@@ -190,6 +190,8 @@ jobs:
       run: |
         set -x
         scripts/cppcheck.sh
+    - name: Check include style of exported headers
+      run: scripts/include-style-check.py --enforce
 
   shellcheck:
     runs-on: ubuntu-24.04
 
@@ -0,0 +1,177 @@
+#!/usr/bin/env python3
+#
+# Check the include style of exported headers
+# Copyright (C) 2026 L. Toniolo
+#
+# This program is free software; you can redistribute it and/or modify it under
+# the terms of the GNU General Public License version 2 or later.
+#
+# The build copies every SRCHEADERS entry into include/, so an exported header
+# exists twice: the source under src/ and the copy every module compiles
+# against. A quoted include searches the includer's own directory first, an
+# angled include does not, so the two forms can reach different copies and both
+# compile.
+#
+# An exported header travels to include/ and must take its siblings with it, so
+# it includes them with quotes and finds the copies beside it wherever it ends
+# up. So does anything else in its directory, wanting the source next to it
+# rather than a stale export. Everything else is a user, builds out of tree
+# where only the exported copies exist, and uses angle brackets.
+#
+import sys
+import os
+import re
+import getopt
+import subprocess
+
+error_on_warning = False
+
+# The script lives in scripts/ and the sources are one level up, so it runs
+# from anywhere.
+topdir = os.path.normpath(os.path.join(os.path.dirname(os.path.realpath(__file__)), ".."))
+
+SUFFIXES = (".c", ".cc", ".cpp", ".h", ".hh", ".comp", ".icomp")
+
+RE_QUOTED = re.compile(r'^[ \t]*#[ \t]*include[ \t]*"([^"]+)"', re.M)
+# A .comp only becomes C below its ;; line.  Above it halcompile takes its own
+# include directive, 'include "bla";', which is not a preprocessor line.
+RE_COMP_QUOTED = re.compile(r'^[ \t]*include[ \t]*"([^"]+)"[ \t]*;', re.M)
+RE_SRCHEADERS = re.compile(r"^SRCHEADERS\s*:=\s*\\\n((?:.*\\\n)*.*)$", re.M)
+
+
+def usage():
+    print("""Check the include style of exported headers.
+Usage:
+  include-style-check.py [-e] [-h] [file...]
+
+Checks every source file under src/ when given no file, which is how CI runs
+it. Named files are useful from a pre-commit hook.
+
+Options:
+  -e|--error    Treat findings as errors (--enforce is accepted as well)
+  -h|--help     This message
+""")
+    sys.exit(2)
+
+
+messages = []
+
+#
+# Collect messages
+#
+def pfind(path, lineno, msg):
+    global messages
+    messages.append((path, lineno, msg))
+
+def flush_messages():
+    kind = "error" if error_on_warning else "warning"
+    for path, lineno, msg in messages:
+        print("{}:{}: {}: {}".format(path, lineno, kind, msg))
+    # Annotate the offending lines when running under CI
+    if os.environ.get("GITHUB_ACTIONS"):
+        for path, lineno, msg in messages:
+            print("::{} file={},line={},title=Include style::{}".format(kind, path, lineno, msg))
+    if not messages:
+        return 0
+    return 1 if error_on_warning else 0
+
+
+def exported_headers():
+    """Map each exported header's basename onto its path under src/, taken
+    from the SRCHEADERS list the build installs into include/."""
+    with open(os.path.join(topdir, "src", "Makefile"), encoding="utf-8", errors="replace") as f:
+        m = RE_SRCHEADERS.search(f.read())
+    if not m:
+        print("No SRCHEADERS list found in src/Makefile", file=sys.stderr)
+        sys.exit(2)
+    headers = {}
+    for line in m.group(1).split("\n"):
+        entry = line.strip().rstrip("\\").strip()
+        if entry:
+            headers[os.path.basename(entry)] = "src/" + entry
+    return headers
+
+
+def tracked_files():
+    """All tracked files under src/, and of those the ones worth reading."""
+    try:
+        out = subprocess.run(["git", "-C", topdir, "ls-files", "src"],
+                             capture_output=True, text=True, check=True).stdout
+    except (OSError, subprocess.CalledProcessError) as err:
+        print(err, file=sys.stderr)
+        sys.exit(2)
+    # A tracked path can be absent from the working tree, deleted but not yet
+    # committed or mid-rebase. The compiler would not see it either, so drop it
+    # rather than fail on the open() below.
+    tracked = {f for f in out.split("\n")
+               if f and os.path.isfile(os.path.join(topdir, f))}
+    return tracked, sorted(f for f in tracked if f.endswith(SUFFIXES))
+
+
+def check_quoted_includes(tracked, files, headers):
+    exported = set(headers.values())
+    for path in files:
+        if path in exported:
+            # An exported header is copied to include/ and has to keep finding
+            # its siblings there, so it includes them with quotes.
+            continue
+        directory = os.path.dirname(path)
+        with open(os.path.join(topdir, path), encoding="utf-8", errors="replace") as f:
+            text = f.read()
+        found = list(RE_QUOTED.finditer(text))
+        if path.endswith((".comp", ".icomp")):
+            head, sep, _ = text.partition("\n;;\n")
+            found += RE_COMP_QUOTED.finditer(head if sep else text)
+            found.sort(key=lambda m: m.start())
+        for m in found:
+            name = m.group(1)
+            source = headers.get(os.path.basename(name))
+            if source is None:
+                continue    # not an exported header, nothing to say about it
+            # A file of that name beside the includer is a different header
+            # that happens to share the basename, not this one.
+            local = os.path.normpath(os.path.join(directory, name))
+            if local != source and local in tracked:
+                continue
+            if directory == os.path.dirname(source):
+                continue        # beside the header, taking the source copy
+            why = "exported by {}".format(os.path.dirname(source))
+            lineno = text[:m.start()].count("\n") + 1
+            pfind(path, lineno,
+                  'include "{}" is {}, use <{}>'.format(name, why, os.path.basename(name)))
+
+
+def main():
+    try:
+        opts, args = getopt.getopt(sys.argv[1:], "eh", ["error", "enforce", "help"])
+    except getopt.GetoptError as err:
+        print(err, file=sys.stderr)
+        usage()
+
+    global error_on_warning
+    for o, unused_a in opts:
+        if o in ("-e", "--error", "--enforce"):
+            error_on_warning = True
+        elif o in ("-h", "--help"):
+            usage()
+
+    if os.environ.get("INCLUDE_STYLE_CHECK_ENFORCE"):
+        error_on_warning = True
+
+    # From here on we collect findings with pfind(). They get flushed when we
+    # are done. The program's return value depends on whether findings are
+    # treated as errors or not.
+    headers = exported_headers()
+    tracked, files = tracked_files()
+    if args:
+        # Named files, as a pre-commit hook would pass them. Anything outside
+        # the set the whole-tree run covers has nothing to say about it.
+        named = {os.path.relpath(os.path.abspath(a), topdir) for a in args}
+        files = [f for f in files if f in named]
+    check_quoted_includes(tracked, files, headers)
+
+    return
+
+if __name__ == "__main__":
+    main()
+    sys.exit(flush_messages())  # Exit value depends on findings being errors