nodejs/node · error · Exception

Unable to parse config file %s%s

Error message

Unable to parse config file %s%s

What it means

Thrown by the analyzer GYP generator's Config.Init when it opens the file pointed to by the config_path generator flag but json.load() raises a ValueError. This means the file exists and is readable, but its contents are not syntactically valid JSON. The exception chains the original parse error so the exact JSON syntax problem is visible.

Source

Thrown at tools/gyp/pylib/gyp/generator/analyzer.py:279

        self.targets = set()
        self.additional_compile_target_names = set()
        self.test_target_names = set()

    def Init(self, params):
        """Initializes Config. This is a separate method as it raises an exception
        if there is a parse error."""
        generator_flags = params.get("generator_flags", {})
        config_path = generator_flags.get("config_path", None)
        if not config_path:
            return
        try:
            f = open(config_path)
            config = json.load(f)
            f.close()
        except OSError:
            raise Exception("Unable to open file " + config_path)
        except ValueError as e:
            raise Exception("Unable to parse config file " + config_path + str(e))
        if not isinstance(config, dict):
            raise Exception("config_path must be a JSON file containing a dictionary")
        self.files = config.get("files", [])
        self.additional_compile_target_names = set(
            config.get("additional_compile_targets", [])
        )
        self.test_target_names = set(config.get("test_targets", []))


def _WasBuildFileModified(build_file, data, files, toplevel_dir):
    """Returns true if the build file |build_file| is either in |files| or
    one of the files included by |build_file| is in |files|. |toplevel_dir| is
    the root of the source tree."""
    if _ToLocalPath(toplevel_dir, _ToGypPath(build_file)) in files:
        if debug:
            print("gyp file modified", build_file)
        return True

View on GitHub (pinned to 1b2de5e052)

Solutions

  1. Validate the config file with a JSON linter (e.g. `python -m json.tool config.json`) to surface the exact syntax error and line.
  2. Ensure all keys are double-quoted, remove trailing commas, and remove any comments — strict JSON only.
  3. Regenerate the config file programmatically with json.dump() rather than hand-writing or string-formatting it.
  4. Check that the file was fully written (no truncation) and has the correct encoding (UTF-8, no BOM).

Example fix

// before (config.json)
{
  files: ['a.cc'],   // unquoted keys + trailing comma
}
// after
{
  "files": ["a.cc"]
}
Defensive patterns

Strategy: validation

Validate before calling

import json, sys
path = 'config.json'
try:
    with open(path) as f:
        json.load(f)
    print('config JSON is valid')
except json.JSONDecodeError as e:
    print(f'invalid JSON: {e}'); sys.exit(1)
except OSError as e:
    print(f'cannot open: {e}'); sys.exit(1)

Type guard

def is_valid_config_dict(obj) -> bool:
    return isinstance(obj, dict) and 'files' in obj

Try / catch

try:
    config.Init(params)
except Exception as e:
    if 'Unable to parse config file' in str(e):
        report_config_error(config_path, e)
    raise

Prevention

When it happens

Trigger: Running gyp with --format=analyzer and a --generator-flag=config_path=<file> where <file> contains malformed JSON (trailing comma, unquoted keys, single quotes, a comment, or a truncated file). json.load succeeds at open() but fails during parsing, landing in the `except ValueError` branch at analyzer.py:278.

Common situations: Hand-editing the analyzer config JSON and introducing a syntax error; generating the config file with a script that emits JS-style object literals (unquoted keys) or trailing commas; a truncated file from an interrupted write; copy-pasting from a Python dict repr instead of valid JSON.

Understand the failure class

Related errors

AI-assisted analysis of nodejs/node@1b2de5e052 (2026-08-13). Data as JSON: /api/errors/de5ea9c2996aaa08. Report an issue: GitHub.