devtools: Add custom `include-order-fixer` to pre-commit.

diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index 9c69e945..497750a0 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -85,3 +85,20 @@ repos:
       entry: '[^a-z_/](?:fgets|fputs|gets|scanf|sprintf)\('
       types: [ c ]
       exclude: ccan|contrib
+
+    - id: include-order-fixer
+      name: Fix Include Order
+      language: python
+      description: Analyzes Core Lightning C source and header files, assesses the order of
+        their include directives according to the published Coding Style Guidelines
+        [here](https://docs.corelightning.org/docs/coding-style-guidelines), automatically
+        applying sorting them. Aims to conform with `make check-includes`.
+      entry: python devtools/include-order-fixer.py
+      stages: [ manual ]
+      pass_filenames: false
+
+    - id: check-includes
+      name: Check Includes
+      language: system
+      entry: make check-includes
+      pass_filenames: false
diff --git a/Makefile b/Makefile
index a5ab6dc8..e2baaa58 100644
--- a/Makefile
+++ b/Makefile
@@ -546,6 +546,11 @@ SRC_TO_CHECK := $(filter-out $(ALL_TEST_PROGRAMS:=.c), $(ALL_NONGEN_SOURCES))
 check-src-includes: $(SRC_TO_CHECK:%=check-src-include-order/%)
 check-hdr-includes: $(ALL_NONGEN_HEADERS:%=check-hdr-include-order/%)
 
+print-src-to-check:
+	@echo $(SRC_TO_CHECK)
+print-hdr-to-check:
+	@echo $(ALL_NONGEN_HEADERS)
+
 # If you want to check a specific variant of quotes use:
 #   make check-source-bolt BOLTVERSION=xxx
 ifeq ($(BOLTVERSION),$(DEFAULT_BOLTVERSION))
diff --git a/common/htlc_state.c b/common/htlc_state.c
index 75b841d3..52d0edc0 100644
--- a/common/htlc_state.c
+++ b/common/htlc_state.c
@@ -138,4 +138,3 @@ int htlc_state_flags(enum htlc_state state)
 	assert(per_state_bits[state]);
 	return per_state_bits[state];
 }
-
diff --git a/devtools/include-order-fixer.py b/devtools/include-order-fixer.py
new file mode 100755
index 00000000..bee5541f
--- /dev/null
+++ b/devtools/include-order-fixer.py
@@ -0,0 +1,394 @@
+#!/usr/bin/env python3
+"""
+Fix include directive ordering in C source and header files.
+
+This script analyzes Core Lightning C source and header files, ensuring
+include directives are sorted according to the Coding Style Guidelines.
+
+Includes ending in "_gen.h" or with any leading whitespace are preserved
+in their original positions. Comments and blank lines are also preserved.
+Includes found more than once are de-duplicated.
+"""
+
+import locale
+import os
+import re
+import subprocess
+import sys
+import tempfile
+
+# Set C locale for sorting to match Makefile behavior
+locale.setlocale(locale.LC_ALL, "C")
+
+
+def parse_makefile_output(output):
+    """Parse Makefile output, handling the 'Building version' line."""
+    lines = output.splitlines()
+    # Skip "Building version" line if present
+    if lines and lines[0].startswith("Building version"):
+        if len(lines) > 1:
+            file_list = lines[1]
+        else:
+            file_list = ""
+    else:
+        file_list = lines[0] if lines else ""
+
+    # Split by spaces and filter out empty strings
+    files = [f for f in file_list.split() if f]
+    return files
+
+
+def get_files_to_check():
+    """Get lists of C source and header files from Makefile targets."""
+    # Get C source files
+    result = subprocess.run(
+        ["make", "print-src-to-check"],
+        capture_output=True,
+        text=True,
+        cwd=os.path.dirname(os.path.dirname(os.path.abspath(__file__))),
+    )
+    if result.returncode != 0:
+        print(
+            f"Error running 'make print-src-to-check': {result.stderr}", file=sys.stderr
+        )
+        sys.exit(1)
+
+    src_files = parse_makefile_output(result.stdout)
+
+    # Get header files
+    result = subprocess.run(
+        ["make", "print-hdr-to-check"],
+        capture_output=True,
+        text=True,
+        cwd=os.path.dirname(os.path.dirname(os.path.abspath(__file__))),
+    )
+    if result.returncode != 0:
+        print(
+            f"Error running 'make print-hdr-to-check': {result.stderr}", file=sys.stderr
+        )
+        sys.exit(1)
+
+    hdr_files = parse_makefile_output(result.stdout)
+
+    # Return files with their types
+    files_with_types = []
+    for f in src_files:
+        files_with_types.append((f, "c"))
+    for f in hdr_files:
+        files_with_types.append((f, "h"))
+
+    return files_with_types
+
+
+def extract_includes(content):
+    """
+    Extract include directives from file content, preserving comments and whitespace.
