diff --git a/Library/Homebrew/api.rb b/Library/Homebrew/api.rb
index 9ab55cbca1..ff26cbcb5a 100644
--- a/Library/Homebrew/api.rb
+++ b/Library/Homebrew/api.rb
@@ -56,6 +56,7 @@ module Homebrew
       retry_count = 0
       url = "#{Homebrew::EnvConfig.api_domain}/#{endpoint}"
       default_url = "#{HOMEBREW_API_DEFAULT_DOMAIN}/#{endpoint}"
+      primary_url = url
 
       if Homebrew.running_as_root_but_not_owned_by_root? &&
          (!target.exist? || target.empty?)
@@ -68,14 +69,13 @@ module Homebrew
         "--speed-time", ENV.fetch("HOMEBREW_CURL_SPEED_TIME"),
         # This is a Curl format token, not a Ruby one.
         # rubocop:disable Style/FormatStringToken
-        "--write-out", "%{stderr}HTTP status: %{http_code}"
+        "--write-out", "%{http_code}"
         # rubocop:enable Style/FormatStringToken
       ]
 
       insecure_download = DevelopmentTools.ca_file_substitution_required? ||
                           DevelopmentTools.curl_substitution_required?
       skip_download = skip_download?(target:, stale_seconds:)
-
       if enqueue
         unless skip_download
           require "download_queue"
@@ -87,22 +87,61 @@ module Homebrew
         return [{}, false]
       end
 
+      etag_path = Pathname("#{target}.etag")
+      etag_url_path = Pathname("#{etag_path}.url")
+      new_etag_path = Pathname("#{target}.etag.new")
+      new_target = Pathname("#{target}.new")
+
       json_data, from_payload_cache = begin
+        unless skip_download
+          require "lock_file"
+          # cmd/update.sh takes the same lock before replacing the body and validators.
+          download_lock ||= DownloadLock.new(target)
+          download_lock.lock_or_wait(quiet: Context.current.quiet?)
+          skip_download = skip_download?(target:, stale_seconds:)
+        end
         download_succeeded = T.let(false, T::Boolean)
+        unconditional_retry = T.let(false, T::Boolean)
         begin
           args = curl_args.dup
-          args.prepend("--time-cond", target.to_s) if target.exist? && !target.empty?
+          # Keep conditional requests in sync with utils/api.sh and cmd/update.sh.
+          use_etag = HOMEBREW_CURL_SUPPORTS_ETAGS && url == primary_url
+          if !unconditional_retry && target.exist? && !target.empty?
+            if use_etag && etag_path.exist? && !etag_path.empty? &&
+               etag_url_path.exist? && etag_url_path.read == url
+              args.prepend("--etag-compare", etag_path.to_s)
+            elsif !HOMEBREW_CURL_SUPPORTS_ETAGS && url == primary_url
+              args.prepend("--time-cond", target.to_s)
+            end
+          end
+          args.prepend("--etag-save", new_etag_path.to_s) if use_etag
           if insecure_download
             opoo DevelopmentTools.insecure_download_warning(endpoint)
             args.append("--insecure")
           end
           unless skip_download
+            FileUtils.rm_f([new_target, new_etag_path])
             ohai "Downloading #{url}" if $stdout.tty? && !Context.current.quiet?
             # Disable retries here, we handle them ourselves below.
-            Utils::Curl.curl_download(*args, url, to: target, retries: 0, show_error: false)
+            result = Utils::Curl.curl_download(*args, url, to: new_target, retries: 0, show_error: false,
+                                              print_stdout: false)
             download_succeeded = true
+            if new_target.exist?
+              FileUtils.rm_f([etag_path, etag_url_path])
+              FileUtils.mv(new_target, target)
+            end
+            if use_etag && new_etag_path.exist? && !new_etag_path.empty?
+              FileUtils.mv(new_etag_path, etag_path)
+              File.write(etag_url_path, url)
+            elsif etag_path.exist? && result&.stdout != "304"
+              FileUtils.rm_f([etag_path, etag_url_path])
+            end
+          end
+        rescue ErrorDuringExecution => e
+          if e.exitstatus == 56 && args && (args.include?("--etag-compare") || args.include?("--time-cond"))
+            unconditional_retry = true
+            retry
           end
-        rescue ErrorDuringExecution
           if url == default_url
             raise unless target.exist?
             raise if target.empty?
@@ -117,6 +156,8 @@ module Homebrew
           end
 
           opoo "#{target.basename}: update failed, falling back to cached version."
+        ensure
+          FileUtils.rm_f([new_target, new_etag_path]) unless skip_download
         end
 
