diff --git a/.gitignore b/.gitignore
index acd44d8d8064a7a9f1560c24d2f5ff6b675e8096..db6dcecb45535b9e6200d772df451e6bcf90b522 100644
--- a/.gitignore
+++ b/.gitignore
@@ -10,7 +10,14 @@
 /.idea
 /cmake-build-*
 
-/.vs
+/.vs/
+/.vscode/
+
+# clangd cache
+/.cache/
+
+# build directories (vscode-cmake-tools, user-defined, ...)
+/build*/
 
 /docs/mkdocs/docs/examples/
 /docs/mkdocs/docs/__pycache__/
@@ -19,3 +26,8 @@
 /docs/docset/JSON_for_Modern_C++.docset/
 /docs/docset/JSON_for_Modern_C++.tgz
 /docs/mkdocs/docs/images/json.gif
+
+# serve_header
+/serve_header.yml
+/localhost.pem
+/localhost-key.pem
diff --git a/Makefile b/Makefile
index 28de14dcea915e9214ab490b6daf3a52ca69eeab..ce26524ad908f6720204affb9b128ac5d0e7ace3 100644
--- a/Makefile
+++ b/Makefile
@@ -251,3 +251,10 @@ update_hedley:
 	$(SED) -i '1s/^/#pragma once\n\n/' include/nlohmann/thirdparty/hedley/hedley.hpp
 	$(SED) -i '1s/^/#pragma once\n\n/' include/nlohmann/thirdparty/hedley/hedley_undef.hpp
 	$(MAKE) amalgamate
+
+##########################################################################
+# serve_header.py
+##########################################################################
+
+serve_header:
+	./tools/serve_header/serve_header.py --make $(MAKE)
diff --git a/tools/serve_header/README.md b/tools/serve_header/README.md
new file mode 100644
index 0000000000000000000000000000000000000000..a95ef20e2630a56a0561c83f1bdc4b74ad0acb5e
--- /dev/null
+++ b/tools/serve_header/README.md
@@ -0,0 +1,91 @@
+serve_header.py
+===============
+
+Serves the `single_include/nlohmann/json.hpp` header file over HTTP(S).
+
+The header file is automatically amalgamated on demand.
+
+![serve_header.py demo](demo.png)
+
+## Prerequisites
+
+1. Make sure these Python packages are installed.
+    ```
+    PyYAML
+    watchdog
+    ```
+    (see `tools/serve_header/requirements.txt`)
+
+2. To serve the header over HTTPS (which is required by Compiler Explorer at this time), a certificate is needed.
+   The recommended method for creating a locally-trusted certificate is to use [`mkcert`](https://github.com/FiloSottile/mkcert).
+   - Install the `mkcert` certificate authority into your trust store(s):
+     ```
+     $ mkcert -install
+     ```
+   - Create a certificate for `localhost`:
+     ```
+     $ mkcert localhost
+     ```
+     The command will create two files, `localhost.pem` and `localhost-key.pem`, in the current working directory. It is recommended to create them in the top level or project root directory.
+
+## Usage
+
+`serve_header.py` has a built-in default configuration that will serve the `single_include/nlohmann/json.hpp` header file relative to the top level or project root directory it is homed in.
+The built-in configuration expects the certificate `localhost.pem` and the private key `localhost-key.pem`to be located in the top level or project root directory.
+
+To start serving the `json.hpp` header file at `https://localhost:8443/json.hpp`, run this command from the top level or project root directory:
+```
+$ make serve_header
+```
+
+Open [Compiler Explorer](https://godbolt.org/) and try it out:
+```cpp
+#include <https://localhost:8443/json.hpp>
+using namespace nlohmann;
+
+#include <iostream>
+
+int main() {
+    // these macros are dynamically injected into the header file
+    std::cout << JSON_BUILD_TIME << " (" << JSON_BUILD_COUNT << ")\n";
+
+    return 0;
+}
+```
+
+> `serve_header.py` dynamically injects the macros `JSON_BUILD_COUNT` and `JSON_BUILD_TIME` into the served header file. By comparing build count or time output from the compiled program with the output from `serve_header.py`, one can be reasonably sure the compiled code uses the expected revision of the header file.
+
+## Configuration
+
+`serve_header.py` will try to read a configuration file `serve_header.yml` in the top level or project root directory, and will fall back on built-in defaults if the file cannot be read.
