What causes "bboxes string input is not valid JSON: {exc}" in Comfy-Org/ComfyUI?

Validate the string parses with json.loads before wiring it in; fix syntax (double quotes, full brackets)

Comfy-Org/ComfyUI · error · ValueError

bboxes string input is not valid JSON: {exc}

Error message

bboxes string input is not valid JSON: {exc}

What it means

boxes_from_input accepts a JSON string, dict, or list. When a non-empty string arrives it must parse as JSON via json.loads; a parse failure (ValueError/TypeError from the JSON decoder) is re-raised with this message including the underlying parser error.

Source

Thrown at comfy_extras/nodes_bounding_boxes.py:244

                "text": element.get("text", "") if etype == "text" else "",
                "desc": element.get("desc", ""),
                "palette": element.get("color_palette", []) or [],
            },
        })
    return boxes


def boxes_from_input(data, width: int, height: int) -> list:
    if data is None:
        return []
    if isinstance(data, str):
        text = data.strip()
        if not text:
            return []
        try:
            data = json.loads(text)
        except (ValueError, TypeError) as exc:
            raise ValueError(f"bboxes string input is not valid JSON: {exc}") from exc
    if isinstance(data, dict):
        if _looks_like_element(data):
            return elements_to_boxes([data], width, height)
        if _looks_like_bbox(data):
            return normalize_incoming_boxes(data)
        raise ValueError(
            "bboxes dict must be a bounding box (x, y, width, height) or an element (with a 'bbox')"
        )
    if not isinstance(data, list):
        raise ValueError(
            "bboxes input must be bounding boxes, elements, or a JSON string, "
            f"got {type(data).__name__}"
        )
    if not data:
        return []
    first = data[0]
    if isinstance(first, list):
        return normalize_incoming_boxes(data)

View on GitHub (pinned to 1c6d8d45b3)

Solutions

Validate the string parses with json.loads before wiring it in; fix syntax (double quotes, full brackets)
Generate the JSON programmatically (json.dumps) instead of string concatenation/f-strings
If the source emits CSV-style boxes, convert to JSON or pass a list/dict directly (the node accepts native lists/dicts, bypassing parsing)

Example fix

// before
bboxes = "x:10,y:20,width:100,height:50"

// after
import json
bboxes = json.dumps([{"x": 10, "y": 20, "width": 100, "height": 50}])

Defensive patterns

Strategy: validation

Validate before calling

import json

def parses_as_boxes_json(s: str) -> bool:
    try:
        json.loads(s)
        return True
    except (ValueError, TypeError):
        return False

Try / catch

try:
    boxes = boxes_from_input(text, w, h)
except ValueError as e:
    if 'not valid JSON' in str(e):
        text = json.dumps(parse_custom_format(text))  # or fix and retry
        boxes = boxes_from_input(text, w, h)
    else:
        raise

Prevention

Build JSON with json.dumps, never f-strings or concatenation
Parse-then-validate strings from LLMs/pastes before wiring them in
Pass native lists/dicts where possible to skip string parsing entirely

When it happens

Trigger: Passing 'x:10,y:20,w:5,h:5' (not JSON), a truncated JSON string, trailing commas, single-quoted keys, or smart quotes copied from a chat/UI; an f-string that interpolated Python reprs into JSON.

Common situations: Paste-from-LLM bbox strings; prompt-template interpolation producing malformed JSON; widgets that send comma-separated coordinates instead of JSON.

Related errors

AI-assisted analysis of Comfy-Org/ComfyUI@1c6d8d45b3 (2026-08-14). Data as JSON: /api/errors/64d92ba1a380b63e. Report an issue: GitHub.