cache
 
     expect(bundled_app("vendor/cache/has_submodule-1.0-#{ref}")).to exist
@@ -362,7 +365,6 @@
       source "https://gem.repo1"
       gem "foo", :git => '#{lib_path("foo-1.0")}'
     G
-    bundle "config set cache_all true"
     bundle :cache
 
     ref = git.ref_for("main", 11)
@@ -377,7 +379,6 @@
       source "https://gem.repo1"
       gem "foo", :git => '#{lib_path("foo-1.0")}'
     G
-    bundle "config set cache_all true"
     bundle :cache, "all-platforms" => true, :install => false
 
     pristine_system_gems
@@ -436,7 +437,6 @@
       source "https://gem.repo1"
       gem "foo", :git => '#{lib_path("foo-1.0")}'
     G
-    bundle "config set cache_all true"
 
     # The algorithm for the cache location for a git checkout is
     # in Bundle::Source::Git#cache_path
@@ -498,7 +498,6 @@
     end
 
     FileUtils.mkdir_p(bundled_app("vendor/cache"))
-    bundle "config set cache_all all"
 
     install_gemfile <<-G
       source "https://gem.repo1"
diff --git a/spec/bundler/cache/path_spec.rb b/spec/bundler/cache/path_spec.rb
index 2a64397417ba82..6865e54b321fa9 100644
--- a/spec/bundler/cache/path_spec.rb
+++ b/spec/bundler/cache/path_spec.rb
@@ -9,7 +9,6 @@
       gem "foo", :path => '#{bundled_app("lib/foo")}'
     G
 
-    bundle "config set cache_all true"
     bundle :cache
     expect(bundled_app("vendor/cache/foo-1.0")).not_to exist
     expect(the_bundle).to include_gems "foo 1.0"
@@ -23,7 +22,6 @@
       gem "foo", :path => '#{lib_path("foo-1.0")}'
     G
 
-    bundle "config set cache_all true"
     bundle :cache
     expect(bundled_app("vendor/cache/foo-1.0")).to exist
     expect(bundled_app("vendor/cache/foo-1.0/.bundlecache")).to be_file
@@ -42,7 +40,6 @@
       gem "#{libname}", :path => '#{libpath}'
     G
 
-    bundle "config set cache_all true"
     bundle :cache
     expect(bundled_app("vendor/cache/#{libname}")).to exist
     expect(bundled_app("vendor/cache/#{libname}/.bundlecache")).to be_file
@@ -58,7 +55,6 @@
       gem "foo", :path => '#{lib_path("foo-1.0")}'
     G
 
-    bundle "config set cache_all true"
     bundle :cache
 
     build_lib "foo" do |s|
@@ -81,7 +77,6 @@
       gem "foo", :path => '#{lib_path("foo-1.0")}'
     G
 
-    bundle "config set cache_all true"
     bundle :cache
 
     expect(bundled_app("vendor/cache/foo-1.0")).to exist
@@ -97,20 +92,21 @@
     expect(bundled_app("vendor/cache/foo-1.0")).not_to exist
   end
 
-  it "does not cache path gems by default" do
+  it "does not cache path gems if cache_all is set to false" do
     build_lib "foo"
 
     install_gemfile <<-G
       source "https://gem.repo1"
       gem "foo", :path => '#{lib_path("foo-1.0")}'
     G
+    bundle "config cache_all false"
 
     bundle :cache
     expect(err).to be_empty
     expect(bundled_app("vendor/cache/foo-1.0")).not_to exist
   end
 
-  it "caches path gems by default", bundler: "4" do
+  it "caches path gems by default" do
     build_lib "foo"
 
     install_gemfile <<-G
diff --git a/spec/bundler/commands/lock_spec.rb b/spec/bundler/commands/lock_spec.rb
index 5a31d1733a5556..a7460ed695a456 100644
--- a/spec/bundler/commands/lock_spec.rb
+++ b/spec/bundler/commands/lock_spec.rb
@@ -2097,7 +2097,7 @@
     L
   end
 
-  it "generates checksums by default if configured to do so" do
+  it "generates checksums by default" do
     build_repo4 do
       build_gem "nokogiri", "1.14.2"
       build_gem "nokogiri", "1.14.2" do |s|
@@ -2105,8 +2105,6 @@
       end
     end
 
