# 400: Invalid Option Symbol

Use this page when you receive a `400 BAD REQUEST` with:

```json
{
  "s": "error",
  "errmsg": "Invalid option symbol. Use valid OCC symbology for the option symbol."
}
```

If you need the broader `400` troubleshooting flow, start here:
- [400: Bad Request](https://www.marketdata.app/docs/api/troubleshooting/bad-request)

## What This Error Means

The endpoint and query string can be valid, but the `optionSymbol` itself is not valid OCC format.

## OCC Option Symbol Format

Expected format:

`UNDERLYING + YYMMDD + C/P + STRIKE`

Example:

- `AAPL271217C00250000`

Field rules:

1. `UNDERLYING`: Root symbol (for example `AAPL`, `SPY`, `INTC`)
2. `YYMMDD`: Expiration date (2-digit year, month, day)
3. `C/P`: `C` for call or `P` for put
4. `STRIKE`: 8 digits, zero-padded, implied 3 decimal places

## Strike Encoding Reference

The strike is represented as an integer with three implied decimal places.

Examples:

- `150` becomes `00150000`
- `32.5` becomes `00032500`
- `32.25` becomes `00032250`

## Common Construction Errors

1. Expiration is not in `YYMMDD`
2. Missing call/put flag (`C` or `P`)
3. Strike field is not 8 digits
4. Strike is not zero-padded
5. Decimal strike is encoded incorrectly
6. Legacy pre-2010 style symbol is used

## Validate Before Calling Quotes/History

Use the lookup endpoint to build/validate a symbol first.

```http
# Build/validate OCC symbol from human-readable contract
GET https://api.marketdata.app/v1/options/lookup/AAPL%201/17/2025%20150%20Call

# Then request data with the returned optionSymbol
GET https://api.marketdata.app/v1/options/quotes/AAPL271217C00250000/
```

## 400 vs 404 for Options

- `400`: the option symbol format is invalid
- `404`: format is valid, but no matching contract/data was found

## Quick Debug Checklist

1. Log the exact `optionSymbol` your client sent
2. Check `YYMMDD`, `C/P`, and 8-digit strike encoding
3. Validate with `options/lookup`
4. Reuse the exact returned symbol in follow-up requests