TradingAgents/docs/agent/decisions/005-langgraph-parallel-reducers.md

type	status	date	agent_author	tags	related_files
decision	active	2026-03-17	claude	langgraph state parallel scanner	tradingagents/agents/utils/scanner_states.py

Context

Phase 1 runs 3 scanners in parallel. All write to shared state fields (sender, etc.). LangGraph requires reducers for concurrent writes — otherwise raises INVALID_CONCURRENT_GRAPH_UPDATE.

The Decision

Added _last_value reducer to all ScannerState fields via Annotated[str, _last_value].

Constraints

• Any LangGraph state field written by parallel nodes MUST have a reducer.

Actionable Rules

• When adding new fields to ScannerState, always use Annotated[type, reducer_fn].
• Test parallel execution paths to verify no concurrent write errors.