         # Only refresh the cache mtime after a successful curl revalidation/download.
@@ -166,6 +207,8 @@ module Homebrew
       else
         [json_data, !skip_download]
       end
+    ensure
+      download_lock&.unlock
     end
 
     sig {
diff --git a/Library/Homebrew/brew.sh b/Library/Homebrew/brew.sh
index b1e854d881..89fd26c75c 100644
--- a/Library/Homebrew/brew.sh
+++ b/Library/Homebrew/brew.sh
@@ -393,6 +393,7 @@ export HOMEBREW_CURL
 export HOMEBREW_BREWED_CURL_PATH
 export HOMEBREW_CURL_WARNING
 export HOMEBREW_SYSTEM_CURL_TOO_OLD
+export HOMEBREW_CURL_SUPPORTS_ETAGS
 export HOMEBREW_GIT
 export HOMEBREW_GIT_WARNING
 export HOMEBREW_MINIMUM_GIT_VERSION
diff --git a/Library/Homebrew/cleanup.rb b/Library/Homebrew/cleanup.rb
index 06af8f62eb..e1fead1cff 100644
--- a/Library/Homebrew/cleanup.rb
+++ b/Library/Homebrew/cleanup.rb
@@ -662,14 +662,15 @@ module Homebrew
       api_internal = cache/"api/internal"
       api_package_files = if scrub? && api_internal.directory?
         current_api_package_basename = Homebrew::API::Internal.cached_packages_json_file_path.basename.to_s
-        # Keep only the current OS's envelope and its `.payload` and
-        # `.payload.index` sidecars and scrub the rest, including orphaned
-        # sidecars and temp files.
+        # Keep only the current OS's envelope and its sidecars and scrub the
+        # rest, including orphaned sidecars and temp files.
         # Keep in sync with the previous-OS-version removal in cmd/update.sh.
         kept_basenames = [
           current_api_package_basename,
           "#{current_api_package_basename}.payload",
           "#{current_api_package_basename}.payload.index",
+          "#{current_api_package_basename}.etag",
+          "#{current_api_package_basename}.etag.url",
         ]
         api_internal.glob("packages.*.jws.json*").reject do |path|
           kept_basenames.include?(path.basename.to_s)
diff --git a/Library/Homebrew/cmd/update.sh b/Library/Homebrew/cmd/update.sh
index 51d027603b..534142fa9c 100644
--- a/Library/Homebrew/cmd/update.sh
+++ b/Library/Homebrew/cmd/update.sh
@@ -9,7 +9,7 @@
 # HOMEBREW_CASK_REPOSITORY, HOMEBREW_CELLAR, HOMEBREW_CORE_DEFAULT_GIT_REMOTE, HOMEBREW_CORE_GIT_REMOTE,
 # HOMEBREW_CORE_REPOSITORY, HOMEBREW_CURL, HOMEBREW_DEV_CMD_RUN, HOMEBREW_FORCE_BREWED_CA_CERTIFICATES,
 # HOMEBREW_FORCE_BREWED_CURL, HOMEBREW_FORCE_BREWED_GIT,
-# HOMEBREW_SYSTEM_CURL_TOO_OLD, HOMEBREW_USER_AGENT_CURL are set by brew.sh
+# HOMEBREW_CURL_SUPPORTS_ETAGS, HOMEBREW_SYSTEM_CURL_TOO_OLD, HOMEBREW_USER_AGENT_CURL are set by brew.sh
 # shellcheck disable=SC2154
 source "${HOMEBREW_LIBRARY}/Homebrew/utils/cmd.sh"
 source "${HOMEBREW_LIBRARY}/Homebrew/utils/api.sh"
@@ -409,6 +409,7 @@ api_curl_download() {
     --fail --compressed --silent \
     --speed-limit "${HOMEBREW_CURL_SPEED_LIMIT}" --speed-time "${HOMEBREW_CURL_SPEED_TIME}" \
     --location --remote-time --output "${cache_path}" \
+    --write-out "%{http_code}" \
     "$@" \
     --user-agent "${HOMEBREW_USER_AGENT_CURL}" \
     "${json_url}"
@@ -423,6 +424,15 @@ fetch_api_file() {
   local cache_path="${api_cache}/${filename}"
   mkdir -p "$(dirname "${cache_path}")"
 
+  # Match DownloadLock in Homebrew::API.fetch_json_api_file (api.rb).
+  local lock_path="${HOMEBREW_PREFIX}/var/homebrew/locks/${cache_path##*/}.download.lock"
+  if ! { mkdir -p "${lock_path%/*}" && exec 201>"${lock_path}"; } || ! _create_lock 201 "API download ${filename}"
+  then
+    echo "Cannot lock ${filename} for download!" >>"${update_failed_file}"
+    exec 201>&-
+    return 1
+  fi
+
   if [[ "${filename}" == "formula.jws.json" ]] || [[ "${filename}" == "internal/packages.$(bottle_tag).jws.json" ]]
   then
     local is_formula_file=1
@@ -443,28 +453,68 @@ fetch_api_file() {
     echo "Checking if we need to fetch ${filename}..."
   fi
 
