langflow-ai/langflow · error · HTTPException

An error occurred while preparing the flow.

Error message

An error occurred while preparing the flow.

What it means

The streaming executor's preparation failure branch: load_graph_for_execution raised json.JSONDecodeError, OSError, or ValueError, and it is mapped to a generic HTTP 500 ('An error occurred while preparing the flow.'). The concrete cause is logged as 'Flow preparation error: {e}' server-side. These are infrastructure/flow-file problems: unreadable or corrupt flow files, invalid JSON, or invalid flow structure.

Source

Thrown at src/backend/base/langflow/agentic/services/flow_executor.py:214

        HTTPException: If flow file not found or execution fails
    """
    flow_path, flow_type = resolve_flow_path(flow_filename)

    try:
        graph = await load_graph_for_execution(
            flow_path,
            flow_type,
            provider,
            model_name,
            api_key_var,
            provider_vars=global_variables,
        )
    except CustomComponentValidationError as e:
        logger.error(f"Flow preparation error: {e}")
        raise HTTPException(status_code=400, detail=str(e)) from e
    except (json.JSONDecodeError, OSError, ValueError) as e:
        logger.error(f"Flow preparation error: {e}")
        raise HTTPException(status_code=500, detail="An error occurred while preparing the flow.") from e

    event_queue: asyncio.Queue[tuple[str, bytes, float] | None] = asyncio.Queue(maxsize=STREAMING_QUEUE_MAX_SIZE)
    event_manager = create_default_event_manager(event_queue)
    execution_result = FlowExecutionResult()

    flow_task = asyncio.create_task(
        _run_graph_with_events(
            graph=graph,
            input_value=input_value,
            global_variables=global_variables,
            user_id=user_id,
            session_id=session_id,
            event_manager=event_manager,
            event_queue=event_queue,
            execution_result=execution_result,
        )
    )

View on GitHub (pinned to 976ec789d2)

Solutions

1. Check backend logs for 'Flow preparation error:' to see which of JSONDecodeError/OSError/ValueError fired.
2. Validate the flow file: JSON parses cleanly (jq/python -m json.tool) and is readable by the server process.
3. Re-deploy or re-export the flow file atomically (write temp + rename) so readers never see a partial file.
4. Fix filesystem permissions on FLOWS_BASE_PATH if OSError.

import json, pathlib

def flow_file_loadable(p: pathlib.Path) -> bool:
    try:
        if p.suffix == '.json':
            json.loads(p.read_text(encoding='utf-8'))
        return p.is_file() and os.access(p, os.R_OK)
    except (json.JSONDecodeError, OSError):
        return False

except HTTPException as e:
    if e.status_code == 500 and 'preparing the flow' in e.detail:
        alert_ops('flow file unreadable/corrupt — check deploy pipeline and permissions')

When it happens

Trigger: Streaming execute where the flow file under FLOWS_BASE_PATH is missing mid-request, has corrupted JSON (truncated write), cannot be read due to permissions (OSError), or yields a ValueError during graph construction.

Common situations: Flow file partially written by a deploy job; file permissions changed on the flows directory; disk-full truncation; concurrent deploy overwriting the file being read; flow JSON exported with a BOM or encoding issue.

Related errors

• An error occurred while executing the flow.
• An internal error occurred while executing the flow.
• Could not load flow module: {flow_path}
• Error loading flow module: {e}
• Error building Component

AI-assisted analysis of langflow-ai/langflow@976ec789d2 (2026-08-14). Data as JSON: /api/errors/8caca7d9604e2088. Report an issue: GitHub.