What causes "Comments are not allowed in strict mode" in antonmedv/fx?

Remove all // and /* */ comments from the input before parsing.

Comments are not allowed in strict mode

Error message

Comments are not allowed in strict mode

What it means

The lenient parser of this library allows // and /* */ comments while skipping whitespace, but strict mode (standard JSON) forbids them. When skipWhitespace encounters '/' and p.strict is true, it panics with this message. It means the document contains a comment, which is not part of the JSON specification.

Source

Thrown at internal/jsonx/json.go:587

	panic(fmt.Sprintf("Unexpected character %q", p.char))
}

func isEndOfValue(ch byte) bool {
	return isWhitespace(ch) || ch == ',' || ch == '}' || ch == ']' || ch == 0 // 0 is EOF
}

func isWhitespace(ch byte) bool {
	return ch == ' ' || ch == '\t' || ch == '\n' || ch == '\r'
}

func (p *JsonParser) skipWhitespace() {
	for {
		switch p.char {
		case ' ', '\t', '\n', '\r':
			p.next()
		case '/':
			if p.strict {
				panic("Comments are not allowed in strict mode")
			}
			p.skipComment()
		default:
			return
		}
	}
}

func (p *JsonParser) skipComment() {
	p.next()
	switch p.char {
	case '/':
		for p.char != '\n' && p.char != 0 {
			p.next()
		}
	case '*':
		for {
			p.next()

View on GitHub (pinned to 4f31cd3a0c)

Solutions

Remove all // and /* */ comments from the input before parsing.
Parse with strict=false if the parser instance is under your control, since the lenient mode supports comments.
Strip comments programmatically with a JSONC stripper before calling Parse.
Rename/rehandle the file as JSONC and use a JSONC-aware tool.

Example fix

// before (input.json)
{
  // retry count
  "retries": 3
}
// after
{
  "retries": 3
}

Defensive patterns

Strategy: validation

Validate before calling

if strings.Contains(string(data), "//") || strings.Contains(string(data), "/*") {
    if strictRequired {
        return errors.New("comments present but strict JSON required; strip them first")
    }
    data = stripJSONCComments(data)
}

Try / catch

defer func() {
    if r := recover(); r != nil {
        if strings.Contains(fmt.Sprint(r), "Comments are not allowed") {
            err = errors.New("file is JSONC; strip comments or use lenient mode")
        }
    }
}()

Prevention

Keep comments in .jsonc files, not .json.
Run a JSONC-to-JSON stripper on hand-edited config files before parsing.
Detect comment patterns in pre-validation and route to a lenient parser.
Educate editors of config files that JSON has no comments.

When it happens

Trigger: Calling Parse with strict=true on JSON containing // line comments or /* block comments */ anywhere whitespace is legal (between elements, after {, etc.).

Common situations: Hand-edited config files (VS Code jsonc, tsconfig.json-style files), .jsonc files saved as .json, tools that emit annotated JSON.

Related errors

AI-assisted analysis of antonmedv/fx@4f31cd3a0c (2026-09-02). Data as JSON: /api/errors/7304eea2cd53c775. Report an issue: GitHub.