{"record":{"id":"8caca7d9604e2088","repo":"langflow-ai/langflow","slug":"an-error-occurred-while-preparing-the-flow","errorCode":null,"errorMessage":"An error occurred while preparing the flow.","messagePattern":"An error occurred while preparing the flow\\.","errorType":"http","errorClass":"HTTPException","httpStatus":500,"severity":"error","filePath":"src/backend/base/langflow/agentic/services/flow_executor.py","lineNumber":214,"sourceCode":"        HTTPException: If flow file not found or execution fails\n    \"\"\"\n    flow_path, flow_type = resolve_flow_path(flow_filename)\n\n    try:\n        graph = await load_graph_for_execution(\n            flow_path,\n            flow_type,\n            provider,\n            model_name,\n            api_key_var,\n            provider_vars=global_variables,\n        )\n    except CustomComponentValidationError as e:\n        logger.error(f\"Flow preparation error: {e}\")\n        raise HTTPException(status_code=400, detail=str(e)) from e\n    except (json.JSONDecodeError, OSError, ValueError) as e:\n        logger.error(f\"Flow preparation error: {e}\")\n        raise HTTPException(status_code=500, detail=\"An error occurred while preparing the flow.\") from e\n\n    event_queue: asyncio.Queue[tuple[str, bytes, float] | None] = asyncio.Queue(maxsize=STREAMING_QUEUE_MAX_SIZE)\n    event_manager = create_default_event_manager(event_queue)\n    execution_result = FlowExecutionResult()\n\n    flow_task = asyncio.create_task(\n        _run_graph_with_events(\n            graph=graph,\n            input_value=input_value,\n            global_variables=global_variables,\n            user_id=user_id,\n            session_id=session_id,\n            event_manager=event_manager,\n            event_queue=event_queue,\n            execution_result=execution_result,\n        )\n    )\n","sourceCodeStart":196,"sourceCodeEnd":232,"githubUrl":"https://github.com/langflow-ai/langflow/blob/976ec789d2886a86de109c044d089d68e96c9a35/src/backend/base/langflow/agentic/services/flow_executor.py#L196-L232","documentation":"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.","triggerScenarios":"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.","commonSituations":"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.","solutions":["Check backend logs for 'Flow preparation error:' to see which of JSONDecodeError/OSError/ValueError fired.","Validate the flow file: JSON parses cleanly (jq/python -m json.tool) and is readable by the server process.","Re-deploy or re-export the flow file atomically (write temp + rename) so readers never see a partial file.","Fix filesystem permissions on FLOWS_BASE_PATH if OSError."],"exampleFix":null,"handlingStrategy":"try-catch","validationCode":"import json, pathlib\n\ndef flow_file_loadable(p: pathlib.Path) -> bool:\n    try:\n        if p.suffix == '.json':\n            json.loads(p.read_text(encoding='utf-8'))\n        return p.is_file() and os.access(p, os.R_OK)\n    except (json.JSONDecodeError, OSError):\n        return False","typeGuard":null,"tryCatchPattern":"except HTTPException as e:\n    if e.status_code == 500 and 'preparing the flow' in e.detail:\n        alert_ops('flow file unreadable/corrupt — check deploy pipeline and permissions')","preventionTips":["Deploy flow files atomically: write to a temp name then rename into FLOWS_BASE_PATH.","Add a post-deploy smoke check that parses every .json flow file.","Ensure the server process owns read permissions on the flows directory."],"tags":["agentic","streaming","http-500","flow-file","io"],"backgroundTag":null,"analyzedSha":"976ec789d2886a86de109c044d089d68e96c9a35","analyzedAt":"2026-08-14T18:23:12.227Z","schemaVersion":2},"datasetVersion":"2026-08-15T22:17:37.221Z"}