FoundationAgents/MetaGPT · error · JSONDecodeError

Expecting ',' delimiter

Error message

Expecting ',' delimiter

What it means

Raised by the custom decoder's object scanner: after successfully reading a key/value pair, the next non-whitespace character was neither '}' (end of object) nor ',' (pair separator). It is the standard 'missing comma between object members' JSON syntax error, surfaced from MetaGPT's tolerant decoder.

Source

Thrown at metagpt/utils/custom_decoder.py:180

        try:
            value, end = scan_once(s, end)
        except StopIteration as err:
            raise JSONDecodeError("Expecting value", s, err.value) from None
        pairs_append((key, value))
        try:
            nextchar = s[end]
            if nextchar in _ws:
                end = _w(s, end + 1).end()
                nextchar = s[end]
        except IndexError:
            nextchar = ""
        end += 1

        if nextchar == "}":
            break
        elif nextchar != ",":
            raise JSONDecodeError("Expecting ',' delimiter", s, end - 1)
        end = _w(s, end).end()
        nextchar = s[end : end + 1]
        end += 1
        if nextchar != '"':
            raise JSONDecodeError("Expecting property name enclosed in double quotes", s, end - 1)
    if object_pairs_hook is not None:
        result = object_pairs_hook(pairs)
        return result, end
    pairs = dict(pairs)
    if object_hook is not None:
        pairs = object_hook(pairs)
    return pairs, end


def py_scanstring(s, end, strict=True, _b=BACKSLASH, _m=STRINGCHUNK.match, delimiter='"'):
    """Scan the string s for a JSON string.

    Args:

View on GitHub (pinned to 11cdf466d0)

Solutions

1. Go to the column given in the error (end - 1) and add the missing ',' between the two members.
2. Escape inner double quotes inside string values (\") so the comma is not consumed as part of the value.
3. Validate strings with a linter/json.parse before passing them to the decoder.
4. For LLM output, re-request or run through a JSON repair step before decoding.

Example fix

// before
'{"name": "a" "value": 1}' -> JSONDecodeError: Expecting ',' delimiter

// after
'{"name": "a", "value": 1}'

import json
def object_members_comma_separated(text: str) -> bool:
    try:
        json.loads(text)
        return True
    except json.JSONDecodeError:
        return False

try:
    obj = decoder.decode(s)
except json.JSONDecodeError as e:
    if e.msg == "Expecting ',' delimiter":
        s2 = s[:e.pos] + ',' + s[e.pos:]  # targeted repair candidate
        obj = decoder.decode(s2)

When it happens

Trigger: Decoding an object like {"a":1 "b":2} (missing comma), {"a":1;"b":2} (semicolon separator), or text where a quoted string value swallowed the comma because of an unescaped inner quote.

Common situations: LLM-generated JSON with omitted commas, hand-edited config strings, or nested quotes inside string values that terminate the string early and leave a stray token before the comma.

Related errors

• Expecting property name enclosed in double quotes
• Expecting ':' delimiter
• Expecting value
• Unterminated string starting at
• Invalid control character {0!r} at

AI-assisted analysis of FoundationAgents/MetaGPT@11cdf466d0 (2026-08-14). Data as JSON: /api/errors/0358f7b98d74cf17. Report an issue: GitHub.