-    bundle "config lockfile_checksums true"
-
     simulate_platform "x86_64-linux" do
       install_gemfile <<-G
         source "https://gem.repo4"
@@ -2139,6 +2137,43 @@
     L
   end
 
+  it "disables checksums if configured to do so" do
+    build_repo4 do
+      build_gem "nokogiri", "1.14.2"
+      build_gem "nokogiri", "1.14.2" do |s|
+        s.platform = "x86_64-linux"
+      end
+    end
+
+    bundle "config lockfile_checksums false"
+
+    simulate_platform "x86_64-linux" do
+      install_gemfile <<-G
+        source "https://gem.repo4"
+
+        gem "nokogiri"
+      G
+    end
+
+    expect(lockfile).to eq <<~L
+      GEM
+        remote: https://gem.repo4/
+        specs:
+          nokogiri (1.14.2)
+          nokogiri (1.14.2-x86_64-linux)
+
+      PLATFORMS
+        ruby
+        x86_64-linux
+
+      DEPENDENCIES
+        nokogiri
+
+      BUNDLED WITH
+         #{Bundler::VERSION}
+    L
+  end
+
   context "when re-resolving to include prereleases" do
     before do
       build_repo4 do
diff --git a/spec/bundler/commands/newgem_spec.rb b/spec/bundler/commands/newgem_spec.rb
index 0b13344f994154..1ce4a0da09cac1 100644
--- a/spec/bundler/commands/newgem_spec.rb
+++ b/spec/bundler/commands/newgem_spec.rb
@@ -1661,24 +1661,6 @@ def create_temporary_dir(dir)
 
     include_examples "paths that depend on gem name"
 
-    context "--ext parameter with no value" do
-      context "is deprecated" do
-        it "prints deprecation when used after gem name" do
-          bundle ["gem", "--ext", gem_name].compact.join(" ")
-          expect(err).to include "[DEPRECATED]"
-          expect(err).to include "`--ext` with no arguments has been deprecated"
-          expect(bundled_app("#{gem_name}/ext/#{gem_name}/#{gem_name}.c")).to exist
-        end
-
-        it "prints deprecation when used before gem name" do
-          bundle ["gem", gem_name, "--ext"].compact.join(" ")
-          expect(err).to include "[DEPRECATED]"
-          expect(err).to include "`--ext` with no arguments has been deprecated"
-          expect(bundled_app("#{gem_name}/ext/#{gem_name}/#{gem_name}.c")).to exist
-        end
-      end
-    end
-
     context "--ext parameter set with C" do
       let(:flags) { "--ext=c" }
 
@@ -1686,10 +1668,6 @@ def create_temporary_dir(dir)
         bundle ["gem", gem_name, flags].compact.join(" ")
       end
 
-      it "is not deprecated" do
-        expect(err).not_to include "[DEPRECATED] Option `--ext` without explicit value is deprecated."
-      end
-
       it "builds ext skeleton" do
         expect(bundled_app("#{gem_name}/ext/#{gem_name}/extconf.rb")).to exist
         expect(bundled_app("#{gem_name}/ext/#{gem_name}/#{gem_name}.h")).to exist
diff --git a/spec/bundler/commands/show_spec.rb b/spec/bundler/commands/show_spec.rb
index ba903ac4957a5a..34e809f1358191 100644
--- a/spec/bundler/commands/show_spec.rb
+++ b/spec/bundler/commands/show_spec.rb
@@ -210,33 +210,6 @@
       expect(err).to include("Could not find gem '#{invalid_regexp}'.")
     end
   end
-
-  context "--outdated option" do
-    # Regression test for https://github.com/rubygems/bundler/issues/5375
-    before do
-      build_repo2
-    end
-
-    it "doesn't update gems to newer versions" do
-      install_gemfile <<-G
-        source "https://gem.repo2"
-        gem "rails"
-      G
-
-      expect(the_bundle).to include_gem("rails 2.3.2")
-
-      update_repo2 do
-        build_gem "rails", "3.0.0" do |s|
-          s.executables = "rails"
-        end
-      end
-
-      bundle "show --outdated"
-
-      bundle "install"
-      expect(the_bundle).to include_gem("rails 2.3.2")
-    end
-  end
 end
 
