diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index da84ad3b02..d7c4be91e1 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -4,3 +4,20 @@ repos:
     hooks:
       - id: end-of-file-fixer
         exclude: '^(xls/contrib/.*|docs_src/bazel_rules_macros.md|.*\.(sh|ir|txt|patch|csv))$'
+  - repo: local
+    hooks:
+      - id: check-cpp-absolute-includes
+        name: Check C++ absolute includes
+        entry: xls/dev_tools/check_cpp_includes.py
+        language: python
+        files: ^xls/.*\.(h|cc)$
+        exclude: ^xls/contrib/
+        types: [text]
+  - repo: https://github.com/pylint-dev/pylint
+    rev: v3.3.4
+    hooks:
+      - id: pylint
+        files: ^xls/dev_tools/check_.*.py$
+        args: [
+          --rcfile=.pylintrc
+        ]
diff --git a/xls/dev_tools/check_cpp_includes.py b/xls/dev_tools/check_cpp_includes.py
new file mode 100755
index 0000000000..0d45a7ddf3
--- /dev/null
+++ b/xls/dev_tools/check_cpp_includes.py
@@ -0,0 +1,75 @@
+#!/usr/bin/env python3
+
+# Copyright 2025 The XLS Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Checks that C++ files only include absolute paths.
+
+This prevents accidental inclusion via relpath even if it happens to work.
+"""
+
+import sys
+import re
+
+ALLOWED_INCLUDE_STARTS = {
+  'xls/',
+  'absl/',
+  'gmock/',
+  'gtest/',
+  'benchmark/',
+  'llvm/',
+  'fuzztest/',
+  'verible/',
+  're2/',
+  'z3/',
+  'google/protobuf/',
+  'grpcpp/',
+  'grpc/',
+  'openssl/',
+  'nlohmann/',
+  'cppitertools/',
+  'ortools/',
+  'external/zstd/',
+  'riegeli/',
+  'tools/cpp/runfiles',
+  'linenoise.h',
+  'libs/json11/',
+  '%s',  # For format strings embedded in files.
+}
+
+def check_file(filename):
+  with open(filename, encoding='utf-8') as f:
+    content = f.read()
+
+  # Look for quoted (non-system) includes
+  includes = re.findall(r'#include\s*"([^"]*)"', content)
+  bad_includes = [
+    inc for inc in includes
+    if not any(inc.startswith(start) for start in ALLOWED_INCLUDE_STARTS)]
+
+  if bad_includes:
+    print(f'{filename}: Found non-absolute includes:')
+    for include in bad_includes:
+      print(f'  {include}')
+    return 1
+  return 0
+
+def main():
+  exit_code = 0
+  for filename in sys.argv[1:]:
+    exit_code |= check_file(filename)
+  sys.exit(exit_code)
+
+if __name__ == '__main__':
+  main()
diff --git a/xls/dev_tools/check_examples_have_targets.py b/xls/dev_tools/check_examples_have_targets.py
index 70beadf200..7a476770b8 100644
--- a/xls/dev_tools/check_examples_have_targets.py
+++ b/xls/dev_tools/check_examples_have_targets.py
@@ -96,7 +96,7 @@ def _to_target(relpath: str) -> str:
 
 
 def _does_file_seem_to_contain_tests(path: str) -> bool:
-  with open(path) as f:
+  with open(path, encoding='utf-8') as f:
     contents = f.read()
   return '#[test' in contents or '#[test_proc' in contents
 
diff --git a/xls/dev_tools/check_header_guards.py b/xls/dev_tools/check_header_guards.py
index 92b2cba0f8..e84a09caa4 100644
--- a/xls/dev_tools/check_header_guards.py
+++ b/xls/dev_tools/check_header_guards.py
@@ -32,7 +32,7 @@ def get_expected_guard(filepath, repo_root):
 
 
 def check_header_guard(filepath, expected_guard):
-  with open(filepath, 'r') as file:
+  with open(filepath, encoding='utf-8') as file:
     lines = file.readlines()
 
   # Check for the presence of the expected header guard.
@@ -76,7 +76,8 @@ def main():
     for file, expected, actual in non_compliant_files:
       print(file)
       print(f'  want: {expected}')
-      print(f'  got: {actual if actual else "None"}')
+      actual_str = actual if actual else 'None'
+      print(f'  got: {actual_str}')
     sys.exit(-1)
   else:
     print('All header files are style compliant.')
diff --git a/xls/jit/jit_channel_queue_benchmark.cc b/xls/jit/jit_channel_queue_benchmark.cc
index 2ec26b266d..8c64669b52 100644
--- a/xls/jit/jit_channel_queue_benchmark.cc
+++ b/xls/jit/jit_channel_queue_benchmark.cc
@@ -19,7 +19,7 @@
 #include <vector>
 
 #include "absl/log/check.h"
-#include "include/benchmark/benchmark.h"
+#include "benchmark/benchmark.h"
 #include "xls/ir/channel.h"
 #include "xls/ir/channel_ops.h"
 #include "xls/ir/package.h"
diff --git a/xls/jit/value_to_native_layout_benchmark.cc b/xls/jit/value_to_native_layout_benchmark.cc
index bd0272f467..df4916e496 100644
--- a/xls/jit/value_to_native_layout_benchmark.cc
+++ b/xls/jit/value_to_native_layout_benchmark.cc
@@ -17,7 +17,7 @@
 #include <random>
 #include <vector>
 
-#include "include/benchmark/benchmark.h"
+#include "benchmark/benchmark.h"
 #include "xls/interpreter/random_value.h"
 #include "xls/ir/ir_parser.h"
 #include "xls/ir/package.h"