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+# Issue #409 Design - KR Session-Aware Exchange Routing
+
+## Context
+- Issue: #409 (bug: KR 세션별 거래소 미분리 - 스크리닝/주문/이중상장 우선순위 미처리)
+- Related runtime observation targets: #318, #325
+- Date: 2026-03-04
+- Confirmed approach: Option 2 (routing module introduction)
+
+## Goals
+1. Ensure domestic screening uses session-specific exchange market code.
+2. Ensure domestic order submission explicitly sets exchange routing code.
+3. Add dual-listing routing priority logic (spread/liquidity aware) with safe fallback.
+4. Keep existing behavior stable for non-KR flows and existing risk/order policy guards.
+5. Enable runtime observability for #409 while monitoring #318/#325 in parallel.
+
+## Non-Goals
+- Replacing current session classification model.
+- Introducing new market sessions or changing session boundaries.
+- Refactoring overseas order flow.
+
+## Architecture
+### New Component
+- Add `KRExchangeRouter` (new module, e.g. `src/broker/kr_exchange_router.py`).
+- Responsibility split:
+  - `classify_session_id`: session classification only.
+  - `KRExchangeRouter`: final domestic exchange selection (`KRX`/`NXT`) for ranking and order.
+  - `KISBroker`: inject resolved routing values into request params/body.
+
+### Integration Points
+- `KISBroker.fetch_market_rankings`
+  - Session-aware market division code:
+    - `KRX_REG` -> `J`
+    - `NXT_PRE`, `NXT_AFTER` -> `NX`
+- `KISBroker.send_order`
+  - Explicit `EXCG_ID_DVSN_CD` is always set.
+- `SmartVolatilityScanner._scan_domestic`
+  - Ensure domestic ranking API path resolves exchange consistently with current session.
+
+## Data Flow
+1. Scanner path:
+   - Determine `session_id`.
+   - `resolve_for_ranking(session_id)`.
+   - Inject `J` or `NX` into ranking API params.
+2. Order path:
+   - Pass `session_id` into order path.
+   - `resolve_for_order(stock_code, session_id)`.
+   - Single listing: session default exchange.
+   - Dual listing: select by spread/liquidity heuristic when data is available.
+   - Data unavailable/error: fallback to session default.
+   - Send order with explicit `EXCG_ID_DVSN_CD`.
+3. Observability:
+   - Log `session_id`, `resolved_exchange`, `routing_reason`.
+
+## Dual-Listing Routing Priority
+- Preferred decision source: spread/liquidity comparison.
+- Deterministic fallback: session-default exchange.
+- Proposed reasons in logs:
+  - `session_default`
+  - `dual_listing_spread`
+  - `dual_listing_liquidity`
+  - `fallback_data_unavailable`
+
+## Error Handling
+- Router does not block order path when auxiliary data is unavailable.
+- Fail-open strategy for routing selection (fallback to session default) while preserving existing API/network error semantics.
+- `send_order` exchange field omission is forbidden by design after this change.
+
+## Testing Strategy
+### Unit
+- Router mapping by session (`KRX_REG`, `NXT_PRE`, `NXT_AFTER`).
+- Dual-listing routing priority and fallback.
+- Broker order body includes `EXCG_ID_DVSN_CD`.
+- Ranking params use session-aware market code.
+
+### Integration/Regression
+- `smart_scanner` domestic calls align with session exchange.
+- Existing order policy tests remain green.
+- Re-run regression sets covering #318/#325 related paths.
+
+### Runtime Observation (24h)
+- Restart program from working branch build.
+- Run runtime monitor for up to 24h.
+- Verify and track:
+  - #409: session-aware routing evidence in logs.
+  - #318: ATR dynamic stop evidence.
+  - #325: ATR/pred_down_prob injection evidence.
+- If anomalies are detected during monitoring, create separate issue tickets with evidence and links.
+
+## Acceptance Criteria
+1. No domestic ranking call uses hardcoded KRX-only behavior across NXT sessions.
+2. No domestic order is sent without `EXCG_ID_DVSN_CD`.
+3. Dual-listing path has explicit priority logic and deterministic fallback.
+4. Tests pass for new and affected paths.
+5. Runtime monitor evidence is collected for #409, #318, #325; anomalies are ticketed.
+
+## Risks and Mitigations
+- Risk: Increased routing complexity introduces regressions.
+  - Mitigation: isolate router, high-coverage unit tests, preserve existing interfaces where possible.
+- Risk: Runtime events for #318/#325 may not naturally occur in 24h.
+  - Mitigation: mark as `NOT_OBSERVED` and keep issue state based on evidence policy; do not force-close without proof.
+
+## Planned Next Step
+- Invoke `writing-plans` workflow and produce implementation plan before code changes.