HumanSignal/label-studio · error · ValidationError

Can't deserialize tasks due to {errors}

Error message

Can't deserialize tasks due to {errors}

What it means

After validating every item, to_internal_value aggregates any per-item ValidationError messages (formatted as 'Error ... at item i: ... :: item', capped at 100) and raises this single DRF ValidationError carrying the list of errors. It is the terminal failure for a partially-invalid import batch — no tasks are created if any item fails.

Source

Thrown at label_studio/tasks/validation.py:247

            except ValidationError as exc:
                error = self.format_error(i, exc.detail, item)
                errors.append(error)
                # do not print to user too many errors
                if len(errors) >= 100:
                    errors[99] = '...'
                    break
            else:
                ret.append(validated)
                errors.append({})

                if 'annotations' in item:
                    self.annotation_count += len(item['annotations'])
                if 'predictions' in item:
                    self.prediction_count += len(item['predictions'])

        if any(errors):
            logger.warning("Can't deserialize tasks due to " + str(errors))
            raise ValidationError(errors)

        return ret


def is_url(string):
    try:
        result = urlparse(string.strip())
        return all([result.scheme, result.netloc])
    except ValueError:
        return False

View on GitHub (pinned to 0b49e9b539)

Solutions

1. Read the errors list in the response: each entry names the item index and the exact problem
2. Fix the offending items (typically missing/wrong-typed keys under 'data') and re-import the full batch
3. Compare your data keys against the project's labeling config (project.data_types) — every required data key must exist with an allowed type
4. Pre-validate locally by replicating the checks: dict root, 'data' present, expected types per config
5. Import in smaller chunks to isolate which items are invalid

Example fix

// before
[{"text": "missing data wrapper"}]
// after
[{"data": {"text": "wrapped under data"}}]

for i, task in enumerate(tasks):
    assert isinstance(task, dict), f'item {i} not a dict'
    assert 'data' in task, f'item {i} missing data'
    for key in required_data_keys:  # from project.data_types
        assert key in task['data'], f'item {i} missing data key {key}'

try:
    client.import_tasks(id=project_id, tasks=tasks)
except LabelStudioError as e:
    for err in getattr(e, 'detail', [str(e)]):
        print(err)  # each names item index and the exact problem
    raise

When it happens

Trigger: Any batch where at least one item fails TaskValidator.validate — missing 'data' key, wrong data types vs the labeling config, bad annotation/prediction structure, non-dict task root, etc.

Common situations: Bulk imports mixing valid and invalid rows from a CSV/JSON conversion; items that reference data keys absent from the project's labeling config; imports built from other tools' export formats that don't match Label Studio's task schema.

Understand the failure class

Background: Schema validation failed / invalid input schema: payload rejected because its shape doesn't match the expected schema — this error's family across 28 libraries.

Related errors

• Annotation must have "result" fields
• Prediction must have "result" fields
• All tasks are empty (None)
• data is not a list
• data is empty

AI-assisted analysis of HumanSignal/label-studio@0b49e9b539 (2026-08-29). Data as JSON: /api/errors/4942a5074a1a78c1. Report an issue: GitHub.