+
+    Returns:
+        tuple: (main_items, trailing_items, include_start_line, blank_line_index, include_end_line)
+        main_items: List of (type, line) tuples in main block where type is 'include', 'comment', or 'blank'
+        trailing_items: List of (type, line) tuples after blank line (to be preserved)
+        include_start_line: Line number where includes start (0-indexed)
+        blank_line_index: Line number of blank line separator (None if no blank line)
+        include_end_line: Line number after last include (0-indexed)
+    """
+    lines = content.splitlines(keepends=True)
+    main_items = []
+    trailing_items = []
+    include_start = None
+    include_end = None
+    blank_line_index = None
+    in_trailing_block = False
+
+    # Pattern to match include directives (with optional leading whitespace)
+    include_pattern = re.compile(r'^\s*#include\s+[<"].*[>"]\s*$')
+    # Pattern to match comments (single-line or start of multi-line)
+    comment_pattern = re.compile(r'^\s*/\*|^\s*//')
+    # Pattern to match continuation lines of multi-line comments
+    comment_continuation_pattern = re.compile(r'^\s*\*|.*\*/')
+
+    in_multiline_comment = False
+
+    for i, line in enumerate(lines):
+        # Check if this line is an include
+        if include_pattern.match(line):
+            in_multiline_comment = False
+            if include_start is None:
+                include_start = i
+            # Preserve the line as-is (including leading whitespace)
+            if in_trailing_block:
+                trailing_items.append(('include', line))
+            else:
+                main_items.append(('include', line))
+            include_end = i + 1
+        elif include_start is not None:
+            # We've seen includes, but this line is not an include
+            if line.strip():
+                # Check if it's a comment start or continuation
+                if comment_pattern.match(line):
+                    # Start of a comment
+                    in_multiline_comment = True
+                    # Check if it's a single-line comment (ends with */)
+                    if '*/' in line:
+                        in_multiline_comment = False
+                    # Preserve comments
+                    if in_trailing_block:
+                        trailing_items.append(('comment', line))
+                    else:
+                        main_items.append(('comment', line))
+                    include_end = i + 1
+                elif in_multiline_comment and comment_continuation_pattern.match(line):
+                    # Continuation of multi-line comment
+                    if in_trailing_block:
+                        trailing_items.append(('comment', line))
+                    else:
+                        main_items.append(('comment', line))
+                    include_end = i + 1
+                    # Check if this line ends the comment
+                    if '*/' in line:
+                        in_multiline_comment = False
+                else:
+                    # Non-blank, non-include, non-comment line - stop here
+                    in_multiline_comment = False
+                    break
+            else:
+                # Blank line
+                # Only treat as separator if we haven't seen one yet
+                # and we'll continue to look for trailing includes
+                if blank_line_index is None:
+                    blank_line_index = i
+                    in_trailing_block = True
+                    # Add this separator blank line to trailing_items
+                    trailing_items.append(('blank', line))
+                    include_end = i + 1
+                elif in_trailing_block:
+                    # We're in trailing block, preserve blank lines here
+                    trailing_items.append(('blank', line))
+                    include_end = i + 1
+                else:
+                    # Blank line in main block (before separator) - preserve it
+                    main_items.append(('blank', line))
+                    include_end = i + 1
+        # If we haven't started collecting includes yet, continue
+
+    if include_start is None:
+        # No includes found
+        return [], [], None, None, None
+
+    # If we marked a blank line as separator but found no trailing includes,
+    # those blank lines should not be treated as trailing - they're just
+    # normal blank lines after the includes that should remain in after_lines
+    if blank_line_index is not None:
+        # Check if we actually have trailing includes (not just blank lines/comments)
+        has_trailing_includes = any(item_type == 'include' for item_type, _ in trailing_items)
+        if not has_trailing_includes:
+            # No trailing includes found, so blank lines aren't a separator
+            # Reset to treat them as normal file content
+            blank_line_index = None
+            trailing_items = []
+            # Recalculate include_end to point to the last include/comment in main_items
+            # Count how many lines we've processed in main_items
+            include_end = include_start + len(main_items)
+
+    return (
+        main_items,
+        trailing_items,
+        include_start,
+        blank_line_index,
+        include_end,
+    )
+
+
+def sort_includes(items, file_type):
+    """
+    Sort includes according to Core Lightning rules.
+
+    For .c files: all includes in alphabetical order
+    For .h files: config.h first (if present), then others alphabetically
+
+    Includes ending in "_gen.h" or with any leading whitespace are preserved
+    in their original positions. Comments and blank lines are also preserved.