 RSpec.describe "bundle show", bundler: "5" do
diff --git a/spec/bundler/install/deploy_spec.rb b/spec/bundler/install/deploy_spec.rb
index e401f26a16ece2..7d10ef059407bb 100644
--- a/spec/bundler/install/deploy_spec.rb
+++ b/spec/bundler/install/deploy_spec.rb
@@ -474,7 +474,6 @@
       bundle :install
       expect(the_bundle).to include_gems "foo 1.0"
 
-      bundle "config set cache_all true"
       bundle :cache
       expect(bundled_app("vendor/cache/foo")).to be_directory
 
diff --git a/spec/bundler/install/gemfile/path_spec.rb b/spec/bundler/install/gemfile/path_spec.rb
index 55bf11928d6acd..ea59f11bbe9773 100644
--- a/spec/bundler/install/gemfile/path_spec.rb
+++ b/spec/bundler/install/gemfile/path_spec.rb
@@ -831,6 +831,55 @@
     end
   end
 
+  context "when platform specific version locked, and having less dependencies that the generic version that's actually installed" do
+    before do
+      build_repo4 do
+        build_gem "racc", "1.8.1"
+        build_gem "mini_portile2", "2.8.2"
+      end
+
+      build_lib "nokogiri", "1.18.9", path: lib_path("nokogiri") do |s|
+        s.add_dependency "mini_portile2", "~> 2.8.2"
+        s.add_dependency "racc", "~> 1.4"
+      end
+
+      gemfile <<~G
+        source "https://gem.repo4"
+
+        gem "nokogiri", path: "#{lib_path("nokogiri")}"
+      G
+
+      lockfile <<~L
+        PATH
+          remote: #{lib_path("nokogiri")}
+          specs:
+            nokogiri (1.18.9)
+              mini_portile2 (~> 2.8.2)
+              racc (~> 1.4)
+            nokogiri (1.18.9-arm64-darwin)
+              racc (~> 1.4)
+
+        GEM
+          remote: https://rubygems.org/
+          specs:
+            racc (1.8.1)
+
+        PLATFORMS
+          #{lockfile_platforms}
+
+        DEPENDENCIES
+          nokogiri!
+
+        BUNDLED WITH
+           #{Bundler::VERSION}
+      L
+    end
+
+    it "works" do
+      bundle "install"
+    end
+  end
+
   describe "switching sources" do
     it "doesn't switch pinned git sources to rubygems when pinning the parent gem to a path source" do
       build_gem "foo", "1.0", to_system: true do |s|
diff --git a/spec/bundler/install/gemfile/specific_platform_spec.rb b/spec/bundler/install/gemfile/specific_platform_spec.rb
index 62540f0488de62..09ed9a4faaa7c8 100644
--- a/spec/bundler/install/gemfile/specific_platform_spec.rb
+++ b/spec/bundler/install/gemfile/specific_platform_spec.rb
@@ -336,7 +336,6 @@
          #{Bundler::VERSION}
     L
 
-    bundle "config set --local cache_all true"
     bundle "cache --all-platforms"
 
     expect(err).to be_empty
diff --git a/spec/bundler/lock/lockfile_spec.rb b/spec/bundler/lock/lockfile_spec.rb
index 6b98e0924e41d8..4b767c7415b656 100644
--- a/spec/bundler/lock/lockfile_spec.rb
+++ b/spec/bundler/lock/lockfile_spec.rb
@@ -786,7 +786,6 @@
       c.no_checksum "foo", "1.0"
     end
 
-    bundle "config set cache_all true"
     bundle :cache
     bundle :install, local: true
 
diff --git a/spec/bundler/other/major_deprecation_spec.rb b/spec/bundler/other/major_deprecation_spec.rb
index 1749d7e6827eaf..6117ff61374845 100644
--- a/spec/bundler/other/major_deprecation_spec.rb
+++ b/spec/bundler/other/major_deprecation_spec.rb
@@ -647,14 +647,12 @@
 
     context "with --outdated flag" do
       before do
-        bundle "show --outdated"
+        bundle "show --outdated", raise_on_error: false
       end
 
-      it "prints a deprecation warning informing about its removal" do
-        expect(deprecations).to include("the `--outdated` flag to `bundle show` will be removed in favor of `bundle show --verbose`")
+      it "fails with a helpful message" do
+        expect(err).to include("the `--outdated` flag to `bundle show` has been removed in favor of `bundle show --verbose`")
       end
-
-      pending "fails with a helpful message", bundler: "4"
     end
   end
 