-  local arg curl_exit_code json_url last_json_url
-  local -a time_cond
+  local etag_path="${cache_path}.etag"
+  local new_etag_path="${cache_path}.etag.new"
+  local new_cache_path="${cache_path}.new"
+  local arg compare_etag_path curl_exit_code http_code json_url last_json_url
+  local -a etag_args conditional_args
   while read -r json_url
   do
-    time_cond=()
-    while read -r arg
-    do
-      time_cond+=("${arg}")
-    done < <(api_time_cond_args "${cache_path}")
-    api_curl_download "${json_url}" "${cache_path}" "${time_cond[@]}"
+    # Keep conditional requests in sync with Homebrew::API.fetch_json_api_file in api.rb.
+    compare_etag_path=""
+    etag_args=()
+    if [[ "${HOMEBREW_CURL_SUPPORTS_ETAGS}" == "1" && -z "${last_json_url}" ]]
+    then
+      compare_etag_path="${etag_path}"
+      etag_args=(--etag-save "${new_etag_path}")
+    fi
+    conditional_args=()
+    if [[ -z "${last_json_url}" ]]
+    then
+      while read -r arg
+      do
+        conditional_args+=("${arg}")
+      done < <(api_conditional_args "${cache_path}" "${compare_etag_path}" "${json_url}")
+    fi
+    rm -f "${new_cache_path}" "${new_etag_path}"
+    http_code="$(api_curl_download "${json_url}" "${new_cache_path}" "${etag_args[@]}" "${conditional_args[@]}")"
     curl_exit_code=$?
     # A conditional request can fail with a receive error (curl exit code 56) when
     # an unconditional request for the same URL succeeds, so retry exactly once.
-    if [[ ${curl_exit_code} -eq 56 ]] && [[ ${#time_cond[@]} -gt 0 ]]
+    if [[ ${curl_exit_code} -eq 56 ]] && [[ ${#conditional_args[@]} -gt 0 ]]
     then
-      api_curl_download "${json_url}" "${cache_path}"
+      rm -f "${new_cache_path}" "${new_etag_path}"
+      http_code="$(api_curl_download "${json_url}" "${new_cache_path}" "${etag_args[@]}")"
       curl_exit_code=$?
     fi
     last_json_url="${json_url}"
     [[ ${curl_exit_code} -eq 0 ]] && break
   done < <(api_urls "${filename}")
 
+  if [[ ${curl_exit_code} -eq 0 ]]
+  then
+    if [[ -f "${new_cache_path}" ]]
+    then
+      rm -f "${etag_path}" "${etag_path}.url"
+      if ! mv -f "${new_cache_path}" "${cache_path}"
+      then
+        echo "Failed to replace ${cache_path}!" >>"${update_failed_file}"
+        rm -f "${new_cache_path}" "${new_etag_path}"
+        exec 201>&-
+        return 1
+      fi
+    fi
+    if [[ -s "${new_etag_path}" ]]
+    then
+      mv -f "${new_etag_path}" "${etag_path}"
+      printf '%s' "${last_json_url}" >"${etag_path}.url"
+    elif [[ "${http_code}" != "304" ]]
+    then
+      rm -f "${etag_path}" "${etag_path}.url"
+    fi
+  fi
+  rm -f "${new_cache_path}" "${new_etag_path}"
+
   if [[ -n ${is_formula_file} ]] && [[ -f "${api_cache}/formula_names.txt" ]]
   then
     mv -f "${api_cache}/formula_names.txt" "${api_cache}/formula_names.before.txt"
@@ -496,6 +546,7 @@ fetch_api_file() {
   else
     echo "Failed to download ${last_json_url}!" >>"${update_failed_file}"
   fi
+  exec 201>&-
 }
 
 homebrew-update() {
@@ -1070,7 +1121,7 @@ EOS
     rm -f "${HOMEBREW_CACHE}"/api/internal/formula.*.jws.json
     rm -f "${HOMEBREW_CACHE}"/api/internal/cask.*.jws.json
 
-    # Remove API files (and their `.payload` and `.payload.index` sidecars)
+    # Remove API files (and their sidecars)
     # from previous OS versions, keeping the current OS's so `brew
     # update-report`'s API data load stays or becomes prewarmed. Keep in
     # sync with `cache_files` in Library/Homebrew/cleanup.rb.
@@ -1080,6 +1131,8 @@ EOS
         "${HOMEBREW_CACHE}/api/internal/packages.$(bottle_tag).jws.json") ;;
         "${HOMEBREW_CACHE}/api/internal/packages.$(bottle_tag).jws.json.payload") ;;
         "${HOMEBREW_CACHE}/api/internal/packages.$(bottle_tag).jws.json.payload.index") ;;
+        "${HOMEBREW_CACHE}/api/internal/packages.$(bottle_tag).jws.json.etag") ;;
+        "${HOMEBREW_CACHE}/api/internal/packages.$(bottle_tag).jws.json.etag.url") ;;
         *) rm -f "${f}" ;;
       esac
     done
diff --git a/Library/Homebrew/global.rb b/Library/Homebrew/global.rb
index 5a8b0014a7..a243110b52 100644
--- a/Library/Homebrew/global.rb
+++ b/Library/Homebrew/global.rb
@@ -25,6 +25,7 @@ HOMEBREW_PROCESSOR = T.let(ENV.fetch("HOMEBREW_PROCESSOR").freeze, String)
 HOMEBREW_PHYSICAL_PROCESSOR = T.let(ENV.fetch("HOMEBREW_PHYSICAL_PROCESSOR").freeze, String)
 
 HOMEBREW_BREWED_CURL_PATH = Pathname.new(ENV.fetch("HOMEBREW_BREWED_CURL_PATH")).freeze
+HOMEBREW_CURL_SUPPORTS_ETAGS = T.let(ENV["HOMEBREW_CURL_SUPPORTS_ETAGS"] == "1", T::Boolean)
 HOMEBREW_USER_AGENT_CURL = T.let(ENV.fetch("HOMEBREW_USER_AGENT_CURL").freeze, String)
 HOMEBREW_USER_AGENT_FAKE_SAFARI =
   # Don't update this beyond 10.15.7 until Safari actually updates their
diff --git a/Library/Homebrew/test/api_spec.rb b/Library/Homebrew/test/api_spec.rb
index bdb93d813f..3111ee8263 100644
--- a/Library/Homebrew/test/api_spec.rb
+++ b/Library/Homebrew/test/api_spec.rb
@@ -2,6 +2,7 @@
 # frozen_string_literal: true
 
 require "api"
+require "lock_file"
 require "openssl"
 
