# Investigation: Bug in Pairing Simulator

## Bug Summary
In the Pairing Simulator page, clicking the "Simulate Pairing" button results in the following error:
`Unexpected token '<', "<!--DOCTYPE "... is not valid JSON`

## Root Cause Analysis
The frontend `PairingSimulator.jsx` makes a POST request to `/api/pedigree/coi` when simulating a pairing. However, the backend `server/routes/pedigree.js` does not define a `/coi` route. Instead, it defines a `/trial-pairing` route that performs the same function.

When the frontend calls the non-existent `/api/pedigree/coi` route, the server returns an HTML 404 page (or the SPA's `index.html` if in production). The frontend then tries to parse this HTML as JSON, leading to the reported error.

Additionally, `PedigreeView.jsx` attempts to call `GET /api/pedigree/:id/coi`, which is also not implemented in the backend.

## Affected Components
- `client/src/pages/PairingSimulator.jsx`: Calls `/api/pedigree/coi` (POST).
- `client/src/pages/PedigreeView.jsx`: Calls `/api/pedigree/:id/coi` (GET).
- `server/routes/pedigree.js`: Missing route definitions for `/coi` and `/:id/coi`.

## Proposed Solution
1. Update `server/routes/pedigree.js` to:
   - Alias `POST /api/pedigree/coi` to the existing `trial-pairing` logic.
   - Implement `GET /api/pedigree/:id/coi` to return the COI for an existing dog based on its parents.
2. Ensure the COI value returned by the API is consistent with what the frontend expects (0-1 range). Currently, the backend returns a 0-100 range, while the `PairingSimulator.jsx` expects 0-1 and multiplies by 100 in the UI.

## Implementation Plan
1.  **Backend Changes**:
    -   Modify `server/routes/pedigree.js` to add `router.post('/coi', ...)` using the same logic as `trial-pairing`.
    -   Add `router.get('/:id/coi', ...)` to `server/routes/pedigree.js`.
    -   Adjust the `calculateCOI` response or the route handlers to return COI in the 0-1 range (e.g. `0.05` for 5%) to match `PairingSimulator.jsx`'s expectation.
2.  **Frontend Cleanup**:
    -   Check if `PedigreeView.jsx` and `pedigreeHelpers.js` need adjustments once the backend returns the 0-1 range. `formatCOI` in `pedigreeHelpers.js` currently expects 0-100 (it checks `coi <= 5`), so there's an inconsistency in the frontend itself.