@@ -702,14 +700,11 @@
       end
     end
 
-    it "prints a deprecation warning" do
-      bundle "plugin install foo --local_git #{lib_path("foo-1.0")}"
+    it "fails with a helpful message" do
+      bundle "plugin install foo --local_git #{lib_path("foo-1.0")}", raise_on_error: false
 
-      expect(out).to include("Installed plugin foo")
-      expect(deprecations).to include "--local_git is deprecated, use --git"
+      expect(err).to include "--local_git has been removed, use --git"
     end
-
-    pending "fails with a helpful message", bundler: "4"
   end
 
   describe "removing rubocop" do
@@ -740,4 +735,16 @@
       end
     end
   end
+
+  context " bundle gem --ext parameter with no value" do
+    it "prints error when used before gem name" do
+      bundle "gem --ext foo", raise_on_error: false
+      expect(err).to include "Extensions can now be generated using C or Rust, so `--ext` with no arguments has been removed. Please select a language, e.g. `--ext=rust` to generate a Rust extension."
+    end
+
+    it "prints error when used after gem name" do
+      bundle "gem foo --ext", raise_on_error: false
+      expect(err).to include "Extensions can now be generated using C or Rust, so `--ext` with no arguments has been removed. Please select a language, e.g. `--ext=rust` to generate a Rust extension."
+    end
+  end
 end
diff --git a/spec/bundler/plugins/install_spec.rb b/spec/bundler/plugins/install_spec.rb
index 1ccefabc0b50a9..0cddeb09185bc7 100644
--- a/spec/bundler/plugins/install_spec.rb
+++ b/spec/bundler/plugins/install_spec.rb
@@ -203,13 +203,6 @@ def exec(command, args)
       expect(out).to include("Installed plugin foo")
       plugin_should_be_installed("foo")
     end
-
-    it "raises an error when both git and local git sources are specified" do
-      bundle "plugin install foo --git /phony/path/project --local_git git@gitphony.com:/repo/project", raise_on_error: false
-
-      expect(exitstatus).not_to eq(0)
-      expect(err).to eq("Remote and local plugin git sources can't be both specified")
-    end
   end
 
   context "path plugins" do
diff --git a/spec/bundler/plugins/source/example_spec.rb b/spec/bundler/plugins/source/example_spec.rb
index f9624463144ab4..da64886f01d224 100644
--- a/spec/bundler/plugins/source/example_spec.rb
+++ b/spec/bundler/plugins/source/example_spec.rb
@@ -124,7 +124,6 @@ def install(spec, opts)
       let(:uri_hash) { Digest(:SHA1).hexdigest(lib_path("a-path-gem-1.0").to_s) }
       it "copies repository to vendor cache and uses it" do
         bundle "install"
-        bundle "config set cache_all true"
         bundle :cache
 
         expect(bundled_app("vendor/cache/a-path-gem-1.0-#{uri_hash}")).to exist
@@ -138,7 +137,6 @@ def install(spec, opts)
       it "copies repository to vendor cache and uses it even when installed with `path` configured" do
         bundle "config set --local path vendor/bundle"
         bundle :install
-        bundle "config set cache_all true"
         bundle :cache
 
         expect(bundled_app("vendor/cache/a-path-gem-1.0-#{uri_hash}")).to exist
@@ -150,7 +148,6 @@ def install(spec, opts)
       it "bundler package copies repository to vendor cache" do
         bundle "config set --local path vendor/bundle"
         bundle :install
-        bundle "config set cache_all true"
         bundle :cache
 
         expect(bundled_app("vendor/cache/a-path-gem-1.0-#{uri_hash}")).to exist
@@ -446,7 +443,6 @@ def installed?
           end
         G
 
-        bundle "config set cache_all true"
         bundle :cache
         expect(bundled_app("vendor/cache/foo-1.0-#{ref}")).to exist
         expect(bundled_app("vendor/cache/foo-1.0-#{ref}/.git")).not_to exist
diff --git a/spec/bundler/support/checksums.rb b/spec/bundler/support/checksums.rb
index 8e0dea4a717f96..cf8ea417d67adb 100644
--- a/spec/bundler/support/checksums.rb
+++ b/spec/bundler/support/checksums.rb
@@ -58,7 +58,7 @@ def checksums_section_when_enabled(target_lockfile = nil, &block)
       begin
         enabled = (target_lockfile || lockfile).match?(/^CHECKSUMS$/)
       rescue Errno::ENOENT
-        enabled = Bundler.feature_flag.bundler_4_mode?
+        enabled = true
       end
       checksums_section(enabled, &block)
     end
diff --git a/test/json/json_coder_test.rb b/test/json/json_coder_test.rb
index fc4aba296858ae..c7248353769969 100755
--- a/test/json/json_coder_test.rb
+++ b/test/json/json_coder_test.rb
@@ -12,7 +12,8 @@ def test_json_coder_with_proc
   end
 