 RSpec.describe Homebrew::API do
@@ -103,6 +104,209 @@ RSpec.describe Homebrew::API do
       expect(target.mtime.to_i).to eq stale_mtime.to_i
     end
 
+    context "with a stale cached file" do
+      let(:target) { cache_dir/"bar.json" }
+      let(:etag_path) { Pathname("#{target}.etag") }
+      let(:etag_url_path) { Pathname("#{etag_path}.url") }
+      let(:new_etag_path) { Pathname("#{target}.etag.new") }
+      let(:curl_args) { [] }
+
+      before do
+        stub_const("HOMEBREW_CURL_SUPPORTS_ETAGS", true)
+        target.write json
+        FileUtils.touch(target, mtime: Time.now - 7200)
+        etag_url_path.write "#{Homebrew::EnvConfig.api_domain}/bar.json"
+      end
+
+      def stub_curl(status:, etag: "")
+        allow(Utils::Curl).to receive(:curl_download) do |*args, to:, **|
+          curl_args.replace(args)
+          File.write(args.fetch(args.index("--etag-save") + 1), etag) if etag && args.include?("--etag-save")
+          to.write json if status == 200
+          SystemCommand::Result.new(["curl"], [[:stdout, status.to_s]],
+                                    instance_double(Process::Status, success?: true, exitstatus: 0),
+                                    secrets: [])
+        end
+      end
+
+      def fetch
+        described_class.fetch_json_api_file("bar.json", target:, stale_seconds: 3600)
+      end
+
+      it "sends the saved ETag instead of the cache mtime and stores the new one" do
+        etag_path.write 'W/"old"'
+        stub_curl(status: 200, etag: 'W/"new"')
+        fetch
+        expect(curl_args).to include("--etag-compare", etag_path.to_s)
+        expect(curl_args).not_to include("--time-cond")
+        expect(etag_path.read).to eq 'W/"new"'
+      end
+
+      it "downloads a fresh body and saves the ETag when none is saved" do
+        etag_url_path.unlink
+        stub_curl(status: 200, etag: 'W/"new"')
+        fetch
+        expect([curl_args.grep(/--(?:time-cond|etag-compare)/), etag_path.read, etag_url_path.read])
+          .to eq [[], 'W/"new"', "#{Homebrew::EnvConfig.api_domain}/bar.json"]
+      end
+
+      it "downloads a fresh body when the saved ETag is empty" do
+        etag_path.write ""
+        stub_curl(status: 200, etag: 'W/"new"')
+        fetch
+        expect(curl_args).not_to include("--time-cond", "--etag-compare")
+      end
+
+      it "keeps the saved ETag, refreshes the mtime and leaves no temp file on a 304" do
+        etag_path.write 'W/"old"'
+        stub_curl(status: 304)
+        fetch
+        expect(etag_path.read).to eq 'W/"old"'
+        expect(new_etag_path).not_to exist
+        expect(target.mtime).to be > Time.now - 60
+      end
+
+      it "ignores a leftover temp ETag from an earlier run" do
+        etag_path.write 'W/"old"'
+        new_etag_path.write 'W/"leftover"'
+        stub_curl(status: 304, etag: nil)
+        fetch
+        expect(etag_path.read).to eq 'W/"old"'
+      end
+
+      it "preserves an in-progress ETag when using a fresh cached file" do
+        FileUtils.touch(target)
+        new_etag_path.write 'W/"new"'
+        fetch
+        expect(new_etag_path).to exist
+      end
+
+      test_each([false, true]) do |corrupt|
+        it "holds the shared download lock when refreshing a #{corrupt ? "corrupt fresh" : "stale"} cache" do
+          if corrupt
+            target.write json_invalid
+            FileUtils.touch(target)
+          end
+          lock = DownloadLock.new(target)
+          allow(Utils::Curl).to receive(:curl_download) do |*_, to:, **|
+            expect { lock.lock }.to raise_error(OperationInProgressError)
+            to.write json
+          end
+          fetch
+        ensure
+          lock&.unlock
+        end
+      end
+
+      it "removes the saved ETag when a 200 carries none" do
+        etag_path.write 'W/"old"'
+        stub_curl(status: 200)
+        fetch
+        expect(etag_path).not_to exist
+      end
+
+      it "keeps the saved ETag when the download fails" do
+        etag_path.write 'W/"old"'
+        allow(Utils::Curl).to receive(:curl_download).and_raise(ErrorDuringExecution.new(["curl"], status: 1))
+        expect { fetch }.to output(/falling back to cached version/).to_stderr
+        expect(etag_path.read).to eq 'W/"old"'
+      end
+
+      it "preserves the cached body, ETag and mtime after a partial download" do
+        etag_path.write 'W/"old"'
+        mtime = target.mtime
+        allow(described_class).to receive(:opoo)
+        allow(Utils::Curl).to receive(:curl_download) do |*args, to:, **|
