WORK_LOG.md

@@ -40,10 +40,6 @@ The routing method selection logic was updated so perpendicular port-angle cases
 mxpic_router/mxpic_router/builder.py
 mxpic_router/tests/test_eda_router_contract.py
 
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 ## 2026-06-04
 
 ### Scope
@@ -93,3 +89,49 @@ Contract tests were added and updated to cover automatic S-bend `Lstart` assignm
 mxpic_router/mxpic_router/builder.py
 mxpic_router/tests/test_eda_router_contract.py
 
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+## 2026-06-05
+
+### Scope
+
+Backend S-bend grouping contract and `Lstart` ordering refinement in `mxpic_router`.
+
+### Summary
+
+Recent work focused on improving automatic S-bend spacing behavior for multi-port routing cases, especially MMI-to-port and resistor-array routing patterns.
+
+### Completed Work
+
+#### 1. Explicit Route Group Metadata Support
+
+Backend support was added for explicit link grouping metadata from the layout YAML.
+
+`LinkSpec` now records the parent bundle name and accepts optional route grouping fields such as `route_group`, `routeGroup`, `group`, `bundle_group`, and `bundleGroup`.
+
+This prepares the backend for future frontend-provided bundle/group selection while preserving the existing geometry-based fallback behavior.
+
+#### 2. S-Bend Stage Grouping Refinement
+
+The S-bend grouping key was refined to avoid mixing routes from different routing stages within the same YAML bundle.
+
+Automatic S-bend routes are now grouped by routing axis, direction, route stage/span, xsection, width, and radius unless an explicit route group is provided.
+
+This prevents separate stages such as component-to-component routes and component-to-port routes from interfering with each other's `Lstart` ordering.
+
+#### 3. Lstart Ordering Correction
+
+The `Lstart` ordering logic was reviewed for resistor-array and MMI-to-port routing cases.
+
+The sorting basis was corrected to use the source-side route coordinate for same-direction fan-out/fan-in routes, and S-bend axis detection was updated to prefer port direction over raw geometric span.
+
+This fixes cases where routes with large vertical offset were incorrectly classified and assigned reversed `Lstart` values.
+
+#### 4. Test Coverage Update
+
+Contract tests were added and updated to cover explicit route group metadata parsing, route-stage separation, MMI-to-port two-output fan-out ordering, width-aware `Lstart` spacing, and preservation of earlier S-bend spacing behavior.
+
+### Modified Files
+
+mxpic_router/mxpic_router/eda_loader.py
+mxpic_router/mxpic_router/builder.py
+mxpic_router/tests/test_eda_router_contract.py