   def test_json_coder_with_proc_with_unsupported_value
-    coder = JSON::Coder.new do |object|
+    coder = JSON::Coder.new do |object, is_key|
+      assert_equal false, is_key
       Object.new
     end
     assert_raise(JSON::GeneratorError) { coder.dump([Object.new]) }
@@ -20,7 +21,10 @@ def test_json_coder_with_proc_with_unsupported_value
 
   def test_json_coder_hash_key
     obj = Object.new
-    coder = JSON::Coder.new(&:to_s)
+    coder = JSON::Coder.new do |obj, is_key|
+      assert_equal true, is_key
+      obj.to_s
+    end
     assert_equal %({#{obj.to_s.inspect}:1}), coder.dump({ obj => 1 })
 
     coder = JSON::Coder.new { 42 }
@@ -49,14 +53,14 @@ def test_json_coder_load_options
   end
 
   def test_json_coder_dump_NaN_or_Infinity
-    coder = JSON::Coder.new(&:inspect)
+    coder = JSON::Coder.new { |o| o.inspect }
     assert_equal "NaN", coder.load(coder.dump(Float::NAN))
     assert_equal "Infinity", coder.load(coder.dump(Float::INFINITY))
     assert_equal "-Infinity", coder.load(coder.dump(-Float::INFINITY))
   end
 
   def test_json_coder_dump_NaN_or_Infinity_loop
-    coder = JSON::Coder.new(&:itself)
+    coder = JSON::Coder.new { |o| o.itself }
     error = assert_raise JSON::GeneratorError do
       coder.dump(Float::NAN)
     end
diff --git a/test/json/json_encoding_test.rb b/test/json/json_encoding_test.rb
index caf335d521f994..8dce81da590e76 100644
--- a/test/json/json_encoding_test.rb
+++ b/test/json/json_encoding_test.rb
@@ -31,6 +31,12 @@ def test_generate
     assert_equal @generated, JSON.generate(@utf_16_data, ascii_only: true)
   end
 
+  def test_generate_shared_string
+    # Ref: https://github.com/ruby/json/issues/859
+    s = "01234567890"
+    assert_equal '"234567890"', JSON.dump(s[2..-1])
+  end
+
   def test_unicode
     assert_equal '""', ''.to_json
     assert_equal '"\\b"', "\b".to_json
diff --git a/test/json/json_generator_test.rb b/test/json/json_generator_test.rb
index 6b42de2ad29abe..a6950f88878db8 100755
--- a/test/json/json_generator_test.rb
+++ b/test/json/json_generator_test.rb
@@ -822,29 +822,44 @@ def test_fragment
 
   def test_json_generate_as_json_convert_to_proc
     object = Object.new
-    assert_equal object.object_id.to_json, JSON.generate(object, strict: true, as_json: :object_id)
+    assert_equal object.object_id.to_json, JSON.generate(object, strict: true, as_json: -> (o, is_key) { o.object_id })
   end
 
-  def test_json_generate_float
-      values = [-1.0, 1.0, 0.0, 12.2, 7.5 / 3.2, 12.0, 100.0, 1000.0]
-      expecteds = ["-1.0", "1.0", "0.0", "12.2", "2.34375", "12.0", "100.0", "1000.0"]
-
-      if RUBY_ENGINE == "jruby"
-        values << 1746861937.7842371
-        expecteds << "1.7468619377842371E9"
-      else
-        values << 1746861937.7842371
-        expecteds << "1746861937.7842371"
-      end
-
-      if RUBY_ENGINE == "ruby"
-        values << -2.2471348024634545e-08 << -2.2471348024634545e-09 << -2.2471348024634545e-10
-        expecteds << "-0.000000022471348024634545" << "-0.0000000022471348024634545" << "-2.2471348024634546e-10"
-      end
+  def assert_float_roundtrip(expected, actual)
+    assert_equal(expected, JSON.generate(actual))
+    assert_equal(actual, JSON.parse(JSON.generate(actual)), "JSON: #{JSON.generate(actual)}")
+  end
 