+          to.write '{"partial":'
+          File.write(args.fetch(args.index("--etag-save") + 1), 'W/"partial"')
+          raise ErrorDuringExecution.new(["curl"], status: 18)
+        end
+
+        fetch
+
+        expect([target.read, etag_path.read, target.mtime, new_etag_path.exist?, Pathname("#{target}.new").exist?])
+          .to eq [json, 'W/"old"', mtime, false, false]
+      end
+
+      test_each([false, true]) do |supports_etags|
+        it "retries a receive error without conditions with ETag support #{supports_etags}" do
+          stub_const("HOMEBREW_CURL_SUPPORTS_ETAGS", supports_etags)
+          etag_path.write 'W/"old"'
+          requests = []
+          allow(Utils::Curl).to receive(:curl_download) do |*args, to:, **|
+            requests << args
+            if args.include?("--etag-compare") || args.include?("--time-cond")
+              raise ErrorDuringExecution.new(["curl"], status: 56)
+            end
+
+            to.write '{"foo":"updated"}'
+            SystemCommand::Result.new(["curl"], [[:stdout, "200"]],
+                                      instance_double(Process::Status, success?: true, exitstatus: 0),
+                                      secrets: [])
+          end
+
+          fetch
+
+          expect([requests.length, target.read, etag_path.exist?]).to eq [2, '{"foo":"updated"}', false]
+        end
+      end
+
+      it "ignores an ETag saved for a different API domain" do
+        etag_path.write 'W/"old"'
+        etag_url_path.write "https://mirror.example/api/bar.json"
+        stub_curl(status: 200, etag: 'W/"new"')
+        fetch
+        expect(curl_args).not_to include("--time-cond", "--etag-compare")
+      end
+
+      it "ignores an ETag whose origin is unknown" do
+        etag_path.write 'W/"old"'
+        etag_url_path.unlink
+        stub_curl(status: 200, etag: 'W/"new"')
+        fetch
+        expect(curl_args).not_to include("--time-cond", "--etag-compare")
+      end
+
+      it "keeps the saved ETag after a 304 with curl older than 7.68.0" do
+        stub_const("HOMEBREW_CURL_SUPPORTS_ETAGS", false)
+        etag_path.write 'W/"old"'
+        stub_curl(status: 304)
+        fetch
+        expect(etag_path).to exist
+      end
+
+      it "uses only --time-cond and removes the saved ETag after a 200 when curl is older than 7.68.0" do
+        stub_const("HOMEBREW_CURL_SUPPORTS_ETAGS", false)
+        etag_path.write 'W/"old"'
+        stub_curl(status: 200)
+        fetch
+        expect(curl_args).to include("--time-cond", target.to_s)
+        expect(curl_args.grep(/etag/)).to be_empty
+        expect(etag_path).not_to exist
+      end
+
+      it "downloads unconditionally from the fallback API domain" do
+        allow(Homebrew::EnvConfig).to receive(:api_domain).and_return("https://example.invalid/api")
+        etag_url_path.write "https://example.invalid/api/bar.json"
+        etag_path.write 'W/"old"'
+        requests = []
+        allow(Utils::Curl).to receive(:curl_download) do |*args, to:, **|
+          requests << args.dup
+          raise ErrorDuringExecution.new(["curl"], status: 1) if requests.length == 1
+
+          to.write json
+          SystemCommand::Result.new(["curl"], [[:stdout, "200"]],
+                                    instance_double(Process::Status, success?: true, exitstatus: 0),
+                                    secrets: [])
+        end
+        fetch
+        expect(requests.fetch(0)).to include("--etag-compare", "--etag-save")
+        expect(requests.fetch(1)).not_to include("--time-cond")
+        expect(requests.fetch(1).grep(/etag/)).to be_empty
+        expect(etag_path).not_to exist
+      end
+    end
+
     it "refreshes the cache mtime when a fallback to the default API domain succeeds" do
       target = cache_dir/"bar.json"
       target.write json
diff --git a/Library/Homebrew/test/cleanup_spec.rb b/Library/Homebrew/test/cleanup_spec.rb
index 3b09bf0345..3fb323e558 100644
--- a/Library/Homebrew/test/cleanup_spec.rb
+++ b/Library/Homebrew/test/cleanup_spec.rb
@@ -874,6 +874,8 @@ RSpec.describe Homebrew::Cleanup do
         api_internal/current_basename,
         api_internal/"#{current_basename}.payload",
         api_internal/"#{current_basename}.payload.index",
+        api_internal/"#{current_basename}.etag",
+        api_internal/"#{current_basename}.etag.url",
       ]
       scrubbed_files = [
         api_internal/"packages.stale.jws.json.payload",
@@ -887,7 +889,7 @@ RSpec.describe Homebrew::Cleanup do
 
