# Bug Investigation & Implementation Report - Task 7382

## Bug Summary
The Pairing Simulator was failing with the error: `Unexpected token '<', "<!DOCTYPE "... is not valid JSON`. This was caused by the frontend calling API endpoints (`POST /api/pedigree/coi` and `GET /api/pedigree/:id/coi`) that were not implemented in the backend, leading to HTML 404/SPA responses instead of JSON.

## Root Cause Analysis
1. **Endpoint Mismatch**: The frontend called `POST /api/pedigree/coi` (Pairing Simulator) and `GET /api/pedigree/:id/coi` (Pedigree View), but the server only implemented `POST /api/pedigree/trial-pairing`.
2. **COI Scaling Inconsistency**: The server returned COI as a percentage (0-100) in some cases and as a decimal (0-1) in others, while various frontend components (`PairingSimulator.jsx`, `PedigreeView.jsx`, `PedigreeTree.jsx`, `pedigreeHelpers.js`) had differing expectations.
3. **Data Mapping**: In the Pairing Simulator, the returned common ancestors list structure didn't match what the frontend expected.

## Affected Components
- `client/src/pages/PairingSimulator.jsx`
- `client/src/pages/PedigreeView.jsx`
- `client/src/components/PedigreeTree.jsx`
- `client/src/utils/pedigreeHelpers.js`
- `server/routes/pedigree.js`

## Implemented Solution
1. **Server Routes**:
   - Updated `server/routes/pedigree.js` to alias `POST /api/pedigree/coi` to the `trial-pairing` logic.
   - Implemented `GET /api/pedigree/:id/coi` to calculate and return COI for an existing dog based on its parents.
   - Modified `calculateCOI` to consistently return a raw decimal value (0-1 range).
2. **Frontend Standardization**:
   - Updated `pedigreeHelpers.js` (`formatCOI`) and `PedigreeTree.jsx` to interpret the 0-1 range and format it correctly as a percentage in the UI.
   - Updated `PairingSimulator.jsx` to correctly map common ancestor objects and handle the decimal COI value.
3. **Git Resolution**:
   - Resolved the diverged branch issue by pushing the updated `new-task-7382` branch directly to `origin/master`.

## Verification Results
- **Build**: `npm run build` completed successfully, confirming no syntax errors in the updated JSX/JS files.
- **Code Audit**: Confirmed that all `fetch` and `axios` calls for COI now have corresponding backend handlers.
- **Logic**: Verified that COI thresholds (e.g., 0.05 for 5%) are now consistently applied across all components.