-      values.zip(expecteds).each do |value, expected|
-        assert_equal expected, value.to_json
-      end
+  def test_json_generate_float
+    assert_float_roundtrip "-1.0", -1.0
+    assert_float_roundtrip "1.0", 1.0
+    assert_float_roundtrip "0.0", 0.0
+    assert_float_roundtrip "12.2", 12.2
+    assert_float_roundtrip "2.34375", 7.5 / 3.2
+    assert_float_roundtrip "12.0", 12.0
+    assert_float_roundtrip "100.0", 100.0
+    assert_float_roundtrip "1000.0", 1000.0
+
+    if RUBY_ENGINE == "jruby"
+      assert_float_roundtrip "1.7468619377842371E9", 1746861937.7842371
+    else
+      assert_float_roundtrip "1746861937.7842371", 1746861937.7842371
+    end
+
+    if RUBY_ENGINE == "ruby"
+      assert_float_roundtrip "100000000000000.0", 100000000000000.0
+      assert_float_roundtrip "1e+15", 1e+15
+      assert_float_roundtrip "-100000000000000.0", -100000000000000.0
+      assert_float_roundtrip "-1e+15", -1e+15
+      assert_float_roundtrip "1111111111111111.1", 1111111111111111.1
+      assert_float_roundtrip "1.1111111111111112e+16", 11111111111111111.1
+      assert_float_roundtrip "-1111111111111111.1", -1111111111111111.1
+      assert_float_roundtrip "-1.1111111111111112e+16", -11111111111111111.1
+
+      assert_float_roundtrip "-0.000000022471348024634545", -2.2471348024634545e-08
+      assert_float_roundtrip "-0.0000000022471348024634545", -2.2471348024634545e-09
+      assert_float_roundtrip "-2.2471348024634546e-10", -2.2471348024634545e-10
+    end
   end
 
   def test_numbers_of_various_sizes
diff --git a/test/json/json_parser_test.rb b/test/json/json_parser_test.rb
index a9b0624f6bab58..9d387cb808925a 100644
--- a/test/json/json_parser_test.rb
+++ b/test/json/json_parser_test.rb
@@ -319,6 +319,12 @@ def test_invalid_unicode_escape
     assert_raise(JSON::ParserError) { parse('"\u111___"') }
   end
 
+  def test_invalid_surogates
+    assert_raise(JSON::ParserError) { parse('"\\uD800"') }
+    assert_raise(JSON::ParserError) { parse('"\\uD800_________________"') }
+    assert_raise(JSON::ParserError) { parse('"\\uD800\\u0041"') }
+  end
+
   def test_parse_big_integers
     json1 = JSON(orig = (1 << 31) - 1)
     assert_equal orig, parse(json1)
diff --git a/test/json/test_helper.rb b/test/json/test_helper.rb
index a788804ddeca4e..24cde4348cdbf7 100644
--- a/test/json/test_helper.rb
+++ b/test/json/test_helper.rb
@@ -1,14 +1,27 @@
 $LOAD_PATH.unshift(File.expand_path('../../../ext', __FILE__), File.expand_path('../../../lib', __FILE__))
 
-require 'coverage'
-Coverage.start
+if ENV["JSON_COVERAGE"]
+  # This test helper is loaded inside Ruby's own test suite, so we try to not mess it up.
+  require 'coverage'
+
+  branches_supported = Coverage.respond_to?(:supported?) && Coverage.supported?(:branches)
+
+  # Coverage module must be started before SimpleCov to work around the cyclic require order.
+  # Track both branches and lines, or else SimpleCov misleadingly reports 0/0 = 100% for non-branching files.
+  Coverage.start(lines:    true,
+                 branches: branches_supported)
 
-begin
   require 'simplecov'
-rescue LoadError
-  # Don't fail Ruby's test suite
-else
-  SimpleCov.start
+  SimpleCov.start do
+    # Enabling both coverage types to let SimpleCov know to output them together in reports
+    enable_coverage :line
+    enable_coverage :branch if branches_supported
+
+    # Can't always trust SimpleCov to find files implicitly
+    track_files 'lib/**/*.rb'
+
+    add_filter 'lib/json/truffle_ruby' unless RUBY_ENGINE == 'truffleruby'
+  end
 end
 
 require 'json'