       described_class.new(scrub: true, cache:).cleanup_cache
 
-      expect((kept_files + scrubbed_files).map(&:exist?)).to eq([true, true, true, false, false, false])
+      expect((kept_files + scrubbed_files).map(&:exist?)).to eq([true, true, true, true, true, false, false, false])
     end
 
     it "cleans up API source files and symlinks at any depth without cleaning directories" do
diff --git a/Library/Homebrew/test/cmd/update_spec.rb b/Library/Homebrew/test/cmd/update_spec.rb
index 05cdb1f0d4..8046d8f143 100644
--- a/Library/Homebrew/test/cmd/update_spec.rb
+++ b/Library/Homebrew/test/cmd/update_spec.rb
@@ -2,6 +2,7 @@
 # frozen_string_literal: true
 
 require "open3"
+require "lock_file"
 
 require "cmd/shared_examples/args_parse"
 require "cmd/update"
@@ -32,10 +33,11 @@ RSpec.describe Homebrew::Cmd::Update do
   def setup_update_utils
     (test_root/"Library/Homebrew/utils").mkpath
     FileUtils.ln_s repository_root/"Library/Homebrew/utils.sh", test_root/"Library/Homebrew/utils.sh"
-    %w[api cmd executables formatter lock tty].each do |name|
+    %w[api cmd curl executables formatter lock tty].each do |name|
       FileUtils.ln_s repository_root/"Library/Homebrew/utils/#{name}.sh",
                      test_root/"Library/Homebrew/utils/#{name}.sh"
     end
+    FileUtils.ln_s repository_root/"Library/Homebrew/utils/lock_sh", test_root/"Library/Homebrew/utils/lock_sh"
   end
 
   it "stops Git from finding the enclosing repository" do
@@ -138,13 +140,15 @@ RSpec.describe Homebrew::Cmd::Update do
         fetch_api_file formula.jws.json "#{update_failed_file}"
       SH
       {
-        "HOMEBREW_API_DEFAULT_DOMAIN" => "https://formulae.example/api",
-        "HOMEBREW_API_DOMAIN"         => nil,
-        "HOMEBREW_CACHE"              => (test_root/"cache").to_s,
-        "HOMEBREW_CURL_SPEED_LIMIT"   => "100",
-        "HOMEBREW_CURL_SPEED_TIME"    => "5",
-        "HOMEBREW_LIBRARY"            => (test_root/"Library").to_s,
-        "HOMEBREW_USER_AGENT_CURL"    => "Homebrew/test",
+        "HOMEBREW_API_DEFAULT_DOMAIN"  => "https://formulae.example/api",
+        "HOMEBREW_API_DOMAIN"          => nil,
+        "HOMEBREW_CACHE"               => (test_root/"cache").to_s,
+        "HOMEBREW_CURL_SPEED_LIMIT"    => "100",
+        "HOMEBREW_CURL_SPEED_TIME"     => "5",
+        "HOMEBREW_CURL_SUPPORTS_ETAGS" => nil,
+        "HOMEBREW_LIBRARY"             => (test_root/"Library").to_s,
+        "HOMEBREW_PREFIX"              => HOMEBREW_PREFIX.to_s,
+        "HOMEBREW_USER_AGENT_CURL"     => "Homebrew/test",
       },
     )
 
