diff --git a/pybind11_mkdoc/mkdoc_lib.py b/pybind11_mkdoc/mkdoc_lib.py
index 0a553d7..ffc5755 100755
--- a/pybind11_mkdoc/mkdoc_lib.py
+++ b/pybind11_mkdoc/mkdoc_lib.py
@@ -552,9 +552,23 @@ def extract(filename, node, prefix, output, file_cache):
     return None
 
 
+def _report_diagnostics(filename, tu):
+    """Print clang diagnostics to stderr and fail if any of them is an error."""
+    errors = 0
+    for diagnostic in tu.diagnostics:
+        sys.stderr.write(diagnostic.format() + "\n")
+        if diagnostic.severity >= cindex.Diagnostic.Error:
+            errors += 1
+    if errors:
+        msg = f"Clang reported {errors} error(s) while parsing {filename}"
+        raise RuntimeError(msg)
+
+
 def _extract_file(filename, parameters):
-    index = cindex.Index(cindex.conf.lib.clang_createIndex(False, True))
+    # Diagnostics are printed by _report_diagnostics, not by libclang itself.
+    index = cindex.Index(cindex.conf.lib.clang_createIndex(False, False))
     tu = index.parse(filename, parameters)
+    _report_diagnostics(filename, tu)
     output = []
     extract(filename, tu.cursor, "", output, {})
     return output
diff --git a/tests/cli_test.py b/tests/cli_test.py
index 0abf36d..7d86c7f 100644
--- a/tests/cli_test.py
+++ b/tests/cli_test.py
@@ -37,3 +37,18 @@ def test_parse_failure_sets_exit_code(tmp_path: Path) -> None:
 
     assert result.returncode != 0
     assert not tf.exists()
+
+
+def test_missing_include_reports_diagnostics(tmp_path: Path) -> None:
+    header = tmp_path / "bad_header.h"
+    header.write_text('#include "does_not_exist.h"\n', encoding="utf-8")
+    tf = tmp_path / "tmp.h"
+    result = subprocess.run(
+        [sys.executable, "-m", "pybind11_mkdoc", "-o", tf, header],
+        check=False,
+        capture_output=True,
+    )
+
+    assert result.returncode != 0
+    assert not tf.exists()
+    assert "does_not_exist.h" in result.stderr.decode()