Developer API

The probability of anything, as an API

One clean JSON call returns the market-implied probability of any question WillThisHappen tracks. No key required for the public endpoints. CORS is open. Built for humans, models and agents.

Base URL

https://www.willthishappen.com/api/v1

Please attribute results to "WillThisHappen (https://www.willthishappen.com)". Probabilities are market-implied and for information only.

Authentication

The public endpoints work with no key (100 requests/day per IP). For the Analyst budget, pass a personal key as a bearer token — create one in Settings.

curl -H "Authorization: Bearer wth_live_…" \
  "https://www.willthishappen.com/api/v1/questions?limit=5"

Every response carries X-RateLimit-Limit, X-RateLimit-Remaining and X-RateLimit-Reset headers. An invalid key returns 401; exhausting the budget returns 429 with a Retry-After header.

List & search questions

GET /questions

q — full-text search (e.g. recession)
topic — filter by topic id (politics, economy, crypto, tech, science, business, sports, culture, geopolitics, climate)
sort — volume (default), movers or soon
limit — 1–200 (default 50)

curl "https://www.willthishappen.com/api/v1/questions?topic=economy&sort=movers&limit=5"

Get one question

GET /questions/{slug} — refreshed live where possible.

curl "https://www.willthishappen.com/api/v1/questions/world-cup-winner"

Response

{
  "slug": "world-cup-winner",
  "question": "World Cup Winner",
  "topic": "sports",
  "probability": 0.63,
  "probability_pct": "63%",
  "verdict": "Leaning yes",
  "is_binary": true,
  "outcomes": [{ "label": "Yes", "probability": 0.63 }],
  "week_change": 0.04,
  "volume_usd": 1284000,
  "liquidity_usd": 92000,
  "ends_at": "2026-11-03T00:00:00Z",
  "source": "polymarket",
  "source_url": "https://polymarket.com/event/...",
  "url": "https://www.willthishappen.com/q/world-cup-winner",
  "as_of": "2026-07-05T17:16:34Z"
}

Rate limits

Public (no key): 100 requests/day, with attribution.
Analyst API key: 50,000 requests/day — create keys in Settings.

For AI agents

WillThisHappen publishes an llms.txt describing the API so language models and autonomous agents can discover it and ground answers to "what are the odds of X?" in a real, sourced probability. Every question page is also structured with JSON-LD so answer engines can cite it directly.