@@ -153,6 +157,182 @@ RSpec.describe Homebrew::Cmd::Update do
     )
   end
 
+  describe "API ETag revalidation" do
+    let(:cache_path) { test_root/"cache/api/formula.jws.json" }
+    let(:etag_path) { test_root/"cache/api/formula.jws.json.etag" }
+    let(:new_etag_path) { test_root/"cache/api/formula.jws.json.etag.new" }
+    let(:args_file) { test_root/"curl-args.txt" }
+    let(:update_failed_file) { test_root/"update_failed.txt" }
+
+    def fetch_with_curl_response(status_code:, body: "", etag: "", saved_etag: '"old"', curl_supports_etags: "1",
+                                 api_domain: nil, saved_url: nil, exit_code: 0)
+      setup_update_utils
+      cache_path.dirname.mkpath
+      cache_path.write "cached"
+      args_file.write ""
+      etag_path.write saved_etag if saved_etag
+      Pathname("#{etag_path}.url").write saved_url || "#{api_domain || "https://formulae.example/api"}/formula.jws.json"
+
+      _stdout, stderr, status = run_update_shell(
+        <<~SH,
+          source "#{update_script}"
+          curl() {
+            printf '%s\\n' "$@" >> "#{args_file}"
+            [[ "${!#}" == "#{api_domain}/"* ]] && return 22
+            local output etag_save
+            while [[ $# -gt 0 ]]
+            do
+              case "$1" in
+                --output) output="$2"; shift ;;
+                --etag-save) etag_save="$2"; shift ;;
+              esac
+              shift
+            done
+            [[ -n "${etag_save}" ]] && printf '%s' '#{etag}' > "${etag_save}"
+            [[ -n '#{body}' ]] && printf '%s' '#{body}' > "${output}"
+            printf '%s' '#{status_code}'
+            return #{exit_code}
+          }
+          fetch_api_file formula.jws.json "#{update_failed_file}"
+        SH
+        {
+          "HOMEBREW_API_DEFAULT_DOMAIN"  => "https://formulae.example/api",
+          "HOMEBREW_API_DOMAIN"          => api_domain,
+          "HOMEBREW_CACHE"               => (test_root/"cache").to_s,
+          "HOMEBREW_CURL_SPEED_LIMIT"    => "100",
+          "HOMEBREW_CURL_SPEED_TIME"     => "5",
+          "HOMEBREW_CURL_SUPPORTS_ETAGS" => curl_supports_etags,
+          "HOMEBREW_LIBRARY"             => (test_root/"Library").to_s,
+          "HOMEBREW_PREFIX"              => HOMEBREW_PREFIX.to_s,
+          "HOMEBREW_USER_AGENT_CURL"     => "Homebrew/test",
+        },
+      )
+
+      conditional_flags = %w[--etag-compare --etag-save --time-cond]
+      {
+        success:   status.success? && stderr.empty? && !update_failed_file.exist?,
+        curl_args: args_file.read.lines(chomp: true).each_cons(2).filter_map do |flag, value|
+          [flag, value] if conditional_flags.include?(flag)
+        end,
+        body:      cache_path.read,
+        etag:      (etag_path.read if etag_path.exist?),
+        temp_etag: new_etag_path.exist?,
+      }
+    end
+
+    it "revalidates with the saved ETag and saves the new one with the new body" do
+      expect(fetch_with_curl_response(status_code: 200, body: "fresh", etag: '"new"')).to eq(
+        success:   true,
+        curl_args: [
+          ["--etag-save", new_etag_path.to_s],
+          ["--etag-compare", etag_path.to_s],
+        ],
+        body:      "fresh",
+        etag:      '"new"',
+        temp_etag: false,
+      )
+    end
+
+    it "keeps the saved ETag when the server answers 304" do
+      expect(fetch_with_curl_response(status_code: 304).except(:curl_args)).to eq(
+        success: true, body: "cached", etag: '"old"', temp_etag: false,
+      )
+    end
+
+    it "removes the saved ETag after a 200 that carries none" do
+      expect(fetch_with_curl_response(status_code: 200, body: "fresh").except(:curl_args)).to eq(
+        success: true, body: "fresh", etag: nil, temp_etag: false,
+      )
+    end
+
+    it "downloads a fresh body without a saved ETag" do
+      expect(fetch_with_curl_response(status_code: 200, body: "fresh", etag: '"new"', saved_etag: nil)).to eq(
+        success:   true,
+        curl_args: [["--etag-save", new_etag_path.to_s]],
+        body:      "fresh",
+        etag:      '"new"',
+        temp_etag: false,
+      )
+    end
+
+    it "uses only the time condition when curl is older than 7.68.0" do
+      expect(fetch_with_curl_response(status_code: 200, body: "fresh", curl_supports_etags: nil)).to eq(
+        success:   true,
+        curl_args: [["--time-cond", cache_path.to_s]],
+        body:      "fresh",
+        etag:      nil,
+        temp_etag: false,
+      )
+    end
+
+    it "downloads unconditionally from the fallback API domain" do
+      result = fetch_with_curl_response(status_code: 200, body: "fresh", etag: '"new"',
+                                        api_domain: "https://mirror.example/api")
+      expect(result).to eq(
+        success:   true,
+        curl_args: [
+          ["--etag-save", new_etag_path.to_s],
+          ["--etag-compare", etag_path.to_s],
+        ],
+        body:      "fresh",
+        etag:      nil,
+        temp_etag: false,
+      )
+    end
+
+    it "preserves the cached body and ETag after a partial download" do
+      expect(fetch_with_curl_response(status_code: 200, body: "partial", etag: '"partial"', exit_code: 18)
+        .except(:curl_args)).to eq(success: false, body: "cached", etag: '"old"', temp_etag: false)
+    end
+
+    it "ignores an ETag saved for a different API domain" do
+      expect(fetch_with_curl_response(status_code: 200, body: "fresh", etag: '"new"',
+                                      saved_url: "https://mirror.example/api/formula.jws.json")[:curl_args])
+        .to eq [["--etag-save", new_etag_path.to_s]]
+      expect(Pathname("#{etag_path}.url").read).to eq "https://formulae.example/api/formula.jws.json"
+    end
+
+    it "keeps the saved ETag after a 304 with curl older than 7.68.0" do
+      expect(fetch_with_curl_response(status_code: 304, curl_supports_etags: nil).except(:curl_args)).to eq(
+        success: true, body: "cached", etag: '"old"', temp_etag: false,
+      )
+    end
+
+    it "does not overwrite a cache being refreshed by Ruby" do
+      DownloadLock.new(cache_path).with_lock do
+        expect(fetch_with_curl_response(status_code: 200, body: "fresh", etag: '"new"').except(:curl_args)).to eq(
+          success: false, body: "cached", etag: '"old"', temp_etag: false,
+        )
+      end
+    end
+  end
+
+  it "redetects curl ETag support when the flag is inherited" do
+    setup_update_utils
+
+    stdout, stderr, status = run_update_shell(
+      <<~SH,
+        source "#{test_root}/Library/Homebrew/utils.sh"
+        source "#{test_root}/Library/Homebrew/utils/curl.sh"
+        HOMEBREW_CURL=curl
+        for version in 7.41.0 7.67.0 7.68.0 8.7.1
+        do
+          curl() { echo "curl ${version} (x86_64-apple-darwin) libcurl/${version}"; }
+          setup-user-agents
+          [[ -n "${HOMEBREW_CURL_SUPPORTS_ETAGS}" ]] && echo "${version}: yes" || echo "${version}: no"
+        done
+      SH
+      {
+        "HOMEBREW_CURL_SUPPORTS_ETAGS" => "1",
+        "HOMEBREW_LIBRARY"             => (test_root/"Library").to_s,
+      },
+    )
+
+    expect([status.success?, stderr, stdout]).to eq(
+      [true, "", "7.41.0: no\n7.67.0: no\n7.68.0: yes\n8.7.1: yes\n"],
+    )
+  end
+
   it "passes all arguments through to delegated upgrades" do
     args_file = test_root/"brew-args.txt"
     brew_wrapper = test_root/"brew-wrapper"
@@ -241,6 +421,58 @@ RSpec.describe Homebrew::Cmd::Update do
     expect(args_file.read).to eq("update-report\n--auto-update\n")
   end
 