+    """
+    if not items:
+        return items
+
+    # Track includes that should be preserved at their positions
+    preserved_positions = {}  # position -> (type, line)
+    regular_includes = []  # list of (position, include_line) tuples to sort
+
+    for pos, (item_type, line) in enumerate(items):
+        if item_type != 'include':
+            # Preserve comments and blank lines at their positions
+            preserved_positions[pos] = (item_type, line)
+        else:
+            # Check if this include should be preserved
+            # (has any leading whitespace, or ends in "_gen.h")
+            stripped = line.lstrip()
+            has_leading_whitespace = line != stripped
+            is_gen_h = '_gen.h"' in line or "_gen.h>" in line
+
+            if has_leading_whitespace or is_gen_h:
+                # Preserve at original position
+                preserved_positions[pos] = (item_type, line)
+            else:
+                # Regular include to be sorted
+                regular_includes.append((pos, line))
+
+    # Separate config.h from other regular includes for header files
+    config_h_pos = None
+    config_h_include = None
+    other_regular = []
+
+    for pos, inc in regular_includes:
+        if file_type == "h" and '"config.h"' in inc:
+            config_h_pos = pos
+            config_h_include = inc
+        else:
+            other_regular.append((pos, inc))
+
+    # Sort other regular includes using C locale (by the include content, not position)
+    other_regular_sorted = sorted(other_regular, key=lambda x: locale.strxfrm(x[1]))
+
+    # Build sorted list of regular includes
+    sorted_regular = []
+    if config_h_include:
+        sorted_regular.append((config_h_pos, config_h_include))
+    sorted_regular.extend(other_regular_sorted)
+
+    # Build result: preserved items at original positions, sorted regular includes elsewhere
+    result = []
+    regular_idx = 0
+
+    for pos in range(len(items)):
+        if pos in preserved_positions:
+            # Use preserved item at its original position
+            result.append(preserved_positions[pos])
+        else:
+            # Use next sorted regular include
+            if regular_idx < len(sorted_regular):
+                _, sorted_inc = sorted_regular[regular_idx]
+                result.append(('include', sorted_inc))
+                regular_idx += 1
+
+    return result
+
+
+def dedupe_include_items(items, seen):
+    """Remove duplicate include lines, keeping the first occurrence.
+
+    Duplicate detection uses a canonical form of include lines (`lstrip()`),
+    so leading whitespace differences do not prevent matching.
+    Non-include items (comments/blanks) are always preserved.
+    """
+    deduped = []
+    for item_type, line in items:
+        if item_type != "include":
+            deduped.append((item_type, line))
+            continue
+
+        key = line.lstrip()
+        if key in seen:
+            continue
+        seen.add(key)
+        deduped.append((item_type, line))
+    return deduped
+
+
+def fix_file_includes(filepath, file_type):
+    """
+    Fix include ordering in a file.
+
+    Returns:
+        bool: True if file was modified, False otherwise
+    """
+    try:
+        with open(filepath, "r", encoding="utf-8", errors="replace") as f:
+            content = f.read()
+    except IOError as e:
+        print(f"Error reading {filepath}: {e}", file=sys.stderr)
+        return False
+
+    # Extract includes
+    main_items, trailing_items, include_start, blank_line_index, include_end = extract_includes(
+        content
+    )
+
+    if include_start is None:
+        # No includes to sort
+        return False
+
+    # Sort only the main includes block (preserving comments, blanks, and whitespace-prefixed includes)
+    sorted_main_items = sort_includes(main_items, file_type)
+
+    # De-duplicate includes across main and trailing blocks, preserving the first occurrence
+    seen_includes = set()
+    sorted_main_items = dedupe_include_items(sorted_main_items, seen_includes)
+    trailing_items = dedupe_include_items(trailing_items, seen_includes)
+
+    # Reconstruct file content
+    lines = content.splitlines(keepends=True)
+    before_lines = lines[:include_start] if include_start > 0 else []
+    after_lines = lines[include_end:] if include_end < len(lines) else []
+
+    # Build the include section: main sorted items + trailing items
+    # Note: blank lines are already included in main_items/trailing_items, and
+    # blank_line_index is just a marker, so we don't need to add it separately
+    include_section = "".join(line for _, line in sorted_main_items)
+    if trailing_items:
+        # Add trailing items (blank line separator is already in trailing_items if present)
+        include_section += "".join(line for _, line in trailing_items)
+
+    # Combine: before + include section + after
+    new_content = "".join(before_lines) + include_section + "".join(after_lines)
+
+    # Check if content actually changed
+    if new_content == content:
+        return False
+
+    # Write back atomically using temp file
+    try:
+        with tempfile.NamedTemporaryFile(
+            mode="w",
+            encoding="utf-8",
+            dir=os.path.dirname(filepath),
+            delete=False,
+            suffix=".tmp",
+        ) as tmp:
+            tmp.write(new_content)
+            tmp_path = tmp.name
+
+        # Atomic rename
+        os.replace(tmp_path, filepath)
+        return True
+    except IOError as e:
+        print(f"Error writing {filepath}: {e}", file=sys.stderr)
+        if os.path.exists(tmp_path):
+            os.unlink(tmp_path)
+        return False
+
+
+def main():
+    """Main entry point."""
+    files_with_types = get_files_to_check()
+
+    modified_files = []
+
+    for filepath, file_type in files_with_types:
+        if not os.path.exists(filepath):
+            # File might not exist (generated files, etc.)
+            continue
+
+        if fix_file_includes(filepath, file_type):
+            modified_files.append(filepath)
+
+    # Exit with appropriate code
+    if modified_files:
+        # Files were modified - exit 1 so pre-commit shows the diff
+        sys.exit(1)
+    else:
+        # No changes needed
+        sys.exit(0)
+
+
+if __name__ == "__main__":
+    main()