A controllable SPX expected-move and cone chart
Recorded in cvforge on June 15, 2026 by ConvexValue · Published July 25, 2026
Expected move and probability cone for SPX with the horizon under your control — defaulting to 30 days, adjustable to whatever window you are trading.
Exact prompt
Create an expected move and probability cone chart for $SPX. I want to see the expected move, 1 and 2 std. It should default to 30 days but I want to be able to control it.
Data cadence
Live U.S. option snapshots, refreshed through the session. Open interest is an end-of-previous-day figure and updates once daily.
Plan requirements
Runs on the free cvforge app and the free data tier (20 requests/hour). No card needed.
Fields used
implied_volatility, delta, strike_price, expiration_date, underlying_price, fair_market_value
How to read it
Asking for a control rather than a fixed window is the difference between a chart and a tool. The same prompt style — “default to X but let me change it” — is the most reliable way to get something reusable out of a first build.
Moving the horizon also teaches the square-root relationship faster than reading about it. Doubling the days does not double the width, and watching the cone respond makes that concrete.
Change it
The prompt is the interface. Ask for a different ticker, a different expiration window, or an extra column in the same sentence style and the agent edits the app in place — you do not start over, and you do not wait for anyone to build it.
Where it went next
The build did not stop at the first prompt. Each follow-up below was typed into the same session, against the same running app.
Follow-up 1
Follow-up prompt 1
Overlay the probability distribution mark the 1 and 2 sigma expected move add a today P&L curve
Limitations
- Snapshot data, not a transaction tape: the build reads the chain as it stands, so it cannot attribute a trade to a buyer or a seller.
- Open interest updates once a day, so an intraday volume/OI comparison is always this session’s volume against yesterday’s open interest.
- Contract coverage is U.S. listed equities, ETFs and indices. Indices use the I: prefix, for example I:SPX.
- Current per-contract valuation is Massive fair market value (FMV), a proprietary estimate. cvforge does not provide option bids, asks, midpoint, quote sizes, NBBO or last trades, and FMV is not an executable quote.
- These are risk-neutral probabilities implied by option prices, not forecasts. They shift with volatility as much as with expectations.
- Anything derived from implied volatility inherits the model behind it; treat the numbers as a market-implied range, not a measurement.
Related demos
- MU below 600 or above 1000: two-sided implied odds
- Comparing expected move: NVDA against MU
- A year of SPX implied cones against realised outcomes
Search focus: adjustable probability cone chart. Related: spx cone with time control, expected move variable horizon. Originally posted on X.