+  it "keeps the current API file's saved ETag when removing other OS versions' API files" do
+    api_internal = test_root/"cache/api/internal"
+    setup_update_utils
+    api_internal.mkpath
+    (test_root/"repository").mkpath
+    %w[
+      packages.arm64_tahoe.jws.json
+      packages.arm64_tahoe.jws.json.etag
+      packages.arm64_tahoe.jws.json.etag.url
+      packages.arm64_sequoia.jws.json
+      packages.arm64_sequoia.jws.json.etag
+      packages.arm64_sequoia.jws.json.etag.url
+    ].each { |name| (api_internal/name).write "x" }
+
+    _stdout, stderr, status = run_update_shell(
+      <<~SH,
+        source "#{update_script}"
+        bottle_tag() { echo arm64_tahoe; }
+        brew() { :; }
+        fetch_api_file() { :; }
+        git_init_if_necessary() { :; }
+        git() {
+          [[ "$1" == "--version" ]] && return 0
+          return 1
+        }
+        lock() { :; }
+        odie() { echo "Error: $*" >&2; exit 1; }
+        ohai() { :; }
+        onoe() { echo "Error: $*" >&2; }
+        safe_cd() { cd "$1" >/dev/null || exit 1; }
+        setup_ca_certificates() { :; }
+        setup_curl() { :; }
+        setup_git() { :; }
+        homebrew-update --auto-update
+      SH
+      {
+        "HOMEBREW_BREW_GIT_REMOTE"     => "https://github.com/Homebrew/brew",
+        "HOMEBREW_CACHE"               => (test_root/"cache").to_s,
+        "HOMEBREW_CELLAR"              => (test_root/"cellar").to_s,
+        "HOMEBREW_LIBRARY"             => (test_root/"Library").to_s,
+        "HOMEBREW_NO_INSTALL_FROM_API" => nil,
+        "HOMEBREW_PREFIX"              => (test_root/"prefix").to_s,
+        "HOMEBREW_REPOSITORY"          => (test_root/"repository").to_s,
+      },
+    )
+
+    expect([status.success?, stderr]).to eq([true, ""])
+    expect(api_internal.children.map { |path| path.basename.to_s }.sort).to eq(
+      %w[packages.arm64_tahoe.jws.json packages.arm64_tahoe.jws.json.etag packages.arm64_tahoe.jws.json.etag.url],
+    )
+  end
+
   it "does not query redirected remote metadata for no-op tap updates" do
     args_file = test_root/"brew-args.txt"
     fetches_file = test_root/"fetches.txt"
diff --git a/Library/Homebrew/utils/api.sh b/Library/Homebrew/utils/api.sh
index f84640bbce..cd5d5db60a 100644
--- a/Library/Homebrew/utils/api.sh
+++ b/Library/Homebrew/utils/api.sh
@@ -25,12 +25,24 @@ api_curlrc_args() {
   fi
 }
 
-api_time_cond_args() {
+# Keep conditional requests in sync with Homebrew::API.fetch_json_api_file in api.rb.
+api_conditional_args() {
   local cache_path="$1"
+  local etag_path="$2"
+  local json_url="$3"
 
   if [[ -s "${cache_path}" ]]
   then
-    echo "--time-cond"
-    echo "${cache_path}"
+    if [[ -n "${etag_path}" ]]
+    then
+      if [[ -s "${etag_path}" && -f "${etag_path}.url" && "$(<"${etag_path}.url")" == "${json_url}" ]]
+      then
+        echo "--etag-compare"
+        echo "${etag_path}"
+      fi
+    else
+      echo "--time-cond"
+      echo "${cache_path}"
+    fi
   fi
 }
diff --git a/Library/Homebrew/utils/curl.sh b/Library/Homebrew/utils/curl.sh
index 182b4c382d..a0c08a9c03 100644
--- a/Library/Homebrew/utils/curl.sh
+++ b/Library/Homebrew/utils/curl.sh
@@ -73,4 +73,10 @@ setup-user-agents() {
   curl_version_output="$("${HOMEBREW_CURL}" --version 2>/dev/null)"
   curl_name_and_version="${curl_version_output%% (*}"
   HOMEBREW_USER_AGENT_CURL="${HOMEBREW_USER_AGENT} ${curl_name_and_version// //}"
+
+  unset HOMEBREW_CURL_SUPPORTS_ETAGS
+  if [[ "$(numeric "${curl_name_and_version##* }")" -ge "$(numeric "7.68.0")" ]]
+  then
+    HOMEBREW_CURL_SUPPORTS_ETAGS=1
+  fi
 }
diff --git a/docs/Tips-and-Tricks.md b/docs/Tips-and-Tricks.md
index 16dcc846dc..4a09901225 100644
--- a/docs/Tips-and-Tricks.md
+++ b/docs/Tips-and-Tricks.md
@@ -227,6 +227,10 @@ sudo chown -R "${USER}" /usr/local/etc
 
 You can configure Homebrew to retrieve bottles and the JSON API from a caching proxy or mirror.
 
+API mirrors should preserve `ETag` headers: with curl 7.68.0 or later, Homebrew revalidates cached JSON using the ETag saved for that URL.
+Without a matching validator or when falling back to another API domain, Homebrew downloads a fresh response.
+Older curl versions use time-based checks on the configured API domain.
+
 For example, in JFrog's Artifactory, accessible at `https://artifacts.example.com`:
 
 - For the bottles, configure a new "remote" repository with `homebrew` as the "repository key" and `https://ghcr.io` (GitHub Packages) as the URL.