+An annotated example configuration can be found in `tools/serve_header/serve_header.yml.example`.
+
+## Serving `json.hpp` from multiple project directory instances or working trees
+
+`serve_header.py` was designed with the goal of supporting multiple project roots or working trees at the same time.
+The recommended directory structure is shown below but `serve_header.py` can work with other structures as well, including a nested hierarchy.
+```
+json/          ⮜ the parent or web server root directoy
+├── develop/   ⮜ the main git checkout
+│   └── ...
+├── feature1/
+│   └── ...      any number of additional
+├── feature2/  ⮜ working trees (e.g., created
+│   └── ...      with git worktree)
+└── feature3/
+    └── ...
+```
+
+To serve the header of each working tree at `https://localhost:8443/<worktree>/json.hpp`, a configuration file is needed.
+1. Create the file `serve_header.yml` in the top level or project root directory of any working tree:
+    ```yaml
+    root: ..
+    ```
+   By shifting the web server root directory up one level, the `single_include/nlohmann/json.hpp` header files relative to each sibling directory or working tree will be served.
+
+2. Start `serve_header.py` by running this command from the same top level or project root directory the configuration file is located in:
+    ```
+    $ make serve_header
+    ```
+
+`serve_header.py` will automatically detect the addition or removal of working trees anywhere within the configured web server root directory.
diff --git a/tools/serve_header/demo.png b/tools/serve_header/demo.png
new file mode 100644
index 0000000000000000000000000000000000000000..b777516087d820cf8bd22e7a9b98ec9c481fc401
Binary files /dev/null and b/tools/serve_header/demo.png differ
diff --git a/tools/serve_header/requirements.txt b/tools/serve_header/requirements.txt
new file mode 100644
index 0000000000000000000000000000000000000000..d32eed3bd5903bead71f6bb5adcf12200020a3f0
--- /dev/null
+++ b/tools/serve_header/requirements.txt
@@ -0,0 +1,2 @@
+PyYAML==6.0
+watchdog==2.1.7
diff --git a/tools/serve_header/serve_header.py b/tools/serve_header/serve_header.py
new file mode 100755
index 0000000000000000000000000000000000000000..666fa57c3d846c103de8f674defc28c6b35cb700
--- /dev/null
+++ b/tools/serve_header/serve_header.py
@@ -0,0 +1,410 @@
+#!/usr/bin/env python3
+
+import contextlib
+import logging
+import os
+import re
+import shutil
+import sys
+import subprocess
+
+from datetime import datetime, timedelta
+from io import BytesIO
+from threading import Lock, Timer
+
+from watchdog.events import FileSystemEventHandler
+from watchdog.observers import Observer
+
+from http import HTTPStatus
+from http.server import ThreadingHTTPServer, SimpleHTTPRequestHandler
+
+CONFIG_FILE = 'serve_header.yml'
+MAKEFILE = 'Makefile'
+INCLUDE = 'include/nlohmann/'
+SINGLE_INCLUDE = 'single_include/nlohmann/'
+HEADER = 'json.hpp'
+
+DATETIME_FORMAT = '%Y-%m-%d %H:%M:%S'
+
+JSON_VERSION_RE = re.compile(r'\s*#\s*define\s+NLOHMANN_JSON_VERSION_MAJOR\s+')
+
+class ExitHandler(logging.StreamHandler):
+    def __init__(self, level):
+        """."""
+        super().__init__()
+        self.level = level
+
+    def emit(self, record):
+        if record.levelno >= self.level:
+            sys.exit(1)
+
+def is_project_root(test_dir='.'):
+    makefile = os.path.join(test_dir, MAKEFILE)
+    include = os.path.join(test_dir, INCLUDE)
+    single_include = os.path.join(test_dir, SINGLE_INCLUDE)
+
+    return (os.path.exists(makefile)
+            and os.path.isfile(makefile)
+            and os.path.exists(include)
+            and os.path.exists(single_include))
+
+class DirectoryEventBucket:
+    def __init__(self, callback, delay=1.2, threshold=0.8):
+        """."""
+        self.delay = delay
+        self.threshold = timedelta(seconds=threshold)
+        self.callback = callback
+        self.event_dirs = set([])
+        self.timer = None
+        self.lock = Lock()
+
+    def start_timer(self):
+        if self.timer is None:
+            self.timer = Timer(self.delay, self.process_dirs)
+            self.timer.start()
+
+    def process_dirs(self):
+        result_dirs = []
+        event_dirs = set([])
+        with self.lock:
+            self.timer = None
+            while self.event_dirs:
+                time, event_dir = self.event_dirs.pop()
+                delta = datetime.now() - time
+                if delta < self.threshold:
+                    event_dirs.add((time, event_dir))
+                else:
+                    result_dirs.append(event_dir)
+            self.event_dirs = event_dirs
+            if result_dirs:
+                self.callback(os.path.commonpath(result_dirs))
+            if self.event_dirs:
+                self.start_timer()
+
+    def add_dir(self, path):
+        with self.lock:
+            # add path to the set of event_dirs if it is not a sibling of
+            # a directory already in the set
+            if not any(os.path.commonpath([path, event_dir]) == event_dir
+               for (_, event_dir) in self.event_dirs):
+                self.event_dirs.add((datetime.now(), path))
+                if self.timer is None:
+                    self.start_timer()
+
+class WorkTree:
+    make_command = 'make'
+
+    def __init__(self, root_dir, tree_dir):
+        """."""
+        self.root_dir = root_dir
+        self.tree_dir = tree_dir
+        self.rel_dir = os.path.relpath(tree_dir, root_dir)
+        self.name = os.path.basename(tree_dir)
+        self.include_dir = os.path.abspath(os.path.join(tree_dir, INCLUDE))
+        self.header = os.path.abspath(os.path.join(tree_dir, SINGLE_INCLUDE, HEADER))
+        self.rel_header = os.path.relpath(self.header, root_dir)
+        self.dirty = True
+        self.build_count = 0
+        t = os.path.getmtime(self.header)
+        t = datetime.fromtimestamp(t)
+        self.build_time = t.strftime(DATETIME_FORMAT)
+
+    def __hash__(self):
+        """."""
+        return hash((self.tree_dir))
+
+    def __eq__(self, other):
+        """."""
+        if not isinstance(other, type(self)):
+            return NotImplemented
+        return self.tree_dir == other.tree_dir
+
+    def update_dirty(self, path):
+        if self.dirty:
+            return
+
+        path = os.path.abspath(path)
+        if os.path.commonpath([path, self.include_dir]) == self.include_dir:
+            logging.info(f'{self.name}: working tree marked dirty')
+            self.dirty = True
+
+    def amalgamate_header(self):
+        if not self.dirty:
+            return
+
+        mtime = os.path.getmtime(self.header)
+        subprocess.run([WorkTree.make_command, 'amalgamate'], cwd=self.tree_dir,
+                       stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL)
+        if mtime == os.path.getmtime(self.header):
+            logging.info(f'{self.name}: no changes')
+        else:
+            self.build_count += 1
+            self.build_time = datetime.now().strftime(DATETIME_FORMAT)
+            logging.info(f'{self.name}: header amalgamated (build count {self.build_count})')
+
+        self.dirty = False
+
+class WorkTrees(FileSystemEventHandler):
+    def __init__(self, root_dir):
+        """."""
+        super().__init__()
+        self.root_dir = root_dir
+        self.trees = set([])
+        self.tree_lock = Lock()
+        self.scan(root_dir)
+        self.created_bucket = DirectoryEventBucket(self.scan)
+        self.observer = Observer()
+        self.observer.schedule(self, root_dir, recursive=True)
+        self.observer.start()
+
+    def scan(self, base_dir):
+        scan_dirs = set([base_dir])
+        # recursively scan base_dir for working trees
+
+        while scan_dirs:
+            scan_dir = os.path.abspath(scan_dirs.pop())
+            self.scan_tree(scan_dir)
+            try:
+                with os.scandir(scan_dir) as dir_it:
+                    for entry in dir_it:
+                        if entry.is_dir():
+                            scan_dirs.add(entry.path)
+            except FileNotFoundError as e:
+                logging.debug('path disappeared: %s', e)
+
+    def scan_tree(self, scan_dir):
+        if not is_project_root(scan_dir):
+            return
+
+        # skip source trees in build directories
+        # this check could be enhanced
+        if scan_dir.endswith('/_deps/json-src'):
+            return
+
+        tree = WorkTree(self.root_dir, scan_dir)
+        with self.tree_lock:
+            if not tree in self.trees:
+                if tree.name == tree.rel_dir:
+                    logging.info(f'adding working tree {tree.name}')
+                else:
+                    logging.info(f'adding working tree {tree.name} at {tree.rel_dir}')
+                url = os.path.join('/', tree.rel_dir, HEADER)
+                logging.info(f'{tree.name}: serving header at {url}')
+                self.trees.add(tree)
+
+    def rescan(self, path=None):
+        if path is not None:
+            path = os.path.abspath(path)
+        trees = set([])
+        # check if any working trees have been removed
+        with self.tree_lock:
+            while self.trees:
+                tree = self.trees.pop()
+                if ((path is None
+                    or os.path.commonpath([path, tree.tree_dir]) == tree.tree_dir)
+                    and not is_project_root(tree.tree_dir)):
+                    if tree.name == tree.rel_dir:
+                        logging.info(f'removing working tree {tree.name}')
+                    else:
+                        logging.info(f'removing working tree {tree.name} at {tree.rel_dir}')
+                else:
+                    trees.add(tree)
+            self.trees = trees
+
+    def find(self, path):
+        # find working tree for a given header file path
+        path = os.path.abspath(path)
+        with self.tree_lock:
+            for tree in self.trees:
+                if path == tree.header:
+                    return tree
+        return None
+
+    def on_any_event(self, event):
+        logging.debug('%s (is_dir=%s): %s', event.event_type,
+                      event.is_directory, event.src_path)
+        path = os.path.abspath(event.src_path)
+        if event.is_directory:
+            if event.event_type == 'created':
+                # check for new working trees
+                self.created_bucket.add_dir(path)
+            elif event.event_type == 'deleted':
+                # check for deleted working trees
+                self.rescan(path)
+        elif event.event_type == 'closed':
+            with self.tree_lock:
+                for tree in self.trees:
+                    tree.update_dirty(path)
+
+    def stop(self):
+        self.observer.stop()
+        self.observer.join()
+
+class HeaderRequestHandler(SimpleHTTPRequestHandler):
+    def __init__(self, request, client_address, server):
+        """."""
+        self.worktrees = server.worktrees
+        self.worktree = None
+        try:
+            super().__init__(request, client_address, server,
+                             directory=server.worktrees.root_dir)
+        except ConnectionResetError:
+            logging.debug('connection reset by peer')
+
+    def translate_path(self, path):
+        path = os.path.abspath(super().translate_path(path))
+
+        # add single_include/nlohmann into path, if needed
+        header = os.path.join('/', HEADER)
+        header_path = os.path.join('/', SINGLE_INCLUDE, HEADER)
+        if (path.endswith(header)
+            and not path.endswith(header_path)):
+            path = os.path.join(os.path.dirname(path), SINGLE_INCLUDE, HEADER)
+
+        return path
+
+    def send_head(self):
+        # check if the translated path matches a working tree
+        # and fullfill the request; otherwise, send 404
+        path = self.translate_path(self.path)
+        self.worktree = self.worktrees.find(path)
+        if self.worktree is not None:
+            self.worktree.amalgamate_header()
+            logging.info(f'{self.worktree.name}; serving header (build count {self.worktree.build_count})')
+            return super().send_head()
+        logging.info(f'invalid request path: {self.path}')
+        super().send_error(HTTPStatus.NOT_FOUND, 'Not Found')
+        return None
+
+    def send_header(self, keyword, value):
+        # intercept Content-Length header; sent in copyfile later
+        if keyword == 'Content-Length':
+            return
+        super().send_header(keyword, value)
+
+    def end_headers (self):
+        # intercept; called in copyfile() or indirectly
+        # by send_head via super().send_error()
+        pass
+
+    def copyfile(self, source, outputfile):
+        injected = False
+        content = BytesIO()
+        length = 0
+        # inject build count and time into served header
+        for line in source:
+            line = line.decode('utf-8')
+            if not injected and JSON_VERSION_RE.match(line):
+                length += content.write(bytes('#define JSON_BUILD_COUNT '\
+                                              f'{self.worktree.build_count}\n', 'utf-8'))
+                length += content.write(bytes('#define JSON_BUILD_TIME '\
+                                              f'"{self.worktree.build_time}"\n\n', 'utf-8'))
+                injected = True
+            length += content.write(bytes(line, 'utf-8'))
+
+        # set content length
+        super().send_header('Content-Length', length)
+        # CORS header
+        self.send_header('Access-Control-Allow-Origin', '*')
+        # prevent caching
+        self.send_header('Cache-Control', 'no-cache, no-store, must-revalidate')
+        self.send_header('Pragma', 'no-cache')
+        self.send_header('Expires', '0')
+        super().end_headers()
+
+        # send the header
+        content.seek(0)
+        shutil.copyfileobj(content, outputfile)
+
+    def log_message(self, format, *args):
+        pass
+
+class DualStackServer(ThreadingHTTPServer):
+    def __init__(self, addr, worktrees):
+        """."""
+        self.worktrees = worktrees
+        super().__init__(addr, HeaderRequestHandler)
+
+    def server_bind(self):
+        # suppress exception when protocol is IPv4
+        with contextlib.suppress(Exception):
+            self.socket.setsockopt(
+                socket.IPPROTO_IPV6, socket.IPV6_V6ONLY, 0)
+        return super().server_bind()
+
+if __name__ == '__main__':
+    import argparse
+    import ssl
+    import socket
+    import yaml
+
+    # exit code
+    ec = 0
+
+    # setup logging
+    logging.basicConfig(format='[%(asctime)s] %(levelname)s: %(message)s',
+                        datefmt=DATETIME_FORMAT, level=logging.INFO)
+    log = logging.getLogger()
+    log.addHandler(ExitHandler(logging.ERROR))
+
+    # parse command line arguments
+    parser = argparse.ArgumentParser()
+    parser.add_argument('--make', default='make',
+                        help='the make command (default: make)')
+    args = parser.parse_args()
+
+    # propagate the make command to use for amalgamating headers
+    WorkTree.make_command = args.make
+
+    worktrees = None
+    try:
+        # change working directory to project root
+        os.chdir(os.path.realpath(os.path.join(sys.path[0], '../../')))
+
+        if not is_project_root():
+            log.error('working directory does not look like project root')
+
+        # load config
+        config = {}
+        config_file = os.path.abspath(CONFIG_FILE)
+        try:
+            with open(config_file, 'r') as f:
+                config = yaml.safe_load(f)
+        except FileNotFoundError:
+            log.info(f'cannot find configuration file: {config_file}')
+            log.info('using default configuration')
+
+        # find and monitor working trees
+        worktrees = WorkTrees(config.get('root', '.'))
+
+        # start web server
+        infos = socket.getaddrinfo(config.get('bind', None), config.get('port', 8443),
+                                   type=socket.SOCK_STREAM, flags=socket.AI_PASSIVE)
+        DualStackServer.address_family = infos[0][0]
+        HeaderRequestHandler.protocol_version = 'HTTP/1.0'
+        with DualStackServer(infos[0][4], worktrees) as httpd:
+            scheme = 'HTTP'
+            https = config.get('https', {})
+            if https.get('enabled', True):
+                cert_file = https.get('cert_file', 'localhost.pem')
+                key_file = https.get('key_file', 'localhost-key.pem')
+                ssl.minimum_version = ssl.TLSVersion.TLSv1_3
+                ssl.maximum_version = ssl.TLSVersion.MAXIMUM_SUPPORTED
+                httpd.socket = ssl.wrap_socket(httpd.socket,
+                    certfile=cert_file, keyfile=key_file,
+                    server_side=True, ssl_version=ssl.PROTOCOL_TLS)
+                scheme = 'HTTPS'
+            host, port = httpd.socket.getsockname()[:2]
+            log.info(f'serving {scheme} on {host} port {port}')
+            log.info('press Ctrl+C to exit')
+            httpd.serve_forever()
+
+    except KeyboardInterrupt:
+        log.info('exiting')
+    except Exception:
+        log.exception('an error occurred:')
+        ec = 1
+    finally:
+        if worktrees is not None:
+            worktrees.stop()
+            sys.exit(ec)
diff --git a/tools/serve_header/serve_header.yml.example b/tools/serve_header/serve_header.yml.example
new file mode 100644
index 0000000000000000000000000000000000000000..42310910ed708968fe24ce992fda9156f0ec3a28
--- /dev/null
+++ b/tools/serve_header/serve_header.yml.example
@@ -0,0 +1,15 @@
+# all paths are relative to the project root
+
+# the root directory for the web server
+# root: .
+
+# configure SSL
+# https:
+#   enabled: true
+# these filenames are listed in .gitignore
+#  cert_file: localhost.pem
+#  key_file: localhost-key.pem
+
+# address and port for the server to listen on
+# bind